The Intergovernmental Panel on Climate Change (IPCC) published a new report this week that presents the latest understanding on the physical science basis of climate change. The report summarises the views and consensus of the world’s leading climate scientists on the physical science basis for future climate change, highlighting how human activity is changing the climate in unprecedented and sometimes irreversible ways. This article provides an overview of the reports key findings.
The new scientific report (“Climate Change 2021: The physical Science Basis”) addresses the most up-to-date physical understanding of the climate system and climate change, bringing together the latest advances in climate science, and combining multiple lines of evidence from paleoclimate, observations, process understanding, and global and regional climate simulations.
The report is presented in several parts, including a technical summary and a summary for policymakers, in addition to full scientific report. It outlines the key findings of the Working Group 1 contribution to the IPCC’s Sixth Assessment Report (AR6) on the physical science basis of climate change, which builds upon previous reports undertaken by the IPCC and incorporates new scientific methods and analysis across four key areas:
1) The current state of climate change
When talking about the Current State of the Climate, the IPCC highlight that “It is unequivocal that human influence has warmed the atmosphere, ocean and land”. Greenhouse gases (GHG), namely carbon dioxide (CO2), methane (CH4) and nitrous dioxide (N2O), have continued to rise since the previous report in 2013 and the rates of these gases are now higher than in any other period in tat least the last 800,000 years.
Historical changes in the climate have shown that global temperature is closely linked to levels of greenhouse gases in the atmosphere, with increased levels of greenhouse gases being followed by increases in temperature, and vice versa. These temperature changes have impacts on the hydrological cycle, influencing rainfall patterns globally.
The IPCC are more confident that heatwaves and heavy rainfall have both become more frequent across most land regions. Glaciers, sea ice and snow cover have all reduced because of climate change and sea level has already risen by 20cm across the globe. Furthermore, climate zones have moved towards the poles in both hemispheres and growing seasons have increased globally by two days on average since the 1950s.
2)Possible Climate Futures
The IPCC project that global surface temperatures will continue to increase until at least 2050 under every modelling scenario they considered for possible climate futures, with warming of 1.5oC being exceeded during the next 100 years unless there are large reductions in GHG emissions.
As the world gets warmer, the changes in the climate are expected to become larger. Global warming is expected to intensify the global water cycle, with more variability and more severe wet and dry events occurring. Furthermore, as the world gets warmer, the ocean and land carbon sinks will become less effective at slowing the rate that carbon dioxide builds up in the atmosphere.
Figure 1: Climate model simulated changes in mean temperature (oC) and mean precipitation (%) at global warming levels of 1.5oC, 2oC and 4oC.These panels were taken from Figure 5 in the Summary for Policymakers part of the report.
3) Climate Information for Risk Assessment and Regional Adaptation
Regarding Climate Information for Risk Assessment and Regional Adaptation, all regions are expected to experience climate change.
Changes in several climatic impact-drivers would be more widespread at 2°C compared to 1.5°C global warming and even more widespread and/or pronounced for higher warming levels, and the study warns of increasingly extreme heatwaves, droughts and flooding, with more frequent and/or severe agricultural and ecological droughts projected.
Low likelihood(but high impact) outcomes, such as ice sheet collapse, abrupt ocean circulation changes, and forest dieback cannot be ruled out and could occur at global and regional scales.
4) Limiting Future Climate change
Since the report in 2013, scientists have improved understanding and estimates of carbon budget to help to limit future climate change. This means limiting greenhouse gas emissions, with an ambition to reach Net Zero. If carbon dioxide emissions become net negative – more are taken up by natural processes than are produced by humans – the IPCC say that global surface temperature increases could be reversed, but other climate changes may still be locked-in.
Whilst the focus will be on supporting natural systems to sequester and store carbon from the atmosphere, the IPCC recognise that technological processes will also play an important part in helping lower atmospheric carbon dioxide concentrations and reverse surface ocean acidification, such as through carbon capture and storage; removing carbon from the atmosphere and storing it underground.
The IPCC also advise that strong reductions in methane emissions will be critical to limit warming and improve air quality. Methane has a much greater climate impact than carbon dioxide, but has a shorter life-span in the atmosphere, meaning that reductions made in methane emissions can have a much larger an quicker impact on reducing atmospheric warming. Agriculture and in particular livestock systems will therefore play an important role in meeting this requirement.
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