Climate Change 2021 – the physical science basis

This blog post is a short summary of key points that are of current relevance to society from the physical science of climate change. It is based on the headline statements of a report published in 2021 by the Intergovernmental Panel on Climate Change (IPCC). The IPCC is a United Nations body responsible for providing impartial assessments of climate science. Its reports inform international negotiations on tackling climate change.

Written by Jonathan Gregory, Matt Palmer and Ed Hawkins
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What would I say to COP26?

The recent 6th Assessment Report of Working Group I of the Intergovernmental Panel on Climate Change (IPCC) is the result of years of diligent assessment of the latest scientific evidence on climate change building on several previous similar assessments. It was written by hundreds of authors, and openly reviewed by thousands of experts. Every word of the Summary for Policymakers was agreed by all 195 parties to the UNFCCC and at COP26.

The report is clear that the world has warmed by around 1.1°C since the industrial revolution, and that this is due to human activities, primarily burning fossil fuels. As a direct consequence, heatwaves and heavy rainfall events have become more frequent and more intense. As the oceans have warmed and the ice sheets have melted, sea levels have risen, increasing the risk of coastal flooding. Our past greenhouse gas emissions have caused changes of the climate which have harmed both human society and ecosystems.

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Projected longer dry spells under climate change occur during dry seasons

The latest Intergovernmental Panel on Climate Change (IPCC) report states that the global water cycle will intensify with continued global warming. This means fewer rainy days, but with more intense rain over many land regions, and more variability generally. More dry days and longer dry spells have the potential to lead to negative impacts on crop yields and food security, as reductions in water availability limit crop growth. The impacts on crops also depend on the timing of these longer dry spells in the annual cycle and future delays in the wet season are also reported in the IPCC report and by my previous research.

Guest post by Caroline Wainwright
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Climate indicators

The recent IPCC AR6 WGI report summarises the state of knowledge of physical climate science, but the final version of the Summary for Policymakers (SPM) did not include a figure showing a range of indicators of our warming planet.

An earlier draft of the SPM included a figure like that below which aimed to put recent changes into a longer context of changes over the past 2000 years, and to show how other climate metrics have changed in recent decades. Many of these time series were shown in disparate places of the report, and have been brought together in this updated graphic which also indicates key milestones and discoveries in climate science.

Indicators (from top to bottom): atmospheric carbon dioxide concentration, ocean heat content, global sea level, global mean surface temperature, global lower tropospheric temperatures, Arctic sea ice amount, Kyoto cherry blossom date, specific humidity over land. Key moments in the history of climate science are indicated: the invention of the efficient steam engine by James Watt in 1790, the identification of the primary greenhouse gases by John Tyndall in 1861, the first estimate of climate sensitivity by Svante Arrhenius in 1896, and the discovery that the world was warming by Guy Callendar in 1938.

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Adding observations to IPCC figures

The figures in the IPCC AR6 WGI SPM are a huge improvement over previous reports. However, one minor quibble is with the lack of observations shown. This brief post makes a figure available which is based on IPCC AR6 WGI SPM Figure 8, but with some observations added to show how global surface temperature and Arctic sea ice area have varied, compared to the model simulations. In my view this is a scientific improvement over the original version.
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IPCC SPM Figures

The Working Group I (WGI) component of the IPCC 6th Assessment Report (AR6) has been released. One key development since AR5 was the involvement of professional graphic designers in creating the figures for the Summary for Policymakers (SPM). As a result, the graphics are clear and usable, having been user-tested through several design iterations. The data underlying the figures are also openly available.
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The value of ‘data rescue’ for understanding record extremes

Record extreme temperature events are increasing in frequency as the climate warms. Several of these records have been surprising, in that they have been far above the previous record event for that location. Longer and earlier records, often possible through data recovery from undigitised archive material, would help quantify the risks of such record extreme temperature events. Continue reading The value of ‘data rescue’ for understanding record extremes