tag:theconversation.com,2011:/ca/topics/tree-rings-6381/articles
Tree rings – The Conversation
2023-06-14T11:55:13Z
tag:theconversation.com,2011:article/203090
2023-06-14T11:55:13Z
2023-06-14T11:55:13Z
Climate change is making trees bigger, but also weaker
<figure><img src="https://images.theconversation.com/files/520322/original/file-20230411-755-s14pif.jpg?ixlib=rb-1.1.0&rect=2%2C0%2C1914%2C1276&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Warmer temperatures could lengthen the growing season of trees and consequently increase their growth rate.</span> <span class="attribution"><span class="source">(Shutterstock)</span></span></figcaption></figure><p>As <a href="https://public.wmo.int/en/media/press-release/eight-warmest-years-record-witness-upsurge-climate-change-impacts">global temperatures rise</a>, trees in colder areas are benefiting from an <a href="https://www.nature.com/articles/ncomms5967">extended growing season</a>. A longer growing season results in thicker growth rings and, as a result, higher overall wood production. </p>
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<img alt="" src="https://images.theconversation.com/files/524152/original/file-20230503-20-rp105s.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/524152/original/file-20230503-20-rp105s.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/524152/original/file-20230503-20-rp105s.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/524152/original/file-20230503-20-rp105s.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/524152/original/file-20230503-20-rp105s.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/524152/original/file-20230503-20-rp105s.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/524152/original/file-20230503-20-rp105s.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 article is part of <em>La Conversation Canada’s</em> series <a href="https://theconversation.com/ca-fr/topics/foret-boreale-138017">The boreal forest: A thousand secrets, a thousand dangers</a></strong></p>
<p><br><em>La Conversation Canada invites you to take a virtual walk in the heart of the boreal forest. In this series, our experts focus on management and sustainable development issues, natural disturbances, the ecology of terrestrial wildlife and aquatic ecosystems, northern agriculture and the cultural and economic importance of the boreal forest for Indigenous peoples. We hope you have a pleasant — and informative — walk through the forest!</em></p>
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<p>However, studies suggest that longer growing seasons contribute to weakening the wood, making trees structurally weaker. The poor quality of wood means that trunks break more easily.</p>
<p>We are forest ecologists who specialize in the anatomy and growth of wood. Let’s examine the most recent scientific studies available to try to map the future of our forests and analyze how the changing growing season is determining the characteristics of the wood produced.</p>
<h2>Wood: What is it?</h2>
<p><a href="https://doi.org/10.1093/aob/mcac110">Wood</a> is the product of the progressive accumulation of cells — xylem cells — in trees. The purpose of this accumulation is to renew the sap transport system and to provide mechanical support for the stem (trunk), branches and leaves. </p>
<p>A tree ring is the product of a <a href="https://theconversation.com/climate-change-is-altering-the-seasonal-rhythm-of-plant-life-cycle-events-181231">growing season</a> which, in temperate and boreal environments, runs from spring to autumn. Each year a new growth ring is formed. The <a href="https://doi.org/10.1093/aob/mcac110">thickness of a ring</a> is dependent on a combination of factors inherent to the tree (its species and genetic factors) and environmental factors (such as soil type, sun exposure, climate and competition between neighbouring trees). </p>
<p>In some species, especially in conifers, it can be quite easy to distinguish the rings from each other. This is due to the fact that during the growing season the tree produces two types of wood, characterized by cells with different forms and functions.</p>
<p>In spring, the tree produces many large, light-coloured cells with a thin cell wall. This part of the annual ring is called “earlywood.” In late summer, growth slows down. The cells become smaller, but their walls become thicker. This “latewood” is the darker portion of the annual ring. </p>
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<a href="https://images.theconversation.com/files/514939/original/file-20230313-23-18zjsu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Large circular piece of wood cutting with tree ring texture pattern and cracks, close-up" src="https://images.theconversation.com/files/514939/original/file-20230313-23-18zjsu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/514939/original/file-20230313-23-18zjsu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/514939/original/file-20230313-23-18zjsu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/514939/original/file-20230313-23-18zjsu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/514939/original/file-20230313-23-18zjsu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/514939/original/file-20230313-23-18zjsu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/514939/original/file-20230313-23-18zjsu.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>
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<span class="caption">The thickness of a tree ring depends on a combination of factors inherent to the tree (species, genetics) and environmental factors (soil type, sun exposure, climate and competition between neighbouring trees).</span>
<span class="attribution"><span class="source">(Shutterstock)</span></span>
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<p>The characteristics of the cells of wood are particularly important and are of great interest in ecological and economic terms. First of all, <a href="https://www.nature.com/articles/nplants2015160">wood cell walls stock most of the carbon assimilated from the atmosphere</a> by trees. Thus, a thicker cell wall means the tree is absorbing a greater amount of carbon. Secondly, the ratio of the number of earlywood cells to latewood cells determines the density of the wood, and, therefore, its potential use and material value.</p>
<h2>Trees are growing faster</h2>
<p>Over the past century, in the <a href="https://www.nature.com/articles/s41467-022-33196-x">temperate regions of North America</a> and <a href="https://www.nature.com/articles/ncomms5967">Europe</a>, trees have shown a faster growth rate, up to 77 per cent higher than in the previous century. This increase is related to the production of thicker growth rings.</p>
<p>At first sight, faster growth could be interpreted as higher biomass production, which would lead to a higher carbon storage capacity and, therefore, a greater contribution of our forests to the fight against climate change. In other words, a higher growth rate could mean that more wood would be available for our different needs. </p>
<p>But <a href="https://www.folger.edu/explore/shakespeares-works/alls-well-that-ends-well/read/">as William Shakespeare wrote</a>: “Oft expectation fails, and most oft there where most it promises.” </p>
<h2>Trees die younger</h2>
<p><a href="https://doi.org/10.1016/j.foreco.2018.07.045">A study by the Technical University of Munich</a> in Germany analyzed the growth rate of trees and the characteristics of their wood over the last century. They found that as the growth rate increased, the density of the wood dropped by eight to 12 per cent. </p>
<p>Furthermore, as wood density decreased, their carbon content also decreased by about 50 per cent. This suggested that the trees extracted less carbon dioxide (CO2) from the atmosphere.</p>
<p>In addition to a reduced capacity to absorb and store atmospheric carbon, reduced wood density <a href="https://doi.org/10.1016/j.foreco.2008.06.025">can weaken the structural strength of the stems</a>. Wood fulfils the important function of supporting trees. Reducing its density is therefore accompanied by a lower resistance to mechanical stresses that might come from wind or the effect of gravity on steep slopes.</p>
<p>To complicate matters further, another recent study has shown an association between <a href="https://www.nature.com/articles/s41467-020-17966-z">growth and lifespan in trees</a>: fast-growing trees have a shorter life expectancy.</p>
<h2>Too much is not enough</h2>
<p>In <a href="https://www.nature.com/articles/s41598-023-31336-x">our latest study</a>, we quantified the relationships between the length of growing season, productivity and wood cell characteristics in balsam fir.</p>
<p>The study confirmed that trees with a longer growing season produce more wood cells and a thicker growth ring. However, higher growth also corresponds to a change in the ratio between the amount of earlywood and latewood. For every day that the growing season length increased, the trees produced one more cell of earlywood.</p>
<p>The increase in the ratio between earlywood and latewood is reflected in the decrease in wood density. This shows that an increase in volume growth does not necessarily correspond to a higher biomass production.</p>
<h2>What does the future hold for our forests?</h2>
<p>The <a href="https://public.wmo.int/en/media/press-release/eight-warmest-years-record-witness-upsurge-climate-change-impacts">global average temperature has exceeded the pre-industrial average by about 1.15°C</a> (1850-1900), and is expected to rise further in the coming years. Warmer temperatures could lengthen the growing season of trees and consequently increase their growth rate.</p>
<p>While, on the one hand, this may lead to an expansion of forests globally, the rate of carbon uptake from forests is likely to decrease. </p>
<p>Although our forests will make a <a href="https://www.nature.com/articles/s41467-022-33196-x">substantial contribution to the fight against climate change</a>, the results of these studies are further evidence that environmental problems cannot be solved without taking direct action on the causes that trigger global change. </p>
<p>In the context of climate change, reducing the anthropogenic emissions that cause global warming is not something we can afford to negotiate or postpone.</p><img src="https://counter.theconversation.com/content/203090/count.gif" alt="La Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Roberto Silvestro was awarded the PBEEE grant by the Fonds de Recherche du Québec - Nature et Technologies (FRQNT).</span></em></p><p class="fine-print"><em><span>Sergio Rossi has received funding from NSERC, FRQNT and MFFP.</span></em></p>
A longer growing season for trees, due to global warming, does not necessarily lead to an increase in wood production.
Roberto Silvestro, PhD Candidate, Biology, Université du Québec à Chicoutimi (UQAC)
Sergio Rossi, Professor, Département des Sciences Fondamentales, Université du Québec à Chicoutimi (UQAC)
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/203185
2023-04-05T20:03:35Z
2023-04-05T20:03:35Z
‘Like blood, then turned into darkness’: how medieval manuscripts link lunar eclipses, volcanoes and climate change
<figure><img src="https://images.theconversation.com/files/519148/original/file-20230403-17-t8wel.jpg?ixlib=rb-1.1.0&rect=445%2C222%2C3456%2C2430&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A diagram of a lunar eclipse from De Sphaera Mundi by Johannes de Sacrobosco, c. 1240 AD.</span> <span class="attribution"><a class="source" href="https://digitalcollections.nypl.org/items/148cf2c0-f054-0138-15e1-0242ac110003">New York Public Library</a></span></figcaption></figure><p>Before humans started heating the planet by burning fossil fuels in the 19th century, Earth had experienced centuries-long widespread cool period known as the Little Ice Age.</p>
<p>Scientists believe this cold spell may have been <a href="https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2011GL050168">triggered, in part, by volcanic eruptions</a> which made the atmosphere hazier, blocking some incoming sunlight. </p>
<p>Records of these eruptions are sparse, and much of our knowledge of them comes from the traces left behind in <a href="https://www.nature.com/articles/s41467-019-08357-0">polar ice</a> and <a href="https://www.sciencedirect.com/science/article/abs/pii/S0277379115301888">tree rings</a>, which are fragmentary and sometimes contradictory.</p>
<p>In a <a href="https://www.nature.com/articles/s41586-023-05751-z">new study published in Nature</a>, an international team of researchers led by Sébastien Guillet at the University of Geneva has found another way to learn about these historical eruptions: by studying descriptions of lunar eclipses in medieval manuscripts.</p>
<h2>Dark eclipses</h2>
<p>The researchers compiled hundreds of records of lunar eclipses from across Europe, the Middle East, and Asia, documenting 187 eclipses between 1100 and 1300. </p>
<p>In particular, they searched for descriptions that provided information on the brightness and colour of the Moon during the eclipse. Most of these turned out to be from European monks or clerics, writing in Latin.</p>
<p>Based on these descriptions, the researchers ranked the colour and brightness of the Moon reported in each total eclipse. The brighter the eclipse, the clearer the atmosphere at the time: darker eclipses indicated a higher level of aerosol particles in the upper atmosphere – a marker of recent volcanic activity.</p>
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<img alt="" src="https://images.theconversation.com/files/519473/original/file-20230405-18-65dlb3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/519473/original/file-20230405-18-65dlb3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/519473/original/file-20230405-18-65dlb3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/519473/original/file-20230405-18-65dlb3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/519473/original/file-20230405-18-65dlb3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/519473/original/file-20230405-18-65dlb3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/519473/original/file-20230405-18-65dlb3.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">
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<span class="caption">During a total lunar eclipse, the Moon turns red due to sunlight refracted by Earth’s atmosphere. A particularly dark eclipse indicates more aerosols in the atmosphere, which is a sign of recent volcanic activity.</span>
<span class="attribution"><span class="source">Chris Harwood / Shutterstock</span></span>
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<p>The next step was to put the eclipse data together with simulations of how aerosol particles behave in the atmosphere, modern satellite observations, and climatic evidence from historical tree ring records. </p>
<p>This allowed the researchers to estimate the timing of the culprit eruptions more precisely than from previous ice core records – and determine which eruptions reached the stratosphere and would be more likely to generate climatic cooling effects.</p>
<h2>What lunar eclipses tell us about the state of the atmosphere</h2>
<p>A total lunar eclipse is a beautiful sight. When the Sun, Earth and Moon align perfectly, our planet blocks direct sunlight from reaching the Moon’s surface. </p>
<p>However, Earth’s atmosphere bends sunlight around our planet. As a result, some sunlight reaches the Moon even during a total eclipse. </p>
<p>Earth’s atmosphere also scatters sunlight - acting as a giant colour filter. The bluer the light, the more it is scattered – which is why the sky is blue in the daytime, and why the Sun appears ruddy at dawn and dusk.</p>
<p>During a total lunar eclipse, the sunlight reaching the Moon has been filtered by Earth’s atmosphere, removing much of the blue and yellow light. The light that reaches the Moon is effectively the sum of all the dawns and all the dusks occurring at that time. </p>
<p>And the state of Earth’s atmosphere at that time controls just how much light is filtered. </p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/mbT50-rppaU?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">NASA video released to explain the total Lunar eclipse seen from the Americas in December 2011.</span></figcaption>
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<h2>How volcanoes affect lunar eclipses</h2>
<p>If you’ve ever seen a sunset during a dust storm, or on a very smoky day, you know the extra particles clogging up the sky can produce deep, vibrant reds and oranges.</p>
<p>Imagine a total lunar eclipse occurring while wildfires rage overseas. The fires would pump smoke and dust into Earth’s atmosphere, making the Moon redder and darker during the eclipse. </p>
<p>Which brings us to the effect of volcanoes. The largest volcanic eruptions pump vast amounts of material into Earth’s stratosphere, where it can remain for many months. </p>
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<p>The spectacular volcanic sunsets seen throughout Australia in the months following the Tongan volcanic eruption of January 2022 are a great example. And that material, once in the stratosphere, will spread around Earth.</p>
<p>What effect does this have on lunar eclipses? It turns out the brightness of the Moon during a lunar eclipse depends the amount of material in our stratosphere. In the months after a large eruption, any lunar eclipse would be markedly darker than normal.</p>
<h2>How volcanoes affect the climate</h2>
<p>Volcanic eruptions can eject huge amounts of ash, sulphur dioxide, and other gases high into the atmosphere. Eruptions can cause either cooling or warming (both temporary). The effect depends on exactly what the volcano spews out, how high the plume reaches, and the volcano’s location.</p>
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Read more:
<a href="https://theconversation.com/climate-explained-how-volcanoes-influence-climate-and-how-their-emissions-compare-to-what-we-produce-125490">Climate explained: how volcanoes influence climate and how their emissions compare to what we produce</a>
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<p>Sulphur dioxide is particularly important. If it reaches the stratosphere, it reacts with water vapour to form a lingering veil of sulphate aerosols. These aerosols, along with the volcanic ash, block and scatter Solar radiation, often leading to cooling at the Earth’s surface.</p>
<p>Large volcanic eruptions, such as the <a href="https://earthobservatory.nasa.gov/images/1510/global-effects-of-mount-pinatubo">1991 Mount Pinatubo eruption</a> in the Philippines and the infamous <a href="https://www.cambridge.org/core/journals/quaternary-research/article/abs/historic-eruptions-of-tambora-1815-krakatau-1883-and-agung-1963-their-stratospheric-aerosols-and-climatic-impact/13CE8FA2B0EF3BE25951FB759F904446">1815 eruption of Tambora</a> in Indonesia, slightly lowered global temperature in the years after the eruption. After Tambora, Europe and North America experienced a “<a href="https://iopscience.iop.org/article/10.1088/1748-9326/ab3a10">year without a summer</a>” in 1816.</p>
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<a href="https://images.theconversation.com/files/519457/original/file-20230405-22-ay5bt0.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A photo from the International Space Station showing white puffy clouds over the ocean and a dark grey plume from a volcanic eruption." src="https://images.theconversation.com/files/519457/original/file-20230405-22-ay5bt0.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/519457/original/file-20230405-22-ay5bt0.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/519457/original/file-20230405-22-ay5bt0.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/519457/original/file-20230405-22-ay5bt0.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/519457/original/file-20230405-22-ay5bt0.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/519457/original/file-20230405-22-ay5bt0.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/519457/original/file-20230405-22-ay5bt0.jpeg?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>
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<span class="caption">The plume of ash and smoke from the 2022 Hunga Tonga-Hunga Ha'apai eruption was visible from the International Space Station.</span>
<span class="attribution"><span class="source">EPA / NASA / Kayla Barron</span></span>
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<p>On the other hand, water vapour and carbon dioxide from volcanic eruptions have a warming effect. It’s only small, as all present-day volcanic emissions produce <a href="https://www.usgs.gov/programs/VHP/volcanoes-can-affect-climate">less than 1%</a> of the carbon dioxide released by human activities. </p>
<h2>The past and future of volcanoes, eclipses, and the climate</h2>
<p>Eyewitness accounts through historical reports and oral traditional knowledge are often overlooked in the study of volcanoes. However, the inclusion of broader sources of knowledge is incredibly valuable to help us understand past impacts of volcanic eruptions on people and the environment.</p>
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Read more:
<a href="https://theconversation.com/when-the-bullin-shrieked-aboriginal-memories-of-volcanic-eruptions-thousands-of-years-ago-81986">When the Bullin shrieked: Aboriginal memories of volcanic eruptions thousands of years ago</a>
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<p>In this study, the combination of historical observations with ice records and climate reconstructions from tree rings has enabled more precise timing of those ancient eruptions. In turn, this has allowed us to better understand their potential impact on the climate during the European Middle Ages. Such information can help us to understand the role these eruptions may have played in the transition to the Little Ice Age.</p>
<p>In the future, volcanoes may have to work a little harder to create a “dark” eclipse. As the atmosphere warms, the altitude of the stratosphere will increase. As a result, it may take a bigger eruption to put significant amounts of aerosols into the upper layer where they will hang around to darken the Moon for future generations!</p><img src="https://counter.theconversation.com/content/203185/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Heather Handley receives funding from the Australian Research Council. She is Co-Founder of Women in Earth and Environmental Sciences Australasia (WOMEESA) and Co-Founder and Director of the Earth Futures Festival.</span></em></p><p class="fine-print"><em><span>Jonti Horner 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>
Medieval monks recorded hundreds of lunar eclipses. Centuries later, their descriptions are helping scientists unravel the role of volcanoes in historical climate change.
Heather Handley, Associate Professor of Volcanology and Geoscience Communication, University of Twente and Adjunct Associate Professor, Monash University
Jonti Horner, Professor (Astrophysics), University of Southern Queensland
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/197788
2023-02-15T16:00:14Z
2023-02-15T16:00:14Z
Was 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 Arizona
Darrell Kaufman, Professor of Earth and Environmental Sciences, Northern Arizona University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/193346
2023-01-04T05:23:48Z
2023-01-04T05:23:48Z
Putting the bones of giant, extinct ‘thunderbirds’ under the microscope reveals how they grew
<figure><img src="https://images.theconversation.com/files/492562/original/file-20221031-24-5lt2a3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An artist's reconstruction of the giant _Dromornis stirtoni_. </span> <span class="attribution"><span class="source">Peter Trusler ©</span></span></figcaption></figure><p>The largest flightless bird found anywhere in the world today is the ostrich. It stands about 2.7 metres tall and can weigh up to 150kg. But millions of years ago ostriches would have been dwarfed by several other flightless bird species. </p>
<p>One was <em>Dromornis stirtoni</em>, nicknamed the thunderbird. It lived in the late Miocene period of Australia, about 8 million years ago. Another, <em>Vorombe titan</em>, lived far more recently in Madagascar, off the coast of southern Africa. It is thought to have gone extinct in the 17th century after encountering humans. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6170582">Fossil finds</a> suggest that both species stood about 3 metres tall and were more than double the mass of ostriches.</p>
<p>Today all that remains of these birds are their fossilised bones and, in the case of <em>Vorombe titan</em>, <a href="https://www.tandfonline.com/doi/pdf/10.1671/0272-4634%282007%2927%5B1%3AODOAES%5D2.0.CO%3B2?needAccess=true">a handful of eggs</a>. This means that scientists know very little about the birds’ biology; for instance, the physical differences between male and female <em>Dromornis stirtoni</em> were unclear – until now.</p>
<p>Recently, my colleagues Trevor Worthy and Warren Handley from Australia’s Flinders University and I investigated the microscopic structure of <em>Dromornis stirtoni’s</em> bones. This allowed us first to pinpoint the size differences between males and females. Then we assessed how quickly the giant birds grew and how this compared to other, later members of its lineage. </p>
<p>We found that <em>Dromornis stirtoni</em> grew far more slowly than the most recent “thunderbird” species, <em>Genyornis newtoni</em>, which in turn grew far more slowly than the emus it lived alongside about 40,000 years ago and which still exist today. Its slow growth made <em>Dromornis stirtoni</em> vulnerable to climatic shifts. </p>
<p>This is an important finding amid the current environmental degradation driven by climate change: it is clear that slow-growing animals will be the most vulnerable to extinction.</p>
<h2>Bones are key</h2>
<p>My involvement in this research stems from the work I have done on the bone histology and growth patterns of a number of extinct species. These include the aepyornithids, including <em>Vorombe titan</em> from Madagascar – the “<a href="https://academic.oup.com/biolinnean/article/130/2/268/5815707?login=false">elephant bird</a>” – and <a href="https://anatomypubs.onlinelibrary.wiley.com/doi/full/10.1002/ar.24282">several Mesozoic birds</a>, as well as their <a href="https://press.jhu.edu/books/title/3496/microstructure-dinosaur-bone">dinosaurian relatives</a>.</p>
<p>Despite being fossilised for millions of years, the microscopic structure of bone (histology) is preserved intact and gives us a unique glimpse at how the animal grew when it was alive. In some animals, growth rings (like tree rings) occur and, just like tree rings, they can give us an idea of the age of the animal, and also information about its health.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/4b7oJrZ1gtY?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Professor Anusuya Chinsamy-Turan discusses her research.</span></figcaption>
</figure>
<p>By applying geometric morphology to <em>Dromornis stirtoni’s</em> bones, we were <a href="https://www.researchgate.net/publication/303839943_Sexual_dimorphism_in_the_late_Miocene_mihirung_Dromornis_stirtoni_Aves_Dromornithidae_from_the_Alcoota_Local_Fauna_of_central_Australia">able to ascertain</a> that females had an average body mass of about 441kg while males clocked in at on average 528kg.</p>
<p>Next, we wanted to understand the species’ growth rates. For this, <a href="https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.25047">we studied</a> the microscopic structure of <em>Dromornis</em> long bones belonging to small hatchlings, subadults and adults. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/fossil-find-reveals-giant-prehistoric-thunder-birds-were-riddled-with-bone-disease-173745">Fossil find reveals giant prehistoric 'thunder birds' were riddled with bone disease</a>
</strong>
</em>
</p>
<hr>
<p>We found that the young individuals experienced rapid growth but that this was followed by a period of arrested growth. In the largest individual at least 15 growth marks were present, suggesting that they needed at least 15 growth cycles (years) to reach adult body size. </p>
<p>We proposed that, as <a href="https://www.sciencedirect.com/science/article/pii/S0944200620301197">in modern kangaroos</a>, the periods of arrested growth most likely corresponded to the hot, dry summer months when they were likely experiencing thermal stress and needed to put their energy into dealing with that rather than into growth.</p>
<h2>Growth patterns</h2>
<p>This was an interesting finding, but we knew that it would have greater value if we could compare it to growth patterns in similar species, and in extant species.</p>
<p>So we looked at the bone microstructure of the most recent member of the “thunderbird” lineage – another giant flightless bird <a href="https://www.mdpi.com/1424-2818/13/5/219">called</a> <em>Genyornis newtoni</em>. It went extinct only about 40,000 years ago.</p>
<p>Our results showed that <em>Genyornis</em>, which was about 2.5 metres tall and probably weighed about 240kg, grew much more quickly than <em>Dromornis</em>. It reached adult body size on average in about one to two years, although it occasionally needed as many as four years. </p>
<p>We propose that this overall fast growth trajectory is likely to have helped <em>Genyornis</em> adapt to the unstable, turbulent times in the Pleistocene (from around 2.5 million years ago to about 12,000 years ago), when the aridification of Australia intensified. Instead of taking so long to grow up they would have been able to reach adult size more quickly and reproduce sooner.</p>
<p>But this relatively rapid growth trajectory was not enough to protect <em>Genyornis</em> from another threat: humans. Humans arrived on the Australian continent <a href="https://academic.oup.com/edited-volume/35424/chapter-abstract/303182141?redirectedFrom=fulltext&login=true;%20https://www.nature.com/articles/ncomms10511">about 55,000 years ago</a>. By 49,000 years ago, they had reached the Flinders Ranges, well within view of Lake Callabonna, where they would have encountered both <em>Genyornis</em> and emus.</p>
<p>Emus are about six times smaller than <em>Genyornis</em>, and have faster growth rates without any interruptions in their growth. In addition, they lay many more eggs during each breeding season. There is <a href="https://www.nature.com/articles/ncomms10496">strong evidence</a> that <em>Genyornis</em> eggs were collected and eaten by Indigenous people. <em>Genyornis</em> simply couldn’t lay enough eggs and its offspring simply didn’t grow fast enough to keep up with this pressure on their species; they were eventually driven to extinction by about 40,000 years ago. </p>
<h2>Recouping numbers</h2>
<p>Emus, of course, still exist. We propose that the rapid growth and reproductive strategy of emus allowed them to recoup their numbers at a faster rate after hard times, which allowed them to survive until today.</p><img src="https://counter.theconversation.com/content/193346/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>This research was funded by the Australian Research Council, ARC Discovery Project grant number, DP 180101913 to T. Worthy, L. Arnold, and A. Chinsamy-Turan and the National Research Foundation, South Africa, grant number, 117716 to A. Chinsamy</span></em></p>
The findings have repercussions today: it is clear that slow-growing animals will be the most vulnerable to extinction amid shifting climates.
Anusuya Chinsamy-Turan, Professor, Biological Sciences Department, University of Cape Town
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/193080
2022-10-25T23:04:46Z
2022-10-25T23:04:46Z
Radioactive traces in tree rings reveal Earth’s history of unexplained ‘radiation storms’
<figure><img src="https://images.theconversation.com/files/491154/original/file-20221023-40716-6xlwd2.jpg?ixlib=rb-1.1.0&rect=52%2C37%2C4940%2C3952&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">University of Queensland</span></span></figcaption></figure><p>In searching for planets and studying their stars, I’ve had the privilege to use some of the world’s great telescopes. However, our team has recently turned to an even larger system to study the cosmos: Earth’s forests. </p>
<p>We analysed radioactive signatures left in tree rings around the world to study mysterious “radiation storms” that have swept over Earth half a dozen times in the past 10,000 years or so.</p>
<p>Our results, published today in <a href="https://doi.org/10.1098/rspa.2022.0497">Proceedings of the Royal Society A</a>, rule out “solar superflares” as the culprit – but the true cause remains unknown.</p>
<h2>A history written in tree rings</h2>
<p>When high-energy radiation strikes the upper atmosphere it turns nitrogen atoms into radioactive carbon-14, or radiocarbon. The radiocarbon then filters through the air and the oceans, into sediments and bogs, into you and me, into animals and plants - including hardwoods with their yearly tree rings. </p>
<p>To archaeologists, radiocarbon is a godsend. After it is created, carbon-14 slowly and steadily decays back into nitrogen – which means it can be used as a clock to measure the age of organic samples, in what is called <a href="https://www.nature.com/articles/s43586-021-00058-7">radiocarbon dating</a>. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-what-is-radiocarbon-dating-and-how-does-it-work-9690">Explainer: what is radiocarbon dating and how does it work?</a>
</strong>
</em>
</p>
<hr>
<p>To astronomers, this is equally valuable. Tree rings give a year-by-year record of high-energy particles called “cosmic rays” <a href="https://www.nature.com/articles/s41561-020-00674-0">going back millennia</a>. </p>
<p>The magnetic fields of Earth and the Sun shield us from cosmic rays shooting through the Galaxy. More cosmic rays reach Earth when these magnetic fields are weaker, and fewer when the fields are stronger.</p>
<p>This means the rise and fall of carbon-14 levels in tree rings encodes a history of <a href="https://sci-hub.se/10.1126/science.207.4426.11">the 11-year cycle of the solar dynamo</a> (which creates the Sun’s magnetic field) and the reversals of <a href="https://www.science.org/doi/full/10.1126/science.abb8677">Earth’s magnetic field</a>. </p>
<h2>Miyake events</h2>
<p>But tree rings also record events we cannot presently explain. In 2012, Japanese physicist Fusa Miyake <a href="https://ui.adsabs.harvard.edu/abs/2012Natur.486..240M/abstract">discovered a spike</a> in the radiocarbon content of tree rings from 774 AD. It was so big that several ordinary years’ worth of cosmic rays must have arrived all at once. </p>
<p>As more teams have joined the search, tree ring evidence has been uncovered of further “Miyake events”: from <a href="https://ui.adsabs.harvard.edu/abs/2013NatCo...4.1748M/abstract">993 AD</a> and <a href="https://www.cambridge.org/core/journals/radiocarbon/article/relationship-between-solar-activity-and-14c-peaks-in-ad-775-ad-994-and-660-bc/EFBDD78DEFAAA02B1CB9C3A24933B912">663 BC</a>, and prehistoric events in <a href="https://www.nature.com/articles/s41467-022-28804-9">5259 BC</a>, <a href="https://ui.adsabs.harvard.edu/abs/2021GeoRL..4893419M/abstract">5410 BC</a>, and <a href="https://www.nature.com/articles/s41467-021-27891-4">7176 BC</a>.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/a-large-solar-storm-could-knock-out-the-power-grid-and-the-internet-an-electrical-engineer-explains-how-177982">A large solar storm could knock out the power grid and the internet – an electrical engineer explains how</a>
</strong>
</em>
</p>
<hr>
<p>These have already led to a revolution in archaeology. Finding one of these short, sharp spikes in an ancient sample <a href="https://royalsocietypublishing.org/doi/10.1098/rspa.2016.0263">pins its date down to a single year</a>, instead of the decades or centuries of uncertainty from ordinary radiocarbon dating. </p>
<p>Among other things, our colleagues have used the 993 AD event <a href="https://www.nature.com/articles/s41586-021-03972-8">to reveal the exact year</a> of the first European settlement in the Americas, the Viking village at L'Anse aux Meadows in Newfoundland: 1021 AD. </p>
<h2>Could huge radiation pulses happen again?</h2>
<p>In physics and astronomy, these Miyake events remain a mystery. </p>
<p>How do you get such a huge pulse of radiation? A flurry of papers have blamed supernovae, <a href="https://ui.adsabs.harvard.edu/abs/2013MNRAS.430...32H">gamma-ray bursts</a>, <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...887..202W/abstract">explosions from magnetised neutron stars</a>, and even <a href="https://www.nature.com/articles/srep03728">comets</a>. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/491521/original/file-20221025-18-a2bs48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A photograph of solar flares emanating from the Sun." src="https://images.theconversation.com/files/491521/original/file-20221025-18-a2bs48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/491521/original/file-20221025-18-a2bs48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=491&fit=crop&dpr=1 600w, https://images.theconversation.com/files/491521/original/file-20221025-18-a2bs48.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=491&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/491521/original/file-20221025-18-a2bs48.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=491&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/491521/original/file-20221025-18-a2bs48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=617&fit=crop&dpr=1 754w, https://images.theconversation.com/files/491521/original/file-20221025-18-a2bs48.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=617&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/491521/original/file-20221025-18-a2bs48.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=617&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Could ‘solar superflares’ be responsible for radiocarbon spikes in tree rings?</span>
<span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/images/pia21958-major-solar-flare">NASA / GSFC / Solar Dynamics Observatory</a></span>
</figcaption>
</figure>
<p>However, the <a href="https://ui.adsabs.harvard.edu/abs/2013A%26A...552L...3U/abstract">most widely accepted explanation</a> is that Miyake events are “solar superflares”. These hypothetical eruptions from the Sun would be perhaps 50–100 times more energetic than the biggest recorded in the modern era, the <a href="https://en.wikipedia.org/wiki/Carrington_Event">Carrington Event</a> of 1859. </p>
<p>If an event like this occurred today, it would <a href="https://astronomy.com/news/2021/09/understanding-just-how-big-solar-flares-can-get">devastate power grids, telecommunications, and satellites</a>. If these occur randomly, around once every thousand years, that is a 1% chance per decade – a serious risk. </p>
<h2>Noisy data</h2>
<p>Our team at UQ set out to sift through all the available tree ring data and pull out the intensity, timing, and duration of Miyake events. </p>
<p>To do this we had to develop software to solve a <a href="https://johncarlosbaez.wordpress.com/2012/07/24/carbon-cycle-box-models/">system of equations</a> that model how radiocarbon filters through the entire <a href="https://en.wikipedia.org/wiki/Carbon_cycle">global carbon cycle</a>, to work out what fraction ends up in trees in what years, as opposed to the oceans, bogs, or you and me. </p>
<p>Working with archaeologists, we have just released the first reproducible, systematic study of <a href="https://github.com/qingyuanzhang3/radiocarbon_workflow/tree/main/data">all 98 trees of published data</a> on Miyake events. We have also released <a href="https://sharmallama.github.io/ticktack">open source modelling software</a> as a platform for future work.</p>
<h2>Storms of solar flares</h2>
<p>Our results confirm each event delivers between one and four ordinary years’ worth of radiation in one go. <a href="https://www.nature.com/articles/s41467-018-05883-1">Earlier research</a> suggested trees closer to Earth’s poles recorded a bigger spike – which is what we would expect if solar superflares are responsible – but our work, looking at a larger sample of trees, shows this is not the case.</p>
<p>We also found these events can arrive at any point in the Sun’s 11-year activity cycle. Solar flares, on the other hand, <a href="https://link.springer.com/article/10.1007/s11207-021-01831-3">tend to happen</a> around <a href="https://arxiv.org/abs/2207.12787v2">the peak of the cycle</a>. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/why-is-the-sun-going-quiet-22155">Why is the sun going quiet?</a>
</strong>
</em>
</p>
<hr>
<p>Most puzzling, a couple of the spikes seem to take longer than can be explained by the slow creep of new radiocarbon through the carbon cycle. This suggests that either the events can sometimes take longer than a year, which is not expected for a giant solar flare, or the growing seasons of the trees are not as even as previously thought.</p>
<p>For my money, the Sun is still the most likely culprit for Miyake events. However, our results suggest we’re seeing something more like a storm of solar flares rather than one huge superflare. </p>
<p>To pin down what exactly happens in these events, we will need more data to give us a better picture of the events we already know about. To obtain this data, we will need more tree rings – and also other sources such as <a href="https://ui.adsabs.harvard.edu/abs/2015NatCo...6.8611M/abstract">ice cores from the Arctic and Antarctic</a>.</p>
<p>This is truly interdisciplinary science. Normally I think about beautifully clean, precise telescopes: it is much harder to understand the complex, interconnected Earth.</p><img src="https://counter.theconversation.com/content/193080/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Benjamin Pope receives funding from the Australian Research Council and the Big Questions Institute. </span></em></p>
Half a dozen times in the past 10,000 years, enigmatic ‘Miyake events’ have showered Earth with cosmic rays.
Benjamin Pope, ARC DECRA Fellow, The University of Queensland
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/177221
2022-03-03T00:36:03Z
2022-03-03T00:36:03Z
As industry lines up to take water from a wild Top End river, trees tell the story of a much drier past
<figure><img src="https://images.theconversation.com/files/449396/original/file-20220302-15-axfhwp.jpg?ixlib=rb-1.1.0&rect=13%2C0%2C4587%2C3055&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>The Northern Territory has some of the most pristine rivers in the world. But amid the big push to develop northern Australia, industries are lining up to take water from these wild waterways.</p>
<p>The agriculture industry – and in particular, large-scale cotton growers – are <a href="https://ntfarmers.org.au/nt-plant-industries-economic-analysis-2020/">seeking</a> up to 5.2 billion litres of water a year from the Daly River near Katherine. But <a href="https://doi.org/10.1029/2021WR030881">our research</a> published today suggests water allocations based on recent gauge data might be too generous.</p>
<p>We found since the 1960s, flows in the Daly have been higher than at any point in the past 600 years. If flows return to lower levels in future, extracting large volumes of water may cause big problems for the river.</p>
<p>Over-allocating water resources can degrade rivers and harm aquatic life that needs wet season flows to thrive. Before granting water rights to big business, authorities must better understand the ancient history of Australia’s river flows.</p>
<figure class="align-center ">
<img alt="water runs over rocks in river" src="https://images.theconversation.com/files/449399/original/file-20220302-15-iimwdb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/449399/original/file-20220302-15-iimwdb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/449399/original/file-20220302-15-iimwdb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/449399/original/file-20220302-15-iimwdb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/449399/original/file-20220302-15-iimwdb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/449399/original/file-20220302-15-iimwdb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/449399/original/file-20220302-15-iimwdb.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">
<figcaption>
<span class="caption">The Daly River has not always enjoyed high flows.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<h2>Going back in time</h2>
<p>Every year when the <a href="https://theconversation.com/explainer-what-is-the-australian-monsoon-69411">monsoon</a> arrives in Northern Australia, rivers spread over the floodplains and sustain a kaleidoscopic variety of plant and animal life.</p>
<p>To sustainably develop the north while keeping river ecosystems healthy, we must get a full picture of high and low flows over time. But in many rivers, including the Daly, we don’t have enough data to do this.</p>
<p>Streamflow gauges have only been in place for the last 60 years in the Northern Territory. However the climate – and subsequent river flows – can vary across decades and generations.</p>
<p>Luckily, tree rings can <a href="https://www.nytimes.com/2019/04/30/science/tree-rings-climate.html">tell us</a> how the climate behaved for hundreds of years before streamflow gauges were installed.</p>
<p>The rings indicate how old a tree is and what the weather was like during each year of its life. For example, a ring wider than others indicates greater-than-average growth, and might reflect a year of high rainfall or warm temperatures.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/floodplains-arent-separate-to-a-river-theyre-an-extension-of-it-its-time-to-change-how-we-connect-with-them-157890">Floodplains aren't separate to a river — they're an extension of it. It's time to change how we connect with them</a>
</strong>
</em>
</p>
<hr>
<figure class="align-center ">
<img alt="A bridge across a river with a flood gauge visible" src="https://images.theconversation.com/files/448501/original/file-20220225-13-pvrpm4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/448501/original/file-20220225-13-pvrpm4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=397&fit=crop&dpr=1 600w, https://images.theconversation.com/files/448501/original/file-20220225-13-pvrpm4.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=397&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/448501/original/file-20220225-13-pvrpm4.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=397&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/448501/original/file-20220225-13-pvrpm4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=499&fit=crop&dpr=1 754w, https://images.theconversation.com/files/448501/original/file-20220225-13-pvrpm4.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=499&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/448501/original/file-20220225-13-pvrpm4.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=499&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Flood gauges, like this one on the Katherine River, provide data over decades, now centuries.</span>
<span class="attribution"><span class="source">Martin Andersen</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>Nature’s weather stations</h2>
<p>Tree rings are examined by drilling a narrow hole in a tree to extract a core – a process that does not harm the tree. All up, we used cores from 63 different sites in our study.</p>
<p>We obtained existing cores from Northern Territory trees, then sought to interpret how the tree rings related to past climate changes. Usually, the correlation between tree rings and streamflow can be used to build a model of past streamflow. But the gauge data for the Daly is too short to develop a reliable model.</p>
<p>So we used a <a href="https://www.sciencedirect.com/science/article/pii/S1364815220309646?via%3Dihub">new method</a> using much older rainfall data. It meant we could develop a much better and longer reconstruction of the river’s flow.</p>
<p>But we faced another challenge. The oldest Northern Territory tree core we had to work with was 250 years old, but we wanted to look further back.</p>
<p>So we also used cores from older trees in Southeast Asia. These trees also experience monsoon rainfalls and so record similar climate variations. </p>
<figure class="align-center ">
<img alt="close up of tree rings" src="https://images.theconversation.com/files/449397/original/file-20220302-21-33b0hf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/449397/original/file-20220302-21-33b0hf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=269&fit=crop&dpr=1 600w, https://images.theconversation.com/files/449397/original/file-20220302-21-33b0hf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=269&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/449397/original/file-20220302-21-33b0hf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=269&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/449397/original/file-20220302-21-33b0hf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=339&fit=crop&dpr=1 754w, https://images.theconversation.com/files/449397/original/file-20220302-21-33b0hf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=339&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/449397/original/file-20220302-21-33b0hf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=339&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Tree rings can tell us how the climate behaved before scientific records began.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<h2>Rivers of change</h2>
<p>Our reconstruction showed over that, with the exception of dry years in 2019 and 2020, flows over the last 57 years were higher than at any point in the past 600 years.</p>
<p>This is because more rainfall is falling during the monsoon season than ever before. <a href="https://www.nature.com/articles/s41598-017-16414-1">Records</a> from ship logs suggest the increase is part of a much longer trend which began in the 1800s. </p>
<p>Theories abound about why monsoon rainfall has increased, and include changes in sea surface <a href="https://journals.ametsoc.org/view/journals/clim/21/12/2007jcli1908.1.xml">temperatures</a> and changes to the timing of the <a href="https://rmets.onlinelibrary.wiley.com/doi/10.1002/joc.1736">monsoon onset</a>. More research is needed into this phenomenon.</p>
<p>However, the increase suggests flows in the Daly River may return to low levels in future. So allocating water to industry based only on data from the past few decades may mean too much water will be extracted and the river’s health will suffer.</p>
<p>Even during these unusually wet decades, we found the annual monsoon - and therefore streamflow – varies a lot from year to year. </p>
<p>In La Niña years, the Daly’s flow is higher than in other years – but during El Niño there is no difference. So, while low flows lead to higher water demand and risk to rivers, we don’t really understand why they occur.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-hydropower-industry-is-talking-the-talk-but-fine-words-wont-save-our-last-wild-rivers-168252">The hydropower industry is talking the talk. But fine words won't save our last wild rivers</a>
</strong>
</em>
</p>
<hr>
<figure class="align-center ">
<img alt="flooded river with trees and roofs of homes" src="https://images.theconversation.com/files/449391/original/file-20220302-23-1j4zrom.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/449391/original/file-20220302-23-1j4zrom.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/449391/original/file-20220302-23-1j4zrom.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/449391/original/file-20220302-23-1j4zrom.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/449391/original/file-20220302-23-1j4zrom.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/449391/original/file-20220302-23-1j4zrom.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/449391/original/file-20220302-23-1j4zrom.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">
<figcaption>
<span class="caption">The Daly River has recently experienced higher streamflow than in the past, including this flooding in 2018.</span>
<span class="attribution"><span class="source">Secure NT</span></span>
</figcaption>
</figure>
<h2>An uncertain future</h2>
<p>Global climate models differ on whether northern Australia will experience higher or lower rainfall in future. So we don’t know if the Daly and other Northern Territory rivers will remain high in decades to come. </p>
<p>Climate models do suggest monsoon rain will become <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL073217">more variable</a>. This means managing river resources in the territory will be even more complex in coming years and needs careful planning. </p>
<p>The proposed water allocation from the Daly River comes on the back of <a href="http://www.bom.gov.au/climate/updates/articles/a037.shtml">record dry years</a> and much lower wet season flows than normal. And it follows <a href="https://www.abc.net.au/news/2022-02-08/indigenous-owners-call-for-nt-government-to-reject-water-licence/100812012">other industry proposals</a> to extract huge amounts of water from territory rivers.</p>
<p>Monsoon rivers are the lifeblood of the Top End. They’re unique, precious and need to be protected. We can’t hold back development, but we can make good decisions to ensure ecosystems are managed sustainably. </p>
<p>Let’s listen to the message of the trees – and make sure these rivers are still healthy in another 600 years.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/victoria-just-gave-2-billion-litres-of-water-back-to-indigenous-people-heres-what-that-means-for-the-rest-of-australia-150674">Victoria just gave 2 billion litres of water back to Indigenous people. Here's what that means for the rest of Australia</a>
</strong>
</em>
</p>
<hr>
<img src="https://counter.theconversation.com/content/177221/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Fiona Johnson receives funding from the Australian Research Council, NSW and Victorian State Government. </span></em></p><p class="fine-print"><em><span>Jonathan Palmer receives funding from the Australian Research Council (ARC).</span></em></p><p class="fine-print"><em><span>Martin Sogaard Andersen receives funding from ARC, NSW State and Federal Government. </span></em></p><p class="fine-print"><em><span>Philippa Higgins 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>
Tree rings are ‘nature’s weather stations’ and reveal far more of the Daly River’s history than scientific records can.
Philippa Higgins, PhD candidate, UNSW Sydney
Fiona Johnson, Associate Professor, School of Civil and Environmental Engineering, UNSW Sydney
Jonathan Palmer, Research Fellow, School of Biological, Earth and Environmental Sciences., UNSW Sydney
Martin Sogaard Andersen, Associate professor, UNSW Sydney
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/170816
2021-11-02T11:14:29Z
2021-11-02T11:14:29Z
How using tree rings to look into the past can teach us about the climate changes we face in the future
<figure><img src="https://images.theconversation.com/files/429373/original/file-20211029-13-1t8bu8b.jpeg?ixlib=rb-1.1.0&rect=6%2C0%2C4035%2C3036&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Tree rings can tell us about periods of drought, warmth and heavy rainfall in the past.</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Cross-section_of_an_Oak_Log_Showing_Growth_Rings.jpg">Rbreidbrown/Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>“The longer you can look back, the farther you can look forward,” Winston Churchill <a href="https://www.oxfordreference.com/view/10.1093/acref/9780191843730.001.0001/q-oro-ed5-00002969">proclaimed</a> to the Royal College of Physicians in 1944, invoking a much older idea known as “<a href="https://www.nationalgeographic.org/encyclopedia/uniformitarianism/">uniformitarianism</a>”.</p>
<p>Coined by geologists James Hutton and Charles Lyell, this is the idea that past processes (like erosion or climate change) that have <a href="https://www.sciencedirect.com/science/article/abs/pii/S2213305414000289?casa_token=8_1H7-Q3hQ0AAAAA:W0j8NUHkWwHGfzIHucmcKWTLWXefSohunlBv3_QEVhx_NrfvI6z8dehnXh2qpMZYJKW5rY6Mjw">altered the Earth</a> over time remain similar, so we can analyse them to understand the consequences of future processes – such as how climate change might shape our planet in the years to come.</p>
<p>This principle of looking to the past to see the future still guides the science of <a href="https://www.elsevier.com/books/paleoclimatology/bradley/978-0-12-386913-5">palaeoclimatology</a>, or the study of past climates. </p>
<p>For example, the <a href="https://jgs.lyellcollection.org/content/178/1/jgs2020-239">geological record</a> tells us there were <a href="https://www.theatlantic.com/science/archive/2018/08/earths-scorching-hot-history/566762/?utm_source=twb">palm trees</a> in Antarctica many millions of years ago, when CO₂ was at 1,000 parts per million in our planet’s atmosphere.</p>
<p>Looking back to <a href="https://www.climatecentral.org/news/the-last-time-co2-was-this-high-humans-didnt-exist-15938">this period</a>, when our planet was experiencing naturally high <a href="https://www.science.org/doi/10.1126/sciadv.aax1874">CO₂ levels</a>, helps us study what life on Earth might look like if our attempts to reach net zero emissions fail and greenhouse gas emission rates continue to rise.</p>
<p>When we want to look into the past, the greatest tools we have are called <a href="https://interactive.carbonbrief.org/how-proxy-data-reveals-climate-of-earths-distant-past/">proxy climate records</a>. These are <a href="https://www.nap.edu/read/11676/chapter/8">natural archives</a> that record variations in climate, including ice cores, lake sediments, corals, cave stalagmites, peat and tree trunks. These archives provide us with climate change data reaching further back than the few hundred years of information we can get from human instrumental weather records. </p>
<p>My research focuses on using data from trees to reconstruct historical climate conditions across the world, from <a href="https://www.nature.com/articles/ngeo2752">Europe</a> to <a href="https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.2011.0037">Malaysia</a> to <a href="https://link.springer.com/article/10.1007/s00382-021-05827-4">China</a> and <a href="https://www.sciencedirect.com/science/article/pii/S0254629915303264">South Africa</a>.</p>
<h2>Rings</h2>
<p>The most useful proxy we have for exploring the climate of the recent past is the annual rings that form in tree trunks. Using tree rings to reconstruct climate is known as <a href="https://www.cambridge.org/core/journals/quaternary-research/article/abs/dendroclimatology-and-dendroecology/07105BC70CFE929901EF8201D4B7580F">dendroclimatology</a>. </p>
<p>In the UK’s temperate climate, trees form a ring because they stop growing in the winter, forming a ring boundary at the start of each cold season. In other climates, the dry season forms the ring boundary. The width of rings, the density of wood and changes in wood chemistry <a href="https://journals.sagepub.com/doi/abs/10.1191/0959683603hl668rp?casa_token=vFwiY-0BE5QAAAAA:4b3nNz2xxkqmTwb4rxX8E-46twKTadlJaTRVi5vvOxW6uoYTPwqvHfpsDdaUzOV4yx7RUVK3EqPx">all record</a> what the weather was like in the year the ring formed. <a href="https://wires.onlinelibrary.wiley.com/doi/epdf/10.1002/wcc.42">Over time</a>, trees build up a record of climate change in their trunks, forming a record in which each value can be tied to a calendar year.</p>
<figure class="align-center ">
<img alt="A cross-section of a tree trunk showing its rings" src="https://images.theconversation.com/files/429337/original/file-20211029-13-1xffq77.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429337/original/file-20211029-13-1xffq77.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429337/original/file-20211029-13-1xffq77.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429337/original/file-20211029-13-1xffq77.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429337/original/file-20211029-13-1xffq77.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429337/original/file-20211029-13-1xffq77.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429337/original/file-20211029-13-1xffq77.jpeg?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">
<figcaption>
<span class="caption">Tree rings provide a detailed archive of the tree’s local climate.</span>
<span class="attribution"><a class="source" href="https://pixabay.com/photos/annual-rings-tree-wood-texture-3212803/">Couleur/Pixabay</a></span>
</figcaption>
</figure>
<h2>Records</h2>
<p>Trees’ presence across most of the globe – and the ready availability of old tree samples in buildings, churches and boats, thanks to their construction from wood – are the reason tree rings are such a powerful data source. By piecing together living trees, dead wood and archaeological samples, our tree ring records stretch back <a href="https://www.nature.com/articles/312150a0">thousands of years</a>. </p>
<p>One of the most important findings from global studies of tree rings has been that, over the <a href="https://wires.onlinelibrary.wiley.com/doi/10.1002/wcc.418">last 2,000 years</a>, there has been no period before the industrial revolution in which the entire planet has warmed (or cooled) <a href="https://www.nature.com/articles/s41586-019-1401-2">at the same time</a>. This is a unique feature of human-driven climate change in the industrial era: almost everywhere on the planet is warming. </p>
<p>We can look at periods of naturally anomalous climate during the last few thousand years to understand how our climate system changes when the planet warms. Exploring medieval records reveals that climate anomalies affect different places <a href="https://michaelmann.net/sites/default/files/articles/medclimopt.pdf">in different ways</a>. </p>
<p>The <a href="https://theconversation.com/climate-explained-what-was-the-medieval-warm-period-155294">Medieval Climate Anomaly</a>, for example, was named for the unusually warm temperatures experienced across Europe in the middle ages – although the eastern tropical Pacific region actually became cooler during this time.</p>
<figure class="align-center ">
<img alt="Stalagmites in a cave" src="https://images.theconversation.com/files/429355/original/file-20211029-21-1oonbro.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429355/original/file-20211029-21-1oonbro.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429355/original/file-20211029-21-1oonbro.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429355/original/file-20211029-21-1oonbro.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429355/original/file-20211029-21-1oonbro.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429355/original/file-20211029-21-1oonbro.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429355/original/file-20211029-21-1oonbro.jpeg?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">
<figcaption>
<span class="caption">Cave stalagmites can act as natural archives, carrying climate records in their deposited layers.</span>
<span class="attribution"><a class="source" href="https://pixabay.com/photos/potholing-room-stalactites-columns-3511263/">Jaworski/Pixabay</a></span>
</figcaption>
</figure>
<h2>Rain</h2>
<p>Tree rings also help us understand past patterns of wet and dry climate phases. In areas where tree growth is sensitive to rainfall, including across <a href="https://www.science.org/doi/10.1126/sciadv.1500561">Europe</a>, the <a href="https://www.science.org/doi/abs/10.1126/science.1185188">Asian monsoon region</a>, <a href="https://climatedataguide.ucar.edu/climate-data/drought-atlases-tree-rings">North America and Mexico</a>, information from thousands of tree rings has been used to build up “atlases” of past changes in rainfall. These reconstructions reveal a stark picture of the relationship between the development of our societies and the stability of our climate. </p>
<figure class="align-center ">
<img alt="An aerial view of a field in England" src="https://images.theconversation.com/files/429338/original/file-20211029-21-1cs02x3.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429338/original/file-20211029-21-1cs02x3.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429338/original/file-20211029-21-1cs02x3.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429338/original/file-20211029-21-1cs02x3.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429338/original/file-20211029-21-1cs02x3.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=565&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429338/original/file-20211029-21-1cs02x3.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=565&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429338/original/file-20211029-21-1cs02x3.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=565&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Wet summers in the UK have historically spelled disaster for local populations.</span>
<span class="attribution"><a class="source" href="https://pixabay.com/photos/sunset-england-britain-uk-tree-3487528/">IanCollis/Pixabay</a></span>
</figcaption>
</figure>
<p>In the dry areas of northern Mexico and the western US, archaeological wood provides a record of drought intensity going back 1200 years. This record reveals “<a href="https://www.google.com/url?q=https://www.nationalgeographic.com/environment/article/megadrought-persists-in-western-us-as-another-extremely-dry-year-develops&sa=D&source=docs&ust=1635523098733000&usg=AOvVaw1FwnYdrxOjb2mhtOryowR-">megadrought</a>” phases, where low rainfall led to narrow tree rings. </p>
<p>There are close correlations between these narrowing rings and historical records showing the collapse of local <a href="https://www.nationalgeographic.com/environment/article/megadrought-persists-in-western-us-as-another-extremely-dry-year-develops">Native American</a> communities during a drought that tree ring records tell us lasted for decades. </p>
<p>UK populations have historically been more affected by wet summers than by dry ones, since they can lead to crop failure. Climate information in ancient British oaks shows when the UK and Europe experienced successive wet, cool summers. </p>
<p>One such event, around 1310, has become known as the <a href="https://dantean.hypotheses.org/">Dantean anomaly</a>, an agricultural crisis that resulted in the loss of one-sixth of the population of Europe as wet summers led to crop failures and famine. </p>
<p>The <a href="https://onlinelibrary.wiley.com/doi/full/10.1002/jqs.3226">stories</a> of past climate that trees hold in their trunks give us a striking insight into how closely social and climate stability are intertwined. These stories of climate conditions long past raise questions about what kind of future we are making for ourselves.</p><img src="https://counter.theconversation.com/content/170816/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Mary Gagen receives funding from Research Councils UK, National Geographic, The EU, The European Social Fund and Welsh Government. </span></em></p>
Tree rings carry a wealth of information, which can be used to uncover climate data from hundreds of years ago.
Mary Gagen, Professor of Physical Geography, Swansea University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/160526
2021-05-17T20:07:49Z
2021-05-17T20:07:49Z
We found a secret history of megadroughts written in tree rings. The wheatbelt’s future may be drier than we thought
<figure><img src="https://images.theconversation.com/files/400933/original/file-20210517-13-27j0fn.jpg?ixlib=rb-1.1.0&rect=6%2C48%2C4065%2C2661&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An almost-dry dam, surrounded by wheat fields, in WA's wheatbelt region</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>Drought over the last two decades has dealt a heavy blow to the wheatbelt of Western Australia, the country’s most productive grain-growing region. Since 2000, winter rainfall has plummeted by almost 20% and <a href="https://link.springer.com/article/10.1007%2Fs10584-020-02666-w">shifted grain-growing areas</a> towards the coast. </p>
<p>Our <a href="https://link.springer.com/article/10.1007%2Fs00382-021-05782-0">recent research</a>, however, found these dry conditions are nothing out of the ordinary for the region. </p>
<p>In fact, after analysing rings in centuries-old tree trunks, we found the region has seen far worse “megadroughts” over the last 700 years. Australia’s instrumental climate records only cover the last 120 or so years (at best), which means these historic droughts may not have previously been known to science.</p>
<p>Our research also found the 20th century was the wettest of the last seven centuries in the wheatbelt. This is important, because it means scientists have likely been underestimating the actual risk of drought – and this will be exacerbated by climate change.</p>
<h2>What we can learn from ancient trees</h2>
<p>We estimate the risk of extreme climate events, such as droughts, cyclones and floods, based on what we know from instrumental climate records from weather stations. Extending climate records by hundreds or even thousands of years means scientists would be able to get a much better understanding of climate variability and the risk of extreme events.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/400931/original/file-20210517-19-1rtemf9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="_Callitris_ trees overlooking a salt lake" src="https://images.theconversation.com/files/400931/original/file-20210517-19-1rtemf9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/400931/original/file-20210517-19-1rtemf9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/400931/original/file-20210517-19-1rtemf9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/400931/original/file-20210517-19-1rtemf9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/400931/original/file-20210517-19-1rtemf9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/400931/original/file-20210517-19-1rtemf9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/400931/original/file-20210517-19-1rtemf9.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"><em>Callitris</em> trees overlooking a salt lake. We pulled a column of wood from these tree trunks to investigate past climate changes in the region.</span>
<span class="attribution"><span class="source">Alison O'Donnell</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Thankfully we can do just that in many parts of the world using proxy records — things like <a href="https://iopscience.iop.org/article/10.1088/1748-9326/10/12/124002">tree rings</a>, <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010PA002050">corals</a>, <a href="https://www.pnas.org/content/112/15/4576">stalagmites</a> and <a href="https://hess.copernicus.org/articles/20/1703/2016/">ice cores</a> in Antarctica. These record evidence of past climate conditions as they grow.</p>
<p>For example, trees typically create a new layer of growth (“growth ring”) around their trunks, just beneath the bark, each year. The amount of growth generally <a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0249959">depends on how much rain falls in the year</a>. The more it rains, the more growth and the wider the ring.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/400930/original/file-20210517-13-1859yc7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/400930/original/file-20210517-13-1859yc7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/400930/original/file-20210517-13-1859yc7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=91&fit=crop&dpr=1 600w, https://images.theconversation.com/files/400930/original/file-20210517-13-1859yc7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=91&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/400930/original/file-20210517-13-1859yc7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=91&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/400930/original/file-20210517-13-1859yc7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=115&fit=crop&dpr=1 754w, https://images.theconversation.com/files/400930/original/file-20210517-13-1859yc7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=115&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/400930/original/file-20210517-13-1859yc7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=115&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Tree rings of <em>Callitris columellaris</em>.</span>
<span class="attribution"><span class="source">Alison O'Donnell</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>We used growth rings of native cypress trees (<em>Callitris columellaris</em>) near a large salt lake at the eastern edge the wheatbelt region. These trees can live for up to 1,000 years, perhaps even longer.</p>
<p>We can examine the growth rings of living trees without cutting them down by carefully drilling a small hole into the trunk and extracting a column (“core”) of wood about the size of a drinking straw. By measuring the ring widths, we developed a timeline of tree growth and used this to work out how much rain fell in each year of a tree’s life.</p>
<p>This method allowed us to reconstruct the last 668 years of autumn-winter rainfall in the wheatbelt. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/400928/original/file-20210517-17-1o6qtn4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A tree trunk with a blue scientific instrument attached" src="https://images.theconversation.com/files/400928/original/file-20210517-17-1o6qtn4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/400928/original/file-20210517-17-1o6qtn4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=476&fit=crop&dpr=1 600w, https://images.theconversation.com/files/400928/original/file-20210517-17-1o6qtn4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=476&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/400928/original/file-20210517-17-1o6qtn4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=476&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/400928/original/file-20210517-17-1o6qtn4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=598&fit=crop&dpr=1 754w, https://images.theconversation.com/files/400928/original/file-20210517-17-1o6qtn4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=598&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/400928/original/file-20210517-17-1o6qtn4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=598&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 tree borer – a hollow drill used to extract ‘cores’ of wood from tree trunks.</span>
<span class="attribution"><span class="source">Alison O'Donnell</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>A history of megadroughts</h2>
<p>One of the most pressing questions for the wheatbelt is whether the decline in autumn-winter rainfall observed in recent decades is unusual or extreme. Our extended record of rainfall lets us answer this question.</p>
<p>Yes, rainfall since 2000 was below the 668-year average — but it was not extremely low.</p>
<p>The last two decades may seem particularly bad because our expectations of rainfall in the wheatbelt are likely based on memories of higher rainfall. But this frequent wet weather has actually been the anomaly. Our tree rings revealed the 20th century was wetter than any other in the last 700 years, with 12% more rain in the autumn-winter seasons on average than the 19th century. </p>
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Read more:
<a href="https://theconversation.com/500-years-of-drought-and-flood-trees-and-corals-reveal-australias-climate-history-51573">500 years of drought and flood: trees and corals reveal Australia's climate history</a>
</strong>
</em>
</p>
<hr>
<p>Before the 20th century, the wheatbelt saw five droughts that were longer and more severe than any we’ve experienced in living memory, or have recorded in instrumental records. This includes two dry periods in the late 18th and 19th centuries that persisted for more than 30 years, making them “megadroughts”. </p>
<p>While the most recent dry period has persisted for almost two decades so far, rainfall during this period is at least 10% higher than it was in the two historical megadroughts.</p>
<p>This suggests prolonged droughts are a natural and relatively common feature of the wheatbelt’s climate.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/400929/original/file-20210517-23-1t3rsak.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/400929/original/file-20210517-23-1t3rsak.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/400929/original/file-20210517-23-1t3rsak.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/400929/original/file-20210517-23-1t3rsak.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/400929/original/file-20210517-23-1t3rsak.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/400929/original/file-20210517-23-1t3rsak.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/400929/original/file-20210517-23-1t3rsak.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/400929/original/file-20210517-23-1t3rsak.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 aerial view of the tree-ring site, home to trees that can live up to 1,000 years.</span>
<span class="attribution"><span class="source">Hannah Etchells</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>So how does human-caused climate change play into this?</p>
<p>It’s likely both natural climate variability and human-caused climate change contributed to the wheatbelt’s recent decline in rainfall. Unfortunately, it’s also likely their combined influence will lead to even less rainfall in the near future.</p>
<h2>What happens now?</h2>
<p>Our findings have important implications for assessing the risk of drought. It’s now clear we need to look beyond these instrumental records to more accurately estimate the risk of droughts for the wheatbelt. </p>
<p>But currently, proxy climate records like tree rings aren’t generally used in drought risk models, as there aren’t many of them in the regions scientists want to research.</p>
<p>Improving risk estimates leads to better informed decisions around preparing for and managing the effects of droughts and future natural disasters.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/to-help-drought-affected-farmers-we-need-to-support-them-in-good-times-as-well-as-bad-101184">To help drought-affected farmers, we need to support them in good times as well as bad</a>
</strong>
</em>
</p>
<hr>
<p>Our findings are a confronting prospect for the future of farming in the wheatbelt.</p>
<p>Australian farmers have shown <a href="https://www.abc.net.au/news/2021-02-22/farmers-in-australias-southwest-adapting-to-less-water/13168666">tremendous innovation</a> in their ability to adapt in the face of drought, with many shifting from livestock to crops. This resilience will be critical as farmers face a drier, more difficult future.</p><img src="https://counter.theconversation.com/content/160526/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alison O'Donnell receives funding from the Australian Research Council. </span></em></p><p class="fine-print"><em><span>Edward Cook receives funding from US National Science Foundation research grants.
</span></em></p><p class="fine-print"><em><span>Pauline Grierson receives funding from the Australian Research Council</span></em></p>
Our research found that in 700 years, the 20th century was the wheatbelt’s wettest. This means all our drought predictions are skewed.
Alison O'Donnell, Research Fellow in Dendroclimatology, The University of Western Australia
Edward Cook, Ewing Lamont Research Professor, Director Of Tree-Ring Lab, Columbia University
Pauline Grierson, Director, West Australian Biogeochemistry Centre, The University of Western Australia
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/153115
2021-01-18T14:44:28Z
2021-01-18T14:44:28Z
Forests go into growth ‘overdrive’ to recover from drought – new study
<figure><img src="https://images.theconversation.com/files/379220/original/file-20210118-23-1rfo20e.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C6000%2C3997&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Tom Ovenden</span>, <span class="license">Author provided</span></span></figcaption></figure><p>One in 12 people could face severe drought every year by 2100, according to <a href="https://www.nature.com/articles/s41558-020-00972-w">a recent study</a>. And water stored on two-thirds of the Earth’s land surface will shrink as the climate warms. As plant ecologists, we’re concerned with what that means for forests – one of the largest carbon sinks and biggest assets the world has in the fight against climate change.</p>
<p>Droughts can stunt forest growth, kill trees and even change how forests function, or <a href="https://theconversation.com/climate-change-having-the-right-combination-of-tree-personalities-could-make-forests-more-resilient-119786">what species</a> they’re made up of. We studied one species in particular to understand how trees have responded to past droughts, and how resilient they’re likely to be. </p>
<p>What we found suggests that some trees could rebound from difficult periods with more vitality than we might have imagined, which could be good news for forests facing a drier future.</p>
<h2>Tree ring records</h2>
<p><a href="https://ec.europa.eu/jrc/en/research-topic/forestry/qr-tree-project/scots-pine">Scots pine</a> is one of the most widely distributed conifer species in the world. In its native range across Europe, it supports a broad range of wildlife such as red squirrels, and is widely grown for its timber.</p>
<p><a href="https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.13576">In our study</a>, we collected tree rings from an experimental pine forest that scientists planted in Scotland in 1935. Trees form these rings in their trunks and, in doing so, record how growth varied according to the climate conditions of each season. Years with a favourable climate generally produce wide tree rings, while bad years with droughts or other stressful weather produce narrow rings. </p>
<p>These rings effectively allow scientists to look back in time. To understand how trees recover from drought, we compared the width of these rings formed in drought years with growth modelled in an average year and throughout their recovery.</p>
<figure class="align-center ">
<img alt="A stand of towering pine trees amid scrub and small trees on a dry heathland." src="https://images.theconversation.com/files/379252/original/file-20210118-19-pysgko.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/379252/original/file-20210118-19-pysgko.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=416&fit=crop&dpr=1 600w, https://images.theconversation.com/files/379252/original/file-20210118-19-pysgko.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=416&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/379252/original/file-20210118-19-pysgko.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=416&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/379252/original/file-20210118-19-pysgko.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=523&fit=crop&dpr=1 754w, https://images.theconversation.com/files/379252/original/file-20210118-19-pysgko.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=523&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/379252/original/file-20210118-19-pysgko.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">
<figcaption>
<span class="caption">Scots Pine trees are a common sight in forests and heathland across Europe.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/dry-heathland-scots-pines-trees-pinus-161967215">Martin Fowler/Shutterstock</a></span>
</figcaption>
</figure>
<h2>Playing catch-up</h2>
<p>We found that even trees of the same age and species growing in the same place took very different lengths of time to recover from drought. On average, the rate of tree growth took four years to recover to levels that might have been expected if no drought had occurred, with most trees taking between one and six years – though some trees still hadn’t recovered this growth rate nine years later.</p>
<p>After digging a little deeper, we found that the size of each tree before the drought, or how fast it was growing at the time, made a significant difference to how resilient it was. Fast growing trees bounced back quicker, but larger trees took a longer time to achieve growth rates that would have been expected if no drought had happened.</p>
<p>What really surprised us was what happened in some trees after growth had recovered. Rather than continuing to grow at rates we might have expected from the temperature and rainfall records in those post-recovery years, the growth of some trees went into overdrive, and these trees actually started growing faster than in our modelled scenario where no drought had occurred. </p>
<p>While this growth “overdrive” was only temporary and didn’t occur in all of the trees we studied, the combined effect of this compensatory growth was powerful. These trees grew so fast that they started to recover some of the trunk girth that had been lost from the forest as a result of the drought. This meant that the total impact of the drought after nine years was much less than after four years, which is normally the post-drought period similar studies consider.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/A5Kjfquci6I?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>Compensatory growth happens elsewhere in nature – it’s been recorded in species of fish, grasses and moths. <a href="https://doi.org/10.1111/jeb.13150">One study</a> found that female greater wax moths deprived of food for 12 hours undergo rapid growth to catch up once food returns, though the cost of this behaviour is a shorter lifespan. </p>
<p>If or how mature trees use compensatory growth to deal with drought is largely unexplored, perhaps because, until now, studies have only captured their short-term response. But <a href="https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.13576">our study</a> clearly demonstrates that this mechanism exists in Scots pine, and can help forests recover a lot of the woody biomass that drought robs them of. </p>
<p>Now we need to see how common this phenomenon is in other species and in other parts of the world. Even if this habit is widespread, the ability of trees to compensate for growth lost during a drought will depend on the climate remaining good for growth long after the drought has ended. Similarly, more frequent and severe droughts could quickly overwhelm any benefits. </p>
<p>Nevertheless, our results do suggest that we may be underestimating how resilient some forests are and overestimating how much future droughts will cost trees. This could have implications for climate change, so scientists will now need to find out more about this mechanism to strengthen their models. And since we found that trees respond differently to drought, having a variety of sizes and growth rates could leave forests in better stead to respond to the challenges the future will bring.</p><img src="https://counter.theconversation.com/content/153115/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tom Ovenden receives funding from Forest Research, The Scottish Forestry Trust and Stirling University. He is affiliated with The Institute of Chartered Foresters and on the Committee for the British Ecological Society's Forest Ecology Group. </span></em></p><p class="fine-print"><em><span>Alistair Jump receives funding from Natural Environment Research Council, The Royal Society, and Scottish Forestry Trust. He is affiliated with TreeLink Stirling and The Centre for Ecological Research and Forestry Applications (Catalonia). </span></em></p>
Trees may be more resilient to future dry spells than scientists first thought.
Tom Ovenden, PhD Candidate - Forest Ecology, Resilience and Biodiversity Conservation, University of Stirling
Alistair Jump, Professor of Plant Ecology, University of Stirling
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/62805
2016-07-21T19:08:32Z
2016-07-21T19:08:32Z
More CO2 won’t help northern forests or stave off climate change
<figure><img src="https://images.theconversation.com/files/131295/original/image-20160720-31151-1i7wxwl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Higher carbon dioxide levels will not result in faster-growing forests -- just the opposite in many places, study finds. </span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/rosskevin756/15372316869/in/album-72157649311044212/">rosskevin756/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/">CC BY-NC-SA</a></span></figcaption></figure><p>We’ve heard the predictions of how greenhouse gas emissions will drive changes in the temperatures and precipitation people experience. But how these changes affect the world’s forests has broad implications for the future as well. </p>
<p>Could warmer winters, and thus longer growing seasons, cause trees to grow faster? If so, perhaps faster tree growth could slow the pace of climate change, since trees suck carbon out of the air as they grow. </p>
<p>Or perhaps hotter summers will mean more drought-like conditions, thereby hampering trees’ ability to grow and thus cause deterioration of our woodlands.</p>
<p>In a <a href="http://doi.wiley.com/10.1111/ele.12650">recent paper</a>, my colleagues and I set out to make a map of how climate change might influence tree growth across the entire continent of North America. To do this, we dug into historical records of tree growth over the period 1900-1950 collected by many dedicated field ecologists over the decades and deposited in the <a href="http://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets/tree-ring">International Tree Ring Data Bank</a>.</p>
<p>What we found was that the daily life of trees across much of North America will become more challenging, despite the potential benefit that rising carbon dioxide concentrations may have for trees. This is contrary to some scientists’ hopes that climate change will strongly benefit northern latitude forests.</p>
<h2>How trees respond to climate</h2>
<p>The first hurdle in predicting future tree growth is to understand how trees in different ecosystems respond to climate fluctuations. </p>
<p>You might guess that in cold northern forests, a little heat might help trees grow, whereas more heat in the desert Southwest is likely the last thing trees there want. This observation motivated previous scientists to formulate a “boreal greening” <a href="https://geog.umd.edu/sites/geog.umd.edu/files/pubs/Alcaraz%20GCB%202009.pdf">hypothesis</a> – that global warming will cause northern boreal forests to grow faster and help mitigate climate change.</p>
<p>We used the historic tree ring data to map the relationship between regional climate and tree growth. Matching each growth ring to the weather patterns in the corresponding year, we can get a sense for how trees respond to climate fluctuations. For instance, we saw that above-average June temperatures caused faster tree growth in places with climates similar to Fairbanks, Alaska, but slower growth in Phoenix-like climates.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/131298/original/image-20160720-31114-1ph4ig1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/131298/original/image-20160720-31114-1ph4ig1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/131298/original/image-20160720-31114-1ph4ig1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=362&fit=crop&dpr=1 600w, https://images.theconversation.com/files/131298/original/image-20160720-31114-1ph4ig1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=362&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/131298/original/image-20160720-31114-1ph4ig1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=362&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/131298/original/image-20160720-31114-1ph4ig1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=455&fit=crop&dpr=1 754w, https://images.theconversation.com/files/131298/original/image-20160720-31114-1ph4ig1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=455&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/131298/original/image-20160720-31114-1ph4ig1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=455&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Boreal forests such as this one in Alaska are projected to enter into a different climatic zone from rising temperatures due to climate change and fare worse in the future.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/akgypsy37/23143600801/in/photolist-op6SLq-eSpdT6-oDyNjY-op72aP-6hXk6m-4QDJxN-op7eMS-764Vb-764Us-764SS-764TK-9WcE6B-AaMaon-Grk79F-GiaQW9-rUu8zc-xRHsNd-GawN66-Jnjy94-fwow7y-op72GF-oFAxUi-abiNea-oFo4ty-4XLcZR-fwoFuq-GiNCt-hwKJK-GEuKty-hxaZf-618NxS-2R2p3C-Bg81d2-fjt1oo-mMiWD-eEARX-hwKCe-hwJLB-dtTqyA-hwJNQ-rGdapV-4F8rZR-AGXFzG-GiaQC3-G2du4A-oAnLJu-z8vQUK-HbEUbQ-xLxYFm-Hpd2yr">akgypsy37/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>As the climate changes, we might expect the response of trees to change as well. For example, in Fairbanks, our models actually predict that, in the future, above-average June temperatures will be bad for tree growth there, which is opposite of the historic relationship. Why? Fairbanks warms up so much that it shifts to a new climatic zone in which additional warming is now a detriment. Other researchers have actually started to see such a <a href="http://www.sciencedirect.com/science/article/pii/S0378112715002212">shift occur on the ground</a> in Alaska. </p>
<p>Once we characterize how trees respond to changes in climate across the continent, we can use the forecasts from the U.N.’s <a href="http://www.ipcc.ch/">Intergovernmental Panel on Climate Change</a> (IPCC) to predict the corresponding change in tree growth across the continent. For each pixel on our map of North America, we projected how forests will change based on both sets of information – the growth-climate relationship we established through the tree ring analysis and the projected changes in climate in the continent.</p>
<h2>Carbon fertilization</h2>
<p>There is one more wrinkle to this puzzle that we examined. The changing climate is driven largely by a buildup of additional carbon dioxide, and plants use carbon dioxide to photosynthesize. Just as we breathe in oxygen to live, plants breathe in carbon dioxide to live. Thus, increased amounts of carbon dioxide might directly speed up tree growth. This is known as “<a href="https://www.sciencedaily.com/releases/2016/04/160426162610.htm">carbon fertilization</a>” because it’s like we are adding fertilizer to the plants through the air to help them grow.</p>
<p>Scientists are <a href="http://www.skepticalscience.com/carbon-fertilization-effect.html">deeply divided</a> about whether carbon fertilization of this type will actually cause increases in growth, and if so, how much. In our paper, we did not attempt to settle this debate. Instead, we just included multiple different possibilities for the strength of carbon fertilization.</p>
<p>To simulate carbon fertilization, we used a neat little trick suggested by Professor <a href="http://science.sciencemag.org/content/278/5342/1411">Graham Farquhar</a> of Australian National University. The trick relies on the fact that as plants breathe in carbon dioxide, water escapes. Think of the pores on leaves as little mouths that open and close to breathe. The more plants need to open their mouths to breathe, the more water escapes. So plants try to keep their mouths as tightly closed as they can. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/131296/original/image-20160720-31134-oxz13r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/131296/original/image-20160720-31134-oxz13r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/131296/original/image-20160720-31134-oxz13r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=301&fit=crop&dpr=1 600w, https://images.theconversation.com/files/131296/original/image-20160720-31134-oxz13r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=301&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/131296/original/image-20160720-31134-oxz13r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=301&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/131296/original/image-20160720-31134-oxz13r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=378&fit=crop&dpr=1 754w, https://images.theconversation.com/files/131296/original/image-20160720-31134-oxz13r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=378&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/131296/original/image-20160720-31134-oxz13r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=378&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Rising CO2 levels were projected to slow growth of trees in the southwestern U.S., along the Rockies, and through interior Canada and Alaska.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/michaelpwilson/15335204150/in/photolist-pn7VVy-dkKiiM-nEgEWk-9fSDNx-88RmKg-91yFTv-ro5upK-n3AntX-oVoDaE-ehZrvY-pzLFk3-njfFkt-oEUiiC-haF2Mk-haAs7x-6bH5XR-ny2bUL-oSVGte-pbEiaW-fu6VVj-77ujW-diPkPo-49CrBT-cvJhH5-faxHJ2-dPndvM-nVMLK8-oVXvrJ-5GL3Ka-fu6VVQ-dRJamH-5456z8-5GL3Bt-5455Lv-8LqXE-oVsujx-nN3Lhi-ngMziH-bC1b7e-5eQRy4-awL15v-ny2mG1-H1hxpA-9hHo9B-8s4Zkx-5GQm4N-nEhHiF-gSYrXA-gSYnTH-ARz6rS">michaelpwilson</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<p>If the concentrations of carbon dioxide floating around in the air are very high, plants need open their mouths only a little bit for a small gulp of air without losing much water. Thus, as we fertilize the plants with carbon in the air, this directly increases the amount of water the plants are able to retain – with more CO2, the leaves’ pores will absorb the gas more efficiently and in the process lose less water. </p>
<p>Instead of trying to simulate more free carbon floating around in the air, we can just pretend that the plants receive more rainwater. The ultimate effect on growth should be essentially the same, because carbon uptake and water retention are directly linked. </p>
<p>In deserts where water is at a premium and plants are highly motivated to keep their mouths shut, a little carbon fertilization (or a little extra rain) should go a long way toward helping plants grow. By contrast, in rainforests where plants can keep their mouths wide open with little cost, carbon fertilization (or extra rain) might not do much to help the plants.</p>
<p>In our study, we simulated carbon fertilization by simply adding more future precipitation into our models. To satisfy those scientists who strongly believe that carbon fertilization will pan out, in some simulations we added extra water in proportion to the amount of extra carbon that is projected to be released into the atmosphere. To satisfy the nay-saying scientists who don’t believe the carbon fertilization effect will pan out, we also ran simulations without any increased water. And we ran simulations at all levels in between.</p>
<h2>Our models’ predictions</h2>
<p>At the end of the day, our maps of how tree growth might respond to climate change are alarming. </p>
<p>Across much of the west and central parts of the continent, we see massive decreases in tree growth rates, with trees growing up to 75 percent slower by the second half of this century. However, in some areas near the continent’s coasts, such as the Pacific Northwest, western Canada and the southeastern United States, we saw some local increases in tree growth rates.</p>
<p>On average, without the carbon fertilization effect, our models project growth rates across the continent to fall by almost 20 percent under the worst-case climate change scenario put forth by the IPCC (this scenario has 6 degrees Celsius of warming forecast across the continent).</p>
<p>We found that it would take a very large carbon fertilization effect (unrealistically large, according to the opinion of several of our study’s co-authors) to offset this slowdown. And across much of the continent, our models projected slower growth rates no matter how large the carbon fertilization effect.</p>
<p>Also, we did not see a large increase in cold northern forest growth rates in our simulations. So, on average, we saw no “boreal greening.” If anything, we saw a slowdown of these forests. This is largely driven by the shift in how trees respond to climates in places like Fairbanks.</p>
<h2>What it means</h2>
<p>The implication of our analysis is that forests do not seem poised to save us from climate change.</p>
<p>Our models suggest that most of our forests will be growing more slowly in the future. This will, of course, have direct impacts on all the ways we and other species rely on trees. But it will also feed back into climate change itself. As global warming causes trees to absorb less carbon, there will be more carbon left in the air to cause faster warming, thus creating an accelerating cycle.</p>
<p>Furthermore, many sustained years of bad growth in trees will likely deplete the resources they need to survive, making them susceptible to severe droughts or insect outbreaks. This may mean that what we project as slower growth may translate into widespread tree death. In other words, the forest picture may be even gloomier than our models suggest.</p>
<p>In our models, we don’t take into account the way forests are changing due to changes in logging practices or forest management. In many areas, forests are regrowing faster simply because we stopped logging them recently. Such factors should be thought of as another layer to add on top of our projections.</p>
<p>This study, like any of its kind, is really our best guess at approximating the future. I think of such forecasts not as hard-and-fast predictions of what will happen, but as reasonable possibilities. There are so many unknowns involved, including the fact that future climates will likely be quite different from any we have seen in the past.</p>
<p>And of course the biggest unknown is how much willpower our human community will bring to the cause of clamping down on greenhouse gas emissions.</p>
<p><em>This article was updated on July 27, 2016 to say higher levels of carbon dioxide in air result in an increase, rather than decrease, in the amount of water plants can retain.</em></p><img src="https://counter.theconversation.com/content/62805/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Noah Charney has received funding from the National Science Foundation, the Massachusetts Natural Heritage and Endangered Species Program, the Switzer Foundation, and the University of Arizona, Tucson.. He is affiliated with Radnor To River. </span></em></p>
Study using tree ring data and climate projections shows that buildup of CO2 will not benefit most northern forests and that growth rates will actually fall.
Noah Charney, Postdoctoral Research Associate of Ecology and Evolutionary Biology, University of Arizona
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/57098
2016-04-06T09:46:13Z
2016-04-06T09:46:13Z
Do prehistoric Pueblo populist revolutions presage American politics today?
<p><em>Inequality. Economic recession. Wage stagnation.</em> These are the buzzwords of the <a href="http://www.nytimes.com/2016/03/27/magazine/how-can-donald-trump-and-bernie-sanders-both-be-populist.html">populist uprisings</a> on both the left and the right during this 2016 election season. Although they’re running strikingly different campaigns, Bernie Sanders and Donald Trump are both capitalizing on anger with the so-called “elites” in the United States – and <a href="http://www.nytimes.com/2016/03/13/opinion/sunday/the-bernie-sanders-revolution.html?_r=0">promising revolutions</a> should they get elected (and predicting <a href="http://www.nytimes.com/politics/first-draft/2016/03/16/donald-trump-warns-of-riots-if-party-blocks-him-at-convention/">riots should Trump be denied</a>). </p>
<p>Obviously this form of populist uprising isn’t new, but could it be something common to all societies at one point or another, or even predictable?</p>
<p>Recently, we’ve been looking at revolutions in the prehistory of the southwestern U.S. – in particular, the history of the Pueblo people. In our <a href="http://doi.org/10.1126/sciadv.1501532">recently published <em>Science Advances</em> paper</a>, along with our colleagues <a href="http://www.public.asu.edu/%7Ekintigh/">Keith Kintigh</a> and <a href="http://cybergis.illinois.edu/about/leadership">Jonathan Rush</a>, we identified four points in Pueblo history between A.D. 500 and 1400 when such revolutions likely occurred.</p>
<p>What we discovered is that in each case, the revolution was instigated by drought that severely affected the primary subsistence base: maize agriculture. Although this may seem remote from today, the idea of an economy collapsing and leading to social upheaval shouldn’t be too hard to imagine. And actually, as of 2013, maize is the world’s <a href="http://faostat3.fao.org/browse/rankings/commodities_by_regions/E">most produced grain by weight</a>. All societies are at risk when their subsistence bases hit very tough times.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/117383/original/image-20160404-27112-1jm1a61.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/117383/original/image-20160404-27112-1jm1a61.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/117383/original/image-20160404-27112-1jm1a61.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=427&fit=crop&dpr=1 600w, https://images.theconversation.com/files/117383/original/image-20160404-27112-1jm1a61.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=427&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/117383/original/image-20160404-27112-1jm1a61.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=427&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/117383/original/image-20160404-27112-1jm1a61.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=537&fit=crop&dpr=1 754w, https://images.theconversation.com/files/117383/original/image-20160404-27112-1jm1a61.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=537&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/117383/original/image-20160404-27112-1jm1a61.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=537&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 tree’s rings give information about when it lived and the climate it encountered.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/robin1966/15031766608">PROMicolo J</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Turning to the trees</h2>
<p>To untangle the relationship between climate and social revolution, we analyzed two large databases built from information about trees.</p>
<p>Most trees add a “ring” of growth for every year they’re alive, and the widths of these rings are related to the changing climate a tree experienced over its lifespan. Tree-ring scientists – dendrochronologists – can compare a sample with a regional “<a href="http://www.ltrr.arizona.edu/skeletonplot/masterchronology.htm">master chronology</a>” and establish the year each tree died (with varying levels of precision).</p>
<p>Started by fellow archaeologist Mike Berry in the 1970s, the first data set we used contains nearly <a href="http://core.tdar.org/dataset/399314/compiled-tree-ring-dates-from-the-southwestern-united-states-restricted">30,000 tree-ring dates</a> collected from archaeological sites throughout the American Southwest. These are samples (roof beams, posts and so on) from structures built by Pueblo people. So if we have a cluster of dates at A.D. 800 from timbers from a pithouse, we can be quite sure building was going on at that location in that very year.</p>
<p>The second tree-based data set we used collects ring-width chronologies from many trees in the same locale. Because trees put on growth rings of varying thickness in response to particular climate conditions, and these can be dated, we can piece together sequences from many trees in a local area to recreate its annual climate over centuries.</p>
<p>For example, pinyon and juniper trees from low elevations in the Southwest may be sensitive to annual variation in rain, while spruce and pines from higher elevations may be more sensitive to temperature variability. In an especially dry year a valley juniper might barely grow; in a wetter year it will put on a wide ring. Depending on the species and elevation of the tree, the width of its rings tells us about the rain or temperature in specific years.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/117377/original/image-20160404-27125-1pteq0e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/117377/original/image-20160404-27125-1pteq0e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/117377/original/image-20160404-27125-1pteq0e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=514&fit=crop&dpr=1 600w, https://images.theconversation.com/files/117377/original/image-20160404-27125-1pteq0e.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=514&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/117377/original/image-20160404-27125-1pteq0e.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=514&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/117377/original/image-20160404-27125-1pteq0e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=646&fit=crop&dpr=1 754w, https://images.theconversation.com/files/117377/original/image-20160404-27125-1pteq0e.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=646&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/117377/original/image-20160404-27125-1pteq0e.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=646&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Balcony House in Mesa Verde National Park in southwest Colorado. Timbers incorporated into the structure are visible.</span>
<span class="attribution"><span class="source">Tim Kohler</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Trees as thermometers and rain gauges</h2>
<p>For this study, we drew on <a href="http://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets/tree-ring">532 of these ring-width chronologies</a> from across the western U.S. We wanted to use these ring-width chronologies to predict – or <em>retrodict</em>, since we are predicting into the past – temperature and precipitation at each of over 2 million spots across the landscape, for A.D. 500–1400.</p>
<p>For each location, we know the spatial and temporal climate patterns from the 20th century based on weather station measurements that were then <a href="http://www.prism.oregonstate.edu">estimated at other points on the landscape</a>. From that, we <a href="http://doi.org/10.1038/ncomms6618">borrow a sophisticated algorithm</a> <a href="http://doi.org/10.2202/1544-6115.1730">from quantitative genomics</a> to mine the ring-width data and reconstruct prehistoric conditions for each spot.</p>
<p>Combining these annual precipitation and temperature reconstructions for each location, we wound up with a map for each year showing where the maize the Pueblo people depended on for survival could be grown without irrigation.</p>
<p>We then compared these two tree-ring-based data sets – construction dates and climate. It’s no surprise that we find that people tended to live <em>where</em> they could farm, and that they built more <em>when</em> they could farm. People were building – and societies were flourishing – where and when the climate allowed them to make a good living growing maize.</p>
<h2>Alternating between exploration and exploitation</h2>
<p>What’s more important is what we don’t see in the data.</p>
<p>In 1982, <a href="https://books.google.com/books?id=kpR1AAAAMAAJ&q=Time,+Space,+and+Transition+in+Anasazi+Prehistory&dq=Time,+Space,+and+Transition+in+Anasazi+Prehistory&hl=en&sa=X&ved=0ahUKEwjEhZuD6PXLAhUDxGMKHbUYBDwQ6AEIHDAA">Mike Berry noted</a> a pronounced pattern. Based on the number of dated timbers through time, there seemed to be a peak in building every 200 years or so. He suggested that the pattern of highs and lows over the centuries represented long periods of relatively good farming conditions during the peaks, and relatively poor growing conditions for maize during the intervening troughs.</p>
<p>But that’s not what we find in our climate reconstruction, which represents the first attempt to figure out growing conditions for maize throughout the Southwest for the Common Era. Instead, we saw that while the peaks did <em>end</em> during somewhat unusual droughts, in general, the troughs weren’t worse times for agriculture than the peaks.</p>
<p>So what explains the pattern of peaks and valleys in the number of dated timbers through time?</p>
<p>We argue that Pueblo societies went through alternating phases of <em>exploration</em> and <em>exploitation</em> of niches that are simultaneously ecological, cultural and organizational. During exploration phases – the valleys of the date distribution – people experimented with farming in new areas of the Southwest, trying out new architectural and ceramic forms, social organizations and ritual practices. When droughts occurred during these periods, they don’t seem to have caused major social upheavals.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/117380/original/image-20160404-18157-uavatz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/117380/original/image-20160404-18157-uavatz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/117380/original/image-20160404-18157-uavatz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=496&fit=crop&dpr=1 600w, https://images.theconversation.com/files/117380/original/image-20160404-18157-uavatz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=496&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/117380/original/image-20160404-18157-uavatz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=496&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/117380/original/image-20160404-18157-uavatz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=623&fit=crop&dpr=1 754w, https://images.theconversation.com/files/117380/original/image-20160404-18157-uavatz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=623&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/117380/original/image-20160404-18157-uavatz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=623&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 extensive settlements at Chaco Canyon in New Mexico exemplify a hierarchical social system that eventually proved unsustainable.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/snowpeak/15588023925">John Fowler</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Eventually people converged on combinations of successful places for growing, along with behaviors (ceremonial, social and political) that allowed their societies to take off. During these exploitation phases – the peaks in building activity – conformity became the norm. More people began living in villages and clusters of these successful villages grew in size. </p>
<p>In the process, though, they became inflexible and eventually unable to cope with climate shifts. In some periods (especially during the Chaco era, by around A.D. 1100) inequality rose, as measured by variability in household size and access to prestige items such as ceramics from far-off places, or even high-quality meat like deer. </p>
<p>And at the end of each peak, these societies underwent revolutionary change, spurred by drought. Villages were abandoned – sometimes violently – and people again began exploring new ways of living. During the first building peak, people stored their maize in underground cists outside their homes; after the revolution they switched to storing corn in aboveground rooms connected with other living spaces. We think attitudes towards food sharing became more restrictive after the first revolution. At the end of the third peak in the mid-1100s, people stopped constructing the large “Great Houses,” an <a href="http://dx.doi.org/10.1126/science.345.6200.991">apparent connection to Mesoamerican culture</a> that had come to dominate regional architecture. And in the most dramatic revolution on record, people at the end of the 1200s completely left the northern Southwest and moved to points south.</p>
<h2>Ingredients for revolutionary change</h2>
<p>While social consensus – agreement on how to live together and be part of a community – might take decades to develop, it can disintegrate in a surprisingly short amount of time. We argue this cycle of slow development of social consensus and rapid breakdown happened at least four times in the Pueblo past between A.D. 500 and 1400.</p>
<p>Inequality can have a pernicious effect on the stability of societies, making them less resilient in the face of environmental or economic challenges. In the Pueblo Southwest, these two were one and the same: the “market” was agricultural production, and “market forces” – which leaders were charged with controlling – were climate.</p>
<p>When ceremonies weren’t working to bring the rains anymore, political and spiritual leaders lost their legitimacy. In at least the last two peaks, inequality in responsibilities and access to resources rose to such a degree that, when things did turn bad, those at the bottom rejected prior ways in what could be considered a “populist” uprising. These were <a href="http://dx.doi.org/10.7183/0002-7316.79.3.444">often violent</a> – the inhabitants of <a href="http://www.jstor.org/stable/1593823">entire villages were massacred</a>, and there is evidence for <a href="http://www.amazon.com/Man-Corn-Cannibalism-Prehistoric-Southwest/dp/0874809681">ritually linked cannibalism</a>. In other research, we’ve found that at times there was also <a href="http://dx.doi.org/10.7183/0002-7316.81.1.74">substantial environmental inequality</a> in these societies. Some people lived in places where maize agriculture was very consistent, and others lived in much more marginal areas.</p>
<h2>Is our society gearing up for revolution now?</h2>
<p>Both of these conclusions have echoes today.</p>
<p>We are living in a period of dramatic economic inequality, aggravated by the recession eight years ago. Officially we <a href="http://economix.blogs.nytimes.com/2010/09/20/the-recession-has-officially-ended/">“recovered” from the Great Recession in 2010</a>, but large numbers of Americans <a href="http://www.nytimes.com/2016/02/25/business/economy/poorest-areas-have-missed-out-on-boons-of-recovery-study-finds.html">haven’t felt that recovery</a> – many are even asking <a href="https://theconversation.com/is-the-american-dream-dead-57095">whether the American dream is dead</a>.</p>
<p>The result is populist unrest – many voters have effectively cast a vote of no confidence in our current leadership and governmental systems. We may now be experiencing what Pueblo society experienced many times, many years ago. Inequality made Pueblo society less resilient in the face of challenges – it remains to be seen how these populist sentiments play out this time around.</p><img src="https://counter.theconversation.com/content/57098/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kyle Bocinsky has received funding from the National Science Foundation. He is a member of the AAAS and Society for American Archaeology. He is registered in the Democratic Party in Colorado. </span></em></p><p class="fine-print"><em><span>Tim Kohler receives funding from the US National Science Foundation. He is a member of the AAAS, the American Anthropological Association, the Society for American Archaeology and the Register of Professional Archaeologists, which lobby for scientific, anthropological, and archaeological research or for maintaining professional standards in the discipline. </span></em></p>
Multiple times over the centuries, climate issues caused Pueblo farming to collapse, taking the establishment down with it. New research suggests there are parallels with American inequalities today.
Kyle Bocinsky, Director of Sponsored Projects at Crow Canyon Archaeological Center and Adjunct Professor of Anthropology, Washington State University
Tim Kohler, Regents Professor of Archaeology and Evolutionary Anthropology, Washington State University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/55730
2016-03-09T11:13:40Z
2016-03-09T11:13:40Z
Shipwreck records and tree rings unveil Caribbean hurricane history – and clues to the future
<figure><img src="https://images.theconversation.com/files/114147/original/image-20160307-31289-d5bb6r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Researchers compared the shipwreck history to tree ring data from slash pines to piece together the hurricane history over past centuries.</span> <span class="attribution"><span class="source">Grant Harley</span>, <span class="license">Author provided</span></span></figcaption></figure><p>The U.S. is currently in a decade-long hurricane “drought”: no major hurricanes of category 3 or higher have made landfall in the U.S. since Hurricane Wilma hit Florida in 2005. With <a href="http://journals.ametsoc.org/doi/abs/10.1175/BAMS-85-5-697">damage costs for U.S. hurricanes</a> from 1970 to 2002 estimated at US$57 billion (in 2015 dollars), this landfall drought is fortuitous for coastal communities and insurance companies alike. </p>
<p>The 10-year hurricane pause is the <a href="http://onlinelibrary.wiley.com/doi/10.1002/2015GL063652/epdf">longest since 1851</a>, the period for which reliable hurricane records are available through the National Hurricane Center’s Hurricane Database <a href="http://www.aoml.noaa.gov/hrd/hurdat/Data_Storm.html">(HURDAT)</a>. The current lull emphasizes how fickle hurricane systems can be, which complicates accurate hurricane predictions under human-caused climate change. </p>
<p><em>Valerie Trouet talked about this research on the <a href="http://academicminute.org/">Academic Minute</a></em>.</p>
<p><audio preload="metadata" controls="controls" data-duration="150" data-image="" data-title="Shipwrecks, Tree Rings and Hurricanes" data-size="2400783" data-source="The Academic Minute for July 21, 2016" data-source-url="http://www.theacademicminute.org" data-license="" data-license-url="">
<source src="https://cdn.theconversation.com/audio/452/07-21-16-arizona-shipwrecks-tree-rings-and-hurricanes.mp3" type="audio/mpeg">
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<div class="audio-player-caption">
Shipwrecks, Tree Rings and Hurricanes.
<span class="attribution"><a class="source" rel="nofollow" href="http://www.theacademicminute.org">The Academic Minute for July 21, 2016</a><span class="download"><span>2.29 MB</span> <a target="_blank" href="https://cdn.theconversation.com/audio/452/07-21-16-arizona-shipwrecks-tree-rings-and-hurricanes.mp3">(download)</a></span></span>
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<p>Is there a way to use the record of past hurricanes to better predict the future?</p>
<p>Climate models for the future <a href="http://www.nature.com/ngeo/journal/v3/n3/abs/ngeo779.html">generally agree</a> that globally, tropical cyclones will become more intense but less frequent in the 21st century. Projections on the regional level, however, vary greatly and have large uncertainties. </p>
<p>For the North Atlantic Basin, the <a href="http://www.nature.com/nclimate/journal/v2/n8/abs/nclimate1530.html">main uncertainties</a> are due to the chaotic nature of the climate system and to our limited understanding of how tropical cyclones – known as hurricanes in the U.S. – respond to changes in how much energy is in the atmosphere. Such changes in the so-called energy budget – the amount of energy that enters the atmosphere and how much is lost to space – can be caused by greenhouse gas emissions, but also by changes in the strength of the sun’s radiation.</p>
<p>In a recently published <a href="http://www.pnas.org/content/early/2016/03/02/1519566113">study</a>, we combined documentary shipwreck data and tree-ring data to extend the tropical cyclone record for the Caribbean back over the last 500 years. These two new proxies and the extended Caribbean tropical cyclone record allow us to look at past big swings in the atmosphere’s energy budget and how tropical cyclones have responded to them. </p>
<p>By providing insight into hurricane dynamics of the past, we aim to give climate scientists better tools for predicting how climate change will affect hurricane intensity and frequency with more regional accuracy.</p>
<h2>Poring over shipwreck records and tree rings</h2>
<p>All three of us are tree-ring scientists. We use the rings in trees and wood to study the climate, the forests and human societies of the past. All three of us remember exactly the moment and the place when the idea for this research project emerged: it was on the last night of the second <a href="https://ameridendro.ltrr.arizona.edu/conferenceDisplay.py?confId=0">Ameridendro</a> conference, held in Tucson, Arizona, in May 2013, and we were sharing conversation and a beer on the patio of <a href="http://hotelcongress.com/">Hotel Congress</a>. </p>
<p>Grant mentioned a set of tree-ring samples that he collected from approximately 250-year old slash pine trees on the Florida Keys.</p>
<p>When hurricanes pass near to the Keys, strong winds and storm surges cause these pines to grow less and to form narrow growth rings. The pine trees are intolerant of salt (storm surge), and strong winds cause branches and needles to break off trees, which results in narrow growth rings for a few years after a storm.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/114150/original/image-20160307-31272-5kt8ol.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/114150/original/image-20160307-31272-5kt8ol.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=209&fit=crop&dpr=1 600w, https://images.theconversation.com/files/114150/original/image-20160307-31272-5kt8ol.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=209&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/114150/original/image-20160307-31272-5kt8ol.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=209&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/114150/original/image-20160307-31272-5kt8ol.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=262&fit=crop&dpr=1 754w, https://images.theconversation.com/files/114150/original/image-20160307-31272-5kt8ol.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=262&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/114150/original/image-20160307-31272-5kt8ol.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=262&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">South Florida slash pine section from Big Pine Key, Florida Keys, U.S. Growth of the tree is from right to left. ‘Bands’ of narrow growth years can be detected throughout the section and represent the growth suppression caused by hurricane disturbance (strong winds, storm surge).</span>
<span class="attribution"><span class="source">Grant Harley</span></span>
</figcaption>
</figure>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/114162/original/image-20160307-31289-7j63d3.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/114162/original/image-20160307-31289-7j63d3.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=902&fit=crop&dpr=1 600w, https://images.theconversation.com/files/114162/original/image-20160307-31289-7j63d3.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=902&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/114162/original/image-20160307-31289-7j63d3.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=902&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/114162/original/image-20160307-31289-7j63d3.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1134&fit=crop&dpr=1 754w, https://images.theconversation.com/files/114162/original/image-20160307-31289-7j63d3.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1134&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/114162/original/image-20160307-31289-7j63d3.jpeg?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">
<figcaption>
<span class="caption">Old, remnant stumps and logs from South Florida slash pines were cut with chainsaws to extend the hurricane suppression chronology back to the early 1700s, Big Pine Key, Florida Keys, U.S.</span>
<span class="attribution"><span class="source">Grant Harley, John Sakulich</span></span>
</figcaption>
</figure>
<p>Marta told us stories of the <a href="http://onlinelibrary.wiley.com/doi/10.1111/j.1095-9270.2012.00361.x/abstract?userIsAuthenticated=false&deniedAccessCustomisedMessage=">dendro-archeological work </a> she’s done on shipwrecks: by tree-ring dating the wood recovered from shipwrecks, information can be inferred about the provenance of the wood, timber procurement for shipbuilding, past forest management practices, and woodworking techniques. She also mentioned a shipwreck database that contained comprehensive information about the when, where and why of past shipwrecks.</p>
<p>All three of us remember the exact moment, because our minds collided and an exhilarating idea arose: could we combine the shipwreck record with the Florida Keys tree-ring record to recreate past tropical cyclone activity?</p>
<p>The first thing we did was to filter the shipwreck database, narrowing in on the Spaniards since they were the first to make the transatlantic voyage in 1492 and thus provided the longest available shipwreck record. We extracted data on Spanish ships that wrecked in the Caribbean during the hurricane season from July through November. We recorded only shipwrecks that were documented to be caused by storms or by unknown causes, not by wars, pirates, fires or poor navigation. </p>
<p>This resulted in a 330-year (from 1495 to 1825) record that showed how many of the 657 Spanish shipwrecks occurred each year. When we compared this shipwreck record to the Florida Keys tree-ring record, we found an astonishing result that allowed us to reconstruct the cyclone record of past centuries.</p>
<p>From Grant’s work, we knew that the Florida Keys trees responded to tropical cyclones since 1851 with bands of narrow rings. </p>
<p>When comparing the two sets of data – the tree-ring and shipwreck records – we discovered that the trees had these narrow rings during years when many ships wrecked in the Caribbean. We thus discovered a way to reconstruct Caribbean tropical cyclones back to 1495.</p>
<h2>A 60-year lull in Caribbean tropical cyclones</h2>
<p>Our next discovery jumped out at us when we plotted the combined data over time of the shipwreck and the tree-ring records. We noticed a very distinct low in the number of shipwrecks from 1645 to 1715, a period known to paleoclimatologists as the <a href="http://solarscience.msfc.nasa.gov/SunspotCycle.shtml">Maunder Minimum</a>. </p>
<p>Sunspot activity – and thus solar irradiance, or the amount of solar energy that reached the Earth – during the Maunder Minimum was at its lowest over documented history, resulting in a period of cool temperatures in the Northern Hemisphere. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/114148/original/image-20160307-31272-7akct4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/114148/original/image-20160307-31272-7akct4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=373&fit=crop&dpr=1 600w, https://images.theconversation.com/files/114148/original/image-20160307-31272-7akct4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=373&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/114148/original/image-20160307-31272-7akct4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=373&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/114148/original/image-20160307-31272-7akct4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=469&fit=crop&dpr=1 754w, https://images.theconversation.com/files/114148/original/image-20160307-31272-7akct4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=469&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/114148/original/image-20160307-31272-7akct4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=469&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Reduced Caribbean tropical cyclone activity during the Maunder Minimum. a) Decadal-scale shipwreck and instrumental (HURDAT) time series sums and tree-growth suppression averages; b) number of years per decade with more than 1 or 4 shipwrecks; c) spectral solar irradiance (Lean, 2000).</span>
</figcaption>
</figure>
<p>By using shipwreck rates as a proxy for past tropical cyclone activity – something that has not been done before – we found that the cool temperatures of the Maunder Minimum coincided with a tropical cyclone drought. But why?</p>
<p>The mechanism that explains the link between the lull in shipwrecks, tropical cyclones and sunspots during the Maunder Minimum involves sea surface temperatures. With cool global temperatures come cool sea surface temperatures. <a href="http://www.nature.com/nature/journal/v436/n7051/abs/nature03906.html">Sea surface temperatures</a> are an important driver of tropical cyclones. We know from 20th-century data that with cooler sea surface temperatures, there are fewer tropical cyclones. In short: cooler sea surface temperatures during the Maunder Minimum (due to less solar irradiance) resulted in fewer tropical cyclones and fewer shipwrecks.</p>
<p>We also found that global climate dynamical patterns, such as the El Nino-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO), likely converged during the Maunder Minimum to enhance the link between the lows in solar irradiance and in tropical cyclones.</p>
<p><a href="http://science.sciencemag.org/content/293/5529/474.short">We know from 20th-century data</a> that these climate patterns play an important role in Caribbean tropical cyclones, with fewer cyclones occurring during positive “El Niño” phases and negative NAO phases. For instance, the continued hurricane drought in 2015 has been attributed to this year’s strong El Niño. </p>
<h2>Caribbean tropical cyclones under a warming future</h2>
<p>Our study provides information on how Caribbean tropical cyclones respond on decadal time scales to changes in the atmosphere’s energy budget. </p>
<p>When looking at model projections for the 21st century, these decadal time scales are of crucial importance. Most of the <a href="http://www.nature.com/nclimate/journal/v2/n8/abs/nclimate1530.html">uncertainty in the model projections</a> for the Caribbean Basin comes from our lack of understanding of how Caribbean hurricane activity responds to changes in the amount of energy in the atmosphere. Our historical record helps us improve that understanding and thus the model projections.</p>
<p>We know that with more densely populated coastal areas, sea level rise and stronger storm surges, 21st-century tropical cyclones will become increasingly destructive and costly. Our continued improvement of understanding how frequent and/or intense future tropical cyclones will be is of the utmost importance, especially with regard to the safety and livelihoods of coastal communities.</p><img src="https://counter.theconversation.com/content/55730/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Valerie Trouet receives funding from the National Science Foundation and the Department of Energy. </span></em></p><p class="fine-print"><em><span>Grant Harley receives funding from the Bureau of Ocean Energy Management.</span></em></p><p class="fine-print"><em><span>Marta Domínguez Delmás receives funding from the European Marie Skodolwska Curie Innovative Training Networks program in the frame of the ForSEAdiscovery project (PITN-2013-GA-607545). For this research, she also received funding from an Agnese N. Haury Visiting Fellowship of the University of Arizona.
Other affiliations include academic affiliation to the University of Huelva, department of History I (Spain)</span></em></p>
In an attempt to better understand hurricanes, researchers recreate hundreds of years of hurricane records with Spanish shipwreck logs and tree ring data.
Valerie Trouet, Associate Professor of Dendrochronology, University of Arizona
Grant Harley, Assistant Professor of Geography, The University of Southern Mississippi
Marta Domínguez Delmás, Research Fellow Dendrochronologist, Universidade de Santiago de Compostela
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/45349
2015-07-30T19:03:17Z
2015-07-30T19:03:17Z
Extreme droughts weaken trees’ ability to soak up carbon
<figure><img src="https://images.theconversation.com/files/90183/original/image-20150729-30846-rekooz.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">High and dry: a water-stressed forest in the US Southwest.</span> <span class="attribution"><span class="source">Leander Anderegg</span>, <span class="license">Author provided</span></span></figcaption></figure><p>There’s a mystery inside trees upon which the fate of coastal cities, threatened by rising sea levels from climate change, may depend. Each year, the Earth’s forests take up about <a href="http://www.sciencemag.org/content/333/6045/988.short">one-quarter</a> of the carbon dioxide emitted by humans, effectively slowing the speed and severity of global warming. They lock up most of this carbon in their stems and keep it there for centuries. This service that they provide to humanity is worth about US$1 trillion <a href="http://www.epa.gov/climatechange/EPAactivities/economics/scc.html">each year</a>. </p>
<p>But there’s no guarantee that trees around the globe will continue to do this in coming decades. Climate change is superfueling droughts, massive wildfires and insect outbreaks, all of which slow trees’ growth, kill trees and release this carbon back into the atmosphere. </p>
<p>Thus, the one-trillion-dollar question that scientists are racing to answer is – what is the fate of trees in a changing climate? </p>
<p>In a study of the impact of droughts on trees, my colleagues and I focused on one aspect of this question, specifically the impact of drought on trees. Our analysis shows that forests do not rebound from extreme drought as quickly as scientists had thought – a finding that could lead to better models for how forests absorb carbon emissions.</p>
<h2>Reading tree history</h2>
<p>Currently, we’re largely flying blind when it comes to forecasting the fate of trees. Our best predictive tools – mathematical models based on plant physiology and climate – <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00579.1">disagree wildly</a> on the future of forests. </p>
<p>Some show that trees continue to grow like crazy and take up carbon through the end of the century. In others, however, drought, temperature, and other ecosystem disturbances dampen tree growth and kill many trees, and ecosystems begin to lose carbon, rapidly accelerating the speed and severity of climate change. Worse still, these models are generally not thought to accurately simulate <a href="http://onlinelibrary.wiley.com/doi/10.1111/nph.12390/full">drought impacts on forests</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/90192/original/image-20150729-30851-i7urak.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/90192/original/image-20150729-30851-i7urak.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/90192/original/image-20150729-30851-i7urak.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=128&fit=crop&dpr=1 600w, https://images.theconversation.com/files/90192/original/image-20150729-30851-i7urak.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=128&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/90192/original/image-20150729-30851-i7urak.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=128&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/90192/original/image-20150729-30851-i7urak.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=161&fit=crop&dpr=1 754w, https://images.theconversation.com/files/90192/original/image-20150729-30851-i7urak.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=161&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/90192/original/image-20150729-30851-i7urak.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=161&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Experts are able to identify when droughts occur from tree rings, such as this cross-section of a 200-year-old tree.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/7147684@N03/5107152640/in/photolist-8MiuxS-8DVmcW-8DSdTp-8opWCH-6eukAj-2MZw5U-8ot2YN-7J44Un-4VCZwg-8sFrY5-8opTkz-8opSHD-8opQTa-8opvNF-8osCB9-8opLWP-8opsjT-8opvie-8opvyz-8opszZ-8osCQh-8osD3u-8opQeT-8ot2a7-8osXrL-8opPok-8ot1JN-8opQDF-8opPNV-8ot4Mj-8opPBe-8osWYL-8ot5A5-8opWqZ-8ot3dA-gjTPA4-8opTHv-8ot725-8opVsZ-8opUP2-8osYtS-8ot4zS-8opUsv-8osZDh-8opMw2-8ot5cw-8ot3F3-8ot3t1-8ot7WN-8osYWQ">Jason Hollinger/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>To unravel part of this mystery, we turned to tree rings. Most trees in the Northern Hemisphere grow annual rings, which are layered around the stem like pages in a book over that tree’s lifetime.</p>
<p>Scientists have known for decades that during drought, trees grow more slowly. But we wanted to know the answer to a simple question: what happens after drought? Do trees recover and continue growing, taking up that carbon from the atmosphere? Or, could damage sustained during drought make them grow more slowly afterwards, even once the drought is over? </p>
<p>Because climate change will lead to more frequent and more severe droughts in <a href="https://theconversation.com/what-historic-megadroughts-in-the-western-us-tell-us-about-our-climate-future-37615">many regions of the world</a>, the recovery of forests after drought will be crucial to determining if they keep taking up carbon dioxide and slowing global warming. </p>
<h2>Drought hangover</h2>
<p>We used an online, publicly available database of tree rings from several thousand forest sites around the globe, with most measurements coming from North America, Europe and parts of Russia. At around 1,300 forest sites that met our criteria, we searched historical climate data to find when and where the severe droughts had occurred. We then looked at surviving trees’ growth after those droughts had finished. </p>
<p>What we found was quite startling. On average, forests took two to four years to recover their previous growth rates after severe drought. This “legacy effect” of drought was most severe in dry forests like the southwestern United States and in pine species. In addition, tree species that took the most risks – that is, those that continue to drink water at a high rate during a drought – also had the largest legacy effects. </p>
<p>Without more detailed studies in individual forests, we weren’t able to pin down exactly why trees grow more slowly after drought, but we do have a few indications. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/90193/original/image-20150729-30871-oldu1z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/90193/original/image-20150729-30871-oldu1z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/90193/original/image-20150729-30871-oldu1z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/90193/original/image-20150729-30871-oldu1z.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/90193/original/image-20150729-30871-oldu1z.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/90193/original/image-20150729-30871-oldu1z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/90193/original/image-20150729-30871-oldu1z.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/90193/original/image-20150729-30871-oldu1z.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">Severe drought has contributed to the widespread bark beetle damage and tree mortality in pine forests in the western US.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/usdagov/4878318165/in/photolist-8r5E2T-bFozj2-a2evFM-6rfJs1-fCFKWC-6aQmvs-qWjMwT-dg6D3o-ryLDXa-df8sML-aopMTA-kocsTZ-aomXxF-aopL1L-aopDw7-q6MY8w-miaksR-bS8rMK-mibtdu-KGj1J-a7uyJT-oXbn5a-8Zghv9-dfF6Nm-n1pGvF-oES7D4-rT2wgV-toXVhq-cWr3kf-jXWKnT-vH3cc8-ad9cWo-4tC21V-fF1xNh-pW8cmK-3QGDmX-6gRjWS-aAgW2J-8vZXgr-6Mi72f-DueZ9-oSF2AP-hFUbnx-o4UbCx-bUphmH-bBpiTH-ah4Q92-d649ob-8jyLLP-dakM3N">US Department of Agriculture</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Most promising was the finding that tree species that take risks during drought seem to recover the slowest. Some species, like the trembling aspen tree, continue to use water and sustain photosynthesis during drought, but they risk damage to their water transport systems and even death. This strategy is probably advantageous in wet times or moderate droughts, helping them outgrow their neighbors and gain more light. But in severe droughts, this strategy can backfire. In fact, in our previous research on aspen trees, we found a lot of drought-driven damage in the wood that persisted for <a href="http://onlinelibrary.wiley.com/doi/10.1111/gcb.12100/abstract">a decade or so</a>. </p>
<p>When we looked at the current mathematical models, we found that all of these models treated drought like a light switch. When the trees were drought-stressed, they grew more slowly. But when the drought was alleviated, the models assumed the switch was flipped back on and they recovered their growth perfectly and immediately. </p>
<p>This indicates that these models are probably missing several key processes to accurately simulate drought impacts on forests. Thus, they may be overestimating how much trees can help slow climate change. </p>
<h2>Gigatons of carbon from the atmosphere</h2>
<p>How much did this effect matter? We did several simple calculations of semi-arid forests’ carbon uptake with and without these drought legacy effects. We found that over a century, semi-arid forests would store 1.6 gigatons of carbon less due to the legacy effect of drought, which is about 25% of <a href="http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html">US annual carbon emissions</a>. This is certainly a very conservative estimate of how much it matters, because it only considers one community of plants in a region, or biome. </p>
<p>Many major questions remain, however. First, do these legacy effects of drought occur in tropical forests, which are among the strongest carbon sinks of all forests? Many tropical tree species don’t make consistent annual rings, which makes tree ring analysis challenging, but some research groups have recently succeeded in measuring tree rings in <a href="http://www.nature.com/ngeo/journal/v8/n1/full/ngeo2313.html">some tropical species</a>.</p>
<p>Second, how sensitive is tree mortality to drought in different regions and ecosystems across the globe? Tree mortality has proven to be incredibly difficult to study, understand and predict, and the community is racing to solve these questions. </p>
<p>Third, how can we improve our predictive models to better simulate drought? In other research, my colleagues and I are attempting to improve these vegetation models by adding the mechanism of drought damage that wounds and kills trees with risky water-use strategies. </p>
<p>Ultimately, the sooner and more aggressively we address human-caused climate change, the lower the risks will be for forests around the globe. </p>
<p>Our research has shone a small ray of light into the secrets at the heart of trees, traveling back in time so that we can understand the future. But many more mysteries remain etched inside these majestic organisms, mysteries that we desperately need to unravel.</p><img src="https://counter.theconversation.com/content/45349/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>William R. L. Anderegg receives research funding from the National Science Foundation (NSF) and the National Oceanic and Atmospheric Administration (NOAA).</span></em></p>
Forests take longer than expected to rebound from droughts, diminishing their role as global carbon sinks.
William R.L. Anderegg, Postdoctoral Research Fellow at Princeton Environmental Institute, Princeton University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/24185
2014-03-10T21:10:42Z
2014-03-10T21:10:42Z
Wet climate helped Genghis Khan conquer Asia
<figure><img src="https://images.theconversation.com/files/43506/original/4zrnz9c8-1394474638.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Genghis Khan's army swept across Asia, fuelled by luscious green grass.</span> <span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Genghis_Khan_The_Exhibition_(5465078899).jpg">William Cho</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>After unifying the Mongols, Genghis Khan led his army from Northern Mongolia in 1211 to the Caspian Sea in 1224, forging the largest contiguous land empire in history. For centuries, <a href="http://www.atmosedu.com/Geol390/articles/CasualitiesOfCCJan2011.pdf">common wisdom</a> held that the Mongols were driven to conquest because of harsh conditions – namely, drought. Our new record, dating back to 900AD with confidence, indicates the opposite. </p>
<p><a href="http://www.pnas.org/cgi/doi/10.1073/pnas.1318677111">Published in PNAS</a>, our new tree-ring data suggests that it was consistently wet from 1211 to 1225CE, a period we are calling the Mongol Pluvial. No years during this period were below the long-term average, which is the only run of moisture conditions in our 1,100-year-long record. Independent tree-ring records of the same area also indicate that this period was warm.</p>
<h2>Grass is life</h2>
<p>On the cool semi-arid steppe of Central Asia, water is life and, in those days, water was energy. The Mongol diet is heavily based on the meat of grazing animals. Their mode of transportation was the short, but Pheidippidic, horse. So, for food and for travel, grass was life. Grass was energy. </p>
<p>An abundance of moisture would seem to provide the horsepower for the rapidly growing Mongol Empire. The Mongol soldier typically had five steeds at their disposal. With a large army, that quickly translates into huge herds and a huge need for grass.</p>
<p>The tree-ring records we collected suggest that the grasslands of central Mongolia were probably productive during this period of unprecedented moisture and mild temperatures. The tree-ring data strongly agrees with satellite estimates of grassland productivity during the modern period. Going back in time, then, the trees would suggest that during its rapid expansionm the Mongol Empire was sitting in a sea of grass – a sea of energy – a potential abundance of life.</p>
<h2>Branch biopsies</h2>
<p>As with many scientific discoveries, our initial finding that Siberian pine trees from the Khorgo lava field in Central Mongolia yielded data that matched the year that Genghis Khan was born, was initially an accidental one. We had collected samples as an aside to another project we were doing (<a href="http://www.ldeo.columbia.edu/%7Eadk/mongoliaFire/">Fire, Climate, and Forest History in Mongolia</a>). When they were finally analysed, it was with great surprise that we learnt of the correlation they showed with Genghis Khan’s empire-building. We had to go back for more samples.</p>
<p>Using tree rings from these, we reconstructed a 1,112-year water-balance record. Trees are sedentary creatures, sitting in one spot for centuries unable to move from fire, drought or other ravages of time. Thus, they have to sit there and take what is thrown at them. </p>
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<a href="https://images.theconversation.com/files/43507/original/xtm2pr89-1394475096.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/43507/original/xtm2pr89-1394475096.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/43507/original/xtm2pr89-1394475096.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=491&fit=crop&dpr=1 600w, https://images.theconversation.com/files/43507/original/xtm2pr89-1394475096.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=491&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/43507/original/xtm2pr89-1394475096.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=491&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/43507/original/xtm2pr89-1394475096.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=617&fit=crop&dpr=1 754w, https://images.theconversation.com/files/43507/original/xtm2pr89-1394475096.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=617&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/43507/original/xtm2pr89-1394475096.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=617&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Rings in the tree trunk reveal facts about the tree’s age and climate at the time.</span>
<span class="attribution"><span class="source">Neil Pederson</span></span>
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<p>For tree-ring scientists like ourselves, changes in the environment are recorded in the structure of the trees – clues about their age – and in the rings of their branches. We seek out the oldest looking trees on the landscape and take small samples, like a biopsy, that are the diameter of a chopstick. And, on dry sites like the Khorgo lava field, we took samples from dead trees to extend the living tree record further back in time by matching the unique patterns of large and small rings.</p>
<p>The rings in one sense can be considered a bar code of information. The information we glean from the bar code made of wood indicates the growth of these trees are severely limited by moisture. We test this hypothesis statistically by comparing ring width measurements and human measures of moisture availability. So, when we see 15 years of above average ring widths in the early 1200s, this indicates wet conditions.</p>
<p>That is our hypothesis, at least, and something we will test in the coming years with historical documents, environmental records from lake sediments, more tree rings, and ecological modelling experiments.</p>
<h2>Cultural transformation</h2>
<p>While this record speaks to a rapid transformation of Eurasian culture during the 13th century, it also speaks about an abrupt transformation in Mongol culture today. Towards the end of our tree-ring record we see a prolonged drought from the end of the 20th century into the beginning of the 21st century. This drought followed the wettest century of the last 11 and occurred during the warmest period of the last 1,100 years in Asia. </p>
<p>The abrupt transition in environmental conditions – one that’s <a href="http://link.springer.com/article/10.1134/S2079096112010076">seen hundreds of lakes and wetlands disappear</a> from the region’s landscape – occurred during the move from a more agriculturally based economy to a more urban one. These severe conditions, in combination with some harsh winters, killed millions of livestock and are thought to be one trigger of a mass migration of Mongols from the countryside into the capital of Ulaanbaatar.</p>
<p>Though we don’t have the data to connect this heat drought to climate change, warming temperatures have stacked the deck towards causing higher amounts of evaporation, so even if the amount of precipitation remains the same, high temperatures will generate a more intense drought. That’s what we observed in the early 21st century and based on past moisture variation in Mongolia and future predictions of warming, we would expect to see similar events in the future.</p><img src="https://counter.theconversation.com/content/24185/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors 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>
After unifying the Mongols, Genghis Khan led his army from Northern Mongolia in 1211 to the Caspian Sea in 1224, forging the largest contiguous land empire in history. For centuries, common wisdom held…
Neil Pederson, Lamont Assistant Research Professor, Columbia University
Amy Hessl, Associate Professor of Geography, West Virginia University
Licensed as Creative Commons – attribution, no derivatives.