Editing IPCC:AR6/WGI/SPM (section)

== A. The Current State of the Climate ==

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''''Since AR5, improvements in observationally based estimates and information from paleoclimate archives provide a comprehensive view of each component of the climate system and its changes to date. New climate model simulations, new analyses, and methods combining multiple lines of evidence lead to improved understanding of human influence on a wider range of climate variables, including weather and climate extremes. The time periods considered throughout this section depend upon the availability of observational products, paleoclimate archives and peer-reviewed studies.'''' '''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.   Expand [[#figure-spm-1|Figures SPM.1]] , [[#figure-spm-2|SPM.2]] Links to chapters 2.2, 2.3, Cross-Chapter Box 2.3, 3.3, 3.4, 3.5, 3.6, 3.8, 5.2, 5.3, 6.4, 7.3, 8.3, 9.2, 9.3, 9.5, 9.6, Cross-Chapter Box 9.1'''
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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 parts per million (ppm) for carbon dioxide (CO <sub>2</sub> ), 1866 parts per billion (ppb) for methane (CH <sub>4</sub> ), and 332 ppb for nitrous oxide (N <sub>2</sub> O) in 2019. <sup>[[#footnote-043|6]]</sup> Land and ocean have taken up a near-constant proportion (globally about 56% per year) of CO <sub>2</sub> emissions from human activities over the past six decades, with regional differences ( ''high confidence'' ). <sup>[[#footnote-042|7]]</sup>   Links to chapters 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 temperature <sup>[[#footnote-041|8]]</sup> in the first two decades of the 21st century (2001–2020) was 0.99 [0.84 to 1.10] °C higher than 1850–1900. <sup>[[#footnote-040|9]]</sup> 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. <sup>[[#footnote-039|10]]</sup>   Links to chapters 2.3, Cross-Chapter Box 2.3 [[#figure-spm-1|Figure SPM.1]]

A.1.3 The ''likely'' range of total human-caused global surface temperature increase from 1850–1900 to 2010–2019 <sup>[[#footnote-038|11]]</sup> 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 <sup>[[#footnote-037|12]]</sup> 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.   Links to chapters 3.3, 6.4, 7.3, TS.2.3, Cross-Section Box TS.1 [[#figure-spm-2|Figure SPM.2]]

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.   Links to chapters 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.   Links to chapters 2.3, 3.4, 8.3, 9.3, 9.5, TS.2.5

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 CO <sub>2</sub> 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.   Links to chapters 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 <sup>–1</sup> between 1901 and 1971, increasing to 1.9 [0.8 to 2.9] mm yr <sup>–1</sup> between 1971 and 2006, and further increasing to 3.7 [3.2 to 4.2] mm yr <sup>–1</sup> between 2006 and 2018 ( ''high confidence'' ). Human influence was ''very likely'' the main driver of these increases since at least 1971.   Links to chapters 2.3, 3.5, 9.6, Cross-Chapter Box 9.1, Box TS.4

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'' ).   Links to chapters 2.3, TS.2.6

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Figure SPM.1 | '''History of global temperature change and causes of recent warming'''

'''Panel (a) Changes in global surface temperature reconstructed from paleoclimate archives''' (solid grey line, years 1–2000) '''and from direct observations''' (solid black line, 1850–2020), both relative to 1850–1900 and decadally averaged. The vertical bar on the left shows the estimated temperature ( ''very likely'' range) during the warmest multi-century period in at least the last 100,000 years, which occurred around 6500 years ago during the current interglacial period (Holocene). The Last Interglacial, around 125,000 years ago, is the next most recent candidate for a period of higher temperature. These past warm periods were caused by slow (multi-millennial) orbital variations. The grey shading with white diagonal lines shows the ''very likely'' ranges for the temperature reconstructions.

'''Panel (b) Changes in global surface temperature over the past 170 years''' (black line) relative to 1850–1900 and annually averaged, compared to Coupled Model Intercomparison Project Phase 6 (CMIP6) climate model simulations (see Box SPM.1) of the temperature response to both human and natural drivers (brown) and to only natural drivers (solar and volcanic activity, green). Solid coloured lines show the multi-model average, and coloured shades show the ''very likely'' range of simulations. (See Figure SPM.2 for the assessed contributions to warming).   Links to chapters 2.3.1, Cross-Chapter Box 2.3, 3.3, TS.2.2, Cross-Section Box TS.1, Figure 1a

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Figure SPM.2 | '''Assessed contributions to observed warming in 2010–2019 relative to 1850–1900'''

'''Panel (a) Observed global warming''' (increase in global surface temperature). Whiskers show the ''very likely'' range.

'''Panel (b) Evidence from attribution studies, which synthesize information from climate models and observations.''' The panel shows temperature change attributed to: total human influence; changes in well-mixed greenhouse gas concentrations; other human drivers due to aerosols, ozone and land-use change (land-use reflectance); solar and volcanic drivers; and internal climate variability. Whiskers show ''likely'' ranges.

'''Panel (c) Evidence from the assessment of radiative forcing and climate sensitivity.''' The panel shows temperature changes from individual components of human influence: emissions of greenhouse gases, aerosols and their precursors; land-use changes (land-use reflectance and irrigation); and aviation contrails. Whiskers show ''very likely'' ranges. Estimates account for both direct emissions into the atmosphere and their effect, if any, on other climate drivers. For aerosols, both direct effects (through radiation) and indirect effects (through interactions with clouds) are considered.   Links to chapters Cross-Chapter Box 2.3, 3.3.1, 6.4.2, 7.3

'''A.2 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.   Expand [[#figure-spm-1|Figure SPM.1]] Links to chapters 2.2, 2.3, Cross-Chapter Box 2.1, 5.1'''
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A.2.1 In 2019, atmospheric CO <sub>2</sub> concentrations were higher than at any time in at least 2 million years ( ''high confidence'' ), and concentrations of CH <sub>4</sub> and N <sub>2</sub> O were higher than at any time in at least 800,000 years ( ''very high confidence'' ). Since 1750, increases in CO <sub>2</sub> (47%) and CH <sub>4</sub> (156%) concentrations far exceed – and increases in N <sub>2</sub> O (23%) are similar to – the natural multi-millennial changes between glacial and interglacial periods over at least the past 800,000 years ( ''very high confidence'' ).   Links to chapters 2.2, 5.1, TS.2.2

A.2.2 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 <sup>[[#footnote-036|13]]</sup> [0.2°C to 1°C relative to 1850–1900] ( ''medium confidence'' ). Prior to that, the next most recent warm period was about 125,000 years ago, when the multi-century temperature [0.5°C to 1.5°C relative to 1850–1900] overlaps the observations of the most recent decade ( ''medium confidence'' ).   Links to chapters 2.3, Cross-Chapter Box 2.1, Cross-Section Box TS.1 [[#figure-spm-1|Figure SPM.1]]

A.2.3 In 2011–2020, annual average Arctic sea ice area reached its lowest level since at least 1850 ( ''high confidence'' ). Late summer Arctic sea ice area was smaller than at any time in at least the past 1000 years ( ''medium confidence'' ). The global nature of glacier retreat since the 1950s, with almost all of the world’s glaciers retreating synchronously, is unprecedented in at least the last 2000 years ( ''medium confidence'' ).   Links to chapters 2.3, TS.2.5

A.2.4 Global mean sea level has risen faster since 1900 than over any preceding century in at least the last 3000 years ( ''high confidence'' ). The global ocean has warmed faster over the past century than since the end of the last deglacial transition (around 11,000 years ago) ( ''medium confidence'' ). A long-term increase in surface open ocean pH occurred over the past 50 million years ( ''high confidence'' ). However, surface open ocean pH as low as recent decades is unusual in the last 2 million years ( ''medium confidence'' ).   Links to chapters 2.3, TS.2.4, Box TS.4

'''A.3 Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since AR5.   Expand [[#figure-spm-3|Figure SPM.3]] Links to chapters 2.3, 3.3, 8.2, 8.3, 8.4, 8.5, 8.6, Box 8.1, Box 8.2, Box 9.2, 10.6, 11.2, 11.3, 11.4, 11.6, 11.7, 11.8, 11.9, 12.3'''
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A.3.1 It is ''virtually certain'' that hot extremes (including heatwaves) have become more frequent and more intense across most land regions since the 1950s, while cold extremes (including cold waves) have become less frequent and less severe, with ''high confidence'' that human-induced climate change is the main driver <sup>[[#footnote-035|14]]</sup> of these changes. Some recent hot extremes observed over the past decade would have been ''extremely unlikely'' to occur without human influence on the climate system. Marine heatwaves have approximately doubled in frequency since the 1980s ( ''high confidence'' ), and human influence has ''very likely'' contributed to most of them since at least 2006.   [[#figure-spm-3|Figure SPM.3]] Links to chapters Box 9.2, 11.2, 11.3, 11.9, TS.2.4, TS.2.6, Box TS.10

A.3.2 The frequency and intensity of heavy precipitation events have increased since the 1950s over most land area for which observational data are sufficient for trend analysis ( ''high confidence'' ), and human-induced climate change is ''likely'' the main driver. Human-induced climate change has contributed to increases in agricultural and ecological droughts <sup>[[#footnote-034|15]]</sup> in some regions due to increased land evapotranspiration <sup>[[#footnote-033|16]]</sup> ( ''medium confidence'' ).   [[#figure-spm-3|Figure SPM.3]] Links to chapters 8.2, 8.3, 11.4, 11.6, 11.9, TS.2.6, Box TS.10

A.3.3 Decreases in global land monsoon precipitation <sup>[[#footnote-032|17]]</sup> from the 1950s to the 1980s are partly attributed to human-caused Northern Hemisphere aerosol emissions, but increases since then have resulted from rising GHG concentrations and decadal to multi-decadal internal variability ( ''medium confidence'' ). Over South Asia, East Asia and West Africa, increases in monsoon precipitation due to warming from GHG emissions were counteracted by decreases in monsoon precipitation due to cooling from human-caused aerosol emissions over the 20th century ( ''high confidence'' ). Increases in West African monsoon precipitation since the 1980s are partly due to the growing influence of GHGs and reductions in the cooling effect of human-caused aerosol emissions over Europe and North America ( ''medium confidence'' ).   Links to chapters 2.3, 3.3, 8.2, 8.3, 8.4, 8.5, 8.6, Box 8.1, Box 8.2, 10.6, Box TS.13

A.3.4 It is ''likely'' that the global proportion of major (Category 3–5) tropical cyclone occurrence has increased over the last four decades, and it is '''very likely''' that the latitude where tropical cyclones in the western North Pacific reach their peak intensity has shifted northward; these changes cannot be explained by internal variability alone ( ''medium confidence'' ). There is ''low confidence'' in long-term (multi-decadal to centennial) trends in the frequency of all-category tropical cyclones. Event attribution studies and physical understanding indicate that human-induced climate change increases heavy precipitation associated with tropical cyclones ( ''high confidence'' ), but data limitations inhibit clear detection of past trends on the global scale.   Links to chapters 8.2, 11.7, Box TS.10

A.3.5 Human influence has ''likely'' increased the chance of compound extreme events <sup>[[#footnote-031|18]]</sup> since the 1950s. This includes increases in the frequency of concurrent heatwaves and droughts on the global scale ( ''high confidence'' ), fire weather in some regions of all inhabited continents ( ''medium confidence'' ), and compound flooding in some locations ( ''medium confidence'' ).   Links to chapters 11.6, 11.7, 11.8, 12.3, 12.4, TS.2.6, Table TS.5, Box TS.10

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Figure SPM.3 | '''Synthesis of assessed observed and attributable regional changes'''

The IPCC AR6 WGI inhabited regions are displayed as '''hexagons''' with identical size in their approximate geographical location (see legend for regional acronyms). All assessments are made for each region as a whole and for the 1950s to the present. Assessments made on different time scales or more local spatial scales might differ from what is shown in the figure. The '''colours''' in each panel represent the four outcomes of the assessment on observed changes. Striped hexagons (white and light-grey) are used where there is ''low agreement'' in the type of change for the region as a whole, and grey hexagons are used when there is limited data and/or literature that prevents an assessment of the region as a whole. Other colours indicate at least ''medium confidence'' in the observed change. The '''confidence level''' for the human influence on these observed changes is based on assessing trend detection and attribution and event attribution literature, and it is indicated by the number of dots: three dots for ''high confidence'' , two dots for ''medium confidence'' and one dot for ''low confidence'' (single, filled dot: limited agreement; single, empty dot: limited evidence).

'''Panel (a) For hot extremes,''' the evidence is mostly drawn from changes in metrics based on daily maximum temperatures; regional studies using other indices (heatwave duration, frequency and intensity) are used in addition. Red hexagons indicate regions where there is at least ''medium confidence'' in an observed increase in hot extremes.

'''Panel (b) For heavy precipitation,''' the evidence is mostly drawn from changes in indices based on one-day or five-day precipitation amounts using global and regional studies. Green hexagons indicate regions where there is at least ''medium confidence'' in an observed increase in heavy precipitation.

'''Panel (c) Agricultural and ecological droughts''' are assessed based on observed and simulated changes in total column soil moisture, complemented by evidence on changes in surface soil moisture, water balance (precipitation minus evapotranspiration) and indices driven by precipitation and atmospheric evaporative demand. Yellow hexagons indicate regions where there is at least ''medium confidence'' in an observed increase in this type of drought, and green hexagons indicate regions where there is at least ''medium confidence'' in an observed decrease in agricultural and ecological drought.

For all regions, Table TS.5 shows a broader range of observed changes besides the ones shown in this figure. Note that Southern South America (SSA) is the only region that does not display observed changes in the metrics shown in this figure, but is affected by observed increases in mean temperature, decreases in frost and increases in marine heatwaves.   Links to chapters 11.9, [[IPCC:Wg1:Chapter:Atlas|Atlas]] 1.3.3, Figure Atlas.2, Table TS.5, Box TS.10, Figure 1

'''A.4 Improved knowledge of climate processes, paleoclimate evidence and the response of the climate system to increasing radiative forcing gives a best estimate of equilibrium climate sensitivity of 3°C, with a narrower range compared to AR5.   Expand Links to chapters 2.2, 7.3, 7.4, 7.5, Box 7.2, 9.4, 9.5, 9.6, Cross-Chapter Box 9.1'''
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A.4.1 Human-caused radiative forcing of 2.72 [1.96 to 3.48] W m <sup>–2</sup> in 2019 relative to 1750 has warmed the climate system. This warming is mainly due to increased GHG concentrations, partly reduced by cooling due to increased aerosol concentrations. The radiative forcing has increased by 0.43 W m <sup>–2</sup> (19%) relative to AR5, of which 0.34 W m <sup>–2</sup> is due to the increase in GHG concentrations since 2011. The remainder is due to improved scientific understanding and changes in the assessment of aerosol forcing, which include decreases in concentration and improvement in its calculation ( ''high confidence'' ).   Links to chapters 2.2, 7.3, TS.2.2, TS.3.1

A.4.2 Human-caused net positive radiative forcing causes an accumulation of additional energy (heating) in the climate system, partly reduced by increased energy loss to space in response to surface warming. The observed average rate of heating of the climate system increased from 0.50 [0.32 to 0.69] W m <sup>–2</sup> for the period 1971–2006 <sup>[[#footnote-030|19]]</sup> to 0.79 [0.52 to 1.06] W m <sup>–2</sup> for the period 2006–2018 <sup>[[#footnote-029|20]]</sup> ( ''high confidence'' ). Ocean warming accounted for 91% of the heating in the climate system, with land warming, ice loss and atmospheric warming accounting for about 5%, 3% and 1%, respectively ( ''high confidence'' ).   Links to chapters 7.2, Box 7.2, TS.3.1

A.4.3 Heating of the climate system has caused global mean sea level rise through ice loss on land and thermal expansion from ocean warming. Thermal expansion explained 50% of sea level rise during 1971–2018, while ice loss from glaciers contributed 22%, ice sheets 20% and changes in land-water storage 8%. The rate of ice-sheet loss increased by a factor of four between 1992–1999 and 2010–2019. Together, ice-sheet and glacier mass loss were the dominant contributors to global mean sea level rise during 2006–2018 ( ''high confidence'' ).   Links to chapters 9.4, 9.5, 9.6, Cross-Chapter Box 9.1

A.4.4 The equilibrium climate sensitivity is an important quantity used to estimate how the climate responds to radiative forcing. Based on multiple lines of evidence, <sup>[[#footnote-028|21]]</sup> The ''very likely'' 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'' ), compared to 1.5°C to 4.5°C in AR5, which did not provide a best estimate.   Links to chapters 7.4, 7.5, TS.3.2

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