Editing IPCC:AR6/WGI/Chapter-3 (section)

=== Human Influence on the Atmosphere and Surface ===

<div id="h2-2-siblings" class="h2-siblings"></div>

'''The''' likely '''range of human-induced warming in glo''' '''bal-me''' '''an surface air temperature (GSAT) in 201''' '''0''' '''–2019 relative to''' '''185''' '''0''' '''–19''' '''00 is 0.8°C–1.3°C, encompassing the observed warming of 0.9°C–1.2°C, while the change attributable to natural forcings''' '''is only −0.1°C to +0.1°C.''' The best estimate of human-induced warming is 1.07°C. Warming can now be attributed since 1850–1900, instead of since 1951 as done in AR5, thanks to a better understanding of uncertainties and because observed warming is larger. The ''likely'' ranges for human-induced GSAT and global mean surface temperature (GMST) warming are equal ( ''medium confidence'' ). Attributing observed warming to specific anthropogenic forcings remains more uncertain. Over the same period, forcing from greenhouse gases <sup>[[#footnote-002|1]]</sup> ''likely'' increased GSAT by 1.0°C–2.0°C, while other anthropogenic forcings including aerosols ''likely'' decreased GSAT by 0.0°C–0.8°C. It is ''very likely'' that human-induced greenhouse gas increases were the main driver <sup>[[#footnote-001|2]]</sup> of tropospheric warming since comprehensive satellite observations started in 1979, and ''extremely likely'' that human-induced stratospheric ozone depletion was the main driver of cooling in the lower stratosphere between 1979 and the mid-1990s. {3.3.1}

'''The CMIP6 model ensemble reproduces the observed historical global surface temperature trend and variability''' '''with biases small enough to support detection and attribution of hu''' '''man-in''' '''duced warming''' ( very high confidence ''').''' The CMIP6 historical simulations assessed in this report have an ensemble mean global surface temperature change within 0.2°C of the observations over most of the historical period, and observed warming is within the 5–95% range of the CMIP6 ensemble. However, some CMIP6 models simulate a warming that is either above or below the assessed 5–95% range of observed warming. CMIP6 models broadly reproduce surface temperature variations over the past millennium, including the cooling that follows periods of intense volcanism ( ''medium confidence'' ). For upper air temperature, there is ''medium confidence'' that most CMIP5 and CMIP6 models overestimate observed warming in the upper tropical troposphere by at least 0.1°C per decade over the period 1979 to 2014. The latest updates to satellite-derived estimates of stratospheric temperature have resulted in decreased differences between simulated and observed changes of global mean temperature through the depth of the stratosphere ( ''medium confidence'' ). {3.3.1}

'''The slower rate of GMST increase observed over 1998–2012 compared to 1951–2012 was a temporary event followed by a strong GMST increase''' ( very high confidence ''').''' Improved observational datasets since AR5 show a larger GMST trend over 1998–2012 than earlier estimates. All the observed estimates of the 1998–2012 GMST trend lie within the 10th–90th percentile range of CMIP6 simulated trends ( ''high confidence'' ). Internal variability, particularly Pacific Decadal Variability, and variations in solar and volcanic forcings partly offset the anthropogenic surface warming trend over the 1998–2012 period ( ''high confidence'' ). Global ocean heat content continued to increase throughout this period, indicating continuous warming of the entire climate system ( ''very high confidence'' ). Since 2012, GMST has warmed strongly, with the past five years (2016–2020) being the warmest five-year period in the instrumental record since at least 1850 ( ''high confidence'' ). {Cross-Chapter Box 3.1, 3.3.1; 3.5.1}

'''It is''' likely '''that human influence has contributed to''' <sup>[[#footnote-000|3]]</sup> '''moistening''' '''in the upper troposphere since 1979.''' Also, there ''is medium confidence'' that human influence contributed to a global increase in annual surface specific humidity, and ''medium confidence'' that it contributed to a decrease in surface relative humidity over mid-latitude Northern Hemisphere continents during summertime. {3.3.2}

'''It is''' likely '''that human influence has contributed to observed large-scale precipitation changes since the mid-20th century.''' New attribution studies strengthen previous findings of a detectable increase in Northern Hemisphere mid- to high-latitude land precipitation ( ''high confidence'' ). Human influence has contributed to strengthening the zonal mean precipitation contrast between the wet tropics and dry subtropics ( ''medium confidence'' ). Yet, anthropogenic aerosols contributed to decreasing global land summer monsoon precipitation from the 1950s to 1980s ( ''medium confidence'' ). There is also ''medium confidence'' that human influence has contributed to high-latitude increases and mid-latitude decreases in Southern Hemisphere summertime precipitation since 1979 associated with the trend of the Southern Annular Mode toward its positive phase. Despite improvements, models still have deficiencies in simulating precipitation patterns, particularly over the tropical ocean ( ''high confidence'' ). {3.3.2, 3.3.3, 3.5.2}

'''Human-induced greenhouse gas forcing is the main driver of the observed changes in hot and cold extremes on the global scale''' ( virtually certain ''') and on most continents''' ( very likely ''').''' It is ''likely'' that human influence, in particular due to greenhouse gas forcing, is the main driver of the observed intensification of heavy precipitation in global land regions during recent decades. There is ''high confidence'' in the ability of models to capture the large-scale spatial distribution of precipitation extremes over land. The magnitude and frequency of extreme precipitation simulated by CMIP6 models are similar to those simulated by CMIP5 models ( ''high confidence'' ). {Cross-Chapter Box 3.2}

'''It is''' likely '''that human influence has contributed to the poleward expansion of the zonal mean Hadley cell in the Southern Hemisphere since the 1980s.''' There is ''medium confidence'' that the observed poleward expansion of the zonal mean Hadley cell in the Northern Hemisphere is within the range of internal variability. The causes of the observed strengthening of the Pacific Walker circulation since the 1980s are not well understood, and the observed strengthening trend is outside the range of trends simulated in the coupled models ( ''medium confidence'' ). While CMIP6 models capture the general characteristics of the tropospheric large-scale circulation ( ''high confidence'' ), systematic biases exist in the mean frequency of atmospheric blocking events, especially in the Euro-Atlantic sector, some of which reduce with increasing model resolution ( ''medium confidence'' ). {3.3.3}

<span id="human-influence-on-the-cryosphere"></span>