tag:theconversation.com,2011:/us/topics/petm-26958/articlesPETM – The Conversation2021-11-01T17:11:07Ztag: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>
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<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>
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<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">
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<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/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>
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<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>
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<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>
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<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/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/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.