tag:theconversation.com,2011:/ca/topics/climate-sensitivity-7265/articlesClimate sensitivity – The Conversation2023-06-20T04:38:20Ztag:theconversation.com,2011:article/2079252023-06-20T04:38:20Z2023-06-20T04:38:20ZIs climate change outpacing our ability to predict extreme heatwaves?<p>When an extreme weather event happens somewhere in the world these days, it’s common to read quotes from climate scientists explaining this is exactly the kind of event we expect to see more often as climate change progresses. Such events are often devastating, but not surprising if you’ve been paying attention to the climate projections issued by scientists for many decades now.</p>
<p>But every so often, an event is so extreme it causes scientists to question our understanding of just how fast climate change is progressing. One such event was the heatwave across the Pacific Northwest region of the United States and Canada in the northern summer of 2021, when temperatures at some locations <a href="https://www.climatehubs.usda.gov/hubs/northwest/topic/2021-northwest-heat-dome-causes-impacts-and-future-outlook">hit 49°C</a> (121°F) – hotter than the all-time record for Texas. </p>
<p>It broke heat records by such a wide margin that scientists were <a href="https://edition.cnn.com/2021/07/20/world/climate-change-extreme-weather-speed-cmd-intl/index.html">quoted</a> in the <a href="https://www.nationalobserver.com/2021/07/21/news/climate-change-scientists-heat-wave-summer">media</a> saying they hadn’t expected to see temperatures so high in the Pacific Northwest until much later this century.</p>
<p>The basic concern for these scientists was that our computer climate models are best at simulating things that span large areas and long time periods, such as the annual average global temperature (what we broadly mean when we say “the climate”). They aren’t as good at simulating smaller-scale things such as an individual storm or hot wind (that is, “the weather”).</p>
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<p>It’s not that our models can’t simulate small-scale weather – they’re basically the same models we use for weather forecasting – it’s just very computationally expensive to have them zoom in and run in “weather mode” to get a highly detailed simulation. It’s feasible for a seven-day weather forecast, but not for a century-long climate simulation. </p>
<p>Given this limitation, the scientists quoted in the media were concerned extreme weather events might be more sensitive to climate change than our models suggest.</p>
<h2>Quantity matters too</h2>
<p>While these concerns around the <em>quality</em> of our model simulations at weather-relevant scales are valid, what’s often overlooked is the <em>quantity</em> of model simulations involved. Given the natural variability in the climate system, scientists prefer not to rely on just one model simulation when making climate projections. Instead, they run a range of century-long simulations – from just a handful up to 50 or more for the most well-resourced modelling groups – and look at the range of possible outcomes.</p>
<p>For climate metrics such as the annual average global temperature, that’s enough simulations to capture the full range of possibilities. It’s a value that doesn’t vary much from year to year because it’s an average over the entire globe, so the climate change signal dominates over natural variability. To use a slightly more technical term, we say it has a high “signal-to-noise” ratio.</p>
<p>In contrast, the weather can vary greatly over relatively short time frames, and therefore has a very low climate signal-to-noise ratio. Something like the hottest day of the year at a given location is especially noisy, because small variations in the alignment of weather patterns can make all the difference between a regular hot day and a record-shattering one. </p>
<p>In this situation, many more simulations would be required to reliably estimate the upper limit on what extreme temperatures are possible.</p>
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Read more:
<a href="https://theconversation.com/weather-and-climate-are-used-interchangeably-they-shouldnt-be-110129">"Weather" and "climate" are used interchangeably. They shouldn't be</a>
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<h2>How many simulations are enough?</h2>
<p>To try and understand how many model simulations would be needed, our <a href="https://doi.org/10.1088/2752-5295/acd714">recently published research</a> used a climate model to simulate 45,000 years’ worth of daily weather at Seattle-Tacoma airport in the Pacific Northwest. </p>
<p>We then went through a process of picking out 1,000 random samples of 100 years of data from this population of 45,000 years, then 1,000 samples of 500 years, 1,000 years, 5,000 years, and so on. For each sample, we wrote down the maximum daily temperature we found (that is, the record temperature produced in each of these sample simulations). </p>
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<img alt="" src="https://images.theconversation.com/files/532493/original/file-20230618-29-iflkh5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/532493/original/file-20230618-29-iflkh5.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=483&fit=crop&dpr=1 600w, https://images.theconversation.com/files/532493/original/file-20230618-29-iflkh5.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=483&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/532493/original/file-20230618-29-iflkh5.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=483&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/532493/original/file-20230618-29-iflkh5.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=607&fit=crop&dpr=1 754w, https://images.theconversation.com/files/532493/original/file-20230618-29-iflkh5.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=607&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/532493/original/file-20230618-29-iflkh5.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=607&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">Distribution of record temperatures at Seattle Tacoma airport for 1000 repeated sub-samples of varying size.</span>
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<p>To our surprise, as the samples got bigger, the record temperatures we found showed little evidence of stabilising. They just continued to grow, indicating even samples spanning several thousand years are insufficient to capture the full range of possible extreme temperatures. </p>
<p>The reason we kept finding hotter days as the sample size grew is that the larger samples included more weather patterns. This meant there was a greater chance of producing a unique pattern with the near-perfect alignment of weather systems to generate even more heat at our fixed location. It turns out the weather patterns that produce the most extreme heat are very unique – and indeed far rarer than we’d expected.</p>
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<span class="caption">The weather pattern for the hottest day at Seattle Tacoma airport (green cross) in the observational record (June 28 2021, left) and our model simulations (right). The similarity between the two suggests extremely hot days in the model are generated by similar weather patterns as in the real world.</span>
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<h2>Luck of the draw</h2>
<p>From this perspective, the record-shattering heat experienced in the Pacific Northwest in 2021 was due not just to the overall trend of global heating, but also to the random shuffling of the weather. And our research suggests the latter factor plays an even larger role in this type of event than many climatologists had suspected. </p>
<p>This means that even though the Pacific Northwest heatwave broke records by such a wide margin, that is not necessarily a sign climate change is happening faster than expected, or that our models are doing a bad job of simulating how climate change increases the likelihood of extreme heatwaves.</p>
<p>It could simply be that our sample sizes are too small. If we had run more model simulations we could have simulated the right chance alignment of weather to generate a record-shattering day, meaning this real-life heatwave wouldn’t then have outstripped climatologists’ predictions to such an extent.</p>
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Read more:
<a href="https://theconversation.com/the-north-american-heatwave-shows-we-need-to-know-how-climate-change-will-change-our-weather-163802">The North American heatwave shows we need to know how climate change will change our weather</a>
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<p>Advances in supercomputers have traditionally been used to run climate models at higher resolution (that is, to zoom in and get closer to “weather mode”). But when it comes to predicting just how extreme the weather can get in a warming world, we might get more bang for our buck by using those advances to run many more simulations as well. That will show us what kind of extreme heat is possible as a rare event now, and what will be more commonplace in the coming decades.</p><img src="https://counter.theconversation.com/content/207925/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>The 2021 Pacific Northwest heatwave outstripped even the most severe climate prections. A new study simulated 45,000 years of weather at Seattle Tacoma airport to try and figure out why.Damien Irving, Climate Data Scientist, CSIROJames Risbey, Researcher, Oceans and Atmosphere, CSIROLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1781602022-04-04T22:49:58Z2022-04-04T22:49:58ZSimulating Earth’s changing climate: why some models exaggerate future warming<figure><img src="https://images.theconversation.com/files/456061/original/file-20220404-26-53lfso.jpg?ixlib=rb-1.1.0&rect=230%2C142%2C7076%2C2530&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock/Dima Zel</span></span></figcaption></figure><p>The latest <a href="https://www.ipcc.ch/report/sixth-assessment-report-working-group-3/">report</a> by the Intergovernmental Panel on Climate Change (<a href="https://www.ipcc.ch/">IPCC</a>), released overnight, shows a viable path to cutting global emissions by half by the end of this decade.</p>
<p>It follows earlier reports in the IPCC’s <a href="https://www.ipcc.ch/">Sixth Assessment</a> round, which reiterate that climate change is unequivocal and ubiquitous, humans are to blame and warming will surpass the Paris target to keep warming below 2°C unless we make deep cuts to emissions.</p>
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<a href="https://theconversation.com/ipcc-says-the-tools-to-stop-catastrophic-climate-change-are-in-our-hands-heres-how-to-use-them-179654">IPCC says the tools to stop catastrophic climate change are in our hands. Here's how to use them</a>
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<p>For its projections of future warming, the IPCC relies heavily on climate models – computer simulations that help us understand how the climate has changed in the past and how it is likely to change in the future under various emissions scenarios. </p>
<p>These models are <a href="https://www.carbonbrief.org/cmip6-the-next-generation-of-climate-models-explained">continuously updated</a> but some new-generation models are “running hot”, showing a notably higher <a href="https://www.carbonbrief.org/explainer-how-scientists-estimate-climate-sensitivity">climate sensitivity</a> than previous ones. </p>
<p>According to the IPCC, our planet’s actual climate sensitivity is unlikely to be as large as these models suggest, which raises the question of why we would use them if their climate sensitivities are likely unrealistic.</p>
<h2>Estimating climate sensitivity</h2>
<p>Climate sensitivity describes how much global temperatures will rise in response to human-caused greenhouse gas emissions. The best estimate is 3°C of warming for a doubling of pre-industrial carbon dioxide levels, with a likely range of 2.5 to 4°C, but ongoing research aims to narrow this range.</p>
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<figcaption><span class="caption">How scientists estimate climate sensitivity.</span></figcaption>
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<p>Several new models, contributed by renowned modelling centres, display climate sensitivities outside this likely range and larger than any models used for the IPCC’s last assessment in 2013. As a consequence, they simulate anomalously large and fast warming during the 21st century.</p>
<p>Critics see climate models generally as flawed attempts at capturing the complexities of the climate system, not good enough as scientific evidence to guide climate policies.</p>
<p>Yes, all climate models have flaws because they are models, not reality. But they are spectacularly successful at capturing past climate change, including the steady march of global warming and the intensification and increasing frequency of floods and droughts that now regularly make headlines. Nevertheless the large sensitivities of some models are a cause of concern.</p>
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Read more:
<a href="https://theconversation.com/the-climate-wont-warm-as-much-as-we-feared-but-it-will-warm-more-than-we-hoped-143175">The climate won't warm as much as we feared – but it will warm more than we hoped</a>
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<p>The story starts in the early 2000s, when various satellite measurements were combined to better describe the Earth’s radiation budget – the balance between incoming solar radiation and reflected outgoing visible light and invisible infrared radiation. </p>
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<img alt="A true-colour satellite image of the Earth." src="https://images.theconversation.com/files/456054/original/file-20220404-21-yuez6j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/456054/original/file-20220404-21-yuez6j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=424&fit=crop&dpr=1 600w, https://images.theconversation.com/files/456054/original/file-20220404-21-yuez6j.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=424&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/456054/original/file-20220404-21-yuez6j.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=424&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/456054/original/file-20220404-21-yuez6j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=533&fit=crop&dpr=1 754w, https://images.theconversation.com/files/456054/original/file-20220404-21-yuez6j.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=533&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/456054/original/file-20220404-21-yuez6j.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=533&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Clouds are difficult to represent in climate models.</span>
<span class="attribution"><span class="source">Planet Observer/Universal Images Group via Getty Images</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<p>Based on this, the earlier IPCC report concluded clouds over the Southern Ocean were poorly represented in models, with insufficient sunlight reflected back into space and too much reaching the surface where it warmed the ocean. <a href="https://journals.ametsoc.org/view/journals/clim/29/11/jcli-d-15-0564.1.xml">Later research</a> found many models simulated ice clouds when in fact they should have been liquid clouds.</p>
<h2>Simulating water and clouds</h2>
<p>This may sound like an elementary problem, but it isn’t. If water comes in very small droplets – as it does in clouds – it can remain liquid down to about -35°C. We call such droplets supercooled. </p>
<p>If the water contains <a href="https://www.pnas.org/content/115/11/2687">impurities</a>, its freezing temperature can be anywhere between 0°C and -35°C. Simulating clouds under all conditions is therefore far from trivial.</p>
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<a href="https://theconversation.com/why-clouds-are-the-missing-piece-in-the-climate-change-puzzle-140812">Why clouds are the missing piece in the climate change puzzle</a>
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<p>Modelling groups generally succeeded in introducing more supercooled liquid clouds into their latest models and at least partly solved this Southern Ocean cloud problem. But this change weakened an important climate feedback: as the climate warms, liquid clouds become more prevalent at the expense of ice clouds.</p>
<p>Liquid clouds are brighter and more reflective than ice clouds, and under progressive global warming more and more incoming sunlight is reflected back into space, counteracting the warming effect. However by replacing ice with supercooled liquid clouds, newer models weaken this cooling effect. This is the leading explanation for the larger climate sensitivity of many new-generation climate models. </p>
<h2>The IPCC’s response</h2>
<p>The latest IPCC report didn’t raise the estimate of the planet’s actual climate sensitivity. It cites observational evidence to make the case, including “historical” warming which is very well understood for the past several decades.</p>
<p>Models with a middle-of-the-road climate sensitivity near 3°C often better reproduce the temperature variations of this historical period than those with a large climate sensitivity. </p>
<p>Further evidence comes from simulations of the Earth’ geological past (thousands to millions of years ago) which saw both much colder and much warmer climates than at present. Geological evidence shows high-sensitivity models exaggerate the temperature swings of this distant past. By the same token, a few very low-sensitivity models are also unlikely to be correct.</p>
<p>The latest report concludes climate sensitivity is now better understood, but it doesn’t go as far as dismissing high-sensitivity models altogether. Instead, it says such models simulate “high risk, low likelihood” futures that cannot be ruled out. </p>
<h2>Refining climate models</h2>
<p>What does the future hold for climate models? Climate sensitivity is the result of a model’s “tuning” whereby parameters are varied systematically until the model produces an acceptable representation of the well observed climate of the past few decades. </p>
<p>Clearly this process requires refinement. Low, medium, and high-sensitivity models have all passed this test, yet these models project quite different magnitudes of warming for this century. </p>
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Read more:
<a href="https://theconversation.com/how-machine-learning-is-helping-us-fine-tune-climate-models-to-reach-unprecedented-detail-165818">How machine learning is helping us fine-tune climate models to reach unprecedented detail</a>
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<p>There is scope for increasing cooperation between institutions, scientific disciplines and countries to rise to this challenge. The latest IPCC report did an excellent job at dealing with this, but clearly better constraining the climate sensitivity in models would further raise confidence in climate projections. </p>
<p>The stakes are high. Climate projections inform expensive and disruptive adaptation and mitigation decisions around the world, including which coastal properties should be abandoned due to rising seas, how quickly we need to wean ourselves off fossil fuels, or how to make agriculture climate resilient and climate neutral while still feeding a growing human population. </p>
<p>Seen against this backdrop, a seemingly innocuous, technical problem in climate modelling takes on outsized importance.</p><img src="https://counter.theconversation.com/content/178160/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Olaf Morgenstern receives funding from the NZ Ministry for Business, Innovation, and Employment under the Deep South National Science Challenge and under the Strategic Science Investment Fund. </span></em></p>The current estimate is that Earth would warm by 1.5°C to 4.5°C if emissions were to double on pre-industrial levels. The range has remained stubbornly wide, despite improved climate modelling.Olaf Morgenstern, Principal Scientist -- Atmosphere and Climate, National Institute of Water and Atmospheric ResearchLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1654302021-08-04T18:42:44Z2021-08-04T18:42:44Z5 things to watch for in the latest IPCC report on climate science<figure><img src="https://images.theconversation.com/files/414584/original/file-20210804-307-1hdsycu.jpg?ixlib=rb-1.1.0&rect=76%2C69%2C4540%2C2526&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Wildfires burn on the island of Evia, north of Athens, Greece, on Aug. 3, 2021, as the country dealt with the worst heat wave in decades. Temperatures reached 41 C in parts of Athens. </span> <span class="attribution"><span class="source">(AP Photo/Michael Pappas) </span></span></figcaption></figure><p>On Aug. 9, the Intergovernmental Panel on Climate Change (IPCC) will release its most comprehensive report on the science of climate change <a href="https://www.ipcc.ch/assessment-report/ar5/">since 2013</a>. It will be the first of four reports released under the IPCC’s latest assessment cycle, with subsequent reports coming in 2022.</p>
<p>Over the past eight years, climate scientists have improved the methods they use to measure different aspects of climate and to model (or project) what might happen in the future. They’ve also been monitoring the changes that have developed right before our eyes. </p>
<p>This updated assessment comes three months before world leaders gather in Glasgow, Scotland, to find ways to avoid the worst effects of climate change and renew their commitments to reduce greenhouse gases. It also comes amid another year of <a href="https://www.cbc.ca/news/science/extreme-weather-2021-1.6128874">severe heat waves, droughts, wildfires, flooding and storms</a>. </p>
<p>The report will provide policy-makers with the best possible information regarding the physical science of climate change, which is essential for long-term planning in many sectors, from infrastructure to energy to social welfare. Here are five things to look for in the new report:</p>
<h2>1. How sensitive is the climate to increasing carbon dioxide?</h2>
<p>Atmospheric carbon dioxide (CO2) levels are higher now than they have been in 800,000 years, <a href="https://research.noaa.gov/article/ArtMID/587/ArticleID/2764/Coronavirus-response-barely-slows-rising-carbon-dioxide">reaching 419 parts per million (ppm) in May 2021</a>. Average global temperature rises with each increase in atmospheric CO2 concentration, but how much it rises depends on many factors. </p>
<p>Climate scientists use models to understand how much warming occurs when CO2 concentrations double from pre-industrial levels — from 260 ppm to 520 ppm — a concept called “<a href="https://www.metoffice.gov.uk/research/climate/understanding-climate/climate-sensitivity-explained">climate sensitivity</a>.” The more sensitive the climate, the faster greenhouse gas emissions must be curbed to stay below 2 C.</p>
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<img alt="Climate sensitivity is greater in CMIP6 than previous model intercomparisons." src="https://images.theconversation.com/files/414193/original/file-20210802-18-19bqb14.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/414193/original/file-20210802-18-19bqb14.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=454&fit=crop&dpr=1 600w, https://images.theconversation.com/files/414193/original/file-20210802-18-19bqb14.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=454&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/414193/original/file-20210802-18-19bqb14.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=454&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/414193/original/file-20210802-18-19bqb14.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=570&fit=crop&dpr=1 754w, https://images.theconversation.com/files/414193/original/file-20210802-18-19bqb14.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=570&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/414193/original/file-20210802-18-19bqb14.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=570&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">Equilibrium climate sensitivity from the last three major climate model intercomparisons. (Note: There was no ‘CMIP4’.)</span>
<span class="attribution"><span class="source">(Data: IPCC, Graph: Alex Crawford)</span></span>
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<p>Older climate models estimated that a doubling of atmospheric CO2 would lead to a temperature increase of <a href="https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter09_FINAL.pdf">2.1 C to 4.7 C</a>. The latest set of climate models, <a href="https://www.carbonbrief.org/cmip6-the-next-generation-of-climate-models-explained">called CMIP6</a>, broadened the range to <a href="https://www.doi.org/10.1126/sciadv.aba1981">1.8 C to 5.6 C</a>, meaning the climate is at least as sensitive to doubling of carbon dioxide as previous models showed, but may, in fact, <a href="https://doi.org/10.1029/2019GL085782">be even more sensitive</a>.</p>
<p>The range is influenced by uncertainties in a number of climate factors, including water vapour and cloud cover, and how they will increase or decrease the effects of warming. Scientists are working to narrow the range in climate projections so that we know more about how quickly we must reduce greenhouse gas emissions to avoid the worst effects of climate change and adapt to others.</p>
<h2>2. What’s going on with clouds?</h2>
<p>Clouds are a wild card in the climate change game. They create <a href="https://climate.nasa.gov/nasa_science/science/">feedbacks to warming</a>, meaning that warming changes cloud cover, but <a href="http://www.climate.be/textbook/chapter4_node8.html">cloud cover can also speed up or slow down warming</a> in different situations. </p>
<p>Clouds reflect about <a href="https://earthobservatory.nasa.gov/features/EnergyBalance">a quarter of incoming sunlight</a> away from the Earth. So, if more warming leads to more clouds, we would expect more sunlight to be reflected, slowing warming. However, clouds also insulate the Earth, trapping the heat given off by the surface. So, increasing cloud cover (like during nighttime) could amplify warming. </p>
<figure class="align-center ">
<img alt="Multiple cloud types" src="https://images.theconversation.com/files/414192/original/file-20210802-22-1kxulbf.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/414192/original/file-20210802-22-1kxulbf.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/414192/original/file-20210802-22-1kxulbf.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/414192/original/file-20210802-22-1kxulbf.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/414192/original/file-20210802-22-1kxulbf.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/414192/original/file-20210802-22-1kxulbf.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/414192/original/file-20210802-22-1kxulbf.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">Cloud feedback properties depend in part on the type and altitude of the cloud.</span>
<span class="attribution"><span class="source">(Alex Crawford)</span></span>
</figcaption>
</figure>
<p><a href="https://e360.yale.edu/features/why-clouds-are-the-key-to-new-troubling-projections-on-warming">Two main issues stand out</a>: First, many factors, including cloud type, altitude and season, determine a cloud’s overall effect on warming. Second, clouds are incredibly difficult to model; how the models treat clouds is key to the range in climate sensitivity.</p>
<h2>3. Did climate change fuel recent extreme weather?</h2>
<p>Since the last IPCC report, our ability to assess global warming’s impact on extreme events has improved immensely. <a href="https://www.ipcc.ch/site/assets/uploads/2018/09/AR6_WGI_outlines_P46.pdf">Chapter 11 of the latest report</a> is devoted to this. </p>
<p>Global warming means <a href="https://www.worldweatherattribution.org/western-north-american-extreme-heat-virtually-impossible-without-human-caused-climate-change/">stronger summer heat waves</a> and <a href="https://climateatlas.ca/map/canada/tropicalnights_2060_85#">more frequent tropical nights</a> (temperatures above 20 C) are occurring in middle latitudes, like Canada and Europe.</p>
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Read more:
<a href="https://theconversation.com/record-breaking-temperatures-mean-we-must-change-the-way-we-talk-about-the-climate-emergency-163627">Record-breaking temperatures mean we must change the way we talk about the climate emergency</a>
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<p><a href="https://news.climate.columbia.edu/2019/09/23/climate-change-impacts-water/">Warmer air can hold more water</a>. This can cause more evaporation from land, and lead to drought and wildfires. In addition, an atmosphere with more water can produce more precipitation and flooding. </p>
<p>Scientists projected decades ago that these changes to the water cycle <a href="https://www.ipcc.ch/report/ar4/wg1/">would occur</a>, but now it’s clear they’re <a href="https://doi.org/10.1038/s41586-018-0673-2">already happening</a>. </p>
<h2>4. Have regional climate projections improved?</h2>
<figure class="align-right ">
<img alt="head of Shamrock glacier, with bare mountain peaks behind it" src="https://images.theconversation.com/files/414197/original/file-20210802-28-1fyf84o.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/414197/original/file-20210802-28-1fyf84o.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/414197/original/file-20210802-28-1fyf84o.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/414197/original/file-20210802-28-1fyf84o.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/414197/original/file-20210802-28-1fyf84o.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/414197/original/file-20210802-28-1fyf84o.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/414197/original/file-20210802-28-1fyf84o.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">Shamrock Glacier, like many other glaciers in Alaska, has been thinning and retreating since the 1950s.</span>
<span class="attribution"><span class="source">(Alex Crawford)</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The climate models evaluated by the IPCC <a href="https://www.climate.gov/maps-data/primer/climate-models">are global models</a>. This is essential to capture the connections between tropics and poles or land and ocean. However, it comes at a cost — the models struggle to simulate many features smaller than 100 kilometres across, like small storms or islands. </p>
<p>Regional relationships can be complex: For example, extreme storms help <a href="https://doi.org/10.1175/JCLI-D-19-0925.1">break up summer Arctic sea ice</a>, but reduced sea ice cover may also <a href="https://doi.org/10.1029/2020JD034366">lead to stronger storms</a>.</p>
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Read more:
<a href="https://theconversation.com/5-ways-climate-driven-ocean-change-can-threaten-human-health-162341">5 ways climate-driven ocean change can threaten human health</a>
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<p>Since the last IPCC report, <a href="https://www.gfdl.noaa.gov/climate-model-downscaling/">techniques for taking this large-scale information and refining it</a> have shown how regional and local climate has changed and could change in the future. Other experiments target regional issues, like the <a href="https://www.wcrp-climate.org/modelling-wgcm-mip-catalogue/cmip6-endorsed-mips-article/1303-modelling-cmip6-pamip">impacts of Arctic sea ice loss</a> on storms.</p>
<h2>5. How will Antarctic ice sheets contribute to sea-level rise?</h2>
<p><a href="https://sealevel.nasa.gov/understanding-sea-level/global-sea-level/overview">Global sea level is rising</a> because <a href="https://www.carbonbrief.org/explainer-how-climate-change-is-accelerating-sea-level-rise">water expands slightly when it warms up</a>, and <a href="https://www.nps.gov/glac/learn/nature/climate-change.htm">mountain glaciers</a> and the <a href="https://nsidc.org/greenland-today/">Greenland Ice Sheet</a> are melting and adding water to the ocean. </p>
<p>But the largest potential source of sea-level rise over the next century is Antarctica. <a href="https://www.carbonbrief.org/studies-shed-new-light-on-antarcticas-future-contribution-to-sea-level-rise">Ice sheet models show</a> that melting of Antarctic ice sheets will add between 14 and 114 centimetres to sea-level rise by 2100. That is a huge range, and it all depends on whether the West Antarctic Ice Sheet remains relatively stable or begins a slow but unstoppable collapse. </p>
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Read more:
<a href="https://theconversation.com/high-tide-flood-risk-is-accelerating-putting-coastal-economies-at-risk-164481">High-tide flood risk is accelerating, putting coastal economies at risk</a>
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<p>How the IPCC communicates this scientific debate will impact how <a href="https://www.ducks.ca/stories/conservator/rising-sea-levels-on-canadas-coasts/">coastal communities plan for sea-level rise</a>. Low-lying cities, like <a href="https://www.cnn.com/2021/08/01/africa/lagos-sinking-floods-climate-change-intl-cmd/index.html">Lagos, Nigeria</a>, could become uninhabitable by the end of the century due to sea-level rise, especially if the higher model estimates prove most prescient.</p>
<figure class="align-center ">
<img alt="Edge of the West Antarctic Ice Sheet" src="https://images.theconversation.com/files/414187/original/file-20210802-19-13frmz4.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/414187/original/file-20210802-19-13frmz4.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/414187/original/file-20210802-19-13frmz4.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/414187/original/file-20210802-19-13frmz4.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/414187/original/file-20210802-19-13frmz4.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/414187/original/file-20210802-19-13frmz4.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/414187/original/file-20210802-19-13frmz4.jpeg?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">The fate of the West Antarctic Ice Sheet lies with the Thwaites Glacier. If the front of the Thwaites breaks up, it would expose an even larger body of ice to warm waters.</span>
<span class="attribution"><span class="source">(Karen Alley)</span></span>
</figcaption>
</figure>
<p>The IPCC report will give policy-makers a better understanding of how climate change is affecting us today. This will be especially helpful for putting short-term adaptation strategies in place. </p>
<p>But as the science has improved, the outlook on future change has become more sobering, and the large uncertainties that remain mean substantial future work for climate scientists.</p><img src="https://counter.theconversation.com/content/165430/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alex Crawford receives funding through the Canada-150, Canada Excellence Research Chair (CERC), and Canada Research Chair (CRC) programs as well as the Natural Sciences and Engineering Research Council (NSERC).</span></em></p>The latest report on climate science comes on the heels of heatwaves, wildfires, flooding and storms. It will help policy-makers act on plans to curb emissions or adapt to climate change.Alex Crawford, Research Associate at the Centre for Earth Observation Science, Clayton H. Riddle Faculty of Environment, Earth, and Resources, University of ManitobaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1431752020-07-22T19:52:19Z2020-07-22T19:52:19ZThe climate won’t warm as much as we feared – but it will warm more than we hoped<figure><img src="https://images.theconversation.com/files/348813/original/file-20200722-31-1tzkaji.jpg?ixlib=rb-1.1.0&rect=71%2C53%2C5919%2C3314&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>We know the climate changes as greenhouse gas concentrations rise, but the exact amount of expected warming remains uncertain. </p>
<p>Scientists study this in terms of “equilibrium climate sensitivity” – the temperature rise for a sustained doubling of carbon dioxide concentrations. Equilibrium climate sensitivity has long been estimated within a likely range of 1.5-4.5°C. </p>
<p>Under our current emissions trajectories, carbon dioxide concentrations in the atmosphere will likely double between 2060 and 2080, relative to concentrations before the industrial revolution. Before that, they had changed little for millennia.</p>
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Read more:
<a href="https://theconversation.com/explainer-what-is-climate-sensitivity-18815">Explainer: what is climate sensitivity?</a>
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<p>A <a href="https://climateextremes.org.au/wp-content/uploads/2020/07/WCRP_ECS_Final_manuscript_2019RG000678R_FINAL_200720.pdf">major new assessment</a> has now calculated a range of 2.6–3.9°C. This implies that <a href="https://theconversation.com/just-how-hot-will-it-get-this-century-latest-climate-models-suggest-it-could-be-worse-than-we-thought-137281">alarmingly high estimates</a> from some recent climate models are unlikely, but also that comfortingly low estimates from other studies are even less likely.</p>
<h2>More warming, greater impacts</h2>
<p>Current and future climate change impacts include heatwaves, changing rainfall and drought patterns, and rising seas. Their severity depends on how much warming takes place. </p>
<p>Human activities are the main determinant of future temperatures, so a world with aggressive emissions control looks very different from a world in which emissions continue to increase.</p>
<p>Even if we knew exactly how emissions would change in the future, the exact amount of warming that would result remains uncertain. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/348817/original/file-20200722-19-19upq9i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Four wind turbines at the end of a road, flanked by pale green-brown grass." src="https://images.theconversation.com/files/348817/original/file-20200722-19-19upq9i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/348817/original/file-20200722-19-19upq9i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/348817/original/file-20200722-19-19upq9i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/348817/original/file-20200722-19-19upq9i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/348817/original/file-20200722-19-19upq9i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/348817/original/file-20200722-19-19upq9i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/348817/original/file-20200722-19-19upq9i.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">Drastic measures are still needed to curb climate change.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
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</figure>
<p>Our new equilibrium climate sensitivity analysis substantially reduces this uncertainty, by combining modern understanding of atmospheric physics with modern, historic, and prehistoric data using robust statistical methods.</p>
<p>The results indicate that substantial warming is much more solidly assured than we thought.</p>
<h2>A matter of probabilities</h2>
<p>In 1979, a <a href="https://www.nap.edu/catalog/12181/carbon-dioxide-and-climate-a-scientific-assessment">farsighted report</a> estimated for the first time that equilibrium climate sensitivity falls somewhere between 1.5°C and 4.5°C. So if carbon dioxide concentrations doubled, global temperatures would eventually increase by somewhere in that range. </p>
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Read more:
<a href="https://theconversation.com/there-are-no-time-travelling-climatologists-why-we-use-climate-models-15347">There are no time-travelling climatologists: why we use climate models</a>
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<p>The width of this range is a problem. If equilibrium climate sensitivity lies at the low end of the range, climate change might be manageable with relatively relaxed national policies. </p>
<p>In contrast, a value near the high end would be catastrophic unless drastic action is taken to reduce emissions and draw carbon dioxide from the atmosphere. </p>
<p>Consequently, narrowing the equilibrium climate sensitivity range has been a key focus of climate science. While recent estimates haven’t really changed, climate scientists have learned a lot about how likely each outcome is. </p>
<p>For example, the <a href="https://www.ipcc.ch/report/ar5/wg1/">2013 Intergovernmental Panel on Climate Change (IPCC) report</a> estimated a minimum two-thirds chance that equilibrium climate sensitivity falls within the 1.5–4.5°C range. This implies there’s a chance of up to one-third that equilibrium climate sensitivity is lower or, worryingly, much higher. </p>
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<a href="https://images.theconversation.com/files/348832/original/file-20200722-31-1chjq5r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="The silhouette of a coal-burning power plant." src="https://images.theconversation.com/files/348832/original/file-20200722-31-1chjq5r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/348832/original/file-20200722-31-1chjq5r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/348832/original/file-20200722-31-1chjq5r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/348832/original/file-20200722-31-1chjq5r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/348832/original/file-20200722-31-1chjq5r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/348832/original/file-20200722-31-1chjq5r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/348832/original/file-20200722-31-1chjq5r.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">There’s only a 17% chance we’ll keep warming under 2°C, in the lowest global emissions scenario.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
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</figure>
<p>Recently, the potential for high climate sensitivities gained further attention after results from new climate models <a href="https://theconversation.com/just-how-hot-will-it-get-this-century-latest-climate-models-suggest-it-could-be-worse-than-we-thought-137281">suggested values</a> in excess of 5°C.</p>
<p>Our new assessment rules out low climate sensitivities, finding only a 5% chance that equilibrium climate sensitivity is below 2.3°C. </p>
<p>On the brighter side, we also find a low chance of it rising above 4.5°C. Constraining the precise probability of high equilibrium climate sensitivity range is difficult and depends to some extent on how the evidence is interpreted. Still, the alarming predictions of the new models appear unlikely.</p>
<p>We also find the chances of the world exceeding the 2°C Paris Accord target by late this century are 17% under the lowest-emission scenario considered by the IPCC, 92% under a scenario that approximates current efforts, and 100% under the highest-emission scenario.</p>
<h2>Why our study is different</h2>
<p>The new assessment uses several strands of evidence. One is the recent, historical past since industrialisation, during which time temperatures have increased by about 1.1°C. </p>
<p>We compared this with knowledge about the natural drivers of climate over this period (such as slight changes in solar output and a few major volcanic eruptions), human-caused increases in atmospheric carbon dioxide and other greenhouse gases, and changes to the land surface. </p>
<p>Second, the assessment uses data for temperature changes and the underpinning natural processes from ice ages and warm periods in pre-historic times. </p>
<p>And third, it uses physical laws and present-day observations to evaluate how the planet responds to change, for example by examining brief warming or cooling episodes.</p>
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Read more:
<a href="https://theconversation.com/why-2-of-global-warming-is-much-worse-for-australia-than-1-5-77548">Why 2℃ of global warming is much worse for Australia than 1.5℃</a>
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<p>One conclusion is especially consistent between all lines of evidence. Unless the equilibrium climate sensitivity is larger than 2°C, we cannot explain either the warming we’ve already seen since industrialisation, the ice ages in Earth’s past, or certain aspects of how weather changes operate today. </p>
<p>This unequivocally demonstrates that relaxed efforts against carbon emissions will not avoid substantial warming.</p>
<h2>This is not the final word</h2>
<p>The new assessment is by no means the last word. It narrows the range, but we still don’t know exactly how hot it’s going to get. </p>
<p>Our assessment will also feed into the upcoming <a href="https://www.ipcc.ch/assessment-report/ar6/">IPCC report</a>, but the panellist will of course make an independent assessment. And further research may narrow the range more in the future.</p>
<p>While high sensitivities are unlikely, they cannot be completely excluded. But whether the temperature rise is moderate or high, the message is the same: drastic measures are needed to curb climate change. </p>
<p>Crucially, the new assessment clearly demonstrates that betting on low sensitivities and failing to implement drastic measures is risky to the point of irresponsibility.</p><img src="https://counter.theconversation.com/content/143175/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Steve Sherwood receives funding from the Australian Research Council.</span></em></p><p class="fine-print"><em><span>Eelco Rohling has received funding from the Australian Research Council and the UK Natural Environment Research Council. </span></em></p><p class="fine-print"><em><span>Katherine Marvel has received funding from NASA, NOAA, and US Department of Energy research grants. </span></em></p>A new assessment found substantial warming is much more solidly assured than we thought.Steven Sherwood, ARC Laureate Fellow, Climate Change Research Centre, UNSW SydneyEelco Rohling, Professor of Ocean and Climate Change, Australian National UniversityKatherine Marvel, Associate Research Scientist, NASALicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1431122020-07-22T16:17:26Z2020-07-22T16:17:26ZJust how sensitive is the climate to increased carbon dioxide? Scientists are narrowing in on the answer<figure><img src="https://images.theconversation.com/files/348920/original/file-20200722-34-tv3ufh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/content/an-astronauts-view-from-space">NASA/Reid Wiseman</a></span></figcaption></figure><p>At the dawn of the industrial revolution, the Earth’s atmosphere contained <a href="https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide">278 parts of CO₂ per million</a>. Today, after more than two and a half centuries of fossil fuel use, that figure is around <a href="https://scripps.ucsd.edu/programs/keelingcurve/">414 parts per million</a> (ppm). If the build-up of CO₂ continues at <a href="https://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/long-range/forecasts/co2-forecast">current rates</a>, by 2080 it will have passed 560 ppm – more than double the level of pre-industrial times. </p>
<p>Exactly how the climate will respond to all this extra CO₂ is one of the central questions in climate science. Just how much will the climate actually change? </p>
<p>A major new international assessment of the Earth’s climate sensitivity, now published in the journal <a href="https://climateextremes.org.au/wp-content/uploads/2020/07/WCRP_ECS_Final_manuscript_2019RG000678R_FINAL_200720.pdf">Reviews of Geophysics</a>, addresses this question. This research has improved our understanding of how much the world will eventually warm if the carbon dioxide in the atmosphere is maintained at double the level of pre-industrial times.</p>
<p>While an exact figure is still not possible, low levels of warming are now found to be far less likely than previously thought. Very high values are slightly less likely too. There is much greater certainty that, if left unchecked, global warming would be high enough to bring very severe impacts and risks worldwide.</p>
<p>The study, which was organised by the World Climate Research Programme (WCRP) and involving many leading climate scientists (including one of us: Tim), looks at a measure called “<a href="https://www.carbonbrief.org/explainer-how-scientists-estimate-climate-sensitivity">equilibrium climate sensitivity</a>”. This refers to how much global average temperatures will increase by in the long-term following a doubling of carbon dioxide concentrations. It can be estimated using three main lines of evidence:</p>
<ol>
<li><p>Temperature measurements made with thermometers from 1850 (when enough global coverage began) to the near present. By comparing temperatures, CO₂ levels and the effect of other climate drivers in the past and present, we can estimate the longer-term changes.</p></li>
<li><p>Evidence from paleoclimate records from the peak of the last ice age 20,000 years ago, when CO₂ was lower than now, and a warm period around 4 million years ago when CO₂ was more comparable to today. We can tell how warm the climate was and how much CO₂ there was in the atmosphere based on the make-up of gases trapped in air bubbles in ancient ice cores.</p></li>
<li><p>Present-day observations – for instance from satellite data – and evidence from climate models, theory and detailed process models that examine the physics of interactions within the climate system.</p></li>
</ol>
<p>Despite its importance, equilibrium climate sensitivity is very uncertain and for many years the standard estimate has been 1.5°C to 4.5°C. In its 5th <a href="https://www.ipcc.ch/assessment-report/ar5/">Assessment Report</a>, the Intergovernmental Panel on Climate Change (IPCC) gave these values as the “likely range”, which meant it considered there was at least a 66% chance that it fell within this range. Or, in other words, it judged there was up to a 33% chance that warming would either be less than 1.5°C or more than 4.5°C.</p>
<p>The new study suggests that this “likely range” has narrowed to, at most, 2.3°C to 4.5°C – or possibly an even narrower range*. The lower end of the range has therefore risen substantially, meaning that scientists are now much more confident that global warming will not be small. </p>
<p><strong>Global warming assessments old and new</strong></p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/348885/original/file-20200722-28-ht6nj7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/348885/original/file-20200722-28-ht6nj7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=274&fit=crop&dpr=1 600w, https://images.theconversation.com/files/348885/original/file-20200722-28-ht6nj7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=274&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/348885/original/file-20200722-28-ht6nj7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=274&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/348885/original/file-20200722-28-ht6nj7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=344&fit=crop&dpr=1 754w, https://images.theconversation.com/files/348885/original/file-20200722-28-ht6nj7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=344&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/348885/original/file-20200722-28-ht6nj7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=344&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Ranges of equilibrium climate sensitivity from the IPCC and the new study.</span>
</figcaption>
</figure>
<h2>We won’t be saved by low sensitivity</h2>
<p>An important implication is that humans would be taking an even bigger risk than previously thought if we relied on low climate sensitivity to allow us to meet the <a href="https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement">Paris Agreement</a> target of keeping global temperatures to “well below” 2°C above pre-industrial levels, and to “pursue efforts” to limit warming to 1.5°C. This is therefore further confirmation that CO₂ emissions need to be rapidly reduced and ultimately reach net zero if the Paris targets are to have a good chance of being met.</p>
<p>According to the study, if CO₂ does reach double the pre-industrial level and stays there (or to be precise, if the total effect of all human impact on greenhouse gases and other climate drivers reaches an equivalent level), then there is up to an 18% chance that temperatures will rise to 4.5°C above pre-industrial levels, and a no more than 5% chance that they will go above 5.7°C. This has important implications for <a href="https://www.theccc.org.uk/publications/third-uk-climate-change-risk-assessment/">climate change risk assessments</a>. In a risk assessment, it is normal to consider outcomes that are possible even if they are not the most likely. </p>
<p>The <a href="https://advances.sciencemag.org/content/6/26/eaba1981.abstract">latest climate models</a> have a wide range of climate sensitivities, with our own <a href="https://www.metoffice.gov.uk/research/approach/modelling-systems/unified-model/climate-models/hadgem3">Met Office models</a> at the high end. This happens because climate sensitivity is not something that scientists input to the models, but rather it emerges from the same complex interactions the models simulate.</p>
<p>This diversity of models lets us understand the regional changes in climate and extreme weather associated with different climate sensitivities, and assess their potential impacts. This includes the high sensitivities that are less likely but still possible. At the other end of the range, seeing the minimum changes we can expect will help inform climate change adaptation measures. </p>
<p>The new study allows a key aspect of climate models, their climate sensitivity, to be seen in the context of other evidence. While there is still more to be done to assess more precisely how the global climate will respond to further increases in greenhouse gases, these advances provide a much more solid base of evidence on which climate change policy can be further developed.</p>
<p><em>*WCRP provides two sets of ranges. The first is based on a “baseline” calculation which represents a single interpretation of the evidence and may be over-confident. The second set of “robust” ranges are designed to bound the range of plausible alternative interpretations of the evidence and statistical modelling assumptions. The numbers quoted in this article are from the robust range. For further details, see <a href="https://climateextremes.org.au/wp-content/uploads/2020/07/WCRP_ECS_Final_manuscript_2019RG000678R_FINAL_200720.pdf">Sherwood et. al, 2020</a></em></p>
<hr>
<p><em>This article was updated on 23 July to correct an error in the first paragraph. At current rates, CO₂ concentrations will pass 560 ppm by 2080, not 2060.</em></p><img src="https://counter.theconversation.com/content/143112/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Richard Betts is employed by the Met Office in the Hadley Centre for Climate Science and Services. This is funded by the UK government through the department of Business, Energy and Industrial Strategy (BEIS) and Department for Environment, Food and Rural Affairs. He is also employed by the University of Exeter where he receives funding from Defra via the Committee on Climate Change in support of his role as technical lead on the UK's 3rd National Climate Change Risk Assessment. </span></em></p><p class="fine-print"><em><span>Jason Lowe is employed by the Met Office in the Hadley Centre for Climate Science and Services. This is funded by the UK government through the department of Business, Energy and Industrial Strategy (BEIS) and Department for Environment, Food and Rural Affairs.</span></em></p><p class="fine-print"><em><span>Timothy Andrews is employed by the Met Office in the Hadley Centre for Climate Science and Services. This is funded by the UK government through the department of Business, Energy and Industrial Strategy (BEIS) and Department for Environment, Food and Rural Affairs. </span></em></p>New study shows low levels of future global warming are far less likely than previously thought. Very high warming is also slightly less likely.Richard Betts, Met Office Head of Climate Impacts Research and Professor, University of ExeterJason Lowe, Principal Fellow, Met Office Hadley Centre and Professor, Priestley International Centre for Climate, University of LeedsTimothy Andrews, Research Scientist in Atmospheric Physics, Met Office Hadley CentreLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1372812020-05-17T19:58:58Z2020-05-17T19:58:58ZJust how hot will it get this century? Latest climate models suggest it could be worse than we thought<p>Climate scientists use mathematical models to project the Earth’s future under a warming world, but a group of the <a href="https://www.carbonbrief.org/cmip6-the-next-generation-of-climate-models-explained">latest models</a> have included unexpectedly high values for a measure called “climate sensitivity”.</p>
<p><a href="https://theconversation.com/explainer-what-is-climate-sensitivity-18815">Climate sensitivity</a> refers to the relationship between changes in carbon dioxide in the atmosphere and warming.</p>
<p>The high values are an unwelcome surprise. If they’re right, it means a hotter future than previously expected – warming of up to 7°C for Australia by 2100 if emissions continue to rise unabated.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/there-are-no-time-travelling-climatologists-why-we-use-climate-models-15347">There are no time-travelling climatologists: why we use climate models</a>
</strong>
</em>
</p>
<hr>
<p><a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019EF001469">Our recent study</a> analyses these climate models (named <a href="https://www.carbonbrief.org/cmip6-the-next-generation-of-climate-models-explained">CMIP6</a>), which were released at the end of last year, and what insights they give for Australia. </p>
<p>These models contain the latest improvements and innovations from some of the world’s leading climate modelling institutes, and will feed into the Intergovernmental Panel on Climate Change (IPCC) <a href="https://www.ipcc.ch/report/sixth-assessment-report-working-group-ii/">Sixth Assessment Report</a> in 2021.</p>
<p>But the new climate sensitivity values raise the question of whether previous climate modelling has underestimated potential climate change and its effects, or whether the new models are overdoing things. </p>
<p>If the high estimate is right, this would require the world to make greater and more urgent emission cuts to meet any given warming target.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/334228/original/file-20200512-66649-1js3956.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/334228/original/file-20200512-66649-1js3956.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/334228/original/file-20200512-66649-1js3956.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=370&fit=crop&dpr=1 600w, https://images.theconversation.com/files/334228/original/file-20200512-66649-1js3956.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=370&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/334228/original/file-20200512-66649-1js3956.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=370&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/334228/original/file-20200512-66649-1js3956.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=465&fit=crop&dpr=1 754w, https://images.theconversation.com/files/334228/original/file-20200512-66649-1js3956.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=465&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/334228/original/file-20200512-66649-1js3956.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=465&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">These higher climate sensitivity values point to the urgent need to cut our greenhouse gas emissions.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<h2>What is climate sensitivity?</h2>
<p>Climate sensitivity is one of the most important factors for climate change, strongly influencing our planning for adaptation and mitigation of greenhouse gas emissions.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-what-is-climate-sensitivity-18815">Explainer: what is climate sensitivity?</a>
</strong>
</em>
</p>
<hr>
<p>It’s a standardised measure of how much the climate responds when carbon dioxide concentrations in the atmosphere double. There are a few indices of climate sensitivity that the scientific community uses, and perhaps the most commonly used is “equilibrium climate sensitivity”. </p>
<p>We can estimate equilibrium climate sensitivity by raising carbon dioxide concentrations in models abruptly and then calculating the warming experienced after 150 years – when the atmosphere and ocean would return to a temperature balance. </p>
<p>In other words, giving the climate a “push” with more carbon emissions and waiting until it settles down into a new state. </p>
<p>The previous generation of models (<a href="https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2012GL051607">CMIP5</a>) had equilibrium climate sensitivity values between 2.1°C to 4.7°C global temperature change. The values for the latest models (CMIP6) are from 1.8°C to 5.6°C. </p>
<p>This includes a cluster of models with sensitivity of 5°C or more, a group of models within the previous range, and two models with very low values at around 2°C. </p>
<h2>What this means for our future</h2>
<p>Higher equilibrium climate sensitivity values mean a hotter future climate than previously expected, for any given scenario of future <a href="https://theconversation.com/reducing-emissions-alone-wont-stop-climate-change-new-research-45493">emissions</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/334201/original/file-20200512-66698-y2ng35.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/334201/original/file-20200512-66698-y2ng35.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/334201/original/file-20200512-66698-y2ng35.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=500&fit=crop&dpr=1 600w, https://images.theconversation.com/files/334201/original/file-20200512-66698-y2ng35.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=500&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/334201/original/file-20200512-66698-y2ng35.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=500&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/334201/original/file-20200512-66698-y2ng35.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=628&fit=crop&dpr=1 754w, https://images.theconversation.com/files/334201/original/file-20200512-66698-y2ng35.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=628&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/334201/original/file-20200512-66698-y2ng35.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=628&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">We’ll see Australian temperature increase in a low and high emissions scenario projections (temperature relative to 1995-2014, range of models shown as coloured bands, observations as a black line).</span>
<span class="attribution"><span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>According to these new models, Australian warming could crack more than 7°C by 2100 under a scenario where greenhouse gas emissions continue to increase through the century. </p>
<p>These higher temperature changes are not currently presented in the <a href="https://www.climatechangeinaustralia.gov.au/en/">national climate projections</a>, as they didn’t occur under the previous generation of models and emission scenarios.</p>
<p>So what does this mean in practice? </p>
<p>Higher climate sensitivity means increases to heat extremes. It would mean we’ll see greater flow-on changes to other climate features, such as extreme rainfall, sea level rise, extreme heatwaves and more, reducing our ability to adapt.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/immediate-phase-out-of-fossil-fuels-could-keep-warming-below-1-5-c-109672">Immediate phase out of fossil fuels could keep warming below 1.5°C</a>
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</em>
</p>
<hr>
<p>High equilibrium climate sensitivity would also mean we need to make bigger cuts to our greenhouse gas emissions for a given global warming target. The Paris Agreement aims to keep global warming well under 2°C since pre-industrial times. </p>
<h2>Should we be worried?</h2>
<p>These are credible models, representing the new generation versions of top performing modelling systems, developed over decades at high-ranking research institutions globally. Their results cannot be rejected out of hand just because we don’t like the answer. </p>
<p>But – we shouldn’t jump on this piece of evidence, throw out all others and assume the results from a subset of new models is the final answer. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/why-2-of-global-warming-is-much-worse-for-australia-than-1-5-77548">Why 2℃ of global warming is much worse for Australia than 1.5℃</a>
</strong>
</em>
</p>
<hr>
<p>The weight and credibility of each piece of evidence must be carefully assessed by the research community, and by scientists putting together the upcoming IPCC assessment. </p>
<p>We’re only just starting to understand the reasons for the high sensitivity in these models, such as how <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL085782">clouds</a> interact with particles in the air. </p>
<p>And there are other lines of evidence underpinning the IPCC estimate of equilibrium climate sensitivity. </p>
<p>These include the warming seen since the last ice age around 20,000 years ago; measurements of warming seen over recent decades from greenhouse gases already emitted; and understanding different climate feedbacks from field experiments and observed natural variability. These other lines of evidence may not support the new model results.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/we-climate-scientists-wont-know-exactly-how-the-crisis-will-unfold-until-its-too-late-133400">We climate scientists won't know exactly how the crisis will unfold until it’s too late</a>
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</em>
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<hr>
<p>Essentially, the jury is still out on the exact value of equilibrium climate sensitivity, high values can’t be <a href="https://link.springer.com/article/10.1007/s00382-019-04991-y">ruled out</a>, and the results from the new models need to be taken seriously.</p>
<p>In any case, the new values are a worrying possibility that no one wants, but one we must still grapple with. As researchers in one <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL083978">study</a> conclude: “what scares us is not that the models’ [equilibrium climate sensitivity] is wrong […] but that it might be right”.</p><img src="https://counter.theconversation.com/content/137281/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michael Grose receives funding from the Australian Government's National Environmental Science Program under the Earth Systems and Climate Change hub</span></em></p><p class="fine-print"><em><span>Julie Arblaster receives funding from the Australian Research Council and the U.S. Department of Energy.</span></em></p>The new values are a worrying outcome that no one wants, but one we must still grapple with.Michael Grose, Climate Projections Scientist, CSIROJulie Arblaster, Associate Professor, Monash UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/370902015-02-03T04:58:00Z2015-02-03T04:58:00ZWhat will a hotter Australia be like? The past gives us some clues<figure><img src="https://images.theconversation.com/files/70897/original/image-20150203-9205-5qirzt.jpg?ixlib=rb-1.1.0&rect=29%2C35%2C3748%2C2142&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Carbon dioxide levels are rising at their fastest rate since the dinosaurs' time.</span> <span class="attribution"><span class="source">iurii/Shutterstock.com</span></span></figcaption></figure><p>The latest <a href="http://www.climatechangeinaustralia.gov.au/media/ccia/2.1.4/cms_page_media/168/CCIA_PROJECTIONS_TECHNICAL_REPORT.pdf">climate projections</a> released last week by the Bureau of Meteorology and CSIRO predict that Australia could potentially warm by between 2.8C and 5.1C by 2090.</p>
<p>Meanwhile, the 2014 <a href="http://www.bom.gov.au/state-of-the-climate/">State of the Climate</a> report, also produced by the Bureau and CSIRO, showed that Australia has already warmed by 0.9C since 1900, and that seven of the ten warmest years have occurred since 1998.</p>
<p>What actually happens will depend largely on the <a href="http://www.climatechangeinaustralia.gov.au/media/ccia/2.1.4/cms_page_media/168/CCIA_PROJECTIONS_TECHNICAL_REPORT.pdf">trends in greenhouse gas emissions</a>. However, we can look to the past to begin to understand what the future may have in store. </p>
<h2>Possible futures</h2>
<p>The new projections hinge on RCPs (<a href="http://www.skepticalscience.com/rcp.php">Representative Concentration Pathways</a>) used by the Intergovernmental Panel on Climate Change (IPCC), which model a range of scenarios for for future greenhouse gas emissions, aerosols and land use changes. Here are the details:</p>
<ol>
<li><p>Scenario A (known as RCP2.6) assumes very strong emission reduction from a peak at 2020 and a CO<sub>2</sub> concentration of around 420 parts per million by 2100. Global mean temperatures would rise by 0.3–1.7C, while in Australia by 2080-99, temperatures would rise by 0.6-1.7C relative to 1986-2005.</p></li>
<li><p>Scenario B (RCP4.5) assumes slower emission reductions that result in CO<sub>2</sub> concentrations of around 540 parts per million by 2100. In Australia by 2080-2099, temperatures would rise by 1.4-2.7C above 1986-2005.</p></li>
<li><p>Scenario C (RCP8.5) assumes a rise in emissions leading to CO<sub>2</sub> concentrations of around 940 parts per million by 2100, leading to global temperature rise of 2.6-4.8C by 2081. In Australia by 2080-2099, temperatures would rise by 2.8-5.1C relative to 1986-2005.</p></li>
</ol>
<p>Warming is projected to be stronger over land relative to the oceans and very strong in the Arctic. Hot days and heat waves are projected to become more frequent and cold days less frequent. Projected rainfall levels are not uniform, with increases at high latitudes and near the equator and decrease in the sub-tropics.</p>
<p>These changes reflect an expansion of the tropics and shrinking of polar circles, implying a shift of the earth’s climate zones toward the poles.</p>
<p>An increase is projected in extreme rainfalls and flooding. Sea levels would rise by 2080-2100 relative to 1986-2005, due to ocean thermal expansion and glacier and ice cap melting by 26-55 cm according to Scenario A and 45-82 cm according to Scenario C.</p>
<h2>So which path are we on?</h2>
<p>Atmospheric CO<sub>2</sub> levels are <a href="http://www.esrl.noaa.gov/gmd/webdata/ccgg/trends/co2_trend_mlo.pdf">currently rising</a> at a rate of 1.97 parts per million per year, as you can see in the graph below. This represents an emissions rate which, if continued to 2100, would lead to an atmospheric CO<sub>2</sub> level of 569 parts per million.</p>
<p>This level, consistent with Scenario B, implies a temperature rise of around 1.4-2.7C.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/70869/original/image-20150202-13069-1mxpp53.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/70869/original/image-20150202-13069-1mxpp53.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/70869/original/image-20150202-13069-1mxpp53.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=458&fit=crop&dpr=1 600w, https://images.theconversation.com/files/70869/original/image-20150202-13069-1mxpp53.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=458&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/70869/original/image-20150202-13069-1mxpp53.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=458&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/70869/original/image-20150202-13069-1mxpp53.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=576&fit=crop&dpr=1 754w, https://images.theconversation.com/files/70869/original/image-20150202-13069-1mxpp53.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=576&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/70869/original/image-20150202-13069-1mxpp53.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=576&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">CO2 has recently risen at rates of around 2 parts per million each year.</span>
<span class="attribution"><span class="source">Andrew Glikson</span></span>
</figcaption>
</figure>
<p>But if we increase our greenhouse gas emissions, CO<sub>2</sub> levels may rise to 940 parts per million, resulting in temperatures between 2.8C and 5.1C. Such levels may be compounded by feedbacks from fires and <a href="http://www.csiro.au/Organisation-Structure/Divisions/Marine--Atmospheric-Research/PermafrostCarbon.aspx">methane release from permafrost</a>. Shallow sediments and bogs could dramatically increase greenhouse gas emissions.</p>
<p>The problem is that we don’t know. As the chart below shows, we are entering uncharted territory.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/70870/original/image-20150202-13054-2e7xqu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/70870/original/image-20150202-13054-2e7xqu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/70870/original/image-20150202-13054-2e7xqu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=392&fit=crop&dpr=1 600w, https://images.theconversation.com/files/70870/original/image-20150202-13054-2e7xqu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=392&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/70870/original/image-20150202-13054-2e7xqu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=392&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/70870/original/image-20150202-13054-2e7xqu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=492&fit=crop&dpr=1 754w, https://images.theconversation.com/files/70870/original/image-20150202-13054-2e7xqu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=492&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/70870/original/image-20150202-13054-2e7xqu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=492&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 current rate of CO2 increase may be faster than following the K-T asteroid impact event which killed the dinosaurs (marked ‘K-T impact’)</span>
<span class="attribution"><span class="source">Andrew Glikson</span></span>
</figcaption>
</figure>
<p>The current rate of CO<sub>2</sub> rise of around 2 parts per million per year exceeds historical rates over the past tens of millions of years – a period that probably includes the mass extinction that killed off the dinosaurs around 65 million years ago. A major unknown remains over the effects of such extreme rates on feedbacks from the land and ocean.</p>
<h2>Why does it matter?</h2>
<p>Given daily temperature variations which can reach as much as <a href="http://en.wikipedia.org/wiki/Diurnal_temperature_variation">50C between day and night</a>, especially in mountain and desert terrains, some may ask why a temperature change of around 5C should be a source for concern?</p>
<p>The answer resides in studies of the evolution of the climate system: a global temperature change of around 5C represents a major shift in state of the climate. It is similar to the shift between glacial states and interglacial states, or following major asteroid impacts and global volcanic or methane release events, with attendant <a href="http://www.springer.com/earth+sciences+and+geography/earth+system+sciences/book/978-94-007-7331-8">mass extinction of species</a>.</p>
<p>Continents and regions dominated by deserts and/or by mountains, where cultivated land areas are marginal may suffer the most. In Australia, a desert and bush-dominated continent, expansion of arid terrains southward in South Australia and Western Australia would severely affect grain and livestock resources.</p>
<p>In these regions as well as southeastern Australia, the Eucalypt-dominated bush, originating under Mediterranean climate conditions, would be increasingly prone to fires.</p>
<p>A problem inherent in many climate models is their limit to the 21st century. Because melting ice and rising seas leg behind temperature, sea levels may continue rising well into the future. Under 2C warming, sea levels would eventually rise by 20-30 metres, as was the case in the Pliocene <a href="http://www.nature.com/ngeo/journal/v4/n5/full/ngeo1118.htm">2.6 – 5.2 million years ago</a>, flooding coastal and river valleys where the bulk of human agriculture is located.</p>
<p>It may be too late to avoid the consequences of the present level of atmospheric Co<sub>2</sub>. However, the eleventh-hour warning by the IPCC, and the latest projections from the Bureau and CSIRO ought to alert civilisation there may still be time to <a href="http://www.eci.ox.ac.uk/publications/downloads/schellnhuber08-pnas.pdf">avert the worst consequences</a>.</p><img src="https://counter.theconversation.com/content/37090/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Glikson does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The latest climate projections released last week by the Bureau of Meteorology and CSIRO predict that Australia could potentially warm by between 2.8C and 5.1C by 2090. Meanwhile, the 2014 State of the…Andrew Glikson, Earth and paleo-climate scientist, Australian National UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/216172014-01-08T04:08:45Z2014-01-08T04:08:45ZHow clouds can make climate change worse than we thought<figure><img src="https://images.theconversation.com/files/38332/original/srdtrcmk-1387494574.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Understanding clouds is crucial to understanding whether temperatures will rise quickly in coming decades.</span> <span class="attribution"><span class="source">Visun Khankasem/Shutterstock.com</span></span></figcaption></figure><p>The amount of global warming we can expect in the future has been a tough question to pin down. A new study that I led with colleagues in France has enabled us to come up with a more accurate analysis based on a better understanding of how clouds behave. It suggests that if fossil fuel use continues unabated we can expect warming of at least 4C by 2100. This is at the high end of the warming range suggested by many existing studies.</p>
<p>Our study ultimately concerns <a href="https://theconversation.com/explainer-what-is-climate-sensitivity-18815">“climate sensitivity”</a> - the amount the planet warms for a given amount of greenhouse gas. Think of it this way: to plan for a pizza party, you would start with an estimate of how much pizza an average person eats, and then multiply by the number of guests you plan to have. By analogy, global warming predictions begin by working out the climate sensitivity, and then scaling it to the amount of carbon dioxide and other warming gases we expect to emit. </p>
<h2>Uncertainty abounds</h2>
<p>The problem is that estimates of climate sensitivity have long ranged from 1.5-4.5C for a doubling of atmospheric carbon dioxide levels. Imagine not knowing whether the average guest will eat two or five slices of pizza. Determining this number more accurately is arguably the biggest question in climate science. </p>
<p>Unrestricted fossil fuel use could bring the carbon dioxide concentration to roughly three times pre-industrial levels by 2100. If climate sensitivity were near the 1.5C level, this would lead to 2-3C of further global warming by the end of the century. But if climate sensitivity were closer to 4.5C, we could expect 5-6C of warming by 2100. And by 2200, the warming in the high-sensitivity case could exceed 10C.</p>
<p>We could probably adapt to 2C of warming, although at the least it would be disruptive to delicate ecosystems like tropical coral reefs. But 6C would be catastrophic. Even 4C would probably be enough to displace tropical human populations, destroy natural ecosystems, force changes to agriculture and our way of life, and lead to eventual loss of the Greenland ice sheet and many metres of sea-level rise. So the amount matters.</p>
<h2>Cloud control</h2>
<p>This is where the clouds come in. Clouds, along with other factors such as sea ice, are affected by warming in ways that then go on to influence climate sensitivity. The question is whether they increase or decrease it. </p>
<p>In most calculations, warming reduces the amount of clouds near the ocean surface, increasing climate sensitivity because these clouds reflect less sunlight and thus allow more warming. This effect is much stronger in some models than others.</p>
<p>Our <a href="http://www.nature.com/nature/journal/v505/n7481/full/nature12829.html">study</a> breaks new ground in two ways. First, it identifies what is causing much of the discrepancy between different models, and why most of them show clouds near the surface thinning out as the climate warms. Second, it strongly discounts the lowest estimates of future warming. I am very happy about the first result, not so happy about the second one.</p>
<p>The key process we have identified is mixing of air between levels near the ocean surface where these clouds form, and levels a few kilometres higher up. If this mixing is strong, it tends to thin out the cloud layer as the atmosphere warms. </p>
<p>On the other hand, if the mixing is very weak, the water lost from low clouds is more than matched by increased ocean evaporation. Thus the clouds are thickened and warming is reduced.</p>
<p>By looking at the present-day atmosphere, we can therefore work out which models will offer the best prediction of the future. </p>
<p>We found that the mixing is strong in the real atmosphere, implying a high climate sensitivity. Of the 43 climate models we examined, every one with realistically strong mixing had sensitivity of more than 3C.</p>
<p>This narrows the expected range of warming to between 3C and 4.5C for a doubling of carbon dioxide. This is a momentous and sobering result, but how reliable is it?</p>
<h2>The big picture</h2>
<p>Some people have <a href="http://environmentalresearchweb.org/cws/article/news/53102">interpreted</a> the recent modest warming rates as evidence of lower climate sensitivity. It would be great if those lower values were definitive, but working them out requires us to understand all the influences on climate in recent decades. For instance, pollution particles called aerosols have changed greatly in recent decades, particularly as a result of Asia’s economic growth, with uncertain effects on climate. </p>
<p>In contrast, researchers looking at prehistoric warming episodes have often found evidence of <a href="http://www.realclimate.org/index.php/archives/2013/01/on-sensitivity-part-i/">higher sensitivity</a>.</p>
<p>As with any single study, our work does not settle the issue of climate sensitivity. But it does open a new door in understanding the role of clouds, and it raises the likelihood that sensitivity is high. If we are lucky, future research may find something that brings our best estimates of warming back down again. However, our new study shows that such a lucky finding would have to involve some process that is currently missing from every climate model - a much taller order than before.</p>
<p>Some media and blog reports have claimed our work shows that climate models are wrong. This misses the point. All models have flaws, but usually these flaws tell us nothing about whether — or in what direction — their future predictions might be off. What we have found is one particular flaw that systematically causes many (but not all) models to underestimate warming.</p>
<p>Our result is a sobering one, but in a logical world it would not alter policy all that much. After all, if you didn’t know whether the average guest would eat two or five pizza slices, you would probably order enough pizza to cover yourself in any case. Given the appalling stakes for being wrong on carbon dioxide, it is crazy not to take the same conservative approach. But so far, the world has evidently been crazy.</p>
<p>Perhaps our result can serve as a reminder that not knowing everything does not justify complacency. Uncertainty may mean the problem is worse than you thought.</p><img src="https://counter.theconversation.com/content/21617/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Steven Sherwood receives funding from the Australian Research Council, the Australian Department of the Environment, and the New South Wales Department of Environment and Climate Change.</span></em></p>The amount of global warming we can expect in the future has been a tough question to pin down. A new study that I led with colleagues in France has enabled us to come up with a more accurate analysis…Steven Sherwood, Director, Climate Change Research Centre, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/188152013-11-06T05:44:48Z2013-11-06T05:44:48ZExplainer: what is climate sensitivity?<figure><img src="https://images.theconversation.com/files/33422/original/gxqvqvxf-1382404923.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">What is the relationship between carbon dioxide and temperature rise?</span> <span class="attribution"><span class="source">Werner Wittersheim</span></span></figcaption></figure><p>Humans are emitting CO<sub>2</sub> and other greenhouse gases into the atmosphere. As these gases build up they trap extra heat and make the climate warmer. But how much warmer?</p>
<p>Scientists have long understood that CO<sub>2</sub> concentrations in the atmosphere contribute to the Earth’s natural greenhouse effect. This understanding pre-dates the 20th century, and was based on the fundamental science of radiation during the 1800s.</p>
<p>A quick study of the geological record confirms this link; past atmospheres with high CO<sub>2</sub> concentrations have always been warmer in the recent geological record.</p>
<p>As our understanding of the enhanced greenhouse effect, and a human influence on climate, has increased - it has become something of a scientific imperative to work out the relationship between carbon dioxide and warming. How much warmer does each unit of CO<sub>2</sub> make us? </p>
<p>This relationship between CO<sub>2</sub> and warming is known as the climate’s “sensitivity” to CO<sub>2</sub>.</p>
<p>Determining climate sensitivity helps us understand our future risks and plan for future climate change.</p>
<h2>Two types of sensitivity</h2>
<p>The climate system is extremely complicated, as is the way humans affect it. To understand the basic processes we simplify the system and consider two different measures of sensitivity.</p>
<p>These measures are called “transient climate response” and “equilibrium climate sensitivity”.</p>
<p>The “transient climate response” is a measure of how much warmer the climate will get when it is still being pushed by human activity.</p>
<p>The equilibrium response shows what the end result is hundreds of years down the track.</p>
<h2>Transient climate response</h2>
<p>The transient climate response is defined by how much the global mean temperature would rise if we were to elevate the CO<sub>2</sub> content of the atmosphere by 1% each year, compounded, from pre-industrial levels (of 280 parts per million) to double that value. At that rate of increase, this would take 70 years.</p>
<p>This measure of sensitivity tells us how much the atmosphere would warm under these circumstances.</p>
<p>It is important to note that the sensitivity to double CO<sub>2</sub> is not a climate change “projection”: scientists aren’t saying this doubling will actually happen in this way. Rather, it is a simplified technique used by scientists to understand how temperature <em>might</em> respond to rising CO<sub>2</sub> levels. </p>
<p>The transient climate response is therefore not directly comparable to the climate projections that the IPCC or other bodies have released.</p>
<p>One can think of sensitivity to double CO<sub>2</sub> as analogous to human sensitivity to a standard, measured dose of a particular drug. Once we know how we react to one dose, we can infer how we might react to more of the same. </p>
<h2>Equilibrium climate sensitivity</h2>
<p>The transient climate response only tells us part of the picture, the amount of warming essentially at the instant CO<sub>2</sub> concentrations have doubled.</p>
<p>However, different parts of the climate system warm at different rates; for example, the atmosphere warms much faster than the oceans.</p>
<p>The oceans average nearly four kilometres in depth and cover more than three-quarters of the Earth’s surface. That’s a colossal amount of water (1.3 billion cubic kilometres, by <a href="http://hypertextbook.com/facts/2001/SyedQadri.shtml">most estimates</a>); hence, it will take a very long time to warm up.</p>
<p>What climate models and the real world show us is that the deeper reaches of the oceans are only beginning to warm in response to elevated CO<sub>2</sub> (see <a href="http://www.nodc.noaa.gov/OC5/3M_HEAT_CONTENT/">here</a> and <a href="http://www.climatechange2013.org/images/uploads/WGIAR5_WGI-12Doc2b_FinalDraft_Chapter03.pdf">here</a>), but will continue to do so for hundreds of years.</p>
<p>In other words, the oceans are not in equilibrium with the atmosphere.</p>
<p>The transient climate response, then, is the warming that the planet experiences once CO<sub>2</sub> levels have doubled, but long before temperature equilibrium between the oceans and the atmosphere is attained.</p>
<p>The equilibrium climate sensitivity asks how much warming would occur if CO<sub>2</sub> remains at twice the initial level and we wait until the atmosphere and the ocean come into temperature balance. </p>
<p>Going back to our medical analogy, the equilibrium response is equivalent to how one should feel after the treatment has run its entire course.</p>
<p>In the climate system, this can take hundreds, if not thousands, of years.</p>
<p>The equilibrium climate sensitivity should be larger than the transient climate response because the warming occurs over a much longer period, and the climate has time to fully respond to the increased CO<sub>2</sub>.</p>
<h2>Estimating climate sensitivity</h2>
<p>There are several ways to estimate climate sensitivity. We can use:</p>
<ul>
<li><p><a href="https://theconversation.com/there-are-no-time-travelling-climatologists-why-we-use-climate-models-15347">climate models</a>. These are mathematical representations of the climate system, based on the laws of physics and chemistry. They contain our best understanding of the physical processes that operate in the atmosphere, oceans, land surface and cryosphere (snow and ice)</p></li>
<li><p>information from distant past climates, such as from the last ice age. Paleoclimate reconstructions can reveal how Earth’s climate reacted to natural “pushes”, such as changes in the planet’s <a href="https://theconversation.com/by-jove-can-climate-change-lead-us-to-life-on-other-planets-18722">orbit</a>, and associated changes in CO<sub>2</sub>. </p></li>
<li><p>data on how Earth responds to the short, sharp climate push it gets from <a href="https://theconversation.com/indonesias-samalas-volcano-may-have-kickstarted-the-little-ice-age-18772">large volcanoes</a>. For instance, in 1991 Mt Pinatubo spewed millions of tonnes of particles into the atmosphere, reflecting back some incoming sunlight and resulting in a two or three year dip in the Earth’s temperatures</p></li>
<li><p>recent climate trends. This helps us evaluate how much the climate has warmed over the last 50 years in response to the CO<sub>2</sub> we’ve already emitted.</p></li>
</ul>
<h2>Putting it all together</h2>
<p>Every six years or so the <a href="http://www.ipcc.ch/">Intergovernmental Panel on Climate Change (IPCC)</a> exhaustively examines all lines of evidence, and puts them together to give its best estimate of how sensitive our climate is to increasing levels of greenhouse gases in our atmosphere.</p>
<p>IPCC reports in 1990, 1992, 1995, 2001, and <a href="https://theconversation.com/topics/ipcc-fifth-assessment-report">the recent report in 2013</a>, all estimated a likely range of equilibrium climate sensitivity of 1.5C to 4.5C (although the 2007 report modified it slightly to 2.0C to 4.5C). This shows a remarkable consistency over more than 20 years.</p>
<p>This means if we kept CO<sub>2</sub> levels at twice initial levels, then waited the hundreds of years for heat in the atmosphere and ocean to balance, the overall warming would be between 1.5C and 4.5C.</p>
<p>The transient climate response is estimated at around 1.0C to 2.5C. This means if we increase atmospheric carbon dioxide from pre-industrial levels by 1% a year until it is doubled (which will take 70 years), we can expect the climate to warm by between 1C and 2.5C.</p>
<h2>What does this mean for global warming?</h2>
<p>Measures of climate sensitivity allow scientists to readily compare climate models with each other and with observed changes. And because equilibrium climate sensitivity has been in use for more than 30 years it can help us track how our estimates have changed over time.</p>
<p>But climate sensitivity does not directly tell us what temperature changes we might expect over the next century. </p>
<p>The best guide for future warming is provided by projections from the current generation of climate models. They indicate warming will depend strongly on the level of future emissions – on the “scenario”.</p>
<p>For a high emission scenario, CO<sub>2</sub> levels increase to more than 900 parts per million by 2100, compared with pre-industrial levels of 280 parts per million. For this scenario, models project end of century warming of 3.2C to 5.4C above an 1850 –1900 baseline. </p>
<p>For a low emission scenario, for which CO<sub>2</sub> levels peak then decline to 420 parts per million, models project warming of 0.9C to 2.3C by 2100.</p><img src="https://counter.theconversation.com/content/18815/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Rob Colman is a Principal Research Scientist at the Bureau of Meteorology which operates under the authority of the Meteorology Act 1955 and provides Australians with environmental intelligence for their safety, sustainability, well-being and prosperity. Rob receives some research funding from the Department of the Environment's Australian Climate Change Science Programme. He does not consult to, own shares in, or receive funding from any company or organisation that would benefit from this article. </span></em></p><p class="fine-print"><em><span>The Bureau of Meteorology provides Australians with environmental intelligence for their safety, sustainability, well-being and prosperity. Our weather, climate and water services include observations, alerts, warnings and forecasts for extreme events. Karl Braganza does not consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.
</span></em></p>Humans are emitting CO2 and other greenhouse gases into the atmosphere. As these gases build up they trap extra heat and make the climate warmer. But how much warmer? Scientists have long understood that…Robert Colman, Leader of the Climate Change Processes Team, Australian Bureau of MeteorologyKarl Braganza, Manager, Climate Monitoring Section, Australian Bureau of MeteorologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/183782013-09-24T00:49:57Z2013-09-24T00:49:57ZCherry pick all the facts you like, but Earth is still warming<figure><img src="https://images.theconversation.com/files/31806/original/qv73r3t6-1379970798.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">No evidence is enough evidence for some people.</span> <span class="attribution"><span class="source">Zak Hussein/PA</span></span></figcaption></figure><p>In the run-up to the publication of the <a href="http://www.climatechange2013.org/">Fifth assessment report</a> of the United Nations <a href="http://www.ipcc.ch/">Intergovernmental Panel on Climate Change</a> (IPCC) there is the usual flurry of activity among those with an interest in discrediting the substantial body of evidence demonstrating human influence on climate change.</p>
<p>Opinion pieces seem to generally follow the line: “the planet isn’t warming but if it is we are not the cause, and if we are the cause then it’s not important and may actually be beneficial …” (as <a href="http://www.skepticalscience.com/5-stages-climate-denial-on-display.html">neatly summarised</a> by the Skeptical Science blog).</p>
<p>Sure enough, the Mail on Sunday published several <a href="http://www.slate.com/blogs/bad_astronomy/2013/09/18/climate_change_denier_article_updated_still_riddled_with_errors.html">inaccurate articles</a> on climate change, which were then pounced upon and republished by various others such as the <a href="http://www.telegraph.co.uk/earth/environment/climatechange/10310712/Top-climate-scientists-admit-global-warming-forecasts-were-wrong.html">Telegraph</a> in the UK and the <a href="http://www.theaustralian.com.au/news/global-warming-forecasts-wrong-says-un-report/story-e6frg6n6-1226720435324">Australian</a>.</p>
<p>One of the most frequently misrepresented elements of climate science is the slowdown in global surface warming over the last 15 years. This fact, taken in isolation, is held as proof that human activity is not causing the climate to change. The <a href="http://www.sciencemediacentre.org/wp-content/uploads/2013/07/SMC-Briefing-Notes-Recent-Slowdown-in-Global-Temperature-Rise.pdf">latest science</a> shows this to be false.</p>
<p>Changes in the <a href="http://www.pnas.org/content/early/2011/06/27/1102467108">sun’s rays</a>, <a href="http://www.sciencemag.org/content/333/6044/866.abstract">volcanic activity</a>, <a href="http://www.nature.com/ngeo/journal/v6/n4/full/ngeo1740.html">particle air pollution</a> and <a href="http://www.sciencemag.org/content/327/5970/1219.abstract">water vapour in the stratosphere</a> may have contributed to the slowed rate of surface warming, reflecting some heat back into the atmosphere and so offsetting some of the heat caused by rising greenhouse gas concentrations.</p>
<p>However, since 2000, heating of the planet has in fact continued at a rate equivalent to more than 250 billion 1KW household electric heaters. This is based upon evidence from thousands of <a href="http://www.argo.ucsd.edu/How_Argo_floats.html">automated ocean buoys</a> that measure down to a depth of nearly 2km, combined with <a href="http://www.nature.com/ngeo/journal/v5/n2/abs/ngeo1375.html">global satellite data</a> measuring radiative energy entering and leaving the planet.</p>
<p><a href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12534.html">Recent research</a> indicates the current slowing in the rate of surface warming is primarily caused by <a href="http://www.met.reading.ac.uk/%7Esgs02rpa/latest.html#Kosaka">natural ocean variability</a> linked to <a href="http://en.wikipedia.org/wiki/La_Ni%C3%B1a">La Niña</a> conditions affecting the Pacific. </p>
<p>During the 1980s and 1990s, heating from greenhouse gases warmed the upper layers of the ocean, which affected global surface temperatures. In the 2000s, changes in ocean circulation have caused this additional heat energy to affect deeper layers beneath the sea surface as demonstrated using <a href="http://onlinelibrary.wiley.com/doi/10.1002/grl.50382/abstract">a combination of simulations and observations</a>. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/31652/original/qw7gmpx8-1379600426.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/31652/original/qw7gmpx8-1379600426.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/31652/original/qw7gmpx8-1379600426.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/31652/original/qw7gmpx8-1379600426.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/31652/original/qw7gmpx8-1379600426.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/31652/original/qw7gmpx8-1379600426.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/31652/original/qw7gmpx8-1379600426.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/31652/original/qw7gmpx8-1379600426.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Illustration of how heating from rising greenhouse gas concentrations has affected surface temperature over recent decades.</span>
<span class="attribution"><span class="source">Richard Allan/University of Reading</span></span>
</figcaption>
</figure>
<p>Warming has therefore not stopped and simulations which capture this <a href="http://www.nature.com/nclimate/journal/v1/n7/full/nclimate1229.html">natural variability</a> indicate that we can expect substantial rates of surface warming to return over the coming decades in response to rising greenhouse gases.</p>
<p>Another line taken by sceptics in the popular media is that climate sensitivity (the amount of global warming caused by a doubling of CO<sub>2</sub> concentrations) is low, in which case adding more greenhouse gas will not have a big effect on society. Previous IPCC reports have noted that the exact level of climate sensitivity <a href="http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch10s10-5.html#box-10-2">is uncertain</a>, ranging from around 1.5-4.5C. </p>
<p>Yet based upon <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI3819.1">observations, basic physics and detailed simulations</a> we know rising temperatures cause increases in water vapour and diminishing surface ice which strongly amplify climate change (known as positive feedback) and explain why the middle of this range is close to 3C. </p>
<p>While <a href="http://www.sciencemediacentre.co.nz/2013/05/20/climate-preditions-tightened-experts-respond/">recent studies</a> have led to discussion of whether climate sensitivity is likely to be at the lowest end of the range, such a value would require substantial negative feedbacks. For example, increased reflection of sunlight by clouds as temperatures rise which, while possible, has <a href="http://www.sciencemag.org/content/325/5939/460.abstract">not been observed</a>.</p>
<p>And anyway, arguing over a few tenths of a degree in climate sensitivity masks the real issue: the damaging climate change expected in response to continued emissions of greenhouse gases, which are currently in line with <a href="http://www.nature.com/nclimate/journal/v3/n1/full/nclimate1783.html">worst case emissions scenarios</a> used by the IPCC.</p>
<p>Other well-trodden <a href="http://skepticalscience.com/argument.php">climate sceptic</a> claims that resurface regularly in the media refer to the warmth of the middle ages compared to today, suggest that Antarctic and Arctic ice is increasing, that CO<sub>2</sub> levels have been higher than today’s, or that a rise in temperature increases CO<sub>2</sub> levels, rather than the other way around. </p>
<p>These claims often confuse weather with climate and mix up processes that play out over hundreds, thousands or even millions of years. Crucially they largely ignore the vast body of evidence pointing to human influence on current and future climate change. </p>
<p>Entrenched views and confirmation bias apply to journalists, the public and scientist alike. But it’s important to consider why an opinion is voiced so fervently. In the case of the Mail on Sunday a hint is offered in an <a href="http://www.dailymail.co.uk/debate/article-2420871/MAIL-ON-SUNDAY-COMMENT-These-climate-fanatics-cloud-debate.html">editorial piece</a> which uses an <a href="http://www.dailymail.co.uk/news/article-2420783/Global-warming-just-HALF-said-Worlds-climate-scientists-admit-computers-got-effects-greenhouse-gases-wrong.html">accompanying article</a> riddled with <a href="http://www.carbonbrief.org/blog/2013/09/scientists-take-the-mail-on-sunday-to-task-over-claim-that-warming-is-half-what-ipcc-expected/">inaccuracies</a> to reaffirm the paper’s apparent agenda against wind farms and in favour of fracking.</p>
<p>The climate is highly complex and there will always be debate over what is causing each lump and bump in the temperature record. The recent, much discussed slowdown in global surface warming is an example. The forthcoming IPCC report will be our best assessment of the science of climate change. </p>
<p>While it certainly cannot be considered perfect, it is built upon a vast body of evidence presented in the <a href="http://www.ipcc.ch/publications_and_data/ar4/wg1/en/contents.html">previous 2007 report</a>, and will further demonstrate a clear connection between human activity and global warming, one that is set to increase in the near future, with ensuing serious consequences.</p><img src="https://counter.theconversation.com/content/18378/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Richard Allan receives funding from the UK Natural Environment Research Council. He is affiliated with the National Centre for Atmospheric Sciences and National Centre for Earth Observation.</span></em></p>In the run-up to the publication of the Fifth assessment report of the United Nations Intergovernmental Panel on Climate Change (IPCC) there is the usual flurry of activity among those with an interest…Richard P. Allan, Professor of Climate Science, University of ReadingLicensed as Creative Commons – attribution, no derivatives.