IPCC - Climate Change 2021: The Physical Science Basis (2021)

The latest report from the IPCC Working Group 1 is the most recent contribution to the IPCC’s sixth assessment report cycle due to be completed in 2022. AR6 Climate Change 2021: The Physical Science Basis analyzes our current scientific understanding of the climate system and change. It evaluates and summarizes the latest advances in climate science using evidence from paleoclimate data, physical observations, and global and regional climate simulations. The report begins by describing the current state of global climate as it relates to human activities. It then analyzes various high to low emissions scenarios and how these may impact our global climate systems in the future. Much of the report’s focus is on changes to precipitation patterns, glaciers, arctic sea ice, ocean warming and acidification, sea-level rise, and surface temperatures around the world. The report concludes by providing recommendations for risk assessment, regional adaptation, and how we can limit future climate change.


I. The Current State of the Climate


It is certain that human activities have warmed the atmosphere, ocean, and land in unprecedented ways that have led to widespread and rapid changes such as increased precipitation patterns, the global retreat of glaciers and arctic sea ice, ocean warming, and acidification, sea-level rise, and shifting climate zones. Since 1850, each of the last four decades has been warmer than the preceding decade. Global surface temperatures have already reached about 1°C of warming above pre industrial (1850) levels. Concentrations of CO2 are currently higher than any time period in the last 2 million years, and methane (CH4) and nitrous oxide (N20) concentrations are higher than any time in the last 800,000 years. Similar patterns are seen for annual average Arctic Sea ice surface area and global mean sea level rise since 1850-1900. The loss of arctic sea ice on land, combined with global warming and thermal expansion, is responsible for the majority of sea-level rise from 1971 to 2018. It is virtually certain that hot extremes are more frequent and intense, while cold-climate extremes are less frequent and less severe. The frequency and intensity of heavy precipitation events and agricultural and ecological droughts have increased since the 1950s. Scientists have unequivocally found that human influence has contributed to all of these on a global scale and that these impacts can be seen in every inhabited region on the globe.


II. Possible Climate Futures


Global surface temperatures are predicted to increase until at least mid-century under all emissions scenarios considered. Specifically, global warming of 1.5°C will be exceeded during the 21st century unless significant reductions in CO2 and other greenhouse gas emissions occur in the coming decades. With every additional degree of global warming, changes in climate extremes will become larger. For example, every additional 0.5°C of global warming will increase the intensity and frequency of hot extremes like heat waves and heavy precipitation, as well as agricultural and ecological droughts in some areas. The impacts of global warming will be seen and experienced differently in different parts of the world. Mid-Latitude, Semi-Arid Regions, and the South American Monsoon regions will see the highest increase in temperature at 1.5 to 2 times the rate of average global warming. The Arctic will experience temperature increases at 3 times the rate of average global warming. The Arctic is also likely to be practically sea ice-free in September at least once before 2050 under all emission scenarios considered. A warmer climate will intensify very wet and very dry climate events, with implications for flooding and drought, but the location and frequency of these events will depend on projected changes in each region. Additionally, the amount and intensity of category 4-5 tropical cyclones will increase around the world with continued global warming. With more CO2 emissions in the atmosphere, ocean and land carbon sinks are predicted to be less effective at slowing the accumulation of greenhouse gases. This means that carbon sicks will store less CO2 and at slower rates, leading to more CO2 in the atmosphere. Therefore, more CO2  emissions will stay in the atmosphere for longer periods of time. Many of these changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets, and global sea level.


III. Climate Information for Risk Assessment and Regional Adaptation


With continued global warming, every region is projected to increasingly experience concurrent and multiple changes in climatic impact drivers. Changes in several climatic impact drivers will be more widespread at 2°C of global warming compared to 1.5°C, and even more widespread for higher warming levels. At 1.5 degrees Celsius of warming, heavy precipitation and flooding will intensify and be more frequent in every continent, more frequent and/or severe agricultural and ecological droughts in all continents except Asia will be seen, meteorological droughts are predicted in some regions, as well as changes in mean precipitation in a small number of regions. At 2°C of global warming and above, the level of confidence in these predictions and the magnitude of these changes increase compared to those at 1.5°C. Many regions will experience an increase in the probability of compound events with higher global warming such as heatwaves and droughts happening at the same time. Extreme weather events that were low-likelihood in the past will likely become more frequent, while high-likelihood events will have longer durations and be more intense.


IV. Limiting Future Climate Change


Limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions. Strong, rapid, and sustained reductions in methane gas emissions would also limit the warming effect resulting from aerosol pollution and would improve air quality. Reaching net-zero human-caused CO2 emissions is needed to stabilize human-induced global warming at any level. If achieved and sustained, surface temperature increases will gradually reverse but other climate changes would continue in their current direction for decades to come. The removal of anthropogenic CO2 emissions from the atmosphere is also a potential solution to the climate crisis. This tool could help us to reach net-zero greenhouse gas emissions or lower surface temperatures in certain regions of the globe, lowering the amount of greenhouse gases in the atmosphere and reversing surface ocean acidification. However, these methods can have various effects on biogeochemical cycles, climate, water availability and quality, food production, and biodiversity that will be important to consider.


The A46 Climate Change 2021: The Physical Science Basis report clearly outlines that humans have contributed to climate changes we currently experience and predict to experience in the future, globally. Although many such changes are inevitable and will continue to progress for years to come, the report provides very clear steps on how we can address and mitigate these impacts around the world.


1. Intergovernmental Panel on Climate Change, Working Group One. A46 Climate Change 2021: The Physical Science Basis, Summary for Policymakers. IPCC [Internet]. 2021 Aug 9  [cited 2021 Aug 11]. Available from: https://www.ipcc.ch/report/ar6/wg1/#SPM