Summary for Policymakers

Abstract

for Policymakers A.1.2Each of the last four decades has been successively warmer than any decade that preceded it since 1850.Global surface temperature 8 in the first two decades of the 21st century (2001-2020) was 0.99 [0.84 to 1.10] °C higher than 1850-1900.9 Global surface temperature was 1.09 [0.95 to 1.20] °C higher in 2011-2020 than 1850-1900, with larger increases over land (1.59 [1.34 to 1.83] °C) than over the ocean (0.88 [0.68 to 1.01] °C).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).Additionally, methodological advances and new datasets contributed approximately 0.1°C to the updated estimate of warming in AR6. 10 {2.3, Cross-Chapter Box 2.3} (Figure SPM.1) A.1.3The likely range of total human-caused global surface temperature increase from 1850-1900 to 2010-2019 11 is 0.8°C to 1.3°C, with a best estimate of 1.07°C.It is likely that well-mixed GHGs contributed a warming of 1.0°C to 2.0°C, other human drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C, natural drivers changed global surface temperature by -0.1°C to +0.1°C, and internal variability changed it by -0.2°C to +0.2°C.It is very likely that well-mixed GHGs were the main driver 12 of tropospheric warming since 1979 and extremely likely that human-caused stratospheric ozone depletion was the main driver of cooling of the lower stratosphere between 1979 and the mid-1990s.{3.3, 6.4, 7.3, TS.2.3, Cross-Section Box TS.1} (Figure SPM.2) A.1.4Globally averaged precipitation over land has likely increased since 1950, with a faster rate of increase since the 1980s (medium confidence).It is likely that human influence contributed to the pattern of observed precipitation changes since the mid-20th century and extremely likely that human influence contributed to the pattern of observed changes in near-surface ocean salinity.Mid-latitude storm tracks have likely shifted poleward in both hemispheres since the 1980s, with marked seasonality in trends (medium confidence).For the Southern Hemisphere, human influence very likely contributed to the poleward shift of the closely related extratropical jet in austral summer.{2.3, 3.3, 8.3, 9.2, TS.2.3, TS.2.4,Box TS.6} A.1.5 Human influence is very likely the main driver of the global retreat of glaciers since the 1990s and the decrease in Arctic sea ice area between 1979-1988 and 2010-2019 (decreases of about 40% in September and about 10% in March).There has been no significant trend in Antarctic sea ice area from 1979 to 2020 due to regionally opposing trends and large internal variability.Human influence very likely contributed to the decrease in Northern Hemisphere spring snow cover since 1950.It is very likely that human influence has contributed to the observed surface melting of the Greenland Ice Sheet over the past two decades, but there is only limited evidence, with medium agreement, of human influence on the Antarctic Ice Sheet mass loss.{2.3, 3.4, 8.3, 9.3, 9.5, TS.2.5} A.1.6It is virtually certain that the global upper ocean (0-700 m) has warmed since the 1970s and extremely likely that human influence is the main driver.It is virtually certain that human-caused CO 2 emissions are the main driver of current global acidification of the surface open ocean.There is high confidence that oxygen levels have dropped in many upper ocean regions since the mid-20th century and medium confidence that human influence contributed to this drop.{2.3, 3.5, 3.6, 5.3, 9.2, TS.2.4} A.1.7 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 (high confidence).Human influence was very likely the main driver of these increases since at least 1971.{2.3, 3.5, 9.6, Cross-Chapter Box 9.1, Box TS.4} 8 The term 'global surface temperature' is used in reference to both global mean surface temperature and global surface air temperature throughout this SPM.Changes in these quantities are assessed with high confidence to differ by at most 10% from one another, but conflicting lines of evidence lead to low confidence in the sign (direction) of any difference in long-term trend.{Cross-Section Box TS.1} 9The period 1850-1900 represents the earliest period of sufficiently globally complete observations to estimate global surface temperature and, consistent with AR5 and SR1.5, is used as an approximation for pre-industrial conditions.10 Since AR5, methodological advances and new datasets have provided a more complete spatial representation of changes in surface temperature, including in the Arctic.These and other improvements have also increased the estimate of global surface temperature change by approximately 0.1°C, but this increase does not represent additional physica