Following up on previous assessment reports, the United Nations Intergovernmental Panel on Climate Change (IPCC) issued an updated assessment report on August 9, 2021. The Report carries dire warnings of the impacts of man-made activities unless immediate steps are taken.
In the summary for policymakers provided in the Report, the authors make a number of significant findings, regarding the increases in green house gas (GHG), sea level rise, increased temperatures and other impacts of climate change, much of which they find was either likely caused or contributed to by man.
“This SPM provides a high–level summary of the understanding of the current state of the climate, including how it is changing and the role of human influence, the state of knowledge about possible climate futures, climate information relevant to regions and sectors, and limiting human–induced
climate change.…”
Among the findings are the following:
“A.1 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. …
A.1.1 Observed increases in well–mixed greenhouse gas (GHG) concentrations since around 1750 are unequivocally caused by human activities. Since 2011 (measurements reported in AR5), concentrations have continued to increase in the atmosphere, reaching annual averages of 410 ppm for carbon dioxide (CO2),
1866 ppb for methane (CH4), and 332 ppb for nitrous oxide (N2O) in 20196. Land and ocean have taken up a near–constant proportion (globally about 56% per year) of CO2 emissions from human activities over the past six decades, with regional differences (high confidence)7. {2.2, 5.2, 7.3, TS.2.2, Box TS.5}…
A.1.2 Each of the last four decades has been successively warmer than any decade that preceded it since 1850. Global surface temperature8 in the first two decades of the 21st century (2001–2020) was 0.99 [0.84–1.10] °C higher than 1850–19009. 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 0.1ºC to the updated estimate of warming in AR610. …
A.1.3 The likely range of total human–caused global surface temperature increase from 1850–1900 to 2010–201911 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 driver12 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. …
A.1.4 Globally 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. ..
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 (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. …
A.1.6 It 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 CO2 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. …
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. …
A.1.8 Changes in the land biosphere since 1970 are consistent with global warming: climate zones have shifted poleward in both hemispheres, and the growing season has on average lengthened by up to two days per decade since the 1950s in the Northern Hemisphere extratropics (high confidence). …”
The Report concludes that, among other things:
“C-2 With further global warming, every region is projected to increasingly experience concurrent and multiple changes in climatic impact–drivers. 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.
C.3 Low–likelihood outcomes, such as ice sheet collapse, abrupt ocean circulation changes, some compound extreme events and warming substantially larger than the assessed very likely range of future warming cannot be ruled out and are part of risk assessment.
D. Limiting Future Climate Change
D.1 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. Strong, rapid and sustained reductions in CH4 emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality.