tag:theconversation.com,2011:/us/topics/paleoclimatology-5493/articlesPaleoclimatology – The Conversation2023-04-13T18:03:53Ztag:theconversation.com,2011:article/2035322023-04-13T18:03:53Z2023-04-13T18:03:53ZWooded grasslands flourished in Africa 21 million years ago – new research forces a rethink of ape evolution<figure><img src="https://images.theconversation.com/files/520610/original/file-20230412-18-2l6ftt.png?ixlib=rb-1.1.0&rect=5%2C808%2C3552%2C3217&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An ape that lived 21 million years ago was used to a habitat that was both grassy and wooded.</span> <span class="attribution"><span class="source">Corbin Rainbolt</span></span></figcaption></figure><p>Human evolution is tightly connected to the environment and landscape of Africa, <a href="https://humanorigins.si.edu/evidence/human-fossils/species/sahelanthropus-tchadensis">where our ancestors first emerged</a>.</p>
<p>According to the traditional scientific narrative, Africa was once a verdant idyll of vast forests stretching from coast to coast. In these lush habitats, around 21 million years ago, the earliest ancestors of apes and humans first evolved traits – including upright posture – that distinguished them from their monkey cousins.</p>
<p>But then, the story went, global climates cooled and dried, and forests began to shrink. By about 10 million years ago, grasses and shrubs that were better able to tolerate the increasingly dry conditions started to take over eastern Africa, replacing forests. The earliest hominins, our distant ancestors, ventured out of the forest remnants that had been home onto the grass-covered savanna. The idea was that this new ecosystem pushed a radical change for our lineage: We became bipedal.</p>
<p>For a long time, researchers have <a href="https://doi.org/10.1038/s41598-020-69378-0">linked the expansion of grasslands in Africa</a> to the evolution of numerous human traits, including walking on two legs, using tools and hunting.</p>
<p>Despite the prominence of this theory, mounting evidence from paleontological and paleoclimatological research undermines it. <a href="https://www.science.org/doi/10.1126/science.abq2834">In two</a> <a href="https://www.science.org/doi/10.1126/science.abq2835">recent papers</a>, our multidisciplinary team of Kenyan, Ugandan, European <a href="https://scholar.google.com/citations?user=DjYvbR8AAAAJ&hl=en">and</a> <a href="https://scholar.google.com/citations?user=W7H_Y0oAAAAJ&hl=en&oi=ao">American</a> <a href="https://scholar.google.com/citations?user=gwZCXkQAAAAJ&hl=en&oi=ao">scientists</a> concluded that it is time finally to discard this version of the evolutionary story.</p>
<p>A decade ago, we began what, at the time, was a unique experiment in paleoanthropology: Several independent research teams joined together to build a regional perspective on the evolution and diversification of early apes. The project, dubbed REACHE, short for Research on Eastern African Catarrhine and Hominoid Evolution, was based on the premise that conclusions drawn from evidence across many locations would be more powerful than interpretations from individual fossil sites. We wondered whether previous researchers had missed the forest for the trees.</p>
<h2>An ape in Uganda 21 million years ago</h2>
<p>Based on the lifestyle of apes alive today, scientists have hypothesized that the very first ones evolved in dense forests, <a href="https://doi.org/10.1086/284139">where they successfully fed on fruit</a>, thanks to a few key anatomical innovations.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/QKuyv6YdBx4?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Chimpanzees move with an upright posture.</span></figcaption>
</figure>
<p>Apes have stable, upright backs. <a href="https://doi.org/10.1007/978-3-319-47829-6_2001-1">Once the back is vertical</a>, an ape no longer has to walk on the top of small branches like a monkey. Instead, it can grab different branches with its arms and legs, distributing its body mass across multiple supports. Apes can even hang below branches, making them less likely to lose their balance. In this way, they are able to access fruits growing on the edges of tree crowns that otherwise might be available only to smaller species.</p>
<p>But was this scenario true for the earliest apes? A 21 million-year-old site in Moroto, Uganda, became an ideal place to investigate this question. There our REACHE team discovered teeth and other remains belonging to <em>Morotopithecus</em>, the oldest ape for which scientists have found fossils from the cranium, teeth and other parts of the skeleton.</p>
<p>Two bones in particular helped us understand how this species moved. A lower backbone found decades ago and curated by the Uganda National Museum had already been noted for its <a href="https://doi.org/10.1006/jhev.1994.1012">bony attachments for back muscles</a>, indicating that <em>Morotopithecus</em> had a stiff lower back, good for climbing upright in the trees.</p>
<p>A discovery of our own confirmed this climbing behavior in a major way. At Moroto we found a fossil ape thigh bone that is short but strong, with a very thick shaft. This kind of bone is characteristic of living apes and helps them climb up and down trees with a vertical torso.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/520652/original/file-20230413-20-yvp928.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="vertebra, partial jaw and femur fossils" src="https://images.theconversation.com/files/520652/original/file-20230413-20-yvp928.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/520652/original/file-20230413-20-yvp928.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=240&fit=crop&dpr=1 600w, https://images.theconversation.com/files/520652/original/file-20230413-20-yvp928.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=240&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/520652/original/file-20230413-20-yvp928.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=240&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/520652/original/file-20230413-20-yvp928.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=302&fit=crop&dpr=1 754w, https://images.theconversation.com/files/520652/original/file-20230413-20-yvp928.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=302&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/520652/original/file-20230413-20-yvp928.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=302&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Three fossilized bones from <em>Morotopithecus</em>: a vertebra, part of a jaw and a femur.</span>
<span class="attribution"><span class="source">L. MacLatchy and J. Kingston</span></span>
</figcaption>
</figure>
<p>Although both skeletal fossils are consistent with the fruit-eating, forest-dwelling ape hypothesis, <a href="https://www.science.org/doi/10.1126/science.abq2835">we found something astonishing</a> when we discovered an ape lower jaw fragment in the same excavation layer. Its molars were elongated, with well-developed shearing crests running between the cusps. These ridges are ideal for slicing leaves but are unlike the low, round, crushing tooth cusps of committed fruit eaters. If ape skeletal adaptations evolved in forests to aid in fruit exploitation, why would the earliest ape showing these locomotor features instead have teeth like a leaf eater’s?</p>
<p>Such inconsistencies between our evidence and the traditional narrative of ape origins led us to question other assumptions: Did <em>Morotopithecus</em> live in a forested habitat at all? </p>
<h2>The environment at Moroto</h2>
<p>To figure out <em>Morotopithecus’</em> habitat, we studied the chemistry of fossil soils – called paleosols – and the microscopic remains of plants they contain in order to reconstruct the ancient climate and vegetation at Moroto.</p>
<p>Trees and most shrubs and nontropical grasses are classified as C₃ plants, based on the type of photosynthesis they perform. Tropical grasses, which rely on a different photosynthetic system, are known as C₄ plants. Importantly, C₃ plants and C₄ plants differ in the proportions of the various carbon <a href="https://www.britannica.com/science/isotope">isotopes</a> they take in. That means carbon isotope ratios preserved in the paleosols can tell us the composition of the ancient vegetation.</p>
<p>We measured three distinct carbon isotope signatures, each providing a different perspective on the plant community: carbon resulting from decomposition of vegetation and soil microbes; carbon resulting from plant waxes; and calcium carbonate nodules formed in soils through evaporation.</p>
<p>Although each proxy gave us slightly different values, <a href="https://www.science.org/doi/10.1126/science.abq2834">they converged on a single remarkable story</a>. Moroto was not a closed forest habitat but rather a relatively open woodland environment. What’s more, we found evidence of abundant C₄ plant biomass – tropical grasses.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/520653/original/file-20230413-16-s1no2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Traditional versus updated view of early ape habitat and evolution" src="https://images.theconversation.com/files/520653/original/file-20230413-16-s1no2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/520653/original/file-20230413-16-s1no2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=484&fit=crop&dpr=1 600w, https://images.theconversation.com/files/520653/original/file-20230413-16-s1no2w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=484&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/520653/original/file-20230413-16-s1no2w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=484&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/520653/original/file-20230413-16-s1no2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=608&fit=crop&dpr=1 754w, https://images.theconversation.com/files/520653/original/file-20230413-16-s1no2w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=608&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/520653/original/file-20230413-16-s1no2w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=608&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">(A) Forested ecosystem traditionally believed to be the habitat of early apes, which ate fruit at the ends of tree branches, compared with (B) new perspective of grassy woodland ecosystem reconstruction, where early apes lived in open habitats and fed on leaves.</span>
<span class="attribution"><span class="source">Figure modified with permission from MacLatchy et al., Science 380, eabq2835 (2023)</span></span>
</figcaption>
</figure>
<p>This discovery was a revelation. C₄ grasses lose less water during photosynthesis than C₃ trees and shrubs do. Today, C₄ grasses dominate seasonally dry savanna ecosystems that <a href="https://unesdoc.unesco.org/ark:/48223/pf0000058054">cover more than half of Africa</a>. But scientists hadn’t thought the levels of C₄ biomass we measured at Moroto had evolved in Africa until 10 million years ago. Our data suggests it happened twice as far back in time, 21 million years ago.</p>
<p>Our colleagues <a href="https://scholar.google.com/citations?user=A5RCBfEAAAAJ&hl=en&oi=ao">Caroline Strömberg</a>, Alice Novello and <a href="https://scholar.google.com/citations?user=Sg0Q5xkAAAAJ&hl=en&oi=ao">Rahab Kinyanjui</a> used another line of evidence to corroborate the abundance of C₄ grasses at Moroto. They analyzed phytoliths, tiny silica bodies created by plant cells, preserved in the paleosols. Their results supported an open woodland and wooded grassland environment for this time and place.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/520540/original/file-20230412-20-hnv0wy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Early Miocene grass phytoliths" src="https://images.theconversation.com/files/520540/original/file-20230412-20-hnv0wy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/520540/original/file-20230412-20-hnv0wy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/520540/original/file-20230412-20-hnv0wy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/520540/original/file-20230412-20-hnv0wy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/520540/original/file-20230412-20-hnv0wy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/520540/original/file-20230412-20-hnv0wy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/520540/original/file-20230412-20-hnv0wy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Example of typical grass phytoliths, extracted from paleosol at one of the sites, some of which indicate the presence of C₄ grass.</span>
<span class="attribution"><span class="source">Alice Novello</span></span>
</figcaption>
</figure>
<p>Taken together, this evidence dramatically contradicts the traditional view of ape origins – that apes evolved upright torsos to reach fruit in forest canopies. Instead, <em>Morotopithecus</em>, the earliest known ape with upright locomotion, consumed leaves and inhabited an open woodland with grassy areas.</p>
<h2>A new, regional view of early ape habitats</h2>
<p>Through the REACHE project, we applied the same approach to reconstruct habitats at eight other fossil sites in Kenya and Uganda, ranging in age from around 16 million to 21 million years old. After all, <em>Morotopithecus</em> is only one of several apes that lived during this time period.</p>
<p>To our surprise, we discovered that the ecological signal measured at Moroto was not unique. Instead, it was part of a broader pattern in eastern Africa during this time. </p>
<p>Our isotopic proxies at each fossil site contributed two significant revelations. First, vegetation types ranged from closed canopy forests to open wooded grasslands. And second, every site had a mixture of C₃ and C₄ vegetation, with some locations having a high proportion of C₄ grass biomass. Phytoliths from the same paleosols again corroborated that abundant C₄ grasses were present at multiple sites. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/520539/original/file-20230412-20-ys57ae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="cartoon depictions of nine paleoenvironments placed on timeline" src="https://images.theconversation.com/files/520539/original/file-20230412-20-ys57ae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/520539/original/file-20230412-20-ys57ae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=274&fit=crop&dpr=1 600w, https://images.theconversation.com/files/520539/original/file-20230412-20-ys57ae.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=274&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/520539/original/file-20230412-20-ys57ae.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=274&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/520539/original/file-20230412-20-ys57ae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=345&fit=crop&dpr=1 754w, https://images.theconversation.com/files/520539/original/file-20230412-20-ys57ae.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=345&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/520539/original/file-20230412-20-ys57ae.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=345&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Paleoenvironments for the nine fossil sites analyzed range from closed canopy forest to more open wooded grassland environments. Inset map shows the geographic location of sites in eastern Africa.</span>
<span class="attribution"><span class="source">Dan Peppe</span></span>
</figcaption>
</figure>
<p>The realization that such a variety of environments, especially open habitats with C₄ grasses, was present at the dawn of the apes forces a reassessment not just of the evolution of apes but of humans and other African mammals. Although some studies had suggested such habitat variation was present across Africa, our project was able to confirm it, repeatedly, within the very habitats that early apes and their animal contemporaries occupied.</p>
<p>Because the timing of the assembly of Africa’s grassland habitats underlies many evolutionary hypotheses, our discovery that they existed much earlier than expected calls for a recalibration of those ideas.</p>
<p>Regarding human origins, our study adds to a growing body of evidence that our divergence from apes – in anatomy, ecology, behavior – cannot be simply explained by the appearance of grassland habitats. Nevertheless, we cautiously remind ourselves that hominin evolution unfolded over many millions of years. It is almost certain that the vast and majestic grasslands of Africa played an important role in some of the many steps along the path to becoming human.</p><img src="https://counter.theconversation.com/content/203532/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Laura M MacLatchy receives funding from the US National Science Foundation, the LSB Leakey Foundation, the Wenner-Gren Foundation and the University of Michigan. </span></em></p><p class="fine-print"><em><span>Dan Peppe receives funding from National Science Foundation, Leakey Foundation, Baylor University.</span></em></p><p class="fine-print"><em><span>Kieran McNulty has received funding from the National Science Foundation, Leakey Foundation, Wenner-Gren Foundation, Leverhulme Foundation, and the University of Minnesota. </span></em></p>Contrary to the idea that apes evolved their upright posture to reach for fruit in the forest canopy, the earliest known ape with this stature, Morotopithecus, lived in more open grassy environments.Laura M. MacLatchy, Professor of Anthropology, University of MichiganDan Peppe, Associate Professor of Geosciences, Baylor UniversityKieran McNulty, Professor of Anthropology, University of MinnesotaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1977882023-02-15T16:00:14Z2023-02-15T16:00:14ZWas Earth already heating up, or did global warming reverse a long-term cooling trend?<figure><img src="https://images.theconversation.com/files/508700/original/file-20230207-21-1bdmxo.jpg?ixlib=rb-1.1.0&rect=166%2C39%2C5107%2C3328&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Natural records suggest a cooling trend was underway thousands of years ago.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/ploughing-scene-wall-painting-tomb-of-sennedjem-valley-of-news-photo/475591523">DeAgostini/Getty Images</a></span></figcaption></figure><p>Over the past century, the Earth’s average temperature has swiftly <a href="https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature">increased by about 1 degree Celsius</a> (1.8 degrees Fahrenheit). The evidence is hard to dispute. It comes from thermometers and other sensors around the world.</p>
<p>But what about the thousands of years before the Industrial Revolution, before thermometers, and before humans warmed the climate by <a href="https://climate.nasa.gov/faq/19/what-is-the-greenhouse-effect/">releasing heat-trapping carbon dioxide from fossil fuels</a>?</p>
<p>Back then, was Earth’s temperature warming or cooling?</p>
<p>Even though scientists know more about the most recent 6,000 years than any other multimillennial interval, studies on this long-term global temperature trend have come to <a href="https://doi.org/10.1073/pnas.1407229111">contrasting conclusions</a>.</p>
<p>To try to resolve the difference, we conducted a comprehensive, global-scale assessment of the existing evidence, including both natural archives, like tree rings and seafloor sediments, and climate models. Our results, <a href="https://www.nature.com/articles/s41586-022-05536-w">published Feb. 15, 2023</a>, suggest ways to improve climate forecasting to avoid missing some important slow-moving, naturally occurring climate feedbacks.</p>
<h2>Global warming in context</h2>
<p>Scientists like us who study past climate, or <a href="https://www.usgs.gov/programs/climate-research-and-development-program/science/paleoclimate-research">paleoclimate</a>, look for temperature data from far back in time, long before thermometers and satellites.</p>
<p>We have two options: We can find information about past climate stored <a href="https://interactive.carbonbrief.org/how-proxy-data-reveals-climate-of-earths-distant-past/">in natural archives</a>, or we can simulate the past using <a href="https://www.carbonbrief.org/qa-how-do-climate-models-work/">climate models</a>.</p>
<p>There are several natural archives that record changes in the climate over time. The growth rings that form each year in <a href="https://scied.ucar.edu/learning-zone/how-climate-works/tree-rings-and-climate">trees</a>, <a href="https://eos.org/editors-vox/stalagmite-layers-reveal-hidden-climate-stories">stalagmites</a> and <a href="https://www.ncei.noaa.gov/news/how-can-corals-teach-us-about-climate">corals</a> can be used to reconstruct past temperature. Similar data can be found in <a href="https://icecores.org/about-ice-cores">glacier ice</a> and in tiny shells found in the <a href="https://www.icm.csic.es/en/news/what-do-marine-sediments-tell-us-about-earths-climate">sediment that builds up over time at the bottom of the ocean</a> or <a href="https://www.earth.ox.ac.uk/2017/01/using-lake-sediments-to-understand-past-climate/">lakes</a>. These serve as substitutes, or proxies, for thermometer-based measurements.</p>
<figure class="align-center ">
<img alt="Illustration shows different types of natural archives and how cores are taken." src="https://images.theconversation.com/files/506970/original/file-20230130-508-h5lwde.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/506970/original/file-20230130-508-h5lwde.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=448&fit=crop&dpr=1 600w, https://images.theconversation.com/files/506970/original/file-20230130-508-h5lwde.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=448&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/506970/original/file-20230130-508-h5lwde.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=448&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/506970/original/file-20230130-508-h5lwde.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=563&fit=crop&dpr=1 754w, https://images.theconversation.com/files/506970/original/file-20230130-508-h5lwde.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=563&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/506970/original/file-20230130-508-h5lwde.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=563&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Trees are the best-known natural archives. Here are several others that hold evidence of past temperature. Cores or other samples from these archives can be used to reconstruct changes over time.</span>
<span class="attribution"><a class="source" href="https://www.victorleshyk.com/">Viktor O. Leshyk</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>For example, changes in the width of tree rings can <a href="https://doi.org/10.1007/s00382-012-1611-x">record temperature fluctuations</a>. If temperature during the growing season is too cold, the tree ring forming that year is thinner that one from a year with warmer temperatures.</p>
<p>Another temperature proxy is found in seafloor sediment, in the remains of tiny ocean-dwelling creatures called <a href="https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/foraminifera/">foraminifera</a>. When a foraminifer is alive, the chemical composition of its <a href="https://blogs.egu.eu/divisions/cl/2017/11/24/forams-the-sea-thermometers-of-the-past/">shell changes depending on the temperature of the ocean</a>. When it dies, the shell sinks and gets buried by other debris over time, forming layers of sediment at the ocean floor. Paleoclimatologists can then extract sediment cores and chemically analyze the shells in those layers to determine their composition and age, sometimes going back millennia.</p>
<figure class="align-center ">
<img alt="Two female scientists aboard a boat examine a sediment core, with the layers clearly visible." src="https://images.theconversation.com/files/506968/original/file-20230130-14-2uvwwp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/506968/original/file-20230130-14-2uvwwp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/506968/original/file-20230130-14-2uvwwp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/506968/original/file-20230130-14-2uvwwp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/506968/original/file-20230130-14-2uvwwp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/506968/original/file-20230130-14-2uvwwp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/506968/original/file-20230130-14-2uvwwp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Ellie Broadman, right, an author of this article, holds a sediment core from a lake on Alaska’s Kenai Peninsula.</span>
<span class="attribution"><span class="source">Emily Stone</span></span>
</figcaption>
</figure>
<p>Climate models, our other tool for exploring past environments, are mathematical representations of the Earth’s climate system. They model relationships among the atmosphere, biosphere and hydrosphere to create our best replica of reality.</p>
<p>Climate models are used to <a href="https://www.carbonbrief.org/analysis-how-well-have-climate-models-projected-global-warming/">study current conditions</a>, <a href="https://www.ipcc.ch/report/emissions-scenarios/?idp=0">forecast changes in the future</a> and <a href="https://pmip.lsce.ipsl.fr/about_us/overview">reconstruct the past</a>. For example, scientists can input the past concentrations of greenhouse gases, which we know from <a href="https://doi.org/10.1038/s43017-022-00351-3">information stored in tiny bubbles in ancient ice</a>, and the model can use that information to simulate past temperature. Modern climate data and details from natural archives are used to test their accuracy.</p>
<p>Proxy data and climate models have different strengths.</p>
<p>Proxies are tangible and measurable, and they often have a well-understood response to temperature. However, they are not evenly distributed around the world or through time. This makes it difficult to reconstruct global, continuous temperatures.</p>
<p>In contrast, climate models are continuous in space and time, but while they are often very skillful, they will never capture every detail of the climate system.</p>
<h2>A paleo-temperature conundrum</h2>
<p>In our <a href="https://www.nature.com/articles/s41586-022-05536-w">new review paper</a>, we assessed climate theory, proxy data and model simulations, focusing on indicators of global temperature. We carefully considered naturally occurring processes that affect the climate, including long-term variations in <a href="https://climate.nasa.gov/news/2948/milankovitch-orbital-cycles-and-their-role-in-earths-climate/">Earth’s orbit around the Sun</a>, greenhouse gas concentrations, <a href="https://scied.ucar.edu/learning-zone/how-climate-works/how-volcanoes-influence-climate">volcanic eruptions</a> and <a href="https://www.climate.gov/news-features/understanding-climate/climate-change-incoming-sunlight">the strength of the Sun’s heat energy</a>.</p>
<p>We also examined important climate feedbacks, such as vegetation and sea ice changes, that can <a href="https://scied.ucar.edu/learning-zone/how-climate-works/albedo-and-climate">influence global temperature</a>. For example, there is strong evidence that <a href="https://doi.org/10.1016/j.quascirev.2013.10.022">less Arctic sea ice</a> and <a href="https://doi.org/10.2307/2997337">more vegetation cover</a> existed during a period around 6,000 years ago than in the 19th century. That would have darkened the Earth’s surface, causing it to absorb more heat.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/507969/original/file-20230202-12383-ugtxhe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/507969/original/file-20230202-12383-ugtxhe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/507969/original/file-20230202-12383-ugtxhe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=323&fit=crop&dpr=1 600w, https://images.theconversation.com/files/507969/original/file-20230202-12383-ugtxhe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=323&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/507969/original/file-20230202-12383-ugtxhe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=323&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/507969/original/file-20230202-12383-ugtxhe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=406&fit=crop&dpr=1 754w, https://images.theconversation.com/files/507969/original/file-20230202-12383-ugtxhe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=406&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/507969/original/file-20230202-12383-ugtxhe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=406&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Some example of foraminifera shells.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Foraminifera_Phototable.jpg">From Anna Tikhonova, Sofia Merenkova, Sergei Korsun and Alexander Matul via Wikimedia</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Our two types of evidence offer different answers regarding the Earth’s temperature trend over the 6,000 years before modern global warming.
Natural archives generally show that Earth’s average temperature roughly 6,000 years ago was warmer by <a href="https://doi.org/10.1038/s41597-020-0530-7">about 0.7 C (1.3 F) compared with the 19th century median</a>, and then cooled gradually until the Industrial Revolution. We found that most evidence points to this result.</p>
<p>Meanwhile, climate models generally show a slight warming trend, corresponding to a gradual increase in carbon dioxide as <a href="https://education.nationalgeographic.org/resource/development-agriculture">agriculture-based societies developed</a> during the millennia after <a href="https://www.livescience.com/40311-pleistocene-epoch.html">ice sheets retreated</a> in the Northern Hemisphere.</p>
<h2>How to improve climate forecasts</h2>
<p>Our assessment highlights some ways to improve climate forecasts.</p>
<p>For example, we found that models would be more powerful if they more fully represented certain climate feedbacks. One <a href="https://doi.org/10.1126/sciadv.abj6535">climate model experiment</a> that included increased vegetation cover in some regions 6,000 years ago was able to simulate the global temperature peak we see in proxy records, unlike most other model simulations, which don’t include this expanded vegetation.</p>
<p>Understanding and better incorporating these and other feedbacks <a href="https://doi.org/10.1029/2019GL085982">will be important</a> as scientists continue to improve our ability to predict future changes.</p><img src="https://counter.theconversation.com/content/197788/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ellie Broadman has received funding from the National Science Foundation, the University of Arizona, and Northern Arizona University.</span></em></p><p class="fine-print"><em><span>Darrell Kaufman receives funding from the National Science Foundation.</span></em></p>Evidence in Earth’s natural archives, from tree rings to seafloor sediments, points to one trend. Some climate models suggest another.Ellie Broadman, Postdoctoral Research Associate in Climate Science, University of ArizonaDarrell Kaufman, Professor of Earth and Environmental Sciences, Northern Arizona UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1868992022-11-28T13:33:32Z2022-11-28T13:33:32ZWe’re decoding ancient hurricanes’ traces on the sea floor – and evidence from millennia of Atlantic storms is not good news for the coast<figure><img src="https://images.theconversation.com/files/490533/original/file-20221019-23-g61gcn.jpg?ixlib=rb-1.1.0&rect=323%2C871%2C3981%2C2433&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Deep 'blue holes,' like this one off Belize, can collect evidence of hurricanes.</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Belize_Blue_Hole_(TMP)_(16912331906).jpg">The TerraMar Project</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>If you look back at the history of Atlantic hurricanes since the late 1800s, it might seem hurricane frequency is on the rise.</p>
<p>The year 2020 had the <a href="https://www.nhc.noaa.gov/data/tcr/index.php?season=2020&basin=atl">most tropical cyclones</a> in the Atlantic, with 31, and 2021 had the <a href="https://www.nhc.noaa.gov/data/tcr/index.php?season=2021&basin=atl">third-highest</a>, <a href="https://www.nhc.noaa.gov/climo/">after 2005</a>. The past decade saw <a href="https://en.wikipedia.org/wiki/List_of_costliest_Atlantic_hurricanes%22%22">five of the six</a> most destructive Atlantic hurricanes in modern history.</p>
<p>Then a year like 2022 comes along, with no major hurricane landfalls until <a href="https://theconversation.com/fiona-was-one-of-canadas-worst-natural-disasters-but-evacuations-prevented-greater-losses-in-atlantic-canada-191319">Fiona</a> <a href="https://www.washingtonpost.com/nation/2022/09/23/hurricane-fiona-puerto-rico-floods/">and</a> <a href="https://theconversation.com/hurricane-ian-flooded-a-hospital-and-forced-evacuations-from-dozens-of-nursing-homes-many-health-facilities-face-rising-risks-from-severe-storms-191648">Ian</a> struck in late September. The Atlantic hurricane season, which ended on Nov. 30, had <a href="https://www.nhc.noaa.gov/archive/2022/">eight hurricanes and 14 named storms</a>. It’s a reminder that small sample sizes can be misleading when assessing trends in hurricane behavior. There is so much natural variability in hurricane behavior year to year and even decade to decade that we need to look much further back in time for the real trends to come clear.</p>
<p>Fortunately, hurricanes leave behind telltale evidence that goes back millennia.</p>
<p>Two thousand years of this evidence indicates that the Atlantic has experienced even stormier periods in the past than we’ve seen in recent years. That’s not good news. It tells <a href="https://www.researchgate.net/profile/Tyler-Winkler-4">coastal oceanographers like me</a> that we may be significantly underestimating the threat hurricanes pose to Caribbean islands and the North American coast in the future. </p>
<h2>The natural records hurricanes leave behind</h2>
<p>When a hurricane nears land, its winds whip up powerful waves and currents that can sweep coarse sands and gravel into marshes and deep coastal ponds, sinkholes and lagoons.</p>
<p>Under normal conditions, fine sand and organic matter like leaves and seeds fall into these areas and settle to the bottom. So when coarse sand and gravel wash in, a distinct layer is left behind.</p>
<p>Imagine cutting through a layer cake – you can see each layer of frosting. Scientists can see the same effect by plunging a long tube into the bottom of these coastal marshes and ponds and pulling up several meters of sediment in what’s known as a sediment core. By studying the layers in sediment, we can see when coarse sand appeared, suggesting an extreme coastal flood from a hurricane.</p>
<p>With these sediment cores, we have been able to document evidence of Atlantic hurricane activity over thousands of years.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/490544/original/file-20221019-17-2xeuk8.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="One sediment core with dates showing high levels of sand deposits and a photo of one section showing the sand layer." src="https://images.theconversation.com/files/490544/original/file-20221019-17-2xeuk8.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/490544/original/file-20221019-17-2xeuk8.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=842&fit=crop&dpr=1 600w, https://images.theconversation.com/files/490544/original/file-20221019-17-2xeuk8.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=842&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/490544/original/file-20221019-17-2xeuk8.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=842&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/490544/original/file-20221019-17-2xeuk8.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1058&fit=crop&dpr=1 754w, https://images.theconversation.com/files/490544/original/file-20221019-17-2xeuk8.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1058&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/490544/original/file-20221019-17-2xeuk8.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1058&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The red dots indicate large sand deposits going back about 1,060 years. The yellow dots are estimated dates from radiocarbon dating of small shells.</span>
<span class="attribution"><span class="source">Tyler Winkler</span></span>
</figcaption>
</figure>
<p>We now have dozens of chronologies of hurricane activity at different locations – including <a href="https://doi.org/10.1002/2014EF000274">New England</a>, the <a href="https://doi.org/10.1038/s41598-020-75874-0">Florida Gulf</a> <a href="https://doi.org/10.1016/j.margeo.2011.07.001">Coast</a>, the <a href="https://doi.org/10.1073/pnas.1519566113">Florida Keys</a> <a href="https://doi.org/10.1038/srep03876">and</a> <a href="https://doi.org/10.1016/j.quascirev.2020.106570">Belize</a> – that reveal decade- to century-scale patterns in hurricane frequency.</p>
<p>Others, including from <a href="https://doi.org/10.1029/2020GL089859">Atlantic</a> <a href="https://doi.org/10.1016/j.margeo.2018.09.012">Canada</a>, <a href="https://doi.org/10.2112/03-0103.1">North Carolina</a>, <a href="https://doi.org/10.1006/qres.2000.2166">northwestern Florida</a>, Mississippi and <a href="https://doi.org/10.1038/nature05834">Puerto Rico</a>, are lower-resolution, meaning it is nearly impossible to discern individual hurricane layers deposited within decades of one another. But they can be highly informative for determining the timing of the most intense hurricanes, which can have significant impacts on coastal ecosystems.</p>
<p>It’s the records from the Bahamas, however, with nearly annual resolution, that are crucial for seeing the long-term picture for the Atlantic Basin.</p>
<h2>Why The Bahamas are so important</h2>
<p>The Bahamas are exceptionally vulnerable to the impacts of major hurricanes because of their geographic location.</p>
<p>In the North Atlantic, 85% of all major hurricanes form in what is known as the <a href="https://www.wunderground.com/blog/JeffMasters/record-atlantic-ssts-continue-in-the-hurricane-main-development-region.html">Main Development Region</a>, off western Africa. Looking just at observed hurricane tracks from the past 170 years, my analysis shows that about 86% of major hurricanes that affect the Bahamas also form in that region, suggesting the frequency variability in the Bahamas may be representative of the basin.</p>
<figure class="align-center ">
<img alt="Satellite view of Atlantic showing tracks of each storm, most starting off Africa, heading west and then curving northward." src="https://images.theconversation.com/files/490696/original/file-20221019-18-y6wbyg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/490696/original/file-20221019-18-y6wbyg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=336&fit=crop&dpr=1 600w, https://images.theconversation.com/files/490696/original/file-20221019-18-y6wbyg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=336&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/490696/original/file-20221019-18-y6wbyg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=336&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/490696/original/file-20221019-18-y6wbyg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=423&fit=crop&dpr=1 754w, https://images.theconversation.com/files/490696/original/file-20221019-18-y6wbyg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=423&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/490696/original/file-20221019-18-y6wbyg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=423&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Atlantic hurricane tracks from 1851 to 2012.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Atlantic_hurricane_tracks.jpg">Nilfanion/Wikimedia</a></span>
</figcaption>
</figure>
<p>A substantial percentage of North Atlantic storms also <a href="https://www.nhc.noaa.gov/climo/images/1851_2017_allstorms.jpg">pass over or near these islands</a>, so these records appear to reflect changes in overall North Atlantic hurricane frequency through time.</p>
<p>By coupling coastal sediment records from the Bahamas with records from sites farther north, we can explore how changes in ocean surface temperatures, ocean currents, global-scale wind patterns and atmospheric pressure gradients affect regional hurricane frequency.</p>
<p>As sea surface temperatures rise, warmer water provides more energy that can fuel <a href="https://www.ipcc.ch/report/ar6/wg2/">more powerful and destructive</a> hurricanes. However, the frequency of hurricanes – how often they form – isn’t necessarily affected in the same way.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/490539/original/file-20221019-25-jyn1fv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Satellite image of a hurricane over The Bahamas, marked on the map, next to Florida." src="https://images.theconversation.com/files/490539/original/file-20221019-25-jyn1fv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/490539/original/file-20221019-25-jyn1fv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/490539/original/file-20221019-25-jyn1fv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/490539/original/file-20221019-25-jyn1fv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/490539/original/file-20221019-25-jyn1fv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/490539/original/file-20221019-25-jyn1fv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/490539/original/file-20221019-25-jyn1fv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Hurricane Dorian sat over the Bahamas as a powerful Category 5 storm in 2019.</span>
<span class="attribution"><a class="source" href="https://earthobservatory.nasa.gov/images/145555/hurricane-dorian-pounds-the-bahamas">Laura Dauphin/NASA Earth Observatory</a></span>
</figcaption>
</figure>
<h2>The secrets hidden in blue holes</h2>
<p>Some of the best locations for studying past hurricane activity are large, near-shore sinkholes known as blue holes.</p>
<p>Blue holes get their name from their deep blue color. They formed when carbonate rock dissolved to form underwater caves. Eventually, the ceilings collapsed, leaving behind sinkholes. The Bahamas has thousands of blue holes, some as wide as a <a href="https://doi.org/10.1016/j.quascirev.2021.107126">third of a mile</a> and as deep as a <a href="http://www.deansbluehole.org/">60-story building</a>.</p>
<p>They tend to have deep vertical walls that can trap sediments – including sand <a href="https://hakaimagazine.com/news/blue-holes-show-hurricane-activity-in-the-bahamas-is-at-a-centuries-long-low/">transported by strong hurricanes</a>. Fortuitously, deep blue holes often have little oxygen at the bottom, which slows decay, helping to preserve organic matter in the sediment through time.</p>
<figure class="align-center ">
<img alt="Images showing the depth of a blue hole" src="https://images.theconversation.com/files/490518/original/file-20221019-21-pw980u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/490518/original/file-20221019-21-pw980u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1032&fit=crop&dpr=1 600w, https://images.theconversation.com/files/490518/original/file-20221019-21-pw980u.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1032&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/490518/original/file-20221019-21-pw980u.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1032&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/490518/original/file-20221019-21-pw980u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1296&fit=crop&dpr=1 754w, https://images.theconversation.com/files/490518/original/file-20221019-21-pw980u.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1296&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/490518/original/file-20221019-21-pw980u.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1296&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Hine’s Blue Hole in the Bahamas is about 330 feet (100 meters) deep. Seismic imaging shows about 200 feet (60-plus meters) of accumulated sediment.</span>
<span class="attribution"><span class="source">Pete van Hengstum; Tyler Winkler</span></span>
</figcaption>
</figure>
<h2>Cracking open a sediment core</h2>
<p>When we bring up a sediment core, the coarse sand layers are often evident to the naked eye. But closer examination can tell us much more about these hurricanes of the past.</p>
<p>I use X-rays to measure changes in the density of sediment, <a href="https://serc.carleton.edu/research_education/geochemsheets/techniques/XRF.html">X-ray fluorescence</a> to examine elemental changes that can reveal if sediment came from land or sea, and sediment textural analysis that examines the grain size.</p>
<p>To figure out the age of each layer, we typically use <a href="https://youtu.be/phZeE7Att_s">radiocarbon dating</a>. By measuring the amount of carbon-14, a radioactive isotope, in shells or other organic material found at various points in the core, I can create a statistical model that predicts the age of sediments throughout the core.</p>
<p>So far, my colleagues and I have published five paleohurricane records with nearly annual detail from blue holes on islands across the Bahamas. </p>
<p>Each record shows periods of significant increase in storm frequency lasting decades and sometimes centuries. </p>
<figure class="align-center ">
<img alt="A map showing hurricane frequency from 1850 to 2019, with parts of Florida, Louisiana and North Carolina showing nine to 10 storms." src="https://images.theconversation.com/files/490500/original/file-20221018-8290-siwwug.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/490500/original/file-20221018-8290-siwwug.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=391&fit=crop&dpr=1 600w, https://images.theconversation.com/files/490500/original/file-20221018-8290-siwwug.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=391&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/490500/original/file-20221018-8290-siwwug.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=391&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/490500/original/file-20221018-8290-siwwug.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=491&fit=crop&dpr=1 754w, https://images.theconversation.com/files/490500/original/file-20221018-8290-siwwug.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=491&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/490500/original/file-20221018-8290-siwwug.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=491&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The red dots show the sites of high-resolution paleohurricane records. The map shows the frequency of hurricanes ranked Category 2 or above from 1850 to 2019.</span>
<span class="attribution"><span class="source">Tyler Winkler</span></span>
</figcaption>
</figure>
<p>The records vary, showing that a single location might not reflect broader regional trends. </p>
<p>For example, <a href="https://doi.org/10.1038/s41598-020-73132-x">Thatchpoint Blue Hole</a> on Great Abaco Island in the northern Bahamas includes evidence of at least 13 hurricanes per century that were Category 2 or above between the years 1500 and 1670. That significantly exceeds the rate of nine per century documented since 1850. During the same period, 1500 to 1670, <a href="https://doi.org/10.1029/2019PA003665">blue holes at Andros Island</a>, just 186 miles (300 kilometers) south of Abaco, documented the lowest levels of local hurricane activity observed in this region during the past 1,500 years.</p>
<h2>Spotting patterns across the Atlantic Basin</h2>
<p>Together, however, these records offer a glimpse of broad regional patterns. They’re also giving us new insight into the ways ocean and atmospheric changes can influence hurricane frequency.</p>
<p>While rising sea surface temperatures provide more energy that can fuel <a href="https://www.ipcc.ch/report/ar6/wg2/">more powerful and destructive</a> hurricanes, their frequency – how often they form – isn’t necessarily affected in the same way. Some studies have predicted the total number of hurricanes will actually decrease in the future.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/490490/original/file-20221018-7213-wkks8.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Eight chronologies of hurricane evidence stacked to show corresponding periods of higher hurricane frequency." src="https://images.theconversation.com/files/490490/original/file-20221018-7213-wkks8.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/490490/original/file-20221018-7213-wkks8.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1135&fit=crop&dpr=1 600w, https://images.theconversation.com/files/490490/original/file-20221018-7213-wkks8.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1135&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/490490/original/file-20221018-7213-wkks8.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1135&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/490490/original/file-20221018-7213-wkks8.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1427&fit=crop&dpr=1 754w, https://images.theconversation.com/files/490490/original/file-20221018-7213-wkks8.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1427&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/490490/original/file-20221018-7213-wkks8.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1427&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Comparing paleohurricane records from several locations shows periods of higher frequency. The highlighted periods cover the Little Ice Age, a time of cooler conditions in the North Atlantic from 1300 to 1850, and the Medieval Warm Period, from 900 to 1250.</span>
<span class="attribution"><span class="source">Tyler Winkler</span></span>
</figcaption>
</figure>
<p>The compiled Bahamian records document substantially higher hurricane frequency in the northern Caribbean during the <a href="https://eos.org/articles/the-little-ice-age-wasnt-global-but-current-climate-change-is">Little Ice Age</a>, around 1300 to 1850, than in the past 100 years.</p>
<p>That was a time when North Atlantic surface ocean temperatures were <a href="https://doi.org/10.1016/j.quascirev.2021.107126">generally cooler</a> <a href="https://doi.org/10.1016/j.margeo.2021.106653">than they are today</a>. But it also coincided with an intensified West African monsoon. The monsoon could have produced more thunderstorms off the western coast of Africa, which act as <a href="https://doi.org/10.1126/science.249.4974.1251">low-pressure seeds for hurricanes</a>.</p>
<p>Steering winds and vertical wind shear likely also affect a region’s hurricane frequency over time. The Little Ice Age active interval observed in most Bahamian records <a href="https://doi.org/10.1002/2014EF000274">coincides with increased</a> <a href="https://www.cambridge.org/core/journals/quaternary-research/article/barrier-island-response-to-late-holocene-climate-events-north-carolina-usa/BE735D1D3E624DF03F33E8FDC90701F8">hurricane strikes</a> along the U.S. Eastern Seaboard from 1500 to 1670, but at the same time it was a <a href="https://doi.org/10.1016/j.margeo.2011.07.001">quieter period in the Gulf</a> of Mexico, <a href="https://doi.org/10.1029/2019PA003665">central Bahamas</a> and <a href="https://doi.org/10.1038/srep03876">southern Caribbean</a>.</p>
<p>Records from sites farther north tell us more about the climate. That’s because changes in ocean temperature and climate conditions are likely far more important to controlling regional impacts in such areas as the Northeastern U.S. and Atlantic Canada, where cooler climate conditions are often unfavorable for storms.</p>
<h2>A warning for the islands</h2>
<p>I am currently developing records of coastal storminess in locations including Newfoundland and Mexico. With those records, we can better anticipate the impacts of future climate change on storm activity and coastal flooding.</p>
<p>In the Bahamas, meanwhile, sea level rise is putting the islands at increasing risk, so even weaker hurricanes can produce damaging flooding. Given that storms are expected to be more intense, any increase in storm frequency could have devastating impacts.</p><img src="https://counter.theconversation.com/content/186899/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tyler Winkler does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A look back at hurricane history suggests we may be significantly underestimating future risks.Tyler Winkler, Postdoctoral Researcher in Oceanography, Woods Hole Oceanographic InstitutionLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1740882021-12-21T20:41:32Z2021-12-21T20:41:32ZWheel of Time is set thousands of years from now, yet it’s still burdened with today’s climate change<figure><img src="https://images.theconversation.com/files/438664/original/file-20211221-167342-1p2ykxt.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The epic fantasy series has been turned into a tv show on Amazon.</span> <span class="attribution"><span class="source">JAN THIJS/AMAZON STUDIOS</span></span></figcaption></figure><p>Wheel of Time, the 14-book epic fantasy now turned into an Amazon Prime TV series, is a medieval-style adventure set in the Third Age of the World of the Wheel. While not explicit in the storyline, <a href="https://www.theoryland.com/listintv.php">notes from the late author</a> suggest that the First Age was actually modern-day Earth, which ended with a dramatic event (perhaps even climate change). From these notes, we estimate the show takes place around 18,000 years from today. </p>
<p>For climate scientists like us, this poses an interesting question: would today’s climate change still be experienced in the World of the Wheel, even after all those centuries?</p>
<p>About a quarter of carbon dioxide emitted today will remain in the atmosphere even 18,000 years from now. According to <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2014GB005074">biogeochemistry models</a>, carbon dioxide levels could be as high as 1,100 parts per million (ppm) at that point. That’s compared with a present-day value of 415ppm. This very high value assumes that the Paris climate goals will be exceeded and that many natural stores of carbon will also be released into the atmosphere (melting permafrost, for instance). </p>
<p>But the high carbon dioxide concentrations do not necessarily mean a warmer climate. That’s because, over such a long period, slow changes in the orbit and tilt of the planet become more important. This is known as the Milankovitch Cycle and each cycle lasts for around 100,000 years. Given that we are currently at the peak of such a cycle, the planet will naturally cool over the next 50,000 years and this is why scientists were <a href="https://theconversation.com/ice-ages-have-been-linked-to-the-earths-wobbly-orbit-but-when-is-the-next-one-70069">once worried about a new ice age</a>.</p>
<p>But will this be enough to offset the warming from the remaining carbon dioxide in the atmosphere? The image below shows a version of the classic <a href="https://theconversation.com/showyourstripes-how-climate-data-became-a-cultural-icon-123457">warming stripes</a>, a ubiquitous symbol of the past 150 years of climate change, but instead applied over <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2014GB005074">1 million years</a>: </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/438658/original/file-20211221-15-frr0ba.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Annotated stripes" src="https://images.theconversation.com/files/438658/original/file-20211221-15-frr0ba.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438658/original/file-20211221-15-frr0ba.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=205&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438658/original/file-20211221-15-frr0ba.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=205&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438658/original/file-20211221-15-frr0ba.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=205&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438658/original/file-20211221-15-frr0ba.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=257&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438658/original/file-20211221-15-frr0ba.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=257&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438658/original/file-20211221-15-frr0ba.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=257&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Warming stripes of Earth (and the World of the Wheel) for a million years. Today’s climate crisis will disrupt the Milankovitch cycle and its effects will last for many thousands of years.</span>
<span class="attribution"><span class="source">Authors modified from Dan Lunt et al</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>You can clearly see the 100,000 year Milankovitch cycles. Anything red can be considered anthropogenic climate change, and the events of the Wheel of Time are well within this period. Even the descending Milankovitch cycle won’t be enough to counteract the increased warming from carbon dioxide, and so the inhabitants of the World of the Wheel would still experience elevated temperatures from a climate crisis that occurred 18,000 years ago. </p>
<h2>Simulating the weather of the World</h2>
<p>However, some of the weather changes from the still-elevated temperatures could be offset by other factors. Those 18,000 years aren’t very long from a geological perspective, so in normal circumstances the landmasses would not change significantly. However, in this fantasy future magical channelers “broke” the world at the end of the Second Age, creating several new supercontinents. </p>
<p>To find out how the climate would work in the World of the Wheel, we used an <a href="https://www.mdpi.com/594132">exoplanet model</a>. This complex computer program uses fundamental principles of physics to simulate the weather patterns on the hypothetical future planet, once we had fed in its topography based on hand-drawn maps of the world, and carbon dioxide levels of <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2014GB005074">830ppm</a> based on one of the high potential future carbon pathways. </p>
<p>According to our model, the World of the Wheel would be warm all over the surface, with temperatures over land never being cold enough for snow apart from on the mountains. No chance of a white Christmas in this future. Here the story and the science diverge, as at times snow is mentioned in the Wheel of Time. The long-term effects of climate change may have surpassed the imagination of its author, the late great Robert Jordan.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/438705/original/file-20211221-21-13bmc6h.gif?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An animated map with arrows" src="https://images.theconversation.com/files/438705/original/file-20211221-21-13bmc6h.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438705/original/file-20211221-21-13bmc6h.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438705/original/file-20211221-21-13bmc6h.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438705/original/file-20211221-21-13bmc6h.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438705/original/file-20211221-21-13bmc6h.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=369&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438705/original/file-20211221-21-13bmc6h.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=369&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438705/original/file-20211221-21-13bmc6h.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=369&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A simulation focused on where The Wheel of Time events take place, showing surface winds (white arrows).</span>
<span class="attribution"><a class="source" href="https://climatearchive.org/wot">climatearchive.org</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>The World of the Wheel would have stronger and wavier high-altitude jet streams than modern-day Earth. This is likely because there are more mountain ranges in the World of the Wheel, which generate atmospheric waves called Rossby waves, causing oscillations in the jet. There is <a href="https://www.nature.com/articles/srep45242?iu=&iap=false&exception=true&cust_params=">some limited evidence</a> that the jet stream gets wavier with climate change as well, although this is likely to be less important than the mountain ranges. The jet would bring moisture from the western ocean on to land, and deposit it north of the Mountains of Dhoom. Surprising then, that this region (The Great Blight) is so desert-like in the books – perhaps there is some magic at play to explain this.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/HidR2KA8r9o?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Our simulation of the World of the Wheel, showing the jet stream (red and yellow arrows), surface winds (white arrows) and cloud cover (white mist). Source: https://climatearchive.org/wot.</span></figcaption>
</figure>
<p>Winds would often revolve around two particularly enormous mountains, Dragonmount and Shayol Ghul, before blowing downslope and reaching far across the land masses. The peak of Dragonmount itself is nearly always surrounded by clouds, and this is because the mountain is so large the winds travelling up it force surface moisture to higher altitudes, thus cooling it, and forming clouds.</p>
<p>The fact winds would be so different from modern-day Earth is predominantly caused by topography, not the underlying increased temperatures from climate change. Nevertheless, in the World of the Wheel, it is clear that despite the extremely long time since carbon polluted the atmosphere, the inhabitants are still exposed to warmer than usual temperatures.</p>
<p>Acknowledging just how long the effects of climate change will persist for should be a catalyst for change. Yet, even after accepting the facts, we face psychological barriers to subsequent personal action, not least because comprehending the timescales of climate change requires a considerable degree of abstraction. But, given the known changes in extreme weather from climate change, and given how long these changes will remain, we must ask ourselves: how would the mysterious and powerful Aes Sedai stop the climate crisis?</p><img src="https://counter.theconversation.com/content/174088/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Emily Ball receives funding from the Natural Environment Research Council. </span></em></p><p class="fine-print"><em><span>Sebastian Steinig receives funding from the Natural Environment Research Council and has been supported by the Jean Golding Institute for data science and data-intensive research at the University of Bristol.</span></em></p><p class="fine-print"><em><span>Dann Mitchell and Rebecca Áilish Atkinson do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>We modelled the climate of the far future planet Earth in which the fantasy series is set.Dann Mitchell, Professor of Climate Science, University of BristolEmily Ball, PhD Candidate, Climate Science, University of BristolRebecca Áilish Atkinson, Research Fellow, Cognitive Psychology, University of SussexSebastian Steinig, Research Associate in Paleoclimate Modelling, University of BristolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1705252021-11-01T17:11:07Z2021-11-01T17:11:07ZHumanity is compressing millions of years of natural change into just a few centuries<figure><img src="https://images.theconversation.com/files/429578/original/file-20211101-19-hfkg25.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The near future may be similar to the mid-Pliocene warm period a few million years ago.</span> <span class="attribution"><span class="source">Daniel Eskridge / shutterstock</span></span></figcaption></figure><p>Many numbers are swirling around the climate negotiations at the UN climate summit in Glasgow, COP26. These include global warming targets of 1.5°C and 2.0°C, recent warming of 1.1°C, remaining CO₂ budget of 400 billion tonnes, or current atmospheric CO₂ of 415 parts per million. </p>
<p>It’s often hard to grasp the significance of these numbers. But the study of ancient climates can give us an appreciation of their scale compared to what has occurred naturally in the past. Our knowledge of ancient climate change also allows scientists to calibrate their models and therefore improve predictions of what the future may hold.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429582/original/file-20211101-17-gr64rc.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/429582/original/file-20211101-17-gr64rc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429582/original/file-20211101-17-gr64rc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=253&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429582/original/file-20211101-17-gr64rc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=253&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429582/original/file-20211101-17-gr64rc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=253&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429582/original/file-20211101-17-gr64rc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=318&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429582/original/file-20211101-17-gr64rc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=318&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429582/original/file-20211101-17-gr64rc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=318&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Recent climate changes in context.</span>
<span class="attribution"><a class="source" href="https://www.ipcc.ch/report/ar6/">IPCC AR6, chapter 2</a></span>
</figcaption>
</figure>
<p>Recent work, summarised in the <a href="https://www.ipcc.ch/report/ar6/wg1/">latest report</a> of the Intergovernmental Panel on Climate Change (IPCC), has allowed scientists to refine their understanding and measurement of past climate changes. These changes are recorded in rocky outcrops, sediments from the ocean floor and lakes, in polar ice sheets, and in other shorter-term archives such as tree rings and corals. As scientists discover more of these archives and get better at using them, we have become increasingly able to compare recent and future climate change with what has happened in the past, and to provide important context to the numbers involved in climate negotiations.</p>
<p>For instance one headline finding in the IPCC report was that global temperature (currently 1.1°C above a pre-industrial baseline) is higher than at any time in at least the past <a href="https://theconversation.com/the-last-time-earth-was-this-hot-hippos-lived-in-britain-thats-130-000-years-ago-53398">120,000 or so years</a>. That’s because the last warm period between ice ages peaked about 125,000 years ago – in contrast to today, warmth at that time was driven not by CO₂, but by changes in Earth’s orbit and spin axis. Another finding regards the rate of current warming, which is faster than at any time in the past 2,000 years – and probably much longer.</p>
<p>But it is not only past temperature that can be reconstructed from the geological record. For instance, tiny gas bubbles trapped in Antarctic ice can record atmospheric CO₂ concentrations back to 800,000 years ago. Beyond that, scientists can turn to microscopic fossils preserved in seabed sediments. These properties (such as the types of elements that make up the fossil shells) are related to how much CO₂ was in the ocean when the fossilised organisms were alive, which itself is related to how much was in the atmosphere. As we get better at using these “proxies” for atmospheric CO₂, recent work has shown that the current atmospheric CO₂ concentration of around 415 parts per million (compared to 280 ppm prior to industrialisation in the early 1800s), is greater than at any time in <a href="https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_02.pdf">at least the past 2 million years</a>. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429581/original/file-20211101-13-1buk2sp.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="chart showing climate changes over history" src="https://images.theconversation.com/files/429581/original/file-20211101-13-1buk2sp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429581/original/file-20211101-13-1buk2sp.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=518&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429581/original/file-20211101-13-1buk2sp.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=518&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429581/original/file-20211101-13-1buk2sp.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=518&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429581/original/file-20211101-13-1buk2sp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=650&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429581/original/file-20211101-13-1buk2sp.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=650&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429581/original/file-20211101-13-1buk2sp.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=650&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An IPCC graphic showing climate changes at various points since 56 million years ago. Note most rows show changes over thousands or millions of years, while the top row (recent changes) is just a few decades.</span>
<span class="attribution"><span class="source">IPCC AR6, chapter 2 (modified by Darrell Kaufman)</span></span>
</figcaption>
</figure>
<p>Other climate variables can also be <a href="https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_02.pdf">compared to past changes</a>. These include the greenhouse gases methane and nitrous oxide (now greater than at any time in at least 800,000 years), late summer Arctic sea ice area (smaller than at any time in at least the past 1,000 years), glacier retreat (unprecedented in at least 2,000 years) sea level (rising faster than at any point in at least 3,000 years), and ocean acidity (unusually acidic compared to the past 2 million years).</p>
<p>In addition, changes predicted by climate models can be compared to the past. For instance an “intermediate” amount of emissions will likely lead to global warming of between 2.3°C and 4.6°C by the year 2300, which is similar to the mid-Pliocene warm period of about 3.2 million years ago. Extremely high emissions would lead to warming of somewhere between 6.6°C and 14.1°C, which just overlaps with the warmest period since the demise of the dinosaurs – the “Paleocene-Eocene Thermal Maximum” kicked off by massive volcanic eruptions about <a href="https://theconversation.com/sudden-global-warming-55m-years-ago-was-much-like-today-35505">55 million years ago</a>. As such, humanity is currently on the path to compressing millions of years of temperature change into just a couple of centuries.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429485/original/file-20211101-15910-11cfiis.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Small animals in a forest" src="https://images.theconversation.com/files/429485/original/file-20211101-15910-11cfiis.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429485/original/file-20211101-15910-11cfiis.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429485/original/file-20211101-15910-11cfiis.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429485/original/file-20211101-15910-11cfiis.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429485/original/file-20211101-15910-11cfiis.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429485/original/file-20211101-15910-11cfiis.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429485/original/file-20211101-15910-11cfiis.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Many mammals, like these horse-ancestors ‘Eohippus’, first appeared after a sudden warm period 55 million years ago.</span>
<span class="attribution"><span class="source">Daniel Eskridge / shutterstock</span></span>
</figcaption>
</figure>
<h2>Distant past can help predict the near future</h2>
<p>For the first time in an IPCC report, the latest report uses ancient time periods to refine projections of climate change. In previous IPCC reports, future projections have been produced simply by averaging results from all climate models, and using their spread as a measure of uncertainty. But for this new report, temperature and rainfall and sea level projections relied more heavily on those models that did the best job of simulating known climate changes. </p>
<p>Part of this process was based on each individual model’s “climate sensitivity” – the amount it warms when atmospheric CO₂ is doubled. The “correct” value (and uncertainty range) of sensitivity is known from a number of different lines of evidence, one of which comes from certain times in the ancient past when global temperature changes were driven by natural changes in CO₂, caused for example by volcanic eruptions or change in the amount of carbon removed from the atmosphere as rocks are eroded away. Combining estimates of ancient CO₂ and temperature therefore allows scientists to estimate the “correct” value of climate sensitivity, and so refine their future projections by relying more heavily on those models with more accurate climate sensitivities.</p>
<p>Overall, past climates show us that recent changes across all aspects of the Earth system are unprecedented in at least thousands of years. Unless emissions are reduced rapidly and dramatically, global warming will reach a level that has not been seen for millions of years. Let’s hope those attending COP26 are listening to messages from the past. </p>
<hr>
<figure class="align-right ">
<img alt="COP26: the world's biggest climate talks" src="https://images.theconversation.com/files/424739/original/file-20211005-17-cgrf2z.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/424739/original/file-20211005-17-cgrf2z.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/424739/original/file-20211005-17-cgrf2z.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/424739/original/file-20211005-17-cgrf2z.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/424739/original/file-20211005-17-cgrf2z.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/424739/original/file-20211005-17-cgrf2z.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/424739/original/file-20211005-17-cgrf2z.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
</figcaption>
</figure>
<p><strong>This story is part of The Conversation’s coverage on COP26, the Glasgow climate conference, by experts from around the world.</strong>
<br><em>Amid a rising tide of climate news and stories, The Conversation is here to clear the air and make sure you get information you can trust. <a href="https://page.theconversation.com/cop26-glasgow-2021-climate-change-summit/"><strong>More.</strong></a></em> </p>
<hr><img src="https://counter.theconversation.com/content/170525/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dan Lunt receives funding from NERC, the Leverhulme Trust, the EU, and SKB/Posiva. </span></em></p><p class="fine-print"><em><span>Darrell Kaufman receives funding from the US National Science Foundation.</span></em></p>What climate changes in the distant past can tell us about the near future.Dan Lunt, Professor of Climate Science, University of BristolDarrell Kaufman, Professor of Earth and Environmental Sciences, Northern Arizona UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1555802021-02-18T20:20:13Z2021-02-18T20:20:13ZEarth’s magnetic field broke down 42,000 years ago and caused massive sudden climate change<figure><img src="https://images.theconversation.com/files/385125/original/file-20210218-20-utxsk3.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C5991%2C3988&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">vchal / shutterstock</span></span></figcaption></figure><p>The world experienced a few centuries of apocalyptic conditions 42,000 years ago, triggered by a reversal of the Earth’s magnetic poles combined with changes in the Sun’s behaviour. That’s the key finding of our new multidisciplinary study, published in <a href="https://science.sciencemag.org/cgi/doi/10.1126/science.abb8677">Science</a>.</p>
<p>This last major geomagnetic reversal triggered a series of dramatic events that have far-reaching consequences for our planet. They read like the plot of a horror movie: the ozone layer was destroyed, electrical storms raged across the tropics, solar winds generated spectacular light shows (auroras), Arctic air poured across North America, ice sheets and glaciers surged and weather patterns shifted violently.</p>
<p>During these events, life on earth was exposed to intense ultraviolet light, Neanderthals and giant animals known as megafauna went extinct, while modern humans sought protection in caves.</p>
<p>The magnetic north pole – where a compass needle points to – does not have a permanent location. Instead, it usually wobbles around close to the geographic north pole – the point around which the Earth spins – over time due to movements within the Earth’s core.</p>
<p>For reasons still not entirely clear, magnetic pole movements can sometimes be more extreme than a wobble. One of the most dramatic of these pole migrations took place some 42,000 years ago and is known as the Laschamps Excursion – named after the village where it was discovered in the French Massif Central.</p>
<p>The Laschamps Excursion has been recognised around the world, including most recently in <a href="https://theconversation.com/we-found-the-first-australian-evidence-of-a-major-shift-in-earths-magnetic-poles-it-may-help-us-predict-the-next-155040">Tasmania, Australia</a>. But up until now, it has not been clear whether such magnetic changes had any impacts on climate and life on the planet. Our new work draws together multiple lines of evidence that strongly suggest the effects were indeed global and far-reaching.</p>
<h2>Ancient trees</h2>
<p>To investigate what happened, we analysed ancient New Zealand <a href="https://theconversation.com/lord-of-the-forest-new-zealands-most-sacred-tree-is-under-threat-from-disease-but-response-is-slow-100447">kauri trees</a> that had been preserved in peat bogs and other sediments for more than 40,000 years. Using the annual growth rings in the kauri trees, we have been able to create a detailed timescale of how Earth’s atmosphere changed over this time. The trees revealed a prolonged spike in atmospheric radiocarbon levels caused by the collapse of Earth’s magnetic field as the poles switched, providing a way of precisely linking widely geographically dispersed records.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/NSig4MyLQ0o?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">How the tree analysis works.</span></figcaption>
</figure>
<p>“The kauri trees are like the Rosetta Stone, helping us tie together records of environmental change in caves, ice cores, and peat bogs around the world,” says professor Alan Cooper, who co-lead this research project.</p>
<p>Using the newly-created timescale, we were able to show that tropical Pacific rain belts and the Southern Ocean westerly winds abruptly shifted at the same time, bringing arid conditions to places like Australia at the same time as a range of megafauna, including giant kangaroos and giant wombats <a href="https://theconversation.com/dna-evidence-proves-climate-change-killed-off-prehistoric-megafauna-45080">went extinct</a>. Further north, the vast Laurentide Ice Sheet rapidly grew across the eastern US and Canada, while in Europe the Neanderthals spiralled into extinction.</p>
<h2>Climate modelling</h2>
<p>Working with a computer programme that simulated the global interactions between chemistry and the climate, we investigated the impact of a weaker magnetic field and changes in the Sun’s strength. Importantly, during the magnetic switch, the strength of the magnetic field plummeted to less than 6% of what it is today. A compass back then would struggle to even find north.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/385076/original/file-20210218-26-193rwr9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A large tree trunk" src="https://images.theconversation.com/files/385076/original/file-20210218-26-193rwr9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/385076/original/file-20210218-26-193rwr9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/385076/original/file-20210218-26-193rwr9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/385076/original/file-20210218-26-193rwr9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/385076/original/file-20210218-26-193rwr9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/385076/original/file-20210218-26-193rwr9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/385076/original/file-20210218-26-193rwr9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An ancient kauri tree log from Ngāwhā, New Zealand.</span>
<span class="attribution"><a class="source" href="http://www.nelsonskaihukauri.co.nz">Nelson Parker</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>With essentially no magnetic field, our planet totally lost its very effective shield against cosmic radiation, and many more of these very penetrating particles from space could access the top of the atmosphere. On top of this, the Sun experienced several “grand solar minima” throughout this period, during which the overall solar activity was generally much lower but also more unstable, sending out numerous massive solar flares that allowed more powerful ionising cosmic rays to reach Earth.</p>
<p>Our models showed that this combination of factors had an amplifying effect. The high energy cosmic rays from the galaxy and also enormous bursts of cosmic rays from solar flares were able to penetrate the upper atmosphere, charging the particles in the air and causing chemical changes that drove the loss of stratospheric ozone. </p>
<p>The modelled chemistry-climate simulations are consistent with the environmental shifts observed in many natural climate and environmental change archives. These conditions would have also extended the dazzling light shows of the aurora across the world – at times, nights would have been as bright as daytime. We suggest the dramatic changes and unprecedented high UV levels caused early humans to seek shelter in caves, explaining the apparent sudden flowering of cave art across the world 42,000 years ago.</p>
<p>It must have seemed like the end of days.</p>
<h2>The Adams Event</h2>
<p>Because of the coincidence of seemingly random cosmic events and the extreme environmental changes found around the world 42,000 years ago, we have called this period the “Adams Event” – a tribute to the great science fiction writer Douglas Adams, who wrote The Hitchhiker’s Guide to the Galaxy and identified “42” as the answer to life, the universe and everything. Douglas Adams really was onto something big, and the remaining mystery is how he knew?</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/Qs1dLe3GsQY?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Paleopocalypse! - A short video on the Adams Event, narrated by Stephen Fry.</span></figcaption>
</figure><img src="https://counter.theconversation.com/content/155580/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris Fogwill receives funding from UKRI and the Australian Research Council. A huge thanks to Professor Alan Cooper, Honorary Researcher at the South Australian Museum, who co-led this study, Adjunct Professor Ken McCracken and Dr Jonathan Palmer at the University of New South Wales, Drew Lorrey at the New Zealand National Institute of Water and Atmospheric Research, Dr Janet Willmshurst at Landcare Research and our co-authors on the published article.
</span></em></p><p class="fine-print"><em><span>Professor Alan Hogg works for University of Waikato in Hamilton, New Zealand. He is an Associate Investigator in a Royal Society of New Zealand Marsden grant - MFP-NIW1803: Dr Andrew Lorrey, NIWA, Auckland, Principal Investigator.</span></em></p><p class="fine-print"><em><span>Chris Turney receives funding fromthe Australian Research Council and is a scientific advisor to cleantech graphite company, CarbonScape (<a href="https://www.carbonscape.com">https://www.carbonscape.com</a>).</span></em></p><p class="fine-print"><em><span>Zoë Thomas receives funding from the Australian Research Council.</span></em></p>Scientists have uncovered evidence of a global paleopocalyspe.Chris Fogwill, Professor of Glaciology and Palaeoclimatology, Head of School Geography, Geology and the Environment and Director of the Institute for Sustainable Futures, Keele UniversityAlan Hogg, Professor, Director, Carbon Dating Laboratory, University of WaikatoChristian Turney, Professor of Earth Science and Climate Change, Director of the Earth and Sustainability Science Research Centre, Director of Chronos 14Carbon-Cycle Facility, and UNSW Director of ARC Centre for Excellence in Australian Biodiversity and Heritage, UNSW SydneyZoë Thomas, ARC DECRA Fellow, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1487032020-10-29T14:55:49Z2020-10-29T14:55:49ZCentral Asia risks becoming a hyperarid desert in the near future<figure><img src="https://images.theconversation.com/files/366451/original/file-20201029-15-fi2ie7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Jakub Czajkowski / shutterstock</span></span></figcaption></figure><p>Around 34 million years ago, sudden climate change caused ecological breakdown in Central Asia. This ancient event, triggered by rapid drops in temperature and atmospheric carbon dioxide, permanently affected biological diversity in the region. Large areas of Mongolia, (geographic) Tibet and north-western China suddenly became hyperarid deserts with little vegetation cover – and stayed that way for almost 20 million years.</p>
<p>This was a surprising finding of <a href="https://advances.sciencemag.org/content/6/41/eabb8227">new research</a> I carried out with colleagues from across Europe and China, in which we reconstructed the past 43 million years of evolutionary history for the steppe, semi-desert and desert ecosystems of Central Asia (the biogeographical and political conceptions of “Central Asia” differ and we use the former: our research area is shown below). </p>
<p>Many scientists had <a href="https://cp.copernicus.org/articles/4/153/2008/">previously thought</a> that this region was forested for much of that time and only grew drier later on, culminating today in massive, exceptionally arid Asian deserts such as the Gobi and Taklimakan.</p>
<figure class="align-center ">
<img alt="image showing a map, some plants and a cross section of some mountains and a desert" src="https://images.theconversation.com/files/366386/original/file-20201029-15-1gs6mtx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/366386/original/file-20201029-15-1gs6mtx.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=424&fit=crop&dpr=1 600w, https://images.theconversation.com/files/366386/original/file-20201029-15-1gs6mtx.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=424&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/366386/original/file-20201029-15-1gs6mtx.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=424&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/366386/original/file-20201029-15-1gs6mtx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=533&fit=crop&dpr=1 754w, https://images.theconversation.com/files/366386/original/file-20201029-15-1gs6mtx.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=533&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/366386/original/file-20201029-15-1gs6mtx.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=533&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The modern Central Asian steppe-desert (A), characteristic plant families (B), and an altitudinal profile illustrating vegetation belts of the steppe subtypes (C).</span>
<span class="attribution"><span class="source">Science Advances 2020; 6: eabb8227</span></span>
</figcaption>
</figure>
<p>We found that fossil pollen combined with mammal fossils, geological and climatic evidence – all preserved inside ancient rocks – told a different tale. Ancient “wet” steppe-deserts that received enough precipitation to maintain high biodiversity already existed during the late Eocene (40 to 34 million years ago), but suddenly became much colder and drier over an event called the Eocene‒Oligocene Transition (EOT). </p>
<p>Scientists already knew that global climate cooling in this period caused the formation of a <a href="https://www.nature.com/articles/nature08447?platform=hootsuite">permanent Antarctic ice-sheet</a>, but what happened on different continents is less clear. Our new study found that the lowlands of Central Asia became hyperarid deserts with little vegetation cover. The lack of food resources meant that larger animals were mainly replaced by small mammals like rodents, rabbits and hares. </p>
<figure class="align-center ">
<img alt="Three bits of fossilised pollen viewed under a microscope" src="https://images.theconversation.com/files/366235/original/file-20201028-15-1q38yll.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/366235/original/file-20201028-15-1q38yll.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=292&fit=crop&dpr=1 600w, https://images.theconversation.com/files/366235/original/file-20201028-15-1q38yll.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=292&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/366235/original/file-20201028-15-1q38yll.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=292&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/366235/original/file-20201028-15-1q38yll.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=367&fit=crop&dpr=1 754w, https://images.theconversation.com/files/366235/original/file-20201028-15-1q38yll.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=367&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/366235/original/file-20201028-15-1q38yll.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=367&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Scanning electron microscope (SEM) images of fossil pollen used to reconstruct the ancient ecosystems of Central Asia. Scale bars represent 5 micrometres (0.005 mm).</span>
<span class="attribution"><span class="source">Carina Hoorn and Fang Han</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>This hyperaridity lasted for millions of years afterwards, and plants only recovered when the climate became temporarily wetter around 15 million years ago. But now, the major species were small, non-woody herbs, not the salt and drought- tolerant shrubs that had dominated before the ecological collapse. Despite large parts of Central Asia being very dry today, these shrubs (<em>Nitraria</em> and <em>Ephedra</em>) never again recovered their position of ecological prominence. We still don’t fully understand why, but it shows that populations can be permanently altered by sudden environmental changes even if widespread extinctions don’t occur. </p>
<p>This finding is particularly relevant today, because atmospheric carbon dioxide levels and climate are again changing rapidly. Given what we now know about the Asian steppe-desert’s climatic and ecological history, it is unlikely that these ecosystems will ever recover their present biological diversity if forced into a new state.</p>
<h2>History repeats itself</h2>
<p>The modern steppe-desert is the largest ecoregion of its kind in the world, hosting a lot more biodiversity than you might expect. Dry-adapted grasses and herbs support an array of wildlife, many of which are endemics (native to, and living only in, that region). These unique flora and fauna have evolved partly as a result of immense geological and climatic diversity: today Central Asia is home to some of the oldest deserts known, as well as the highest mountains outside of the Himalayas.</p>
<figure class="align-center ">
<img alt="Flat grassy land with snowy mountains in the background" src="https://images.theconversation.com/files/366212/original/file-20201028-17-1hw5ipq.jpg?ixlib=rb-1.1.0&rect=0%2C9%2C6252%2C3809&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/366212/original/file-20201028-17-1hw5ipq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=367&fit=crop&dpr=1 600w, https://images.theconversation.com/files/366212/original/file-20201028-17-1hw5ipq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=367&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/366212/original/file-20201028-17-1hw5ipq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=367&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/366212/original/file-20201028-17-1hw5ipq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=461&fit=crop&dpr=1 754w, https://images.theconversation.com/files/366212/original/file-20201028-17-1hw5ipq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=461&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/366212/original/file-20201028-17-1hw5ipq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=461&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Meadow steppes in the Qilian Mountains of northern China, surrounded by alpine steppe and tundra. Topographic growth in the Tibetan region over many millions of years has created new high-elevation ecosystems for cold-tolerant biota to thrive.</span>
<span class="attribution"><span class="source">Xiaoming Wang / imaggeo.egu.eu</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Ancient climate change and geological forces have shaped the steppe-desert through time. The collision of India with Asia, formation of the Tibetan Plateau and uplift of the Himalaya, Altai and Hangay mountain ranges created extreme altitudinal variation, as well as distinct <a href="https://www.nationalgeographic.org/encyclopedia/rain-shadow/">rain shadows</a> of dry land on the downwind side. This generated a mosaic of habitats, and in turn, an astonishing number of species who call the region home.</p>
<p>But now the steppe-desert’s biodiversity is under severe threat from human-induced climate change and land degradation. Growing seas of sand are claiming native steppes, <a href="https://www.nytimes.com/interactive/2016/10/24/world/asia/living-in-chinas-expanding-deserts.html">imposing desertification</a> at unprecedented rates. Evidence from the past shows us that this is a sign of impending ecosystem breakdown – and it will cause irreversible changes and loss of biodiversity if allowed to continue.</p>
<h2>Claimed by the desert</h2>
<p>Desertification in Asia has major implications for humans too. It now threatens almost half a billion people, many of whom are finding it increasingly difficult to make a living in communities dominated by agriculture. Crops are ravaged by drought, livestock are losing grazing pastures, and deserts are growing towards the cities. </p>
<figure class="align-left ">
<img alt="Large sand dunes" src="https://images.theconversation.com/files/366251/original/file-20201028-23-aiirbj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/366251/original/file-20201028-23-aiirbj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=800&fit=crop&dpr=1 600w, https://images.theconversation.com/files/366251/original/file-20201028-23-aiirbj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=800&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/366251/original/file-20201028-23-aiirbj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=800&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/366251/original/file-20201028-23-aiirbj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1005&fit=crop&dpr=1 754w, https://images.theconversation.com/files/366251/original/file-20201028-23-aiirbj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1005&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/366251/original/file-20201028-23-aiirbj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1005&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Sand sea of the Taklimakan Desert. Similarly hyperarid deserts may have spread across Central Asia in the past as a result of rapid climate change.</span>
<span class="attribution"><span class="source">Matthias Alberti / imaggeo.egu.eu</span></span>
</figcaption>
</figure>
<p>Model predictions from the <a href="https://www.ipcc.ch/site/assets/uploads/2018/02/WGIIAR5-Chap24_FINAL.pdf">Intergovernmental Panel on Climate Change (IPCC)</a> and recent climate records show that interior Asia is fast becoming one of the hottest and driest places on the planet. Major predicted changes include highly reduced vegetation cover and rapid, severe species losses, along with more unreliable rainfall and high dust emissions generated by widespread desertification and erosion. </p>
<p>This new hyperarid desert ecosystem phase would resemble the inhospitable, barren landscapes that spread 34 million years ago. Lessons from the past make it clear that current human-induced global changes must be urgently halted in order to preserve the Asian steppe, which has now become one of the world’s most endangered habitats.</p><img src="https://counter.theconversation.com/content/148703/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Natasha Barbolini receives funding from the European Research Council (grant MAGIC 649081), the Swedish Research Council (grant VR 2017-03985), and the Bolin Centre for Climate Research (grant RA6_2019_12). </span></em></p>We found evidence of irreversible ecological breakdown millions of years ago – this time round, we should heed the warning signs.Natasha Barbolini, Senior postdoctoral fellow in palaeoecology, Stockholm UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1447842020-09-03T12:27:17Z2020-09-03T12:27:17ZVideo: How ancient ice cores show ‘black swan’ events in history – even pandemics<p><em><a href="https://byrd.osu.edu/people/thompson.3">Lonnie Thompson</a> and <a href="https://byrd.osu.edu/people/thompson.4">Ellen Mosley-Thompson</a> at The Ohio State University have been studying ice cores from around the world for over 30 years. They collect, <a href="https://byrd.osu.edu/research/facilities/cold-storage-ice-core">store</a> and study ice cores to understand the history of the Earth’s climate and preserve them for future scientists. In this interview, they explain how <a href="https://youtu.be/ZHOdqb9ViLw">ice cores preserve evidence</a> of rare but impactful changes in Earth’s history often called “black swan” events, as well as smaller environmental changes and why it is necessary to preserve the ice cores and the glaciers they come from.</em></p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/ZHOdqb9ViLw?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">What can ice cores tell us about historical black swan events?</span></figcaption>
</figure>
<h2>How do ice cores help in understanding the past?</h2>
<p>Ice cores are columns of ice drilled through glaciers that are highly versatile and detailed recorders of Earth’s climate and environment that cover hundreds to many thousands of years. </p>
<p>They store anything that is found in the atmosphere, such as atmospheric gases, pollen, microbes, emissions from volcanic eruptions, dust and salts carried by dust storms from deserts and salt flats, agricultural and grazing lands. They even can record ocean spray along with pollutants from human activities such as lead, mercury and <a href="https://www.epa.gov/radiation/radionuclides">radioactive nuclides</a> from thermonuclear bomb tests. </p>
<p>Ice also preserves records of past temperature in the changing isotopic composition of water, and provides histories of snowfall by the thicknesses of ice that has formed each year.</p>
<figure class="align-center ">
<img alt="Three ice cores recovered from different depths." src="https://images.theconversation.com/files/355642/original/file-20200831-19-2fbq7h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/355642/original/file-20200831-19-2fbq7h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=175&fit=crop&dpr=1 600w, https://images.theconversation.com/files/355642/original/file-20200831-19-2fbq7h.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=175&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/355642/original/file-20200831-19-2fbq7h.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=175&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/355642/original/file-20200831-19-2fbq7h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=220&fit=crop&dpr=1 754w, https://images.theconversation.com/files/355642/original/file-20200831-19-2fbq7h.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=220&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/355642/original/file-20200831-19-2fbq7h.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=220&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Relatively young and shallow snow becomes packed into coarse and granular crystals called firn (top: 53 meters deep). Older and deeper snow is compacted further (middle: 1,836 meters). At the bottom of a core (lower: 3,050 meters), rocks, sand and silt discolor the ice. (Photographs courtesy U.S. National Ice Core Laboratory)</span>
<span class="attribution"><a class="source" href="https://earthobservatory.nasa.gov/features/Paleoclimatology_IceCores">U.S. National Ice Core Laboratory</a></span>
</figcaption>
</figure>
<h2>How can studying ice cores help learn about historical events?</h2>
<p>Ice cores provide independent histories of past climate and environmental change that can often be compared to written and archaeological records of human history. This is especially true in the lower latitudes where earlier cultures rose and fell. For example, ice cores from the <a href="https://news.osu.edu/documenting-the-demise-of-quelccaya-the-worlds-largest-tropical-ice-cap/">Quelccaya ice cap</a> in the southern Peruvian Andes provide a nearly 2,000 year-by-year history of tropical climate that has helped anthropologists study how changes in temperature, and annual precipitation and drought patterns tracked the rise and fall of ancient Andean civilizations. For example, a major drought, recorded by precipitation (snowfall) and dust records in the Quelccaya cores, may have played a role in the <a href="https://www.smithsonianmag.com/science-nature/chronicling-the-ice-156979331/">demise of the Tiwanaku civilization</a> around the year 1000. </p>
<p>Abrupt global events and “black swans,” or rare but impactful events, have been observed using ice core-derived paleoclimate information from high elevation tropical mountains. For example, evidence of the so-called “<a href="https://doi.org/10.1177/097194580701000203">East India Drought</a>” in the late 18th century was noticed in ice cores from both the Peruvian Andes and the Himalayas. This drought was partially responsible for <a href="https://doi.org/10.1007/s00382-019-04694-4">million of deaths in India</a>. This was a time when several successive El Niños occurred and were linked with the failure of the monsoon rains and decreasing precipitation in parts of tropical South America. Severe droughts were also documented in Egypt, Java, Australia, Mexico and the Caribbean. Great <a href="https://doi.org/10.1177/097194580701000203">social upheavals</a>, including four civil wars, occurred around the world. </p>
<p>Further back in time, several tropical ice core records contain evidence of <a href="https://www.wired.com/beyond-the-beyond/2018/08/collapse-civilizations-worldwide-defines-youngest-unit-geologic-time-scale/">a major worldwide drought</a> about 4,200 years ago. This occurred during the rapid decline of the Akkadian Empire in Mesopotamia, the Harappan civilization in the Indus Valley, the so-called Old Kingdom in Egypt and the Longshan Culture in East China. </p>
<h2>What kind of evidence would the current pandemic leave in the ice?</h2>
<p>Some <a href="https://doi.org/10.1002/2017GH000064">ice core records show</a> that during the mid-1300s there was less lead in the atmosphere, possibly related to the sharp drop in mining and smelting activities. This coincided with the appearance of the plague known as the “Black Death” in Europe and Asia. This decrease in human industrial activity is analogous to what is happening now during the current COVID-19 pandemic. Throughout the world people are traveling less, resulting in a <a href="https://theconversation.com/covid-19-shutdowns-are-clearing-the-air-but-pollution-will-return-as-economies-reopen-134610">reduction in emissions of carbon dioxide, nitrogen dioxide and sulfur dioxide</a> into the atmosphere. Future glaciologists will likely see decreases in these gases and their chemical derivatives in ice cores.</p>
<h2>As glaciers around the world recede due to climate change, how will it affect our ability to study the past?</h2>
<p>Ice cores stored in freezer facilities become extremely important for future research as these unique archives of our past melt away on our warming Earth. The world’s ice is melting at an accelerating rate and this ice melt has already led to the major shrinking or loss of the smaller and very sensitive mountain glaciers in the Tropics such as some glaciers on Kilimanjaro and virtually all glaciers in Papua, Indonesia (New Guinea), where soon <a href="https://doi.org/10.1073/pnas.1822037116">all the ice is likely to disappear</a>. </p>
<figure class="align-center ">
<img alt="A 3d render of the Puncak Jaya glacier in Indonesia" src="https://images.theconversation.com/files/355645/original/file-20200831-14-1jxznmw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/355645/original/file-20200831-14-1jxznmw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/355645/original/file-20200831-14-1jxznmw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/355645/original/file-20200831-14-1jxznmw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/355645/original/file-20200831-14-1jxznmw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/355645/original/file-20200831-14-1jxznmw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/355645/original/file-20200831-14-1jxznmw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A 3D render of a glacier on the Puncak Jaya peak in Indonesia.</span>
<span class="attribution"><span class="source">Google Earth / Maxar Technologies</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<h2>What are the possible impacts of receding glaciers in the areas that you study?</h2>
<p><a href="https://doi.org/10.1038/s41598-018-33698-z">As mountain glaciers disappear</a> and the streams and rivers that arise from them are affected, nearby communities, and to a lesser extent communities further downstream, face the greatest economic and societal consequences, including disruptions to agriculture, hydropower generation, urban water supply and tourism. In many places such as the Andes and Himalayas, glaciers have deep historical, cultural, and even spiritual meaning to the people who live in their shadows. </p>
<p>For example, since our first studies of the Quelccaya ice cap in southern Peru in 1974, we have been interacting with people in the local communities just to the west. Since the mid-1970s, Quelccaya has lost almost 40% of its area.<strong>link</strong> During the dry season, many of the grasslands that feed the herds of alpacas, llamas and sheep of the people in Phinaya, a local semi-nomadic pastoralist community, can be irrigated only with the water that runs off the ice cap and other glaciated peaks that are part of their territorial domain. </p>
<p>Quelccaya is also considered a very important apu, or sacred mountain, local deity, and ancestor. We encountered similar beliefs in Bolivia and in Papua, Indonesia (New Guinea).</p>
<p>The melting of mountain glaciers also presents hazards to local communities. Melting ice forms new lakes along the glacier margins, and the water is held back by natural dams which often fail. For example, we have mapped the <a href="https://news.osu.edu/documenting-the-demise-of-quelccaya-the-worlds-largest-tropical-ice-cap/">retreat of Quelccaya’s Qori Kalis</a> outlet glacier since 1978. A lake started forming in this valley in 1991 and grew to cover 84 acres and to be 200 ft deep. In March 2006, an avalanche from the ice cap fell into the lake, causing the lake to overtop the moraine dam and drown grazing alpaca along the outlet stream.</p>
<figure class="align-center ">
<img alt="A satellite image of lakes formed by melting glaciers in Bhutan" src="https://images.theconversation.com/files/355635/original/file-20200831-20-wxnr2o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/355635/original/file-20200831-20-wxnr2o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=334&fit=crop&dpr=1 600w, https://images.theconversation.com/files/355635/original/file-20200831-20-wxnr2o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=334&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/355635/original/file-20200831-20-wxnr2o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=334&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/355635/original/file-20200831-20-wxnr2o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=420&fit=crop&dpr=1 754w, https://images.theconversation.com/files/355635/original/file-20200831-20-wxnr2o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=420&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/355635/original/file-20200831-20-wxnr2o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=420&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">This ASTER image shows the lakes left behind by retreating glaciers in the Bhutan-Himalaya.</span>
<span class="attribution"><a class="source" href="https://earthobservatory.nasa.gov/features/GLIMS">Jeffrey Kargel / USGS/NASA</a></span>
</figcaption>
</figure>
<p>[<em>Deep knowledge, daily.</em> <a href="https://theconversation.com/us/newsletters/the-daily-3?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=deepknowledge">Sign up for The Conversation’s newsletter</a>.]</p><img src="https://counter.theconversation.com/content/144784/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lonnie Thompson receives funding from the NSF Paleoclimatology program and The Ohio State University</span></em></p><p class="fine-print"><em><span>Ellen Mosley-Thompson receives funding from NSF Paleoclimatology program and The Ohio State University</span></em></p>Ice cores can preserve evidence of ‘black swan’ events like pandemics and droughts, but the glaciers from which they are collected are disappearing.Lonnie Thompson, Distinguished University Professor, Earth Sciences, The Ohio State UniversityEllen Mosley-Thompson, Distinguished University Professor, Geography (Atmospheric Sciences), Senior Research Scientist, The Ohio State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1261412019-11-12T14:47:19Z2019-11-12T14:47:19ZThe East Asian monsoon is many millions of years older than we thought<figure><img src="https://images.theconversation.com/files/301317/original/file-20191112-178532-a5cjd9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Tanya Bill / Shutterstock</span></span></figcaption></figure><p>The East Asian monsoon covers much of the largest continent on Earth leading to rain in the summer in Japan, the Koreas and lots of China. Ultimately, more than 1.5 billion people depend on the water it provides for agriculture, industry and hydroelectric power. </p>
<p>Understanding the monsoon is essential. That is why colleagues and I recently reconstructed its behaviour throughout its 145m-year history, in order to better understand how it acts in response to changes in geography or the wider climate in the very long term, and what that might mean for the future. </p>
<p>Our study, published in the journal <a href="https://advances.sciencemag.org/content/5/10/eaax1697/tab-article-info">Science Advances</a> indicates that the East Asian monsoon is much older and more varied than previously thought. Until quite recently the general consensus was that the monsoon came into being around <a href="https://www.sciencedirect.com/science/article/pii/S2468265917301142">23m years ago</a>, some time after the Tibetan Plateau was formed.</p>
<p>However, we show that it has been ever present for at least the past 145m years (except during the Late Cretaceous: the era of <em>T. Rex</em>), regardless of whether there was a Tibetan Plateau or how much CO₂ was in the atmosphere. </p>
<h2>What is a monsoon?</h2>
<p>At its most simple level a monsoon is a highly seasonal distribution in precipitation leading to a distinct “wet” and “dry” seasons – the word even derives from the Arabic “mausim”, translated as “season”. </p>
<p>The East Asian monsoon is a “sea breeze monsoon”, the most common type. They form because land and sea heat up at different rates, so high pressure forms over the sea and low pressure over land which results in wind blowing onshore in the summer.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/301322/original/file-20191112-178516-1o4ntuo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/301322/original/file-20191112-178516-1o4ntuo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/301322/original/file-20191112-178516-1o4ntuo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=353&fit=crop&dpr=1 600w, https://images.theconversation.com/files/301322/original/file-20191112-178516-1o4ntuo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=353&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/301322/original/file-20191112-178516-1o4ntuo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=353&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/301322/original/file-20191112-178516-1o4ntuo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=444&fit=crop&dpr=1 754w, https://images.theconversation.com/files/301322/original/file-20191112-178516-1o4ntuo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=444&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/301322/original/file-20191112-178516-1o4ntuo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=444&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">It’s the world’s largest, highest plateau.</span>
<span class="attribution"><span class="source">Rashevskyi Viacheslav / shutterstock</span></span>
</figcaption>
</figure>
<p>Although The Tibetan Plateau is not strictly needed to form the East Asian monsoon it can serve to enhance it. At 5km or more above sea level, the plateau simply sits much higher in the atmosphere and thus the air above it is heated much more than the same air would be at a lower elevation (consider the ground temperature in Tibet compared to the freezing air 5km above your head). As that Tibetan air is warmer than the surrounding cold air it rises and acts as a heat “pump”, sucking more air in to replace it and enhancing the monsoon circulation. </p>
<h2>Changes over the (millions of) years</h2>
<p>We found the intensity of the monsoon has varied significantly over the past 145m years. At first, it was around 30% weaker than today. Then, during the Late Cretaceous 100-66m years ago, a huge inland sea covered much of North America and weakened the Pacific trade winds. This caused East Asia to become very arid due to the monsoon disappearing. </p>
<p>However, rainfall patterns changed substantially after the Indian tectonic plate collided into the Asian continent around 50m years ago, forming the Himalayas and the Tibetan Plateau. As the land rose up, so did the strength of the monsoon. Our results suggest that 5-10m years ago there were “super-monsoons” with rainfall 30% stronger than today. </p>
<p>But how can we be sure that such changes were caused by geography, and not elevated carbon dioxide concentrations? To test this, we again modelled the climate for all different time periods (roughly every 4m years) and increased or reduced the amount of CO₂ in the atmosphere to see what effect this had on the monsoon. In general, irrespective of time period chosen, the monsoon showed little sensitivity (-1% to +13%) to changes in CO₂ compared to the impact of changes in regional geography. </p>
<h2>Climate models are working</h2>
<p>The monsoon in East Asia is mainly a result of its favourable geographic position and regional topography – though our work shows that CO₂ concentrations do have an impact, they are secondary to tectonics. </p>
<p>The past can help us better understand how the monsoon will behave as the climate changes – but its not a perfect analogue. Although rainfall increased almost every time CO₂ doubled in the past, each of these periods was unique and dependent on the specific geography at the time. </p>
<p>The reassuring thing is that climate models are showing agreement with geological data through the past. That means we have greater confidence that climate models are able to accurately predict how the monsoon will respond over the next century as humans continue to emit more CO₂ into the atmosphere.</p><img src="https://counter.theconversation.com/content/126141/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alex Farnsworth receives funding from The Natural Environment Research Council (NERC).</span></em></p>Scientists have reconstructed the monsoon over 145m years – and found it predates the Himalayas.Alex Farnsworth, Postdoctoral Research Associate in meteorology, University of BristolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1135182019-05-09T10:37:37Z2019-05-09T10:37:37ZDeep sea carbon reservoirs once superheated the Earth – could it happen again?<figure><img src="https://images.theconversation.com/files/273412/original/file-20190508-183077-p58kfz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Droplets rising from the Champagne vent on the ocean floor in the Mariana Islands. Fluids venting from the site contain dissolved carbon dioxide. </span> <span class="attribution"><a class="source" href="https://oceanexplorer.noaa.gov/explorations/04fire/logs/april10/media/bubbles.html">NOAA Ocean Explorer</a></span></figcaption></figure><p>As concern grows over human-induced climate change, many scientists are looking back through Earth’s history to events that can shed light on changes occurring today. Analyzing how the planet’s climate system has changed in the past improves our understanding of how it may behave in the future.</p>
<p>It is now clear from these studies that abrupt warming events are <a href="https://www.britannica.com/science/climate-change/Abrupt-climate-changes-in-Earth-history">built into Earth’s climate system</a>. They have occurred when disturbances in carbon storage at Earth’s surface released greenhouse gases into the atmosphere. One of the grand challenges for <a href="https://scholar.google.com/citations?user=0-0jvDwAAAAJ&hl=en">climate scientists like me</a> is to determine where these releases came from before humans were present, and what triggered them. Importantly, we want to know if such an event could happen again.</p>
<p>In a recently published study, my colleagues <a href="http://rses.anu.edu.au/people/katie-harazin">Katie Harazin</a>, <a href="https://portal.research.lu.se/portal/en/persons/nadine-b-quintana-krupinski(3cc6c619-0e19-492f-b33b-94ca161bebf7).html">Nadine Krupinski</a> and I discovered that at the end of the last glacial era, about 20,000 years ago, carbon dioxide was <a href="https://doi.org/10.1088/1748-9326/aafe28">released into the ocean from geologic reservoirs</a> located on the seafloor when the oceans began to warm. </p>
<p>This finding is a potential game-changer. Naturally occurring reservoirs of carbon in the modern ocean could be disturbed again, with potentially serious effects to Earth’s oceans and climate.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/273388/original/file-20190508-183109-wr9bpt.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/273388/original/file-20190508-183109-wr9bpt.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/273388/original/file-20190508-183109-wr9bpt.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=202&fit=crop&dpr=1 600w, https://images.theconversation.com/files/273388/original/file-20190508-183109-wr9bpt.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=202&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/273388/original/file-20190508-183109-wr9bpt.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=202&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/273388/original/file-20190508-183109-wr9bpt.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=253&fit=crop&dpr=1 754w, https://images.theconversation.com/files/273388/original/file-20190508-183109-wr9bpt.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=253&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/273388/original/file-20190508-183109-wr9bpt.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=253&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Earth has cycled between ice ages (low points) and warm interglacial periods over the past 800,000 years. But current climatic warming is occurring much faster than past warming events.</span>
<span class="attribution"><a class="source" href="https://earthobservatory.nasa.gov/features/GlobalWarming/page3.php">NASA</a></span>
</figcaption>
</figure>
<h2>The past is prologue</h2>
<p>One of the best-known examples of a rapid warming caused by release of geologic carbon is the <a href="https://doi.org/10.1038/353225a0">Paleocene-Eocene Thermal Maximum</a>, or PETM, a major global warming event that occured about 55 million years ago. During the PETM, the Earth warmed by 9 to 16 degrees Fahrenheit (5 to 9 degrees Celsius) within about 10,000 years. </p>
<p>Climate scientists now consider the PETM to be an <a href="https://doi.org/10.1038/ngeo2681">analog for environmental changes taking place today</a>. The PETM happened over a longer period and without human involvement, but it shows that there is inherent instability in the climate system if carbon from geologic reservoirs is released rapidly. </p>
<p>Scientists also know that atmospheric carbon dioxide levels rose rapidly at the end of <a href="https://commons.wikimedia.org/wiki/File:Atmospheric_CO2_with_glaciers_cycles.png">each of the late Pleistocene ice ages</a>, helping to warm the climate. During the most recent warming episode, 17,000 years ago, the Earth warmed by 9 to 13 degrees Fahrenheit (5 to 7 degrees Celsius).</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/ldLBoErAhz4?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The Paleocene-Eocene Thermal Maximum warmed the planet so dramatically that tropical rain forests extended northward to the Arctic.</span></figcaption>
</figure>
<p>However, hundreds of scientific studies have failed to establish what caused the rapid carbon dioxide increases that ended each ice age. Researchers agree that the ocean must be involved because it acts as a large carbon capacitor, <a href="https://interactiveoceans.washington.edu/story/Carbon_Cycle">regulating the amount of carbon that resides in the atmosphere</a>. But they are still searching for clues to understand what influences the amount of carbon in the ocean during abrupt climate changes.</p>
<h2>Lakes on the ocean floor</h2>
<p>Over the past two decades, ocean scientists have discovered that there are reservoirs of liquid and solid carbon dioxide accumulating at the bottom of the ocean, within the rocks and sediments on the margins of active <a href="https://oceanservice.noaa.gov/facts/vents.html">hydrothermal vents</a>. At these sites, volcanic magma from within the Earth meets superheated water, producing plumes of carbon dioxide-rich fluids that filter through crevices in the Earth’s crust, migrating upward towards the surface.</p>
<p>When a plume of this fluid meets cold seawater, the carbon dioxide can solidify into a form called hydrate. The hydrate forms a cap that traps carbon dioxide within the rocks and sediments and keeps it from entering the ocean. But at temperatures above roughly 48 degrees Fahrenheit (9 degrees Celsius), hydrate will melt, releasing buoyant liquid or gaseous carbon dioxide directly into the overlying water. </p>
<p>Scientists have thus far documented reservoirs of liquid and hydrate carbon dioxide in the western Pacific near Taiwan and in the <a href="https://www.whoi.edu/news-release/co2-pools">Aegean Sea</a>. In shallower waters, where ocean temperatures are warmer and pressure is lower, researchers have observed pure carbon dioxide <a href="https://phys.org/news/2018-02-carbon-dioxide-leakage-seabed.html">emanating directly from sediments as a gas</a> and rising to the ocean’s surface.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/273377/original/file-20190508-183106-1iccetn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/273377/original/file-20190508-183106-1iccetn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/273377/original/file-20190508-183106-1iccetn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=377&fit=crop&dpr=1 600w, https://images.theconversation.com/files/273377/original/file-20190508-183106-1iccetn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=377&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/273377/original/file-20190508-183106-1iccetn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=377&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/273377/original/file-20190508-183106-1iccetn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=474&fit=crop&dpr=1 754w, https://images.theconversation.com/files/273377/original/file-20190508-183106-1iccetn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=474&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/273377/original/file-20190508-183106-1iccetn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=474&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Nearly pure carbon dioxide bubbles rise from sediments that blanket an active hydrothermal system in the western tropical Pacific.</span>
<span class="attribution"><a class="source" href="https://iopscience.iop.org/article/10.1088/1748-9326/aafe28">Photos by Roy Price, courtesy of Jan Amend</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>A climate wild card</h2>
<p>These discoveries are changing scientists’ understanding of the marine carbon system. Climate scientists have not included deep sea carbon reservoirs in current models that explore the potential impacts of future warming, because little is known about the size and distribution of these carbon sources. </p>
<p>In fact, there is virtually no data that documents how much carbon dioxide is currently being released from these reservoirs into the ocean. This makes the geologic history critically important: It confirms that these types of reservoirs have the capacity to release vast amounts of carbon when they are disturbed.</p>
<p>Analogous carbon reservoirs have also been identified in terrestrial environments. In 1979, Indonesia’s Dieng volcano <a href="https://doi.org/10.1016/0377-0273(89)90058-9">suffocated 142 people</a> when it released nearly pure carbon dioxide. In 1986, a carbon dioxide reservoir at the bottom of Lake Nyos in Cameroon <a href="http://volcano.oregonstate.edu/silent-deadly">erupted</a>, killing 1,700 local villagers and hundreds of animals. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/273385/original/file-20190508-183109-ijlwoy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/273385/original/file-20190508-183109-ijlwoy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/273385/original/file-20190508-183109-ijlwoy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=902&fit=crop&dpr=1 600w, https://images.theconversation.com/files/273385/original/file-20190508-183109-ijlwoy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=902&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/273385/original/file-20190508-183109-ijlwoy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=902&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/273385/original/file-20190508-183109-ijlwoy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1134&fit=crop&dpr=1 754w, https://images.theconversation.com/files/273385/original/file-20190508-183109-ijlwoy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1134&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/273385/original/file-20190508-183109-ijlwoy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1134&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Cow suffocated by carbon dioxide in the 1986 Lake Nyos eruption.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Lake_Nyos_disaster#/media/File:Cow_killed_by_Lake_Nyos_gasses.jpg">USGS/Jack Lockwood</a></span>
</figcaption>
</figure>
<p>Carbon dioxide is also venting around Mammoth Mountain, California, at spots where magma rises through Earth’s crust and stalls at shallow depths. High concentrations of carbon dioxide in the soil have <a href="https://volcanoes.usgs.gov/volcanoes/long_valley/field_guides_horseshoe_lake.html">killed more than 100 acres of trees</a>. Scientists are working to identify and characterize <a href="https://www.smithsonianmag.com/science-nature/defusing-africas-killer-lakes-88765263/">other sites on land</a> where such releases could occur.</p>
<p>It is much more challenging to quantify the carbon dioxide stored in ocean reservoirs. Vast regions of the seafloor contain sites of active volcanism and hydrothermal venting, but scientists know virtually nothing about how much carbon dioxide is accumulating in surrounding rocks and sediments. In my view, there is an urgent need to study marine settings where carbon dioxide is likely accumulating, and then to assess how susceptible they may be to destabilization. </p>
<h2>Warming oceans, increasing risk</h2>
<p>This is not an endeavor that should be deferred. Earth’s oceans are warming rapidly, and climate models project that they will warm fastest near the poles, where deep currents form that <a href="https://oceanservice.noaa.gov/facts/conveyor.html">carry warming waters downward from the surface</a>. </p>
<p>As these warm waters sink into the ocean’s interior, they transport excess heat towards sites where carbon dioxide reservoirs can form. Those warmer waters will eventually destabilize the hydrate seals that keep liquid carbon dioxide trapped.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/273379/original/file-20190508-183080-bnph48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/273379/original/file-20190508-183080-bnph48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/273379/original/file-20190508-183080-bnph48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=358&fit=crop&dpr=1 600w, https://images.theconversation.com/files/273379/original/file-20190508-183080-bnph48.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=358&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/273379/original/file-20190508-183080-bnph48.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=358&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/273379/original/file-20190508-183080-bnph48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=450&fit=crop&dpr=1 754w, https://images.theconversation.com/files/273379/original/file-20190508-183080-bnph48.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=450&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/273379/original/file-20190508-183080-bnph48.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=450&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A very large, slow current called the thermohaline circulation carries warm water to Earth’s polar regions, where it cools and sinks to the deep oceans.</span>
<span class="attribution"><a class="source" href="http://www.grida.no/resources/6918">Maphoto/Riccardo Pravettoni via GRID-Arendal</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>One such reservoir occurs in the western Pacific west of the <a href="https://en.wikipedia.org/wiki/Okinawa_Trough">Okinawa Trough</a> in the East China Sea. The temperature of the bottom waters at this location is 37 to 39 degrees Fahrenheit (3 to 4 degrees Celsius), which means the hydrate cap is within about 4-5 degrees Celsius of its melting point. </p>
<p>Importantly, warm hydrothermal fluids are rising from below the carbon dioxide reservoir toward the surface. As the oceans continue to warm, the temperature difference between cold ocean waters and warmer hydrothermal fluids will decrease. This will cause the hydrate to thin, potentially to a point where it will no longer keep liquid carbon dioxide from escaping.</p>
<p>To date there has been no research to assess whether these ocean carbon dioxide reservoirs are vulnerable to rising ocean temperatures. But Earth’s pre-historic record clearly demonstrates that geologic reservoirs can be destabilized – and that when they are, it leads to rapid increases in atmospheric carbon dioxide and global warming. In my view, this represents an important unknown risk that cannot be ignored.</p><img src="https://counter.theconversation.com/content/113518/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lowell D. Stott receives funding from the National Science Foundation. </span></em></p>Thousands of years ago, carbon gases trapped on the seafloor escaped, causing drastic warming that helped end the last ice age. A scientist says climate change could cause this process to repeat.Lowell D. Stott, Professor, USC Dornsife College of Letters, Arts and SciencesLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1126722019-03-29T14:56:28Z2019-03-29T14:56:28ZLast of the giants: What killed off Madagascar’s megafauna a thousand years ago?<figure><img src="https://images.theconversation.com/files/265740/original/file-20190325-36264-n5hk55.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A modern mouse lemur *Microcebus* sits upon the cranium of an extinct *Megaladapis* lemur.</span> <span class="attribution"><a class="source" href="https://www.daovanhoang.com">Dao Van Hoang www.daovanhoang.com</a></span></figcaption></figure><p>Giant 10-foot-tall elephant birds, with eggs eight times larger than an ostrich’s. Sloth lemurs bigger than a panda, weighing in at 350 pounds. A puma-like predator called the giant fosa.</p>
<p>They sound like characters in a child’s fantasy book, but along with dozens of other species, they once really roamed the landscape of Madagascar. Then, after millions of years of evolution in the middle of the Indian Ocean, the populations crashed in just a couple of centuries.</p>
<p>Scientists know that over the past 40,000 years, most of Earth’s megafauna – that is, animals human-size or larger – have gone extinct. Woolly mammoths, sabre tooth tigers and countless others no longer roam the planet.</p>
<p>What’s remarkable about the megafaunal crash in Madagascar is that it occurred not tens of thousands of years ago but just over 1,000 years ago, between A.D. 700 and 1000. And while some small populations survived a while longer, the damage was done in a relatively short amount of time. Why?</p>
<p>Over the last three years, new investigations into climate and land use patterns, human genetic diversity on the island and the dating of hundreds of fossils have fundamentally changed scientists’ understanding of the human and natural history of Madagascar. <a href="https://scholar.google.com/citations?user=M6TfcNkAAAAJ&hl=en&oi=ao">As two</a> <a href="https://scholar.google.com/citations?user=DUkXIeAAAAAJ&hl=en&oi=ao">paleoclimatologists and</a> <a href="https://scholar.google.com/citations?user=wAfh3EYAAAAJ&hl=en&oi=ao">a paleontologist</a>, we brought together this research with new evidence of megafaunal butchery. In doing so we’ve created <a href="https://doi.org/10.1016/j.jhevol.2019.03.002">a new theory</a> of how, why and when these Malagasy megafauna went extinct.</p>
<h2>Climate at the time of the crash</h2>
<p>The first job is to understand exactly when the megafauna died out.</p>
<p>Radiocarbon dating of <a href="https://doi.org/10.1111/ecog.02376">over 400 recent fossils</a> demonstrates that animals under 22 pounds lived on Madagascar throughout the last 10,000 years. For animals over 22 pounds, there are abundant fossils up to 1,000 years ago, but relatively few since. The biggest decline in number of large animals occurred rapidly between A.D. 700 and 1000 – practically instantaneous given the long history of their existence on the island. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/265741/original/file-20190326-36267-10yh2bt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/265741/original/file-20190326-36267-10yh2bt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/265741/original/file-20190326-36267-10yh2bt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=805&fit=crop&dpr=1 600w, https://images.theconversation.com/files/265741/original/file-20190326-36267-10yh2bt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=805&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/265741/original/file-20190326-36267-10yh2bt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=805&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/265741/original/file-20190326-36267-10yh2bt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1011&fit=crop&dpr=1 754w, https://images.theconversation.com/files/265741/original/file-20190326-36267-10yh2bt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1011&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/265741/original/file-20190326-36267-10yh2bt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1011&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Malagasy graduate student and team member Peterson Faina with stalagmites in a cave in Madagascar.</span>
<span class="attribution"><span class="source">Laurie Godfrey</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>What was the climate doing at that time? One popular theory for the megafaunal extinction has blamed <a href="https://doi.org/10.1111/j.1365-2699.2009.02203.x">island-wide drying</a>. To test this idea, our team has been exploring the caves of Madagascar, collecting and analyzing stalagmites. As stalagmites grow upwards from the cave floor, layer by layer, differences in the chemistry of each layer document changes in the climate outside the cave.</p>
<p>By analyzing chemical composition and comparing ratios of various isotopes in these stalagmites, we created new <a href="https://doi.org/10.1016/j.quascirev.2017.03.017">high-resolution records of changes in the Malagasy ecosystems and climate</a>. We found minor fluctuations in the strength of the summer rains throughout the last 2,000 years, but no significant drying over that period. In fact, A.D. 780-960 was one of the wettest periods of the last 2,000 years. <a href="https://doi.org/10.1111/ecog.02376">Chemical analyses of fossils</a> back up this claim.</p>
<p>So it looks like there was no significant drying around the time the megafauna disappeared.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/265742/original/file-20190326-36279-1j4dim6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/265742/original/file-20190326-36279-1j4dim6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/265742/original/file-20190326-36279-1j4dim6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/265742/original/file-20190326-36279-1j4dim6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/265742/original/file-20190326-36279-1j4dim6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/265742/original/file-20190326-36279-1j4dim6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/265742/original/file-20190326-36279-1j4dim6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/265742/original/file-20190326-36279-1j4dim6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Many of the forests that originally existed on Madagascar are now replaced by more open, human-modified landscapes, like this palm savanna at Anjohibe.</span>
<span class="attribution"><span class="source">Laurie Godfrey</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Instead, the <a href="https://doi.org/10.1016/j.quascirev.2016.01.007">stalagmite records</a> indicated a rapid and dramatic change in the landscape. Changing ratios of the isotopes carbon-12 to carbon-13 reveal a switch from forests to grassland right around A.D. 900, the same time as the megafaunal population crash. Clearly something big happened around this time.</p>
<h2>Cut marks and evidence of butchery</h2>
<p>With no significant change in the climate, some point to the <a href="https://doi.org/10.1073/pnas.1534700100">arrival of humans</a> on the island as a possible cause of the megafauna population crash. It seems logical that once people arrived on Madagascar, they might have hunted the big animals into extinction. New data suggest that this timing doesn’t add up, though.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/265925/original/file-20190326-36264-1gtfa8j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/265925/original/file-20190326-36264-1gtfa8j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/265925/original/file-20190326-36264-1gtfa8j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=736&fit=crop&dpr=1 600w, https://images.theconversation.com/files/265925/original/file-20190326-36264-1gtfa8j.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=736&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/265925/original/file-20190326-36264-1gtfa8j.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=736&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/265925/original/file-20190326-36264-1gtfa8j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=924&fit=crop&dpr=1 754w, https://images.theconversation.com/files/265925/original/file-20190326-36264-1gtfa8j.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=924&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/265925/original/file-20190326-36264-1gtfa8j.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=924&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">One of two chop marks on the head of a femur of an extinct lemur, Pachylemur. This individual’s hind limb was removed from the trunk at the hip joint, probably with a machete.</span>
<span class="attribution"><span class="source">Lindsay Meador</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p><a href="https://doi.org/10.1126/sciadv.aat6925">According to new dates on fossil bones</a> with cut marks on them, humans arrived on Madagascar 10,500 years ago, much earlier than previously believed. But whoever these early people were, there’s no genetic evidence of them left on the island. <a href="https://doi.org/10.1073/pnas.1704906114">New analysis of the human genetic diversity</a> in modern Madagascar suggests the current population derives primarily from two waves of migration: first from Indonesia 3,000 to 2,000 years ago, and later from mainland Africa 1,500 years ago.</p>
<p>So it seems that people lived alongside the megafauna for thousands of years. How did the humans interact with the large animals?</p>
<p>Our new study found dozens of fossils with butchery marks. Cut and chop marks provide compelling evidence as to which species people were hunting and eating. Evidence of butchery of animals that are now extinct continues right up to the time of the megafaunal crash. Some people on Madagascar hunted and ate the megafauna for millennia without a population crash.</p>
<h2>Evidence for a change in land use</h2>
<p>If there was no obvious climate shift and humans lived alongside and sustainably hunted the megafauna for up to 9,000 years, what could have triggered the population crash?</p>
<p>The abrupt land use change might hold some clues. The transition from a forest-dominated ecosystem to a grassland-dominated ecosystem appears to be widespread. Scientists have identified this switch not only in the chemical signature of stalagmites but also in pollen grains buried in layers of <a href="https://doi.org/10.1016/0033-5894(87)90038-X">mud at the bottom of lakes</a>. Ancient lake sediments reveal two other changes occurred at the same time as the shift to grass species: an increase in charcoal from fires and an increase in the fungus <em>Sporormiella</em>, which is associated with the dung of large herbivores <a href="https://doi.org/10.1073/pnas.1534700100">such as cows</a>.</p>
<p>Evidence for simultaneous increases in grassland, fires, and cows and other domesticated animals points to a sudden change in Malagasy lifestyle: the introduction of cattle husbandry and slash-and-burn agriculture known locally as <a href="https://www.madamagazine.com/en/english-tavy-kahlschlag-einer-insel/">Tavy</a>. Here, forests are cut down to make space for rice paddies, and grassland burned to promote the growth of nutritious seedlings for cow fodder.</p>
<p>This move away from foraging and hunting toward farming meant the land could support more people. The result was a rapid rise in the size of the human population – and that’s what we conclude spelled disaster for the megafauna.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/266562/original/file-20190329-70982-kfun1k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/266562/original/file-20190329-70982-kfun1k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/266562/original/file-20190329-70982-kfun1k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/266562/original/file-20190329-70982-kfun1k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/266562/original/file-20190329-70982-kfun1k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/266562/original/file-20190329-70982-kfun1k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/266562/original/file-20190329-70982-kfun1k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/266562/original/file-20190329-70982-kfun1k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Some Malagasy farmers plow agricultural fields in the traditional way.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/malagasy-farmers-plowing-agricultural-field-traditional-266475566">Damian Ryszawy/Shutterstock.com</a></span>
</figcaption>
</figure>
<p>Here lies the contradiction of the situation: Hunting megafauna for survival became less important as people could rely on their agriculture and livestock. But cut marks on fossil bones indicate that hunting didn’t altogether stop just because people had other food sources. It turns out that the impact on the megafauna of larger human populations hunting just to supplement their diet was greater than the impact of smaller human populations relying more heavily on the native animals as a vital food source.</p>
<p>Bringing together new data on land use changes, climatic histories, genetics, fossil ages and butchery of the megafauna, we call this change “<a href="https://doi.org/10.1016/j.jhevol.2019.03.002">the subsistence shift hypothesis</a>.” Both the habitat loss and increase in human population arose out of a fundamental change in the way humans lived on Madagascar, from a more nomadic hunter-gatherer lifestyle to an agrarian society. We argue that it was this reorganization on Madagascar around A.D. 700-1000 that led to the crash in the megafaunal population.</p>
<p>Small populations of megafauna lived on in isolated pockets for another few centuries, but their fate was likely already sealed. The majority of the giant birds and animals that were once common across our planet have gone extinct. Many of the remaining giants, such as elephants and rhinos, are threatened or endangered. Will they go the same way as the Malagasy megafauna, casualties of humans’ changing lifestyles?</p><img src="https://counter.theconversation.com/content/112672/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Nick Scroxton receives funding from the National Science Foundation. </span></em></p><p class="fine-print"><em><span>Laurie Godfrey receives funding from the National Science Foundation BCS 1750598. </span></em></p><p class="fine-print"><em><span>Stephen Burns receives funding from the US National Science Foundation (grant AGS‐1702891/1702691).</span></em></p>A series of new studies sheds light on the population crash and extinction of the giant birds, lemurs and more that roamed the island until around A.D. 700-1000.Nick Scroxton, Postdoctoral Research Scholar in Paleoclimatology, UMass AmherstLaurie Godfrey, Emeritus Professor of Anthropology, UMass AmherstStephen Burns, Professor of Geosciences, UMass AmherstLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/823542017-08-30T17:10:41Z2017-08-30T17:10:41ZVolcanic emissions caused the warmest period in past 56m years – new study<figure><img src="https://images.theconversation.com/files/183980/original/file-20170830-23718-3yvsca.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Natali Snailcat / shutterstock</span></span></figcaption></figure><p>To predict what type of Earth lies ahead of us, we scientists usually turn to complex computer simulations. But how can we test whether these models are remotely accurate? Perhaps the best solution is to turn to instances in the geological past when Earth’s climate experienced similarly rapid warming. One such event is the <a href="http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-040610-133431">Palaeocene-Eocene Thermal Maximum</a> (PETM) that occurred 56m years ago.</p>
<p>In our <a href="http://nature.com/articles/doi:10.1038/nature23646">latest research</a>, we have identified the cause of this well-known warm period. Its links to present day climate change are clear.</p>
<p>Just prior to the PETM, Earth looked very different than it does today. The polar regions were devoid of ice sheets, with <a href="https://www.nature.com/nature/journal/v488/n7409/full/nature11300.html">temperate or even subtropical forests along the coastlines of Antarctica</a>, and Arctic Canada resembling the swamplands of modern Florida. The deep oceans were about 10°C warmer than today, and warm climate zones were all shifted polewards.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/183991/original/file-20170830-11295-cfkg9g.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/183991/original/file-20170830-11295-cfkg9g.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/183991/original/file-20170830-11295-cfkg9g.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=175&fit=crop&dpr=1 600w, https://images.theconversation.com/files/183991/original/file-20170830-11295-cfkg9g.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=175&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/183991/original/file-20170830-11295-cfkg9g.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=175&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/183991/original/file-20170830-11295-cfkg9g.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=220&fit=crop&dpr=1 754w, https://images.theconversation.com/files/183991/original/file-20170830-11295-cfkg9g.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=220&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/183991/original/file-20170830-11295-cfkg9g.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=220&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Global temperatures since the beginning of complex life on Earth. The PETM is the highest spike in the green line – it hasn’t been nearly as hot since.</span>
<span class="attribution"><a class="source" href="https://de.wikipedia.org/wiki/Datei:All_palaeotemps.svg">Glen Fergus / wiki</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Against the background of this “greenhouse world”, the planet warmed by at least a further <a href="http://www.sciencedirect.com/science/article/pii/S0012825213001207">5°C over a few thousand years</a> at the onset of the PETM. Life in the deep sea suffered disproportionately; many species went extinct and parts of the deep ocean <a href="http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-040610-133431">became anoxic</a> (oxygen depleted). On land, the water cycle strengthened, leading to both floods and droughts. It took about 150,000 years for Earth’s climate to naturally recover from this “fever” and regain some sort of equilibrium.</p>
<p>Here’s the really worrying part: 5°C over a few thousand years is breakneck speed in geological terms, but is still nothing compared to our current rate of warming. In fact, if we keep burning fossil fuels at our current rate, the <a href="http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_AnnexII_FINAL.pdf">worst-case scenarios</a> suggest we could hit 5°C by the end of the century.</p>
<h2>Blame the volcanoes</h2>
<p>So what can the PETM tells us about the future? It has long been suspected that the warm period was triggered by increasing greenhouse gas concentrations in the atmosphere. These gases absorb and trap solar heat, which is why any significant increase unavoidably leads to global warming. </p>
<p>We know there was a huge release of “new” carbon into the atmosphere and oceans at the time, thanks to analysis of 56m-year-old sediments. Yet where this carbon came from has always been disputed. Carbon can be emitted as carbon dioxide or methane (aka CH₄) and both are greenhouse gases. Some say the PETM carbon was <a href="http://onlinelibrary.wiley.com/doi/10.1029/95PA02087/abstract">methane from marine sediments</a>, while others have advocated methane from <a href="https://www.nature.com/nature/journal/v484/n7392/full/nature10929.html">thawing Antarctic permafrost</a> or the impact of a <a href="http://science.sciencemag.org/content/354/6309/225">large comet</a> releasing carbon from rocks.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/184037/original/file-20170830-24257-1o35r1c.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/184037/original/file-20170830-24257-1o35r1c.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/184037/original/file-20170830-24257-1o35r1c.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1245&fit=crop&dpr=1 600w, https://images.theconversation.com/files/184037/original/file-20170830-24257-1o35r1c.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1245&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/184037/original/file-20170830-24257-1o35r1c.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1245&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/184037/original/file-20170830-24257-1o35r1c.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1565&fit=crop&dpr=1 754w, https://images.theconversation.com/files/184037/original/file-20170830-24257-1o35r1c.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1565&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/184037/original/file-20170830-24257-1o35r1c.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1565&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Mid-Atlantic Ridge is the longest mountain range in the world but is almost entirely underwater.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Mid-atlantic_ridge_map.png">USGS / wiki</a></span>
</figcaption>
</figure>
<p>In our study recently published in <a href="http://nature.com/articles/doi:10.1038/nature23646">Nature</a>, we identified the distinctive chemical fingerprint of this carbon – it pointed not to methane, but to emissions from intense and prolonged volcanic activity. We also show that atmospheric CO₂ levels more than doubled in less than 25,000 years. </p>
<p>This makes sense: at the same time, Greenland and North America were drifting away from Europe, creating the North Atlantic Ocean and a string of <a href="http://science.sciencemag.org/content/316/5824/587">volcanic activity</a> along what is now the Mid-Atlantic Ridge.</p>
<p>We found more than 10,000 Gigatonnes of carbon must have been released into the atmosphere by volcanic activity during the PETM, which is an order of magnitude higher than all <a href="http://www.nature.com/nature/journal/v517/n7533/abs/nature14016.html">currently-accessible fossil fuel reserves</a> taken together. </p>
<p>But the rate of emissions would have been at least 20 times slower than <a href="https://www.earth-syst-sci-data.net/8/605/2016/essd-8-605-2016.pdf">today</a>. Given how much CO₂ was released, the resulting global warming was about what we would predict based on calculations of current climate sensitivity. </p>
<p>So what would volcanoes large enough to affect the climate like this actually look like, in practice? We could imagine a series of sky-blackening eruptions along the lines of <a href="http://www.sciencedirect.com/science/article/pii/S0012821X14001125">Laki in Iceland</a> which caused temperatures to drop <a href="https://www.scientificamerican.com/article/how-do-volcanoes-affect-w/">across the Northern Hemisphere</a> when it erupted in the 18th century. But, given we know the PETM volcanism largely took place under water and at a slower pace, perhaps the best modern equivalent would be the “black smokers” still found today in the deep North Atlantic – but lots of them.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/n2vPZsTRBaE?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">This, but for thousands of years.</span></figcaption>
</figure>
<p>The carbon released by these vents would bubble up to the surface and kick off a cycle that would eventually affect the oceans themselves. First, extreme PETM warmth led to faster weathering of rocks and soil, which meant more nutrients like phosphorus were being washed into the sea. This in turn stimulated plankton growth. When the plankton died they drifted down to the seafloor and gradually stored that same carbon in <a href="http://www.nature.com/ngeo/journal/v3/n12/full/ngeo1014.html">deep marine sediments</a>. </p>
<p>While this chain of events aided the removal of carbon from the ancient atmosphere it also led to oxygen starvation in some parts of the deep sea – analogous to the “<a href="https://theconversation.com/nutrient-pollution-voluntary-steps-are-failing-to-shrink-algae-blooms-and-dead-zones-81249">dead zones</a>” that form today in areas like the Gulf of Mexico where an excess of nutrients is washed into warm water.</p>
<p>We found the PETM was caused by massive carbon emissions from Earth’s interior. It thus has many parallels to today, where we are ratcheting up CO₂ levels in our atmosphere and oceans by burning fossil fuels that have been buried for millions of years. This extra carbon is, in effect, permanent on human timescales. The PETM is giving us an increasingly clearer picture of what Earth will be like if we carry on, and take our planet to places it has not been in at least 56m years.</p><img src="https://counter.theconversation.com/content/82354/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>This study was funded by a UK Ocean Acidification Research Program NERC/DEFRA/DECC grant (NE/H017518/1). </span></em></p><p class="fine-print"><em><span>Gavin Foster receives funding from the Natural Environment Research Council (NERC).</span></em></p><p class="fine-print"><em><span>Philip Sexton receives funding from the Natural Environment Research Council (NERC).</span></em></p>This sudden, 150,000-year long temperature spike has many parallels with modern climate change.Marcus Gutjahr, Senior Scientist in the field Marine Biogeochemistry and Paleoceanography, GEOMAR Helmholtz Centre for Ocean Research KielGavin Foster, Professor of Isotope Geochemistry, University of SouthamptonPhilip Sexton, Senior Lecturer in Earth Sciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/736482017-04-04T15:04:41Z2017-04-04T15:04:41ZWe are heading for the warmest climate in half a billion years, says new study<figure><img src="https://images.theconversation.com/files/163722/original/image-20170403-21969-1cyjtuf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption"></span> <span class="attribution"><span class="source">Neo Studio / shutterstock</span></span></figcaption></figure><p>Carbon dioxide concentrations are heading towards values not seen in the past 200m years. The sun has also been gradually getting stronger over time. Put together, these facts mean the climate may be heading towards warmth not seen in the past half a billion years.</p>
<p>A lot has happened on Earth since 500,000,000BC – continents, oceans and mountain ranges have come and gone, and complex life has evolved and moved from the oceans onto the land and into the air. Most of these changes occur on very long timescales of millions of years or more. However, over the past 150 years global temperatures have increased by about 1°C, ice caps and glaciers have retreated, polar sea-ice has melted, and sea levels have risen. </p>
<p>Some will point out that Earth’s climate has <a href="https://skepticalscience.com/humans_survived_previous_changes.html">undergone similar changes before</a>. So what’s the big deal?</p>
<p>Scientists can seek to understand past climates by looking at the evidence locked away in rocks, sediments and fossils. What this tells us is that yes, the climate has changed in the past, but the current speed of change is <a href="http://journals.sagepub.com/doi/full/10.1177/2053019616688022">highly unusual</a>. For instance, carbon dioxide hasn’t been added to the atmosphere as rapidly as today for at least the past <a href="http://www.nature.com/ngeo/journal/v9/n4/abs/ngeo2681.html">66m years</a>. </p>
<p>In fact, if we continue on our current path and exploit all convention fossil fuels, then as well as the rate of CO₂ emissions, the absolute climate warming is also likely to be unprecedented in at least the past 420m years. That’s according to a new study we have published in <a href="http://nature.com/articles/doi:10.1038/ncomms14845">Nature Communications</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/163834/original/image-20170404-5725-qfbx8d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/163834/original/image-20170404-5725-qfbx8d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/163834/original/image-20170404-5725-qfbx8d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=348&fit=crop&dpr=1 600w, https://images.theconversation.com/files/163834/original/image-20170404-5725-qfbx8d.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=348&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/163834/original/image-20170404-5725-qfbx8d.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=348&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/163834/original/image-20170404-5725-qfbx8d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=437&fit=crop&dpr=1 754w, https://images.theconversation.com/files/163834/original/image-20170404-5725-qfbx8d.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=437&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/163834/original/image-20170404-5725-qfbx8d.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=437&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Life in the planet’s last greenhouse period, the Eocene.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Eocene_Jay_Matternes.jpg">Jay Matternes / Smithsonian Museum</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>In terms of geological time, 1°C of global warming isn’t particularly unusual. For much of its history the planet was significantly warmer than today, and in fact more often than not Earth was in what is termed a “greenhouse” climate state. During the last greenhouse state 50m years ago, global average temperatures were 10-15°C warmer than today, the polar regions were ice-free, <a href="https://theconversation.com/how-ancient-warm-periods-can-help-predict-future-climate-change-58036">palm trees grew on the coast of Antarctica</a>, and alligators and turtles wallowed in swamp-forests in what is now the frozen Canadian Arctic. </p>
<p>In contrast, despite our current warming, we are still technically in an “icehouse” climate state, which simply means there is ice on both poles. The Earth has naturally cycled between these two climate states every 300m years or so.</p>
<p>Just prior to the industrial revolution, for every million molecules in the atmosphere, about 280 of them were CO₂ molecules (280 parts-per-million, or ppm). Today, due primarily to the burning of fossil fuels, concentrations are about 400 ppm. In the absence of any efforts to curtail our emissions, burning of conventional fossil fuels will cause CO₂ concentrations to be around 2,000ppm by the year 2250. </p>
<p>This is of course a lot of CO₂, but the geological record tells us that the Earth has experienced similar concentrations several times in the past. For instance, our new compilation of data shows that during the Triassic, around 200m years ago, when dinosaurs first evolved, Earth had a greenhouse climate state with atmospheric CO₂ around 2,000-3,000ppm. </p>
<p>So high concentrations of carbon dioxide don’t necessarily make the world totally uninhabitable. The dinosaurs thrived, after all. </p>
<p>That doesn’t mean this is no big deal, however. For a start, there is no doubt that humanity will face major socio-economic challenges dealing with the <a href="http://advances.sciencemag.org/content/1/8/e1500589">dramatic and rapid climate change</a> that will result from the rapid rise to 2,000 or more ppm. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/163723/original/image-20170403-21963-1j6wrmn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/163723/original/image-20170403-21963-1j6wrmn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/163723/original/image-20170403-21963-1j6wrmn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/163723/original/image-20170403-21963-1j6wrmn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/163723/original/image-20170403-21963-1j6wrmn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/163723/original/image-20170403-21963-1j6wrmn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/163723/original/image-20170403-21963-1j6wrmn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/163723/original/image-20170403-21963-1j6wrmn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">If we burnt all fossil fuel reserves the vast Antarctic ice sheet may disappear.</span>
<span class="attribution"><span class="source">vladsilver / shutterstock</span></span>
</figcaption>
</figure>
<p>But our new study also shows that the same carbon concentrations will cause more warming in future than in previous periods of high carbon dioxide. This is because the Earth’s temperature does not just depend on the level of CO₂ (or other greenhouse gases) in the atmosphere. All our energy ultimately comes from the sun, and due to the way the sun generates energy through nuclear fusion of hydrogen into helium, its brightness has increased over time. Four and a half billion years ago when the Earth was young the sun was around 30% less bright.</p>
<p>So what really matters is the combined effect of the sun’s changing strength and the varying greenhouse effect. Looking through geological history we generally found that as the sun became stronger through time, atmospheric CO₂ gradually decreased, so both changes cancelled each other out on average. </p>
<p>But what about in the future? We found no past time period when the drivers of climate, or <a href="http://ossfoundation.us/projects/environment/global-warming/radiative-climate-forcing">climate forcing</a>, was as high as it will be in the future if we burn all the readily available fossil fuel. Nothing like it has been recorded in the rock record for at least 420m years. </p>
<p>A central pillar of geological science is the <a href="http://www.uniformitarianism.net/">uniformitarian principle</a>: that “the present is the key to the past”. If we carry on burning fossil fuels as we are at present, by 2250 this old adage is sadly no longer likely to be true. It is doubtful that this high-CO₂ future will have a counterpart, even in the vastness of the geological record.</p><img src="https://counter.theconversation.com/content/73648/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gavin Foster receives funding from the Natural Environment Research Council (NERC). </span></em></p><p class="fine-print"><em><span>Dan Lunt receives funding from the Natural Environment Research Council (NERC).</span></em></p><p class="fine-print"><em><span>Dana Royer does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The sun is more powerful today than when we last had similar levels of carbon in the atmosphere.Gavin Foster, Professor of Isotope Geochemistry, University of SouthamptonDana Royer, Professor of Earth and Environmental Sciences, Wesleyan UniversityDan Lunt, Professor of Climate Science, University of BristolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/655702016-09-16T12:39:15Z2016-09-16T12:39:15ZSorry David Attenborough, we didn’t evolve from ‘aquatic apes’ – here’s why<figure><img src="https://images.theconversation.com/files/138054/original/image-20160916-6332-1t9bve0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Michael Rosskothen / shutterstock</span></span></figcaption></figure><p>Occasionally in science there are theories that refuse to die despite the overwhelming evidence against them. The “aquatic ape hypothesis” is one of these, now championed by Sir David Attenborough in his recent BBC Radio 4 series <a href="http://www.bbc.co.uk/programmes/b07v0hhm">The Waterside Ape</a>. </p>
<p>The hypothesis suggests that everything from walking upright to our lack of hair, from holding our breath to eating shellfish could be because an aquatic phase in our ancestry. Since the theory was first suggested more than 55 years ago, huge advances have been made in the study of human evolution and our story is much more interesting and complicated than suggested by the catch-all aquatic ape hypothesis.</p>
<p>In 1960, marine biologist Alister Hardy published an article in New Scientist, titled: <a href="http://www.riverapes.com/original/AAH/Hardy/Hardy1960.pdf">Was man more aquatic in the past?</a> He re-told the familiar tale of the evolution of land animals from ancient fish, and then considered the return of various groups of reptiles, birds and mammals to an aquatic existence: ichthyosaurs and plesiosaurs, crocodiles, sea-snakes, penguins, whales, dolphins and porpoises, manatees and dugongs, and seals – as well as polar bears, otters and water voles, who hunt in water. Then he suggested that many of the unique characteristics of humans and their ancestors, marking them out as different from the other apes, could be explained as adaptations to spending time in water.</p>
<p>Hardy put forward all sorts of features which could be explained as “aquatic adaptations”: our swimming ability – and our enjoyment of it; loss of body hair, as well as an arrangement of body hair that he supposed may have reduced resistance in the water; curvy bodies; and the layer of fat under our skin. He even suggested that our ability to walk upright may have developed through wading, with the water helping to support body weight. </p>
<p>For Hardy, this aquatic phase would have occupied the gap in the fossil record that then existed – between around 4m and 7m years ago. He sensibly concluded his paper saying that this was all only speculation – a “hypothesis to be discussed and tested against further lines of evidence”.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/138062/original/image-20160916-6311-akf9m1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/138062/original/image-20160916-6311-akf9m1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/138062/original/image-20160916-6311-akf9m1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/138062/original/image-20160916-6311-akf9m1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/138062/original/image-20160916-6311-akf9m1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/138062/original/image-20160916-6311-akf9m1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/138062/original/image-20160916-6311-akf9m1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/138062/original/image-20160916-6311-akf9m1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A modern day aquatic ape. But there’s no trace of true ‘aquatic apes’ in our ancestry.</span>
<span class="attribution"><span class="source">dotshock/Shutterstock</span></span>
</figcaption>
</figure>
<p>In the 50-odd years since the presentation of this hypothesis, it has enjoyed a certain fame – or perhaps notoriety. The writer Elaine Morgan championed it in her book <a href="http://souvenirpress.co.uk/product/aquatic-ape-hypothesis/">The Aquatic Ape</a>, and developed the hypothesis further, marshalling a seemingly impressive range of characteristics to support it, including breath control and diet. It seems such a tantalising and romantic idea – but a closer look at the evidence reveals it to be little more than that.</p>
<h2>Pouring cold water on it</h2>
<p>All the suggested anatomical and physiological adaptations can be explained by other hypotheses, which fit much better with <a href="https://www.amazon.co.uk/dp/B00JIV9R3I/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1">what we actually know</a> about the ecology of ancient hominins. Hairlessness, for instance, is only a feature of fully aquatic mammals such as whales and dolphins. Semi-aquatic mammals such as otters and water voles are extremely furry. Sexual selection and adaptations to heat loss better explain our pattern of body hair. Sexual selection may also explain our body fat distribution, which differs between the sexes. Voluntary breath control is more likely to be related to speech than to diving. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/138064/original/image-20160916-6340-1afo308.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/138064/original/image-20160916-6340-1afo308.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/138064/original/image-20160916-6340-1afo308.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=423&fit=crop&dpr=1 600w, https://images.theconversation.com/files/138064/original/image-20160916-6340-1afo308.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=423&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/138064/original/image-20160916-6340-1afo308.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=423&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/138064/original/image-20160916-6340-1afo308.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=532&fit=crop&dpr=1 754w, https://images.theconversation.com/files/138064/original/image-20160916-6340-1afo308.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=532&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/138064/original/image-20160916-6340-1afo308.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=532&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Otters evolved from land animals but never lost their fur.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/slobirdr/15800171602/">Gregory </a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The diet of many of our ancestors certainly included marine resources – where people lived on the shores of lakes or the sea. But this was a relatively late development in human evolution, and humans can also survive and thrive on food obtained entirely on land. Compared with other animals, we are not actually that good at swimming, and our skin leaks as well, letting in water so that our fingers become prune-like after a long bath. </p>
<p>What about walking on two legs? That’s something all apes do a bit of – while wading in water, certainly, but also while reaching for fruit, performing aggressive displays or simply moving around in trees. If we evolved from ancestors who already stood up in trees, we don’t need an extraordinary explanation for why we ended up standing on the ground rather than <a href="http://onlinelibrary.wiley.com/doi/10.1111/jzo.12112/abstract">running around on all fours</a>.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/138057/original/image-20160916-6323-12629mz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/138057/original/image-20160916-6323-12629mz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/138057/original/image-20160916-6323-12629mz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=901&fit=crop&dpr=1 600w, https://images.theconversation.com/files/138057/original/image-20160916-6323-12629mz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=901&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/138057/original/image-20160916-6323-12629mz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=901&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/138057/original/image-20160916-6323-12629mz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1133&fit=crop&dpr=1 754w, https://images.theconversation.com/files/138057/original/image-20160916-6323-12629mz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1133&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/138057/original/image-20160916-6323-12629mz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1133&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Hands up if you can stand on dry land.</span>
<span class="attribution"><span class="source">Sergey Uryadnikov / shutterstock</span></span>
</figcaption>
</figure>
<p>Since Hardy and Morgan’s hypothesis was advanced, many of the gaps in the human fossil record have been filled, with <a href="http://rstb.royalsocietypublishing.org/content/371/1698/20150244">at least 13 new species</a> found since 1987. We have also made great strides in reconstructing <a href="http://dx.doi.org/10.1016/j.quascirev.2014.06.012">the environment in which our ancestors lived</a>. And we know that species as far as part in time as <em>Sahelanthropus tchadensis</em> 7m years ago and <em>Homo erectus</em> 2m years ago all lived in <a href="http://dx.doi.org/10.1016/B978-0-08-095975-7.01213-4">forested or open woodland environments</a>. While some of these woods included wetland, this was just part of the mosaic of habitats that our ancestors learned to survive in, and there is absolutely no trace of a hominin ancestor as aquatic as that described by Hardy and Morgan. </p>
<p>We also have evidence our ancestors had to survive <a href="http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0076750">periods of extremely dry climate</a> with little or no aquatic resources. Coping with these highly variable, patchwork environments required <a href="https://www.researchgate.net/publication/258810494_Hominin_evolution_in_settings_of_strong_environmental_variability">behavioural flexibility and co-operation</a>, and our large brains and ultra-social nature likely emerged as a result. This flexibility ultimately led to the invention of culture and technology.</p>
<p>Recent proponents of the aquatic ape hypothesis have pointed to much later watery adaptations, including early archaeological sites where humans have been shown to be exploiting coastal resources. But these don’t have much to say about the origins of bipedalism, more than 6m years before – they just demonstrate the behavioural flexibility of later hominins.</p>
<h2>Too extravagant and too simple</h2>
<p>The original idea, and certainly Elaine Morgan’s elaboration of it, became an <a href="http://www.sciencedirect.com/science/article/pii/S0047248497901469">umbrella hypothesis</a> or a “Theory of Everything”; both far too extravagant and too simple an explanation. It attempts to provide a single rationale for a huge range of adaptations - which we know arose at different times in the course of human evolution. Traits such as <a href="https://www.scribd.com/document/216676020/Challenges-to-Human-Uniqueness-Bipedalism-Birth-and-Brains">habitual bipedalism</a>, <a href="http://dx.doi.org/10.1098/rstb.2014.0064">big brains and language</a> didn’t all appear at once – instead, their emergence is spread over millions of years. It’s nonsense to lump them all together as if they require a single explanation.</p>
<p>Despite the evidence stacked up against the theory, it is <a href="http://onlinelibrary.wiley.com/doi/10.1002/evan.21405/abstract">strangely tenacious</a>. It has become very elastic, and its <a href="http://www.ted.com/talks/elaine_morgan_says_we_evolved_from_aquatic_apes?language=en">proponents</a> will seize hold of any mentions of water, fish or shellfish in human evolution, and any archaeological sites found near coasts, rivers and lakes as supporting evidence. But we must always build our hypotheses on, and test them against, the hard evidence: the fossils, comparative anatomy and physiology, and genetics. In that test, the aquatic ape has failed – again and again. </p>
<p>It is a great shame the BBC recently indulged this implausible theory as it distracts from the emerging story of human evolution that is both more complex and more interesting. Because at the end of the day science is about evidence, not wishful thinking.</p>
<hr>
<p><em>Scientist supporters of the waterside ape hypothesis have responded to this article <a href="http://aquatic-human-ancestor.org/evidence/waterside-ape-bbc-r4-response-to-critics.html">here</a>.</em></p><img src="https://counter.theconversation.com/content/65570/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alice Roberts has made several programmes about human evolution for the BBC.</span></em></p><p class="fine-print"><em><span>Mark Maslin is a Professor at University College London, a Royal Society Industrial Fellow, Founding Director of Rezatec Ltd, Director of The London NERC Doctoral Training Partnership and a member of Cheltenham Science Festival Advisory Committee. He is an unpaid member of the Sopra-Steria CSR Board. He has received funding in the past from the NERC, EPSRC, ESRC, Royal Society, DIFD, DECC, BIS, FCO, Innovate UK, Carbon Trust, UK Space Agency, European Space Agency, Leverhulme Trust, WWF, JLT Re, Channel 4, RICS, British Council, and CAFOD.</span></em></p>David Attenborough’s latest BBC documentary indulges wishful thinking over evidence.Alice Roberts, Professor of Public Engagement in Science, University of BirminghamMark Maslin, Professor of Palaeoclimatology, UCLLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/583572016-05-20T13:04:36Z2016-05-20T13:04:36ZHow events in Panama created the modern world (millions of years ago)<figure><img src="https://images.theconversation.com/files/121773/original/image-20160509-20590-eb381q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">stockmdm / shutterstock</span></span></figcaption></figure><p>Panama has been in the news for <a href="https://theconversation.com/uk/topics/panama-papers">other reasons</a> recently, but paleoclimatologists have long known of its significance. In fact, the formation of this thin strip of land between North and South America may be one of the key events in Earth’s history.</p>
<p>Today, the Panama Canal is one of the world’s most important shipping lanes, allowing large ships to go straight between the Atlantic and Pacific Oceans while avoiding longer and often more dangerous routes around the tip of South America. </p>
<p>But this isn’t the first time there has been an equatorial connection between our two largest oceans. More than 20m years ago, Panama and much of Central America was under water or, more accurately, much of this land mass was yet to form, and water flowed freely between these two climatically important ocean basins.</p>
<p>Once you zoom out to geological timescales of millions of years, it becomes apparent that plate tectonics – continents drifting around the Earth’s surface – have a huge impact on the climate. The position of continents naturally affects the shape and size of the oceans, and crucially, the currents that flow within them that transport energy (in the form of heat) to different parts of the globe.</p>
<p>In this case, the shallowing and eventual closure of the Panama “gateway” and creation of the Isthmus of Panama, where both continents become connected by land, strengthened what we call the “<a href="http://people.oregonstate.edu/%7Eschmita2/pdf/S/schmittner07agu_intro.pdf">Atlantic Meridional Overturning Circulation</a>”, a current that transports warm equatorial waters north and is responsible for Europe’s mild winters. This played a crucial role in <a href="http://www.ncbi.nlm.nih.gov/pubmed/17791515">shaping our modern climate system</a>. </p>
<h2>How Panama closed</h2>
<p>None of this happened overnight. What’s now Panama lies on the boundaries of the South and North American, Caribbean and Cocos plates, and collisions between these huge masses around <a href="http://www.pnas.org/content/112/19/6110.full">23-25m years ago</a> led to rises in the seafloor and the formation of underwater volcanoes. Over time, these eventually emerged above the water line. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/123367/original/image-20160520-4481-125e2da.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/123367/original/image-20160520-4481-125e2da.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=354&fit=crop&dpr=1 600w, https://images.theconversation.com/files/123367/original/image-20160520-4481-125e2da.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=354&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/123367/original/image-20160520-4481-125e2da.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=354&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/123367/original/image-20160520-4481-125e2da.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=445&fit=crop&dpr=1 754w, https://images.theconversation.com/files/123367/original/image-20160520-4481-125e2da.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=445&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/123367/original/image-20160520-4481-125e2da.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=445&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Volcán Barú is Panama’s highest point.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/arkeldiary/25555317046/">Edwin Rios</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>By 7m to 11m years ago, the volcanoes had grown, and the sea had become shallow enough that deep water exchange between the Pacific and Atlantic is thought to have ceased, leading to a reorganisation of oceanic currents. Finally, around 3m years ago, a land bridge emerged, connecting both North and South America. Or so the story goes … </p>
<h2>Why timing matters</h2>
<p>Establishing the timeline is crucial in piecing together the Panama gateway’s climatic role, yet new evidence suggests the closure was earlier than previously thought.</p>
<p>Fossils of both terrestrial and marine species suggest closure may now even date back to around 20m years ago, when there were periods of land chains above sea level, although likely not fully connected. This is <a href="http://www.pnas.org/content/112/19/6110.full">17m years earlier</a> than first suggested. Although land species were moving between the continents as far back as around 20m years ago, it wasn’t significant until 6m years ago, implying that there must have still been inhibitors to mass migration of species between these land masses before then. </p>
<p>The recent removal of sediment for the new expansion of the Panama Canal unearthed <a href="https://www.researchgate.net/publication/258646051_Evidence_for_middle_Eocene_and_younger_land_emergence_in_Central_Panama_Implications_for_Isthmus_closure">20m year old fossilised trees</a>, adding further weight to an earlier closure hypothesis. The same study also suggests that the isthmus may have even formed an <a href="https://www.researchgate.net/publication/258646051_Evidence_for_middle_Eocene_and_younger_land_emergence_in_Central_Panama_Implications_for_Isthmus_closure">uninterrupted chain of land</a> between the Late Eocene (20m-30m years ago) to the Late Miocene (11m-5m years ago), further compounding the long and complex history of this climate-altering event.</p>
<h2>Changing oceans, changing climates</h2>
<p>The shoaling and eventual closure of the Panama gateway was a key reason for <a href="http://link.springer.com/article/10.1007%2Fs00382-007-0265-6#/page-1">glaciation in the northern hemisphere</a>. As winds in the atmosphere often respond to what is occurring in the ocean below, changes in ocean circulation that developed with the shallowing and closure of the Panama seaway led to <a href="http://www.nature.com/articles/srep12252">warm, moist winds blowing northwards</a>. This fresh water held in the atmosphere was deposited at high latitudes where glaciers and ice sheets could form.</p>
<p>Analysis of geological data has shown that despite an open Panama gateway (no isthmus) the strength of the Atlantic Meridional Ocean Circulation <a href="http://www.nature.com/articles/srep12252">was unaffected</a> during the Pliocene (around 2-5m years ago). This casts doubt on the key importance of complete isolation between the Atlantic and Pacific.</p>
<p>Indeed, conventional geologic theory on the role of the Panama seaway <a href="http://www.pnas.org/content/112/19/6110.full">is being</a> <a href="http://link.springer.com/article/10.1007%2Fs00382-007-0265-6#/page-1">challenged</a>. Some argue that greater heat transport towards the poles (in the form of a stronger gulf stream) may have actually <a href="http://www.nature.com/articles/srep12252">hindered ice growth</a> and delayed glaciation in the northern hemisphere. One <a href="http://link.springer.com/article/10.1007%2Fs00382-007-0265-6#/page-1">climate modelling study</a> suggested though that the increased moisture transport outweighs the increase in temperature, supporting the hypothesis that the closure intensified northern hemisphere glaciations through changes in oceanic circulation.</p>
<p>The closure of Panama, and the end of deep water currents flowing between oceans, is not solely responsible for kicking off the ice ages. Other primary factors include decreasing amounts of carbon dioxide in the atmosphere, changes in the Earth’s axis, and long-term “wobbles” which meant the poles received less sun. </p>
<p>However, we can say the transformation of Panama from seaway to isthmus played a crucial role in shaping our present day climate. It’s a great example of how geography and climate are linked: one small strip of newly-created land changed the planet forever.</p><img src="https://counter.theconversation.com/content/58357/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alex Farnsworth receives funding from NERC and EPSRC. This article does not reflect the views of the research councils.</span></em></p><p class="fine-print"><em><span>Emma Stone receives funding from the European Research Council. </span></em></p>Closing the passage between the Atlantic and Pacific oceans strengthened the gulf stream and helped kick off ice ages.Alex Farnsworth, Postdoctoral Research Associate in meteorology, University of BristolEmma Stone, Research Associate in Climatology, University of BristolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/580362016-04-25T15:06:03Z2016-04-25T15:06:03ZHow ancient warm periods can help predict future climate change<figure><img src="https://images.theconversation.com/files/119989/original/image-20160425-22364-78df22.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Eocene_Jay_Matternes.jpg">Jay Matternes / Smithsonian Museum</a></span></figcaption></figure><p>Several more decades of increased carbon dioxide emissions could lead to <a href="https://theconversation.com/tipping-point-how-we-predict-when-antarcticas-melting-ice-sheets-will-flood-the-seas-56125">melting ice sheets</a>, <a href="https://theconversation.com/earths-sixth-mass-extinction-has-begun-new-study-confirms-43432">mass extinctions</a> and <a href="http://link.springer.com/article/10.1007/s10584-016-1661-x">extreme weather</a> becoming the norm. We can’t yet be certain of the exact impacts, but we can look to the past to predict the future.</p>
<p>We could start with the last time Earth experienced CO<sub>2</sub> levels comparable to those expected in the near future, a period 56m to 34m years ago known as the <a href="http://www.bbc.co.uk/nature/history_of_the_earth/Eocene">Eocene</a>. </p>
<p>The Eocene began as a period of extreme warmth around 10m years after the final dinosaurs died. Alligators lived in the <a href="http://www.colorado.edu/news/releases/2010/08/24/new-study-shows-how-tortoises-alligators-thrived-high-arctic-some-50">Canadian Arctic</a> while palm trees grew along the East Antarctic coastline. Over time, the planet gradually cooled, until the Eocene was brought to a close with the formation of a large ice sheet on Antarctica. </p>
<p>During the Eocene, carbon dioxide (CO<sub>2</sub>) concentrations in the atmosphere were much higher than today, with estimates usually ranging between 700 and 1,400 parts per million (ppm). As these values are similar to those anticipated by the end of this century (<a href="http://www.ipcc.ch/report/ar5/wg1/">420 to 935ppm</a>), scientists are increasingly using the Eocene to help predict future climate change.</p>
<p>We’re particularly interested in the link between carbon dioxide levels and global temperature, often referred to as “<a href="https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_SPM_FINAL.pdf">equilibrium climate sensitivity</a>” – the temperature change that results from a doubling of atmospheric CO<sub>2</sub>, once fast climate feedbacks (such as water vapour, clouds and sea ice) have had time to act.</p>
<p>To investigate climate sensitivity during the Eocene we generated new estimates of CO<sub>2</sub> throughout the period. Our study, written with colleagues from the Universities of Bristol, Cardiff and Southampton, is published in <a href="http://nature.com/articles/doi:10.1038/nature17423">Nature</a>. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/120007/original/image-20160425-22375-wjggug.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120007/original/image-20160425-22375-wjggug.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/120007/original/image-20160425-22375-wjggug.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=897&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120007/original/image-20160425-22375-wjggug.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=897&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120007/original/image-20160425-22375-wjggug.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=897&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120007/original/image-20160425-22375-wjggug.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1127&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120007/original/image-20160425-22375-wjggug.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1127&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120007/original/image-20160425-22375-wjggug.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1127&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Reconstruction of the 40m year old planktonic foraminifer <em>Acarinina mcgowrani</em>.</span>
<span class="attribution"><span class="source">Richard Bizley (www.bizleyart.com) and Paul Pearson, Cardiff University</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>As we can’t directly measure the Eocene’s carbon dioxide levels, we have to use “proxies” preserved within sedimentary rocks. Our study utilises <a href="http://www.ucmp.berkeley.edu/fosrec/Wetmore.html">planktonic foraminifera</a>, tiny marine organisms which record the chemical composition of seawater in their shells. From these fossils we can figure out the acidity level of the ocean they lived in, which is in turn affected by the concentration of atmospheric CO<sub>2</sub>.</p>
<p>We found that CO<sub>2</sub> levels approximately halved during the Eocene, from around 1,400ppm to roughly 770ppm, which explains most of the <a href="http://onlinelibrary.wiley.com/doi/10.1002/2014PA002723/full">sea surface cooling</a> that occurred during the period. This supports previously unsubstantiated theories that carbon dioxide was responsible for the extreme warmth of the early Eocene and that its decline was responsible for the subsequent cooling.</p>
<p>We then estimated global mean temperatures during the Eocene (again from proxies such as fossilised leaves or marine microfossils) and accounted for changes in vegetation, the position of the continents, and the lack of ice sheets. This yields a climate sensitivity value of 2.1°C to 4.6°C per doubling of CO<sub>2</sub>. This is similar to that predicted for our own warm future (<a href="http://www.climatechange2013.org/images/report/WG1AR5_Chapter10_FINAL.pdf">1.5 to 4.5°C</a> per doubling of CO<sub>2</sub>). </p>
<p>Our work reinforces <a href="http://www.nature.com/nature/journal/v518/n7537/abs/nature14145.html">previous findings</a> which looked at sensitivity in more recent time intervals. It also gives us confidence that our Eocene-like future is well mapped out by current climate models. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/120015/original/image-20160425-22375-13ri5d7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120015/original/image-20160425-22375-13ri5d7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/120015/original/image-20160425-22375-13ri5d7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120015/original/image-20160425-22375-13ri5d7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120015/original/image-20160425-22375-13ri5d7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120015/original/image-20160425-22375-13ri5d7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120015/original/image-20160425-22375-13ri5d7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120015/original/image-20160425-22375-13ri5d7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Fossil foraminifera from Tanzania – their intricate shells capture details of the ocean 33-50m years ago.</span>
<span class="attribution"><span class="source">Paul Pearson, Cardiff University</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p><a href="https://theconversation.com/profiles/richard-pancost-117512">Rich Pancost</a>, a paleoclimate expert and co-author on both studies, explains: “Most importantly, the collective research into Earth history reveals that the climate can and has changed. And consequently, there is little doubt from our history that transforming fossil carbon underground into carbon dioxide in the air – as we are doing today – will significantly affect the climate we experience for the foreseeable future.”</p>
<p>Our work also has implications for other elements of the climate system. Specifically, what is the impact of higher CO<sub>2</sub> and a warmer climate upon the water cycle? A <a href="http://www.clim-past.net/12/455/2016/cp-12-455-2016.pdf">recent study</a> investigating environmental change during the early Eocene – the warmest interval of the past 65m years – found an increase in global precipitation and evaporation rates and an increase in heat transport from the equator to the poles. The latter is consistent with leaf <a href="http://geode.colorado.edu/%7Eeberlej/Eberle%20and%20Greenwood%20Arctic%20Eocene.pdf">fossil evidence</a> from the Arctic which suggests that high precipitation rates were common.</p>
<p>However, changes in the water cycle are likely to vary between regions. For example, low to mid latitudes likely became drier overall, but with more intense, seasonal rainfall events. Although very few studies have investigated the water cycle of the Eocene, understanding how this operates during past warm climates could provide insights into the mechanisms which will govern future changes.</p><img src="https://counter.theconversation.com/content/58036/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gordon Inglis received funding from the RCUK (NERC). He is currently funded by the ERC.</span></em></p><p class="fine-print"><em><span>Eleni Anagnostou received funding from NERC. This article does not reflect the views of the research councils.</span></em></p>As carbon dioxide emissions continue to increase, scientists are looking to the past.Gordon Inglis, Postdoctoral Research Associate in Organic Geochemistry, University of BristolEleni Anagnostou, Postdoctoral Research Fellow, Ocean and Earth Science, University of SouthamptonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/533982016-01-20T16:39:32Z2016-01-20T16:39:32ZThe last time Earth was this hot hippos lived in Britain (that’s 130,000 years ago)<figure><img src="https://images.theconversation.com/files/108748/original/image-20160120-26085-8u1e79.jpg?ixlib=rb-1.1.0&rect=193%2C0%2C1636%2C1154&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Modern Africa...or prehistoric Britain?</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/indigomood/7506286354/">anson chu</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>It’s official: 2015 was the warmest year <a href="http://www.theguardian.com/environment/2016/jan/20/2015-smashes-record-for-hottest-year-final-figures-confirm">on record</a>. But those global temperature records only date back to 1850 and become increasingly uncertain the further back you go. Beyond then, we’re reliant on signs left behind in tree rings, ice cores or rocks. So when was the Earth last warmer than the present?</p>
<p>The Medieval Warm Period is often cited as the answer. This spell, beginning in roughly 950AD and lasting for three centuries, saw <a href="https://www.pdx.edu/honors/sites/www.pdx.edu.honors/files/Prentice.pdf">major changes</a> to population centres across the globe. This included the collapse of the Tiwanaku civilisation in South America due to increased aridity, and the colonisation of Greenland by the Vikings.</p>
<p>But that doesn’t tell the whole story. Yes, some regions were warmer than in recent years, but others were substantially colder. Across the globe, averaged temperatures then were in fact <a href="http://www.pnas.org/content/early/2008/09/02/0805721105">cooler than today</a>.</p>
<p>To reach a point when the Earth was significantly warmer than today we’d need to go back 130,000 years, to a time known as the Eemian.</p>
<p>For about 1.8m years the planet had fluctuated between a series of ice ages and warmer periods known as “interglacials”. The Eemian, which lasted around 15,000 years, was the most recent of these interglacials (before the one we’re currently in). </p>
<p>Although global annual average temperatures were approximately 1 to 2˚C warmer than preindustrial levels, high latitude regions were <a href="http://www.sciencedirect.com/science/article/pii/S0277379114003382">several degrees warmer</a> still. This meant ice caps melted, Greenland’s ice sheet was reduced and the West Antarctic ice sheet may have collapsed. The sea level was at least <a href="http://science.sciencemag.org/content/349/6244/aaa4019.full">6m higher than today</a>.</p>
<p>Across Asia and North America forests extended much further north than today and straight-tusked elephants (now extinct) and <a href="http://www.sciencedirect.com/science/article/pii/S0016787809000054">hippopotamuses</a> were living as far north as the British Isles.</p>
<p>How do we know all this? Well, scientists can estimate the temperature changes at this time by looking at chemicals found in ice cores and marine sediment cores and studying pollen buried in layers deep underground. Certain isotopes of oxygen and hydrogen in ice cores can determine the temperature in the past while pollen tells us which plant species were present and therefore gives us an indication of climatic conditions suitable for that species. </p>
<p>We know from air bubbles in ice cores drilled on Antarctica that greenhouse gas concentrations in the Eemian were not dissimilar to preindustrial levels. However <a href="http://www.ipcc.ch/report/ar5/wg1/">orbital conditions were very different</a> – essentially there were much larger latitudinal and seasonal variations in the amount of solar energy received by the Earth. </p>
<p>So although the Eemian was warmer than today the driving mechanism for this warmth was fundamentally different to present-day climate change, which is down to greenhouses gases. To find a warm period caused predominantly by conditions more similar to today, we need to go even further back in time.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/108746/original/image-20160120-26096-1h6ymw5.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/108746/original/image-20160120-26096-1h6ymw5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/108746/original/image-20160120-26096-1h6ymw5.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=193&fit=crop&dpr=1 600w, https://images.theconversation.com/files/108746/original/image-20160120-26096-1h6ymw5.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=193&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/108746/original/image-20160120-26096-1h6ymw5.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=193&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/108746/original/image-20160120-26096-1h6ymw5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=243&fit=crop&dpr=1 754w, https://images.theconversation.com/files/108746/original/image-20160120-26096-1h6ymw5.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=243&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/108746/original/image-20160120-26096-1h6ymw5.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=243&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The past 540 million years. Note the Eemian spike and the Miocene Optimum.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:All_palaeotemps.png">Glen Fergus / wiki</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>As climate scientists, we’re particularly interested in the Miocene (around 23 to 5.3 million years ago), and in particular a spell known as the Miocene-Climate Optimum (11-17 million years ago). Around this time CO<sub>2</sub> values (350-400ppm) were similar to today and it therefore potentially serves as an appropriate analogue for the future. </p>
<p>During the Optimum, those carbon dioxide concentrations were the <a href="http://www.sciencedirect.com/science/article/pii/S0012821X12002919">predominant driver of climate change</a>. Global average temperatures were <a href="http://onlinelibrary.wiley.com/doi/10.1029/2008GL036571/abstract">2 to 4˚C warmer</a> than preindustrial values, sea level was around <a href="http://www.sciencedirect.com/science/article/pii/S0012821X12002919">20m higher</a> and there was an expansion of tropical vegetation. </p>
<p>However, during the later Miocene period CO<sub>2</sub> declined to below preindustrial levels, but global temperatures remained significantly warmer. What kept things warm, if not CO<sub>2</sub>? We still don’t know exactly – it may have been orbital shifts, the development of modern ocean circulation or even big geographical changes such as the <a href="http://www.whoi.edu/oceanus/feature/how-the-isthmus-of-panama-put-ice-in-the-arctic">Isthmus of Panama narrowing and eventually closing off</a> – but it does mean direct comparison with the present day is problematic.</p>
<p>Currently orbital conditions are suitable to trigger the next glacial inception. We’re due another ice age. However, as pointed out in a recent study in <a href="http://www.nature.com/nature/journal/v529/n7585/full/nature16494.html">Nature</a>, there’s now so much carbon in the atmosphere the likelihood of this occurring is massively reduced over the next 100,000 years.</p><img src="https://counter.theconversation.com/content/53398/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Emma Stone receives funding from the European Research Council. </span></em></p><p class="fine-print"><em><span>Alex Farnsworth receives funding from the Natural Environmental Research Council (NERC).</span></em></p>But it’s been millions of years since carbon in the atmosphere last warmed the planet to this extent.Emma Stone, Research Associate in Climatology, University of BristolAlex Farnsworth, Postdoctoral Researcher in Climatology, University of BristolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/400972015-04-13T18:35:29Z2015-04-13T18:35:29ZSnowball Earth: new study shows Antarctic climate even gripped the tropics<figure><img src="https://images.theconversation.com/files/77768/original/image-20150413-24294-gcctr7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Welcome to your summer holidays, 750m years ago.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/elisfanclub/9549491022">Eli Duke</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>New details of a nightmare period on Earth with surface conditions as frigid as present-day central Antarctica at the equator have been revealed thanks to the publication of a study of ancient glacier water.</p>
<p>The research, by an international team led by Daniel Herwartz, is published in the journal <a href="http://www.pnas.org/content/early/2015/04/08/1422887112.abstract">Proceedings of the National Academy of Sciences</a> and shows that even tropical regions were once covered in snow and ice.</p>
<p>In the most recent ice age, the last glacial advance (ending about 12,000 years ago) ice sheets extended across Europe at the latitude of southernmost England and reached south of the Great Lakes in North America. Beyond lay tundra, woolly mammoths and so on – but the equatorial belt of tropical rainforest was still there. Much more drastic situations occurred far earlier in Earth’s history however, and it is these that are supported by the new study. The periods have been dubbed “<a href="http://www.snowballearth.org/overview.html">Snowball Earth</a>”.</p>
<p>This term first rose to prominence in the 1990s on the back of decades of geological observations of rocks deposited by glaciers, on land and at sea, during much of the period lasting from about 720m to 630m years ago at locations from across the globe which – at the time – were on the equator or no more than 40 degrees from it. </p>
<p>This was not easy for geologists to establish, because organisms that would leave large, easily recognisable fossils (so useful for relative dating) had not yet evolved. Moreover, the distribution of continents was different and has to be deduced by measuring traces of the Earth’s magnetic field captured into the rocks when they formed (paleomagnetism). Over the period in question, a single super-continent known as <a href="http://www.peripatus.gen.nz/Paleontology/Rodinia.html">Rodinia</a> was beginning to break apart, but still straddled the equator.</p>
<h2>Turning to snow</h2>
<p>Why the climate should ever veer so extremely as to become caught in Snowball Earth conditions is a complex matter. On the one hand the Sun was 20-30% fainter than it is now, and thus provided less heat. However, the ancient atmosphere had much more carbon dioxide in it than now, so there would have been a more effective “greenhouse effect” to trap heat and keep the planet warm. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/77772/original/image-20150413-24312-1xiichh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/77772/original/image-20150413-24312-1xiichh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/77772/original/image-20150413-24312-1xiichh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=534&fit=crop&dpr=1 600w, https://images.theconversation.com/files/77772/original/image-20150413-24312-1xiichh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=534&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/77772/original/image-20150413-24312-1xiichh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=534&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/77772/original/image-20150413-24312-1xiichh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=671&fit=crop&dpr=1 754w, https://images.theconversation.com/files/77772/original/image-20150413-24312-1xiichh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=671&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/77772/original/image-20150413-24312-1xiichh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=671&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Rodinia contained most of the world’s continental crust.</span>
<span class="attribution"><a class="source" href="http://en.wikipedia.org/wiki/File:Rodinia_reconstruction.jpg">John Goodge</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/">CC BY-NC-SA</a></span>
</figcaption>
</figure>
<p>Changes in the Earth’s orbit, or in the tilt of its axis, might have tipped the balance – these are the likely cause of more recent glaciations – but it is possible that full Snowball Earth conditions can be initiated only when a super-continent lies across the equator. As land reflects more solar heat than the oceans, equatorial Rodinia maximised the amount of heat bounced back into space rather than <a href="https://theconversation.com/heat-accumulating-deep-in-the-atlantic-has-put-global-warming-on-hiatus-30805">absorbed into the seas</a> and kept on Earth.</p>
<p>Snowball Earth conditions gripped Rodinia at least twice, in an older episode known as the Surtian and in a younger episode known as the Marinoan. Herwartz and his team studied rock samples from the Dabie-Sulu belt in modern day eastern China. Back in Surtian and Marinoan times this region was between 15 and 35 degrees north, the same sort of latitude as present-day Mexico, India or the Sahara. </p>
<p>The team also studied rocks from a much older proposed Snowball Earth episode, about 2.2 billion years ago, sampled in Karelia, in present-day northwest Russia close to the border with Finland. These too were at a low latitude at the time in question.
0</p>
<h2>Snowball hunting</h2>
<p>The researchers’ key innovation was to analyse oxygen left behind by ancient glacial water as it reacted with rocks to form new minerals. They used this to work out the prevailing surface temperatures.</p>
<p>Oxygen comes in three stable forms, or “isotopes”. Nearly 99.8% of oxygen atoms are oxygen-16 (made of 8 protons and 8 neutrons). Most of the remainder is oxygen-18 (8 protons and 10 neutrons), but there are also traces of oxygen-17 (8 protons and 9 neutrons). </p>
<p>A water molecule containing a heavier isotope of oxygen has the same chemical properties as a water molecule containing the lighter oxygen-16 but will <a href="http://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance/">evaporate less readily and condense more quickly</a>. This means that the oceans lose water molecules containing oxygen-16 at a faster rate, and rain (or snow) falling far from the ocean will be poorer than average in the heaver isotopes. The ratios of the different oxygen isotopes in the recent geological past can be used as a proxy for global temperature, or to estimate how much water from the oceans has been removed and stored in glaciers.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/77784/original/image-20150413-24325-3pkk7h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/77784/original/image-20150413-24325-3pkk7h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/77784/original/image-20150413-24325-3pkk7h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=463&fit=crop&dpr=1 600w, https://images.theconversation.com/files/77784/original/image-20150413-24325-3pkk7h.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=463&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/77784/original/image-20150413-24325-3pkk7h.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=463&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/77784/original/image-20150413-24325-3pkk7h.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=582&fit=crop&dpr=1 754w, https://images.theconversation.com/files/77784/original/image-20150413-24325-3pkk7h.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=582&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/77784/original/image-20150413-24325-3pkk7h.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=582&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Today you’ll have to go to Antarctica or Greenland to find a climate that chilly.</span>
<span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Annual_Average_Temperature_Map.jpg">Robert Rohde / NOAA / UEA</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>It is far from simple to get at the information locked in ancient rocks of the kind studied by Herwartz’s team, but by including the extremely rare oxygen-17 isotope in their study, they were able to show that both of the heavier isotopes must have been rarer than expected in the glacial water that had reacted with the rock. The difference in the depletion of oxygen-17 compared to the depletion in oxygen-18 enabled them to demonstrate likely mean annual surface temperatures of as much as 40°C below zero.</p>
<p>Such low temperatures imply that the oceans would have been deeply frozen too, supporting the full Snowball Earth model. But if the land where the rocks originated was so cold because it was several kilometres above sea level, the possibility that the planet was more of a <a href="http://www.giss.nasa.gov/research/briefs/sohl_01/">Slushball Earth</a>, with open seawater near the equator, cannot entirely be ruled out.</p><img src="https://counter.theconversation.com/content/40097/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>More evidence that planet Earth has gone through various frozen phases.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/373912015-02-09T20:00:53Z2015-02-09T20:00:53ZMining conquistadors caused air pollution 200 years before the industrial revolution<figure><img src="https://images.theconversation.com/files/71494/original/image-20150209-24697-g2c4dn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Conquer Chile? But we don't have the carbon budget.</span> <span class="attribution"><span class="source">Fundación de Santiago (1888) by Pedro Lira</span></span></figcaption></figure><p>When the Spanish conquered South America in the 16th century they took over the Incas’ mines and soon began to pump clouds of lead dust over the Andes. The silver the conquistadors sent back home made them wealthy. It also made them the world’s first industrial-scale toxic metal air polluters – perhaps causing us to rethink the timing of the moment when humans truly began to change the environment.</p>
<p>Formal recognition of the Anthropocene epoch, the “Age of Humans”, will acknowledge the occurrence of an unprecedented impact of human activities on Earth. As scientists, we’ve begun using the term informally, especially in regard to anthropogenic (“human-caused”) climate change. Officially, though, we all live in the Holocene, the epoch named by geologists to mark the end of the last ice age.</p>
<p>To officially say that we live in the Anthropocene – that is, declare the Holocene over and the Anthropocene already underway – we would have to draw an unequivocal line between the two. We’d have to agree on a point in time when human impacts on the environment became large enough to warrant an official change in scientific nomenclature. Some would assign it to the start of agriculture 11,000 years ago, while others tie it to the <a href="https://theconversation.com/first-atomic-bomb-test-may-mark-the-beginning-of-the-anthropocene-36912">advent of the nuclear era in 1945</a>, but most recognise the Anthropocene as beginning with the industrial revolution (1780s-1830s).</p>
<p>However we now have evidence, from an ice core of the Quelccaya Ice Cap in Peru, of anthropogenic pollution of the South American atmosphere that precedes the industrial revolution by around 240 years. The discovery by my colleagues and I, published in the <a href="http://www.eurekalert.org/emb_releases/2015-02/osu-eeo020415.php">Proceedings of the National Academy of Sciences</a>, underscores the difficulty in defining the onset of the Anthropocene.</p>
<h2>In search of the earliest pollution</h2>
<p>While we have plenty of information from around the world about pollution during the industrial period, pre-industrial pollution records are very rare. We have to look to special places on Earth where atmospheric chemicals would have been preserved chronologically, such as lake sediments or the accumulated snow on an ice cap.</p>
<p>Quelccaya is one of those places. The largest ice sheet in the tropics is a <a href="http://www.nytimes.com/2013/04/05/world/americas/1600-years-of-ice-in-perus-andes-melted-in-25-years-scientists-say.html?_r=0">fast-melting</a> poster child for global warming. It’s also a perfect place to learn more about the past climate and environment – the ice core we drilled there in 2003 contained more than 1,200 years of accumulated atmospheric chemistry. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/71501/original/image-20150209-24691-k9zncb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/71501/original/image-20150209-24691-k9zncb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/71501/original/image-20150209-24691-k9zncb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/71501/original/image-20150209-24691-k9zncb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/71501/original/image-20150209-24691-k9zncb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/71501/original/image-20150209-24691-k9zncb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/71501/original/image-20150209-24691-k9zncb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/71501/original/image-20150209-24691-k9zncb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A millennia of climate history.</span>
<span class="attribution"><span class="source">Paolo Gabrielli</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>South America has a rich history of mining and metallurgy. We wondered, would the ice record evidence of ancient metallurgical activity? Air pollution would have to have existed on a truly continental scale to drift on the air from the heart of South American metallurgy in Bolivia across the Andes and onto Quelccaya, some 800 km away.</p>
<p>It did. The story of South American metallurgy – from the rise of the Inca Empire to the Spanish conquest and even the industrial stagnation that followed the end of Spanish rule – is written in the ice.</p>
<h2>An empire built on pollution</h2>
<p>Like the native peoples before them, the Inca gathered metal ore from outcrops or exposed veins and smelted it in <a href="http://www.sciencedirect.com/science/article/pii/S0305440312004141">primitive wind-driven furnaces</a> called <em>huayra</em>. The Quelccaya core first records evidence of pollution from Inca metallurgy around 1480 in the form of trace amounts of bismuth, likely released into the atmosphere during the creation of bismuth bronze, an alloy which has been recovered from the Inca citadel at Machu Picchu. Remarkably, no increases of other trace elements are apparent in the Quelccaya ice record during that period, indicating that the well-known metallurgic activities performed during the Inca reign had a negligible impact on the South American atmosphere.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/71508/original/image-20150209-24682-1xi6db0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/71508/original/image-20150209-24682-1xi6db0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/71508/original/image-20150209-24682-1xi6db0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=417&fit=crop&dpr=1 600w, https://images.theconversation.com/files/71508/original/image-20150209-24682-1xi6db0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=417&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/71508/original/image-20150209-24682-1xi6db0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=417&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/71508/original/image-20150209-24682-1xi6db0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=523&fit=crop&dpr=1 754w, https://images.theconversation.com/files/71508/original/image-20150209-24682-1xi6db0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=523&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/71508/original/image-20150209-24682-1xi6db0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=523&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Cerro Rico, near the Bolivian city of Potosi, was known as the ‘mountain that eats men’.</span>
<span class="attribution"><a class="source" href="http://socialhistory.org/en/today/04-10/potosi-silver-mines">Mining in Potosí (1596), by Theodoor de Bry</a></span>
</figcaption>
</figure>
<p>The Spanish conquistadors lead by Francisco Pizarro defeated the Incas in 1532, starting the colonial period of South America. Silver smelting quickly became the most important industrial activity on the continent, and the Spanish used imported and inefficient Castilian stone furnaces as well as thousands of local <em>huayras</em> as silver extraction spread across Bolivia and Peru. Increases in lead levels in the Quelccaya ice core date to approximately 1540 and document this initial phase of Spanish metallurgy.</p>
<p>In 1572, the Spanish introduced a new technique called amalgamation, which allowed them to process even low-quality ores that contained much more lead than silver. This cold technique involved grinding the ore into powder, which could easily have become airborne. We believe this accounts for the sudden and dramatic spike in lead concentrations in the ice core starting around that time.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/71514/original/image-20150209-24691-1e97gqp.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/71514/original/image-20150209-24691-1e97gqp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/71514/original/image-20150209-24691-1e97gqp.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=221&fit=crop&dpr=1 600w, https://images.theconversation.com/files/71514/original/image-20150209-24691-1e97gqp.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=221&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/71514/original/image-20150209-24691-1e97gqp.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=221&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/71514/original/image-20150209-24691-1e97gqp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=278&fit=crop&dpr=1 754w, https://images.theconversation.com/files/71514/original/image-20150209-24691-1e97gqp.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=278&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/71514/original/image-20150209-24691-1e97gqp.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=278&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Lead concentrations spike during Spanish rule (pink) and drop off after.</span>
<span class="attribution"><span class="source">Uglietti et al.</span></span>
</figcaption>
</figure>
<p>Even the independence war of 1833, which marked the end of Spanish rule, is recorded in the ice. Elsewhere in the world, the industrial revolution was booming – and air pollution growing. But at Quelccaya, lead levels fell and remained low for years after the war, likely due to army destruction of mines in Bolivia and Peru and the post-war lack of infrastructure.</p>
<p>The ice provides a detailed record of more than 1,000 years of South American history that can inform discussions of the Anthropocene timeline. Did it spread out through South America with the trace bits of pollution from the Incas’ bismuth bronze? Or the lead concentrations from increased smelting upon the Spanish arrival? Or perhaps the more dramatic pollution created in the era of amalgamation marks the turning point.</p>
<p>This discovery suggests that our new epoch emerged sporadically through space and time, at different points during human history. Only as we connect the Quelccaya ice core to records elsewhere on Earth can we assemble a clearer picture of the dawn of the Anthropocene.</p><img src="https://counter.theconversation.com/content/37391/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Paolo Gabrielli receives funding from National Science Foundation (USA) and Autonomous Province of Bolzano (Italy).</span></em></p>When the Spanish conquered South America in the 16th century they took over the Incas’ mines and soon began to pump clouds of lead dust over the Andes. The silver the conquistadors sent back home made…Paolo Gabrielli, Research Scientist, Byrd Polar and Climate Research Center and School of Earth Sciences, The Ohio State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/197392013-11-05T06:12:15Z2013-11-05T06:12:15ZGlobal warming finally reaches the last Arctic region<figure><img src="https://images.theconversation.com/files/34243/original/xsdnhrrp-1383312757.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Hudson Bay Lowlands are staying greener for longer as temperatures rise.</span> <span class="attribution"><span class="source">K. Rühland</span></span></figcaption></figure><p>Lakes of the <a href="http://www.pc.gc.ca/docs/v-g/nation/sec4.aspx">Hudson Bay Lowlands</a>, in northeast Canada, are showing evidence of abrupt change in one of the last Arctic regions of the world to have experienced global warming, according to Canadian research <a href="http://rspb.royalsocietypublishing.org/content/280/1772/20131887.full">published</a> in the Proceedings of the Royal Society B journal.</p>
<p>The research team consisting of Drs Kathleen Rühland, John Smol, and Neal Michelutti from <a href="http://post.queensu.ca/%7Epearl/people.htm">Queen’s University Ontario</a>, Dr <a href="http://desc.ca/people/Andrew_Paterson">Andrew Paterson</a> of Ontario’s Ministry of the Environment, and <a href="http://www3.laurentian.ca/livingwithlakes/about/staff/bill-keller/">Bill Keller</a> from the Laurentian University Ontario, retrieved sediment cores from lakes around the western shoreline of Hudson Bay and looked for changes in the microscopic algae that settle at the lake bottom after death.</p>
<p>These algae, known as diatoms, are at the base of the food chain and are an important component of lake ecosystems. When they die and fall to the lake bed they leave behind an environmental archive in the sediment layers that continually accumulate year after year. By examining the changes through time, researchers can trace the environmental history of the region.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/34228/original/5gyst3wj-1383309330.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/34228/original/5gyst3wj-1383309330.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/34228/original/5gyst3wj-1383309330.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/34228/original/5gyst3wj-1383309330.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/34228/original/5gyst3wj-1383309330.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/34228/original/5gyst3wj-1383309330.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/34228/original/5gyst3wj-1383309330.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Diatoms, tiny phytoplankton that hold clues to the climate of the past.</span>
<span class="attribution"><span class="source">Wipeter</span></span>
</figcaption>
</figure>
<p>The Hudson Bay Lowlands were one of the last holdouts against the trend of global warming in the Arctic, but has in a very short period succumbed. In contrast to most of the Arctic, the lowlands maintained relatively stable temperatures until at least the mid-1990s. The region has been an Arctic <a href="http://www.eoearth.org/view/article/155685/">refugium</a> from warming due to the persistence of sea ice on Hudson Bay, the largest northern inland sea, that provides natural cooling.</p>
<p>Previous <a href="http://post.queensu.ca/%7Elow/Intro%20to%20Paleolimnology%20page.html">paleolimnological</a> work (the study of lake histories) in the region found that the biological communities of lakes around Hudson Bay had remained stable for hundreds of years – unlike the dramatic shifts in aquatic biota that were observed throughout most of the Arctic in response to warming.</p>
<p>But in only a couple of decades, air temperatures in the region have increased at a pace and magnitude that are exceptional – even by Arctic standards. <a href="http://onlinelibrary.wiley.com/doi/10.1029/2009JC005334/abstract">Recent studies</a> by climate researchers on Hudson Bay have been reporting reductions in sea ice that have seen the open-water period lengthen by about three weeks compared to the 1990s. The melting sea ice has accelerated the warming trend of the region, quickly creating a positive feedback response that has increased the warming yet further.</p>
<p>We found that, for the first time in over 200 years, the lakes are showing signs of climate change. The diatom records showed relatively stable and simple assemblages consisting of benthic (bottom dwelling) species throughout the record until the last two decades. At that point there was a distinct shift to more diverse assemblages that now include open water diatoms. These diatom changes are very similar <a href="http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2008.01670.x/pdf">to those we have found</a> in lakes and ponds throughout <a href="http://www.pnas.org/content/102/12/4397">circumpolar regions</a> in response to rising air temperatures and less ice. But despite arriving much later in the Hudson Bay Lowlands, the speed and magnitude of the warming taking place here is extraordinary.</p>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/34244/original/fwtxy6cg-1383312925.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/34244/original/fwtxy6cg-1383312925.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=582&fit=crop&dpr=1 600w, https://images.theconversation.com/files/34244/original/fwtxy6cg-1383312925.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=582&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/34244/original/fwtxy6cg-1383312925.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=582&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/34244/original/fwtxy6cg-1383312925.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=731&fit=crop&dpr=1 754w, https://images.theconversation.com/files/34244/original/fwtxy6cg-1383312925.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=731&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/34244/original/fwtxy6cg-1383312925.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=731&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Polar bears living without ice: not a happy bear.</span>
<span class="attribution"><span class="source">Jason Sheath</span></span>
</figcaption>
</figure>
<p>The response of freshwater ecosystems to this very rapid and pronounced warming carries many implications for the future of this Arctic ecosystem.</p>
<p>It is home to the world’s southern-most population of <a href="http://www.cbc.ca/news/canada/manitoba/climate-change-leaves-some-hudson-bay-polar-bears-starving-1.1204260">polar bears</a>, who depend upon permafrost and on sea ice on the bay for survival. In just two decades the response to warming can be detected at both ends of the food chain, among primary producers at the bottom and polar bears at the top. The effects are felt even among humans, with extreme and unpredictable weather conditions now commonplace in the region making it more difficult for First Nations people follow their traditional fishing and hunting routes.</p>
<p>Our findings suggest that ecological tipping points have been crossed and that, sadly, we are witnessing the loss of Arctic ecosystems as we know them. The Hudson Bay Lowlands are the southern-most Arctic region in the world, and therefore on the front lines of rising temperatures. One of the world’s largest peatland areas, the lowlands are a <a href="http://www.mnr.gov.on.ca/stdprodconsume/groups/lr/@mnr/@farnorth/documents/document/stdprod_086879.pdf">significant reservoir</a> of the world’s carbon. If global warming were to increase the release of that carbon into the atmosphere, the repercussions would be of world-wide importance.</p><img src="https://counter.theconversation.com/content/19739/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kathleen Rühland does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Lakes of the Hudson Bay Lowlands, in northeast Canada, are showing evidence of abrupt change in one of the last Arctic regions of the world to have experienced global warming, according to Canadian research…Kathleen Rühland, Research Scientist, Queen's University, OntarioLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/140742013-05-13T04:35:17Z2013-05-13T04:35:17ZAs carbon dioxide hits a new high, there’s still no Planet B<figure><img src="https://images.theconversation.com/files/23601/original/5kjrmvn3-1368407457.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The extreme rate at which greenhouse gases and temperatures are rising is leading to extensive fires.</span> <span class="attribution"><span class="source">AAP Image/Kim Foale</span></span></figcaption></figure><p>On May 9, 2013, the <a href="http://www.noaa.gov/">National Oceanic and Atmospheric Administration</a> in the US recorded CO<sub>2</sub> levels in the atmosphere at of <a href="http://keelingcurve.ucsd.edu/special-note-on-may-9-2013-reading/">400 parts per million</a>. This signifies a return to the atmospheric conditions similar to those of the Pliocene, which ended about 2.6 million years ago.</p>
<h2>The tropical Pliocene</h2>
<p>Global Pliocene temperatures were on average about 2–4°C warmer than pre-industrial temperatures. Those temperatures drove an intense hydrological cycle with extreme evaporation and precipitation. It led to extensive rain forests, lush savannas (now occupied by deserts), small ice caps (about two-thirds of the present) and sea levels about <a href="http://www.giss.nasa.gov/research/features/199704_pliocene/page2.html">25 meters higher than at present</a>.</p>
<p>Life abounded during the Pliocene. But such conditions mean agriculture would hardly be possible. The tropical Pliocene had intense alternating downpours and heat waves. Regular river flow and temperate Mediterranean-type climates which allow extensive farming could hardly exist under those conditions.</p>
<p>After the Pliocene, the earth’s climate shifted gradually into the Pleistocene. During the following 2 million years, glacial-interglacial periods required species to adapt to rapid climate shifts. These shifts included abrupt warming events actually within glacial periods, where regional warming by several degrees occurred over periods as short as several decades to a century. </p>
<p>Basic physics and chemistry, as well as the geological record, tell us that greenhouse gases are the key factor determining the current climate trend. Current greenhouse gas rise rates exceed those of the Pleistocene cycles by more than an order of magnitude. These rates, which during 2012-2013 reached 2.89 ppm CO<sub>2</sub> per year, exceeding any recorded for the last 65 million years, would hardly allow species to adapt to changing climate conditions (see Figure 1).</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/23425/original/pdds7cfk-1368078099.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/23425/original/pdds7cfk-1368078099.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/23425/original/pdds7cfk-1368078099.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=367&fit=crop&dpr=1 600w, https://images.theconversation.com/files/23425/original/pdds7cfk-1368078099.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=367&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/23425/original/pdds7cfk-1368078099.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=367&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/23425/original/pdds7cfk-1368078099.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=461&fit=crop&dpr=1 754w, https://images.theconversation.com/files/23425/original/pdds7cfk-1368078099.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=461&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/23425/original/pdds7cfk-1368078099.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=461&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Figure 1 The current rise in greenhouse emissions is the highest in the past 65 million years, way higher than during the rapid warming periods known as Dansgaard-Oeschger cycles (D-O cycles).</span>
<span class="attribution"><span class="source">Andrew Glikson</span></span>
</figcaption>
</figure>
<h2>Will this mean mass extinction?</h2>
<p>The current CO<sub>2</sub> ppm per year rise rate surpasses CO<sub>2</sub> and temperature rise rates during mass extinctions about 55 million years ago and 65 million years ago.</p>
<p>Fifty-five million years ago, large-scale release of methane drove atmospheric CO<sub>2</sub> to near-1800 ppm and temperature rise to about 5°C over a period of 10,000 years. (<a href="http://www.nature.com/nature/journal/v451/n7176/full/nature06588.html">That’s</a> 0.18 ppm CO<sub>2</sub>year and 0.0005°C/year.)</p>
<p>Sixty-five million years ago, the K-T asteroid impact resulted in a rise of more than 2000ppm CO<sub>2</sub> and about 7.5°C over a period of about 10,000 years (or <a href="http://www.pnas.org/content/99/12/7836.full">about</a> 0.2 ppm/year and 0.00075°C per year). About 4500 billion tons of carbon was released from impacted carbonates and shale, from ignited bushfires and from ocean warming. The CO<sub>2</sub> rise rate was an order of magnitude lower than current rate of 3ppm/year.</p>
<h2>What will the world look like?</h2>
<p>The current rise in greenhouse gases is <a href="http://www.ipcc-wg2.gov/SREX/">enhancing the hydrological cycle</a>, with ensuing <a href="http://www.aph.gov.au/Parliamentary_Business/Committees/Senate_Committees?url=ec_ctte/extreme_weather/index.htm">floods, heat waves and droughts</a>.</p>
<p>If we burned all the earth’s known fossil fuel reserves it would lead to atmospheric CO<sub>2</sub> levels of around 800 to 1000ppm, high or total melting of the polar ice caps, sea level rise on the scale of tens of meters and disruption of the biosphere on a scale analogous to <a href="http://www.astrobio.net/interview/2553/under-a-green-sky">recorded mass extinctions</a>.</p>
<p>At the same time as CO<sub>2</sub> emissions, sulphur dioxide (SO<sub>2</sub>) is being released, mainly from coal burning. This sulphur is increasing the reflection of the atmosphere and thus regulates changes in temperature, as shown in Figure 2 for the periods following 1950, 1975 and 2001. The trend of rising temperatures slowed in 1950 and 2001 when sulphur emissions increased. Likewise, when clean air policies were introduced in 1975, slowing sulphur emissions, a fast-rising temperature trend resumed. The current rise in coal burning and sulphur emissions are locking the world into a Catch-22 cycle.</p>
<p>Carbon emissions may be self-limiting. It is likely that, before atmospheric CO<sub>2</sub> reaches 500ppm, extreme weather events would disrupt industrial and transport fossil fuel-combusting systems enough to lead to reduction of emissions. However, the feedback processes like methane release, forest bushfires and warming oceans will drive CO<sub>2</sub> levels further.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/23502/original/kphx2bmg-1368161512.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/23502/original/kphx2bmg-1368161512.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/23502/original/kphx2bmg-1368161512.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/23502/original/kphx2bmg-1368161512.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/23502/original/kphx2bmg-1368161512.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/23502/original/kphx2bmg-1368161512.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/23502/original/kphx2bmg-1368161512.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/23502/original/kphx2bmg-1368161512.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Sulphur emissions have moderated temperature rises.</span>
<span class="attribution"><span class="source">Temperature: GISS/NASA; Sulphur: SJ Smith</span></span>
</figcaption>
</figure>
<h2>When will we act?</h2>
<p>The land, oceans and biosphere are now in extreme danger, but it doesn’t seem to be driving the global community to the urgent measures required for a meaningful attempt to arrest the current trend. With <a href="http://www.theage.com.au/national/greenhouse-gases-in-new-danger-zone-20130428-2imjm.html">few exceptions</a>, the accelerating rate of atmospheric CO<sub>2</sub> hardly rates a mention on the pages of the global media, preoccupied as it is with short-term economic forecast, daily exchange rates, share market fluctuations and sports results.</p>
<p>In Australia the language has changed from “<a href="http://www.youtube.com/watch?v=CqZvpRjGtGM">the greatest moral issue of our generation</a>” to controversy over a “carbon tax”, diverting the public attention from the climate to a hip-pocket nerve. While we debate the ways to bring about a meaningless 5% reduction in local emissions, we simultaneously develop infrastructure to export hundreds of millions of tons of coal. It all ends up in the same atmosphere.</p>
<p>As Carl Sagan <a href="http://www.goodreads.com/work/quotes/1816628-pale-blue-dot-a-vision-of-the-human-future-in-space">reminded us</a>, on seeing a photograph of Earth taken from Voyager 1 as it left the Solar System</p>
<blockquote>
<p>That’s here. That’s home. That’s us. On it everyone you know, everyone you love, everyone you’ve ever heard of, every human being who ever was, lived out their lives … Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity — in all this vastness — there is no hint that help will come from elsewhere to save us from ourselves.</p>
</blockquote>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/23567/original/4wq9db2n-1368400770.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/23567/original/4wq9db2n-1368400770.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/23567/original/4wq9db2n-1368400770.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=479&fit=crop&dpr=1 600w, https://images.theconversation.com/files/23567/original/4wq9db2n-1368400770.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=479&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/23567/original/4wq9db2n-1368400770.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=479&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/23567/original/4wq9db2n-1368400770.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=602&fit=crop&dpr=1 754w, https://images.theconversation.com/files/23567/original/4wq9db2n-1368400770.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=602&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/23567/original/4wq9db2n-1368400770.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=602&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The pale blue dot of our Earth has no replacement.</span>
<span class="attribution"><span class="source">Voyager 1, NASA</span></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/14074/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Glikson does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>On May 9, 2013, the National Oceanic and Atmospheric Administration in the US recorded CO2 levels in the atmosphere at of 400 parts per million. This signifies a return to the atmospheric conditions similar…Andrew Glikson, Earth and paleo-climate scientist, Australian National UniversityLicensed as Creative Commons – attribution, no derivatives.