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

== Executive Summary ==

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The evidence for human influence on recent climate change strengthened from the IPCC Second Assessment Report to the IPCC Fifth Assessment Report, and is now even stronger in this assessment. The IPCC Second Assessment Report (SAR, 1995) concluded ‘the balance of evidence suggests that there is a discernible human influence on global climate’. In subsequent assessments (TAR, 2001; AR4, 2007; and AR5, 2013), the evidence for human influence on the climate system was found to have progressively strengthened. The AR5 concluded that human influence on the climate system is clear, evident from increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and physical understanding of the climate system. This chapter updates the assessment of human influence on the climate system for large-scale indicators of climate change, synthesizing information from paleo records, observations and climate models. It also provides the primary evaluation of large-scale indicators of climate change in this Report, complemented by fitness-for-purpose evaluation in subsequent chapters.

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=== Synthesis Across the Climate System ===

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'''It is unequivocal that human influence has warmed the''' '''atmosphere, ocean and land since pre-industrial times.''' Combining the evidence from across the climate system increases the level of confidence in the attribution of observed climate change to human influence and reduces the uncertainties associated with assessments based on single variables. Large-scale indicators of climate change in the atmosphere, ocean, cryosphere and at the land surface show clear responses to human influence consistent with those expected based on model simulations and physical understanding. {3.8.1}

'''For most large-scale indicators of climate change, the''' '''simulated recent mean climate from the latest generation Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models underpinning this assessment has improved compared to the Coupled Model Intercomparison Project Phase 5 (CMIP5) models assessed in AR5''' ( high confidence ''').''' Some differences from observations remain, for example in regional precipitation patterns. High-resolution models exhibit reduced biases in some but not all aspects of surface and ocean climate ( ''medium confidence'' ), and most Earth system models, which include biogeochemical feedbacks, perform as well as their lower-complexity counterparts ( ''medium confidence'' ). The multi-model mean captures most aspects of observed climate change well ( ''high confidence'' ). The multi-model mean captures the proxy-reconstructed global-mean surface air temperature (GSAT) change during past high- and low-CO <sub>2</sub> climates ( ''high confidence'' ) and the correct sign of temperature and precipitation change in most assessed regions in the mid-Holocene ( ''medium confidence'' ). The simulation of paleoclimates on continental scales has improved compared to AR5 ( ''medium confidence'' ), but models often underestimate large temperature and precipitation differences relative to the present day ( ''high confidence'' ). {3.8.2}

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

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'''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}

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=== Human Influence on the Cryosphere ===

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'''It is''' very likely '''that anthropogenic forcing, mainly due to''' '''greenhouse gas increases, was the main driver of Arctic sea ice loss since the late 1970s.''' There is new evidence that increases in anthropogenic aerosols have offset part of the greenhouse gas-induced Arctic sea ice loss since the 1950s ( ''medium confidence'' ). In the Arctic, despite large differences in the mean sea ice state, loss of sea ice extent and thickness during recent decades is reproduced in all CMIP5 and CMIP6 models ( ''high confidence'' ). By contrast, global climate models do not generally capture the small observed increase in Antarctic sea ice extent during the satellite era, and there is ''low confidence'' in attributing the causes of this change. {3.4.1}

'''It is''' very likely '''that human influence contributed to the''' '''observed reductions in Northern Hemisphere spring snow cover''' '''since 1950.''' The seasonal cycle in Northern Hemisphere snow cover is better reproduced by CMIP6 than by CMIP5 models ( ''high confidence'' ). Human influence was ''very likely'' the main driver of the recent global, near-universal retreat of glaciers. It is ''very likely'' that human influence has contributed to the observed surface melting of the Greenland Ice Sheet over the past two decades, and there is ''medium confidence'' in an anthropogenic contribution to recent overall mass loss from the Greenland Ice Sheet. However, there is only ''limited evidence'' , with ''medium agreement'' , of human influence on Antarctic Ice Sheet mass balance through changes in ice discharge. {3.4.2, 3.4.3}

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=== Human Influence on the Ocean ===

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'''It is''' extremely likely '''that human influence was the main driver''' '''of the ocean heat content increase observed since the 1970s, which extends into the deeper ocean''' ( very high confidence ''').''' Since AR5, there is improved consistency between recent observed estimates and model simulations of changes in upper (&lt;700 m) ocean heat content, when accounting for both natural and anthropogenic forcings. Updated observations and model simulations show that warming extends throughout the entire water column ( ''high confidence'' ), with CMIP6 models simulating 58% of industrial-era heat uptake (1850–2014) in the upper layer (0–700 m), 21% in the intermediate layer (700–2000 m) and 22% in the deep layer (&gt;2000 m). The structure and magnitude of multi-model mean ocean temperature biases have not changed substantially between CMIP5 and CMIP6 ( ''medium confidence'' ). {3.5.1}

'''It is''' extremely likely '''that human influence has contributed''' '''to observed near-surface and subsurface ocean salinity''' '''changes since the mid-20th century.''' The associated pattern of change corresponds to fresh regions becoming fresher and salty regions becoming saltier ( ''high confidence'' ). Changes to the coincident atmospheric water cycle and ocean-atmosphere fluxes (evaporation and precipitation) are the primary drivers of the observed basin-scale salinity changes ( ''high confidence'' ). The observed depth-integrated basin-scale salinity changes have been attributed to human influence, with CMIP5 and CMIP6 models able to reproduce these patterns only in simulations that include greenhouse gas increases ( ''medium confidence'' ). The basin-scale changes are consistent across models and intensify through the historical period ( ''high confidence'' ). The structure of the biases in the multi-model mean has not changed substantially between CMIP5 and CMIP6 ( ''medium confidence'' ). {3.5.2}

'''Combining the attributable contributions from glaciers,''' '''i''' '''ce-she''' '''et surface mass balance and thermal expansion, it is''' very likely '''that human influence was the main driver of the observed global mean sea level rise since at least 1971.''' Since AR5, studies have shown that simulations that exclude anthropogenic greenhouse gases are unable to capture the sea level rise due to thermal expansion (thermosteric) during the historical period and that model simulations that include all forcings (anthropogenic and natural) most closely match observed estimates. It is ''very likely'' that human influence was the main driver of the observed global mean thermosteric sea level increase since 1970. {3.5.3, 3.5.1, 3.4.3}

'''While observations show that the Atlantic Meridional Overturning Circulation (AMOC) has weakened from the mid-2000s to the mid-2010s''' ( high confidence ) '''and the Southern Ocean upper overturning cell has strengthened since the 1990s''' ( low confidence '''), observational records are too short to determine the relative contributions of internal variability, natural forcing, and anthropogenic forcing to these changes''' ( high confidence ''').''' No changes in Antarctic Circumpolar Current transport or meridional position have been observed. The mean zonal and overturning circulations of the Southern Ocean and the mean overturning circulation of the North Atlantic (the Atlantic Meridional Overturning Circulation, AMOC) are broadly reproduced by CMIP5 and CMIP6 models. However, biases are apparent in the modelled circulation strengths ( ''high confidence'' ) and their variability ( ''medium confidence'' ). {3.5.4}

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=== Human Influence on the Biosphere ===

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'''The main driver of the observed increase in the amplitude of the seasonal cycle of atmospheric CO''' <sub>2</sub> '''is enhanced fertilization of plant growth by the increasing concentration of atmospheric CO''' <sub>2</sub> '''(''' medium confidence) '''.''' However, there is only ''low confidence'' that this CO <sub>2</sub> fertilization has also been the main driver of observed greening because land management is the dominating factor in some regions. Earth system models simulate globally averaged land carbon sinks within the range of observation-based estimates ( ''high confidence'' ), but global-scale agreement masks large regional disagreements. {3.6.1}

'''It is''' virtually certain '''that the uptake of anthropogenic CO''' <sub>2</sub> '''was the main driver of the observed acidification of the global''' '''surface open ocean.''' The observed increase in CO <sub>2</sub> concentration in the subtropical and equatorial North Atlantic since 2000 is ''likely'' associated in part with an increase in ocean temperature, a response that is consistent with the expected weakening of the ocean carbon sink with warming. Consistent with AR5 there is ''medium confidence'' that deoxygenation in the upper ocean is due in part to human influence. There is ''high confidence'' that Earth system models simulate a realistic time evolution of the global mean ocean carbon sink. {3.6.2}

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=== Human Influence on Modes of Climate Variability ===

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'''It is''' very likely '''that human influence has contributed to the observed trend towards the positive phase of the Southern Annular Mode (SAM) since the 1970s and to the associated''' '''strengthening and southward shift of the Southern''' '''Hemispheric extratropical jet in austral summer.''' The influence of ozone forcing on the SAM trend has been small since the early 2000s compared to earlier decades, contributing to a weaker SAM trend observed over 2000–2019 ( ''medium confidence'' ). Climate models reproduce the summertime SAM trend well, with CMIP6 models outperforming CMIP5 models ( ''medium confidence'' ). By contrast, the cause of the Northern Annular Mode (NAM) trend towards its positive phase since the 1960s and associated northward shifts of the Northern Hemispheric extratropical jet and storm track in boreal winter is not well understood. Models reproduce the observed spatial features and variance of the SAM and NAM very well ( ''high confidence'' ). {3.3.3, 3.7.1, 3.7.2}

'''Human influence has not affected the principal tropical modes''' '''of interannual climate variability or their associated regional''' '''teleconnections beyond the range of internal variability''' ( high confidence ''').''' Further assessment since AR5 confirms that climate and Earth system models are able to reproduce most aspects of the spatial structure and variance of the El Niño–Southern Oscillation and Indian Ocean Basin and Dipole modes ( ''medium confidence'' ). However, despite a slight improvement in CMIP6, some underlying processes are still poorly represented. In the Tropical Atlantic basin, which contains the Atlantic Zonal and Meridional modes, major biases in modelled mean state and variability remain. {3.7.3 to 3.7.5}

'''There is''' medium confidence '''that anthropogenic and volcanic aerosols contributed to observed changes in the Atlantic Multi-decadal Variability (AMV) index and associated regional teleconnections since the 1960s, but there is''' low confidence '''in the magnitude of this influence.''' There is ''high confidence'' that internal variability is the main driver of Pacific Decadal Variability (PDV) observed since pre-industrial times, despite some modelling evidence for potential human influence. Uncertainties remain in quantification of the human influence on AMV and PDV due to brevity of the observational records, limited model performance in reproducing related sea surface temperature (SST) anomalies despite improvements from CMIP5 to CMIP6 ( ''medium confidence'' ), and limited process understanding of their key drivers. {3.7.6, 3.7.7}

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