Climate Change 2021: The Physical Science Basis. Summary For Policymakers

Generated Summary

This document is the Summary for Policymakers (SPM) from the IPCC’s Sixth Assessment Report (AR6) on the physical science basis of climate change. It synthesizes key findings from Working Group I (WGI), building upon the AR5 and the 2018–2019 IPCC Special Reports. The SPM assesses the current state of the climate, human influence, climate futures, and climate information for regions and sectors, aiming to inform policy decisions. The report uses scientific understanding to formulate key findings as statements of fact or with assessed confidence levels. The assessment covers scientific literature accepted for publication by 31 January 2021. The document presents key findings regarding observed and projected climate changes, emphasizing the role of human activities. It examines changes in temperature, precipitation, sea level, and extreme weather events across various regions and timeframes. The report also addresses the relationship between cumulative CO2 emissions and global warming, discussing scenarios for future climate change based on different emission pathways. The analysis utilizes climate models (CMIP6) and observational data to provide insights into climate processes, the impacts of human activities, and the potential for limiting future climate change.

Key Findings & Statistics

• In 2019, atmospheric CO2 concentrations reached annual averages of 410 ppm, methane (CH4) at 1866 ppb, and nitrous oxide (N2O) at 332 ppb.
• Global surface temperature in 2001-2020 was 0.99 [0.84-1.10] °C higher than in 1850-1900.
• Global surface temperature was 1.09 [0.95 to 1.20] °C higher in 2011–2020 than 1850–1900.
• The estimated increase in global surface temperature since AR5 is principally due to further warming since 2003–2012 (+0.19 [0.16 to 0.22] °C).
• The likely range of total human-caused global surface temperature increase from 1850–1900 to 2010-2019 is 0.8°C to 1.3°C, with a best estimate of 1.07°C.
• Global mean sea level increased by 0.20 [0.15 to 0.25] m between 1901 and 2018. The average rate of sea level rise was 1.3 [0.6 to 2.1] mm yr¯¹ between 1901 and 1971, increasing to 1.9 [0.8 to 2.9] mm yr¯¹ between 1971 and 2006, and further increasing to 3.7 [3.2 to 4.2] mm yr¯¹ between 2006 and 2018.
• In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years (high confidence).
• Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years (high confidence).
• Temperatures during the most recent decade (2011–2020) exceed those of the most recent multi-century warm period, around 6500 years ago [0.2°C to 1°C relative to 1850–1900] (medium confidence).
• The equilibrium climate sensitivity is between 2°C (high confidence) and 5°C (medium confidence). The AR6 assessed best estimate is 3°C with a likely range of 2.5°C to 4°C (high confidence).
• Under the very low GHG emissions scenario (SSP1-1.9), global surface temperature averaged over 2081–2100 is very likely to be higher by 1.0°C to 1.8°C compared to 1850–1900.
• Under the intermediate scenario (SSP2-4.5), global surface temperature averaged over 2081–2100 is very likely to be higher by 2.1°C to 3.5°C compared to 1850–1900.
• Under the very high GHG emissions scenario (SSP5-8.5), global surface temperature averaged over 2081–2100 is very likely to be higher by 3.3°C to 5.7°C compared to 1850–1900.
• Global warming of 1.5°C relative to 1850-1900 would be exceeded during the 21st century under the intermediate, high and very high scenarios considered in this report (SSP2-4.5, SSP3-7.0 and SSP5-8.5, respectively).
• The best estimate for observed warming in 2010–2019 relative to 1850–1900 (see Figure SPM.2, panel a) is indicated in the darker column in the total bar.
• The range of equilibrium climate sensitivity is between 2°C (high confidence) and 5°C (medium confidence). The AR6 assessed best estimate is 3°C with a likely range of 2.5°C to 4°C (high confidence).
• Each 1000 GtCO2 of cumulative CO2 emissions is assessed to likely cause a 0.27°C to 0.63°C increase in global surface temperature with a best estimate of 0.45°C.

Other Important Findings

• It is unequivocal that human influence has warmed the atmosphere, ocean, and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere, and biosphere have occurred.
• Human influence has warmed the climate at a rate that is unprecedented in at least the last 2000 years.
• Human-induced climate change is already affecting many weather and climate extremes in every region across the globe.
• Projected changes in extremes are larger in frequency and intensity with every additional increment of global warming.
• Many changes in the climate system become larger in direct relation to increasing global warming. They include increases in the frequency and intensity of hot extremes, marine heatwaves, and heavy precipitation, agricultural and ecological droughts in some regions, and proportion of intense tropical cyclones, as well as reductions in Arctic sea ice, snow cover and permafrost.
• The scale of recent changes across the climate system as a whole and the present state of many aspects of the climate system are unprecedented over many centuries to many thousands of years.
• Under scenarios with increasing CO2 emissions, the ocean and land carbon sinks are projected to be less effective at slowing the accumulation of CO2 in the atmosphere.
• If global warming exceeds the assessed very likely range for a given GHG emissions scenario, global and regional changes in many aspects of the climate system, such as regional precipitation and other CIDs, would also exceed their assessed very likely ranges (high confidence).
• Scenarios with very low or low GHG emissions (SSP1-1.9 and SSP1-2.6) lead within years to discernible effects on greenhouse gas and aerosol concentrations, and air quality, relative to high and very high GHG emissions scenarios (SSP3-7.0 or SSP5-8.5).
• From a physical science perspective, 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.
• Scenarios with very low or low GHG emissions (SSP1-1.9 and SSP1-2.6) would lead to substantially smaller changes in a range of CIDs beyond 2040 than under high and very high GHG emissions scenarios (SSP3-7.0 and SSP5-8.5).

Limitations Noted in the Document

• The assessment covers scientific literature accepted for publication by 31 January 2021, which may not include the most recent findings.
• There is uncertainty associated with internal climate variability, which can modulate human-caused changes, particularly at regional scales and in the near term.
• Projections of climate change rely on climate models, which have inherent uncertainties. Some CMIP6 models simulate a warming that is either above or below the assessed very likely range of observed warming.
• The assessment acknowledges low-likelihood, high-impact outcomes related to ice sheet instability processes, characterized by deep uncertainty.
• Data limitations inhibit clear detection of past trends on the global scale for some extreme events, such as the frequency of all-category tropical cyclones.
• The report acknowledges the inherent uncertainty in the assessment of climate sensitivity.
• The role of CDR methods on biogeochemical cycles and climate have wide-ranging effects, which can either weaken or strengthen the potential to remove CO2.
• The assessment relies on the illustrative scenarios (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5), which do not cover all possible scenarios.

Conclusion

The IPCC AR6 WGI report delivers a comprehensive assessment of the current state of the climate, emphasizing the undeniable impact of human activities. The findings underscore that human influence has led to unprecedented warming of the atmosphere, ocean, and land, resulting in widespread and rapid changes across the climate system. A key takeaway is the near-linear relationship between cumulative CO2 emissions and global warming, underscoring the necessity of limiting cumulative CO2 emissions to meet specific warming targets. The report highlights the increasing frequency and intensity of extreme weather events, such as heatwaves, heavy precipitation, and droughts, with these changes becoming more pronounced with each increment of global warming. The findings also emphasize that many of these changes are irreversible on timescales ranging from centuries to millennia. Furthermore, the report provides detailed regional assessments, revealing the diverse ways in which climate change is affecting different parts of the world. The analysis of future climate scenarios underscores the importance of deep and sustained reductions in greenhouse gas emissions to limit global warming and its associated impacts. The projected changes in the climate system require urgent action. The report emphasizes that achieving net-zero CO2 emissions is a crucial step in stabilizing global temperatures, but the scale and nature of these future changes depend on the extent of emissions reductions achieved. The information provided in the SPM is designed to inform decision-makers about the risks of climate change and the urgency of taking actions to reduce the impacts of human activities on the climate system, as well as potential pathways to mitigate these impacts. The report’s findings are a call for significant and immediate action to address climate change, emphasizing the importance of a global response to limit future warming and its associated impacts.