Editing IPCC:AR6/WGI/TS (section)

== TS.2 Large-scale Climate Change: Mean Climate, Variability and Extremes ==

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This section summarizes knowledge about observed and projected large-scale climate change (including variability and extremes), drivers and attribution of observed changes to human activities. It describes observed and projected large-scale changes associated with major components of the climate system: atmosphere, ocean (including sea level change), land, biosphere and cryosphere, and the carbon, energy and water cycles. In each subsection, reconstructed past changes, observed and attributed recent changes, and projected near- and long-term changes to mean climate, variability and extremes are presented, where possible, in an integrated way. See Section TS.1.3.1 for information on the scenarios used for projections.

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=== TS.2.1 Changes Across the Global Climate System ===

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'''In addition to global surface temperature (Cross-Section Box TS.1), a wide range of indicators across all components of the climate system are changing rapidly (Figure TS.7), with many at levels unseen in millennia. The observed changes provide a coherent picture of a warming world, many aspects of which have now been formally attributed to human influence, and human influence on the atmosphere, ocean, and land components of the climate system, taken together, is assessed as unequivocal for the first time in an IPCC assessment report (Table TS.1, Figure TS.7).''' '''It is ''virtually certain'' that global surface temperature rise and associated changes can be limited through rapid and substantial reductions in global GHG emissions. Continued GHG emissions greatly increase the likelihood of potentially irreversible changes in the global climate system (Box TS.9), in particular with respect to the contribution of ice sheets to global sea level change (''high confidence''). Links to chapters 2.3, 3.8, 4.3, 4.6, 4.7, 7.2–7.4, Cross-Chapter Box 7.1, 9.2–9.6'''

Earth system model simulations of the historical period since 1850 are only able to reproduce the observed changes in key climate indicators when anthropogenic forcings are included (Figure TS.7). Taken together with numerous formal attribution studies across an even broader range of indicators and theoretical understanding, this underpins the unequivocal attribution of observed warming of the atmosphere, ocean, and land to human influence (Table TS.1). Links to chapters 2.3, 3.8

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'''Figure TS.7 |''' '''Simulated and observed changes compared to the 1850–1900 average in key large-scale indicators of climate change across the climate system, for continents, ocean basins and globally up to 2014.''' ''The intent of this figure is to compare the observed and simulated changes over the historical period for a range of variables and regions, with and without anthropogenic forcings, for attribution.'' Black lines show observations, orange lines and shading show the multi-model mean and 5–95th percentile ranges for Coupled Model Intercomparison Project Phase 6 (CMIP6) historical simulations including anthropogenic and natural forcing, and green lines and shading show corresponding ensemble means and 5–95th percentile ranges for CMIP6 natural-only simulations. Observations after 2014 (including, for example, a strong subsequent decrease of Antarctic sea ice area that leads to no significant overall trend since 1979) are not shown because the CMIP6 historical simulations end in 2014. A 3-year running mean smoothing has been applied to all observational time series. Links to chapters 3.8, Figure 3.41

'''Table TS.1 |''' '''Assessment of observed changes in large-scale indicators of mean climate across climate system components and their attribution to human influence.''' The colour coding indicates the assessed confidence in/likelihood of the human contribution as a driver or main driver<sup>[[#footnote-002|19]]</sup> (main driver is specified in that case) where available (see colour key). Otherwise, explanatory text is provided in cells with white background. The relevant chapter section with more detailed information is listed in each table cell.

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Future climate change across a range of atmospheric, cryospheric, oceanic and biospheric indicators depends upon future emissions pathways. Outcomes for a broad range of indicators increasingly diverge through the 21st century across the different SSPs (Section TS.1.3.1, Figure TS.8). Due to the slow response of the deep ocean and ice sheets, this divergence continues long after 2100, and 21st century emissions choices will have implications for GMSL rise for centuries to millennia. Furthermore, it is ''likely'' that at least one large volcanic eruption will occur during the 21st century. Such an eruption would reduce global surface temperature for several years, decrease land precipitation, alter monsoon circulation and modify extreme precipitation, at both global and regional scales. Links to chapters 4.3, 4.7, 9.4, 9.6, Cross-Chapter Box 4.1

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'''Figure TS.8 |''' '''Observed, simulated and projected changes compared to the 1995–2014 average in four key indicators of the climate system through to 2100 differentiated by Shared Socio-economic Pathway (SSP) scenario.''' ''The intent of this figure is to show how future emissions choices impact key, iconic large-scale indicators and to highlight that our collective choices matter.'' Past simulations are based on the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensemble. Future projections are based on the assessed ranges based upon multiple lines of evidence for '''(a)''' global surface temperature (Cross-Section Box TS.1) and '''(b)''' global ocean heat content and the associated thermosteric sea level contribution to global mean sea level change (right-hand axis) using a climate model emulator (Cross-Chapter Box 7.1), and CMIP6 simulations for '''(c)''' Arctic September sea ice and '''(d)''' global land precipitation. Projections for SSP1-1.9 and SSP1-2.6 show that reduced greenhouse gas emissions lead to a stabilization of global surface temperature, Arctic sea ice area and global land precipitation over the 21st century. Projections for SSP1-2.6 show that emissions reductions have the potential to substantially reduce the increase in ocean heat content and thermosteric sea level rise over the 21st century but that some increase is unavoidable. The brackets in the x axis in panel (a) indicate assessed 20-year-mean periods. Links to chapters 4.3, Figure 4.2, 9.3, 9.6, Figure 9.6

Observational records show changes in a wide range of climate extremes that have been linked to human influence on the climate system (Table TS.2). In many cases, the frequency and intensity of future changes in extremes can be directly linked to the magnitude of future projected warming. Changes in extremes have been widespread over land since the 1950s, including a ''virtually certain'' global increase in extreme air temperatures and a ''likely'' intensification in global-scale extreme precipitation. It is ''extremely likely'' that human influence is the main contributor to the observed increase (decrease) in the likelihood and severity of hot (cold) extremes (Table TS.2). The frequency of extreme temperature and precipitation events in the current climate will change with warming, with warm extremes becoming more frequent (''virtually certain''), cold extremes becoming less frequent (''extremely likely'') and precipitation extremes becoming more frequent in most locations (''very likely'') . Links to chapters 9.6.4, 11.2, 11.3, 11.4, 11.6, 11.7, 11.8, 11.9, Box 9.2

'''Table TS.2 | Summary table on observed changes in extremes, their attribution since 1950 (except where stated otherwise), and projected changes at +1.5°C, +2°C and +4°C of global warming, on global and continental scales.''' An increase in warm/hot extremes refers to warmer and/or more frequent hot days and nights and warm spells/heatwaves, over most land areas. A decrease in cold extremes refers to warmer and/or fewer cold days and nights and cold spells/cold waves, over most land areas. Drought events are relative to a predominant fraction of land area. For tropical cyclones, observed changes and attribution refer to Categories 3–5, while projected changes refer to Categories 4–5. Tables 11.1 and 11.2 are more detailed versions of this table, containing, in particular, information on regional scales. In general, higher warming levels also imply stronger projected changes for indicators where the confidence level does not depend on the warming level and the table does not explicitly quantify the global sensitivity. ''See also Box TS.10.'' Links to chapters 9.6, Box 9.2, 11.3, 11.7

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=== TS.2.2 Changes in the Drivers of the Climate System ===

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'''Since 1750, changes in the drivers of the climate system are dominated by the warming influence of increases in atmospheric GHG concentrations and a cooling influence from aerosols, both resulting from human activities. In comparison there has been negligible long-term influence from solar activity and volcanoes. Concentrations of CO 2 , methane (CH 4), and nitrous oxide (N 2 O) have increased to levels unprecedented in at least 800,000 years, and there is high confidence that current CO 2 concentrations have not been experienced for at least 2 million years. Global mean concentrations of anthropogenic aerosols peaked in the late 20th century and have slowly declined since in northern mid-latitudes, although they continue to increase in South Asia and East Africa (high confidence).''' '''The total anthropogenic effective radiative forcing (ERF) in 2019, relative to 1750, was 2.72 [1.96 to 3.48] W m <sup>–2</sup> (''medium confidence'') and has ''likely'' been growing at an increasing rate since the 1970s. Links to chapters 2.2, 6.4, 7.2, 7.3'''

Solar activity since 1900 was high but not exceptional compared to the past 9000 years (''high confidence''). The average magnitude and variability of volcanic aerosols since 1900 has not been unusual compared to at least the past 2500 years (''medium confidence''). However, sporadic strong volcanic eruptions can lead to temporary drops in global surface temperature lasting 2–5 years. Links to chapters 2.2.1, 2.2.2, 2.2.8, Cross-Chapter Box 4.1

Atmospheric CO <sub>2</sub> concentrations have changed substantially over millions of years (Figure TS.1). Current levels of atmospheric CO <sub>2</sub> have not been experienced for at least 2 million years (''high confidence'' , Figure TS.9a). Over 1750–2019, CO <sub>2</sub> increased by 131.6 ± 2.9 ppm (47.3%). The centennial rate of change of CO <sub>2</sub> since 1850 has no precedent in at least the past 800,000 years (Figure TS.9), and the fastest rates of change over the last 56 million years were at least a factor of four lower (''low confidence'') than over 1900–2019. Several networks of high-accuracy surface observations show that concentrations of CO <sub>2</sub> have exceeded 400 ppm, reaching 409.9 (± 0.3) ppm in 2019 (Figure TS.9c). The ERF from CO <sub>2</sub> in 2019 (relative to 1750) was 2.16 Wm <sup>–2</sup>. Links to chapters 2.2.3, 5.1.2, 5.2.1, 7.3

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'''Figure TS.9 |''' '''Changes in well-mixed greenhouse gas (WMGHG) concentrations and effective radiative forcing (EFR).''' ''The intent of this figure is to show that the changes of the main drivers of climate system over the industrial period are exceptional in a long-term context.'' (a) Changes in carbon dioxide (CO <sub>2</sub>) from proxy records over the past 3.5 million years. (b) Changes in all three WMGHGs from ice core records over the Common Era. (c) Directly observed WMGHG changes since the mid-20th century. (d) Evolution of ERF and components since 1750. Further details on data sources and processing are available in the associated FAIR data table. Links to chapters 2.2, Figures 2.3, 2.4 and 2.10

By 2019, concentrations of CH <sub>4</sub> reached 1866.3 (± 3.3) ppb (Figure TS.9c). The increase since 1750 of 1137 ± 10 ppb (157.8%) far exceeds the range over multiple glacial–interglacial transitions of the past 800,000 years (''high confidence''). In the 1990s, CH <sub>4</sub> concentrations plateaued, but started to increase again around 2007 at an average rate of 7.6 ± 2.7 ppb yr <sup>–1</sup> (2010–2019; ''high confidence''). There is ''high confidence'' that this recent growth is largely driven by emissions from fossil fuel exploitation, livestock, and waste, with ENSO driving multi-annual variability of wetland and biomass burning emissions. In 2019, ERF from CH <sub>4</sub> was 0.54 Wm <sup>–2</sup>. Links to chapters 2.2.3, 5.2.2, 7.3

Since 1750, N <sub>2</sub> O increased by 62.0 ± 6.0 ppb, reaching a level of 332.1 (± 0.4) ppb in 2019. The increase since 1750 is of comparable magnitude to glacial–interglacial fluctuations of the past 800,000 years (Figure TS.9c). N <sub>2</sub> O concentration trends since 1980 are largely driven by a 30% increase in emissions from the expansion and intensification of global agriculture (''high confidence''). By 2019 its ERF was 0.21 W m <sup>–2</sup>. Links to chapters 2.2.3, 5.2.3

Halogenated gases consist of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and other gases, many of which can deplete stratospheric ozone and warm the atmosphere. In response to controls on production and consumption mandated by the Montreal Protocol on Substances that Deplete the Ozone Layer and its amendments, the atmospheric abundances of most CFCs have continued to decline since AR5. Abundances of HFCs, which are replacements for CFCs and HCFCs, are increasing (''high confidence''), though increases of the major HCFCs have slowed in recent years. The ERF from halogenated components in 2019 was 0.41 Wm <sup>–2</sup>. Links to chapters 2.2.4, 6.3.4, 7.3.2

Tropospheric aerosols mainly act to cool the climate system, directly by reflecting solar radiation, and indirectly by enhancing cloud reflectance. Ice cores show increases in aerosols across the Northern Hemisphere mid-latitudes since 1700 and reductions since the late 20th century (''high confidence''). Aerosol optical depth (AOD), derived from satellite- and ground-based radiometers, has decreased since 2000 over the mid-latitude continents of both hemispheres, but increased over South Asia and East Africa (''high confidence''). Trends in AOD are more pronounced from sub-micrometre aerosols for which the anthropogenic contribution is particularly large. Global carbonaceous aerosol budgets and trends remain poorly characterized due to limited observations, but black carbon (BC), a warming aerosol component, is declining in several regions of the Northern Hemisphere (''low confidence''). Total aerosol ERF in 2019, relative to 1750, is −1.1 [−1.7 to −0.4] W m <sup>−2</sup> (''medium confidence'') and ''more likely than not'' became less negative since the late 20th century, with ''low confidence'' in the magnitude of post-2014 changes due to conflicting evidence (Section TS.3.1). Links to chapters 2.2.6, 6.2.1, 6.3.5, 6.4.1, 7.3.3

There is ''high confidence'' that tropospheric ozone has been increasing from 1750 in response to anthropogenic changes in ozone precursor emissions (nitrogen oxides, carbon monoxide, non-methane volatile organic compounds, and methane), but with ''medium confidenc'' e in the magnitude of this change, due to limited observational evidence and knowledge gaps. Since the mid-20th century, tropospheric ozone surface concentrations have increased by 30–70% across the Northern Hemisphere (''medium confidence''); since the mid-1990s, free tropospheric ozone has increased by 2–7% per decade in most northern mid-latitude regions and 2–12% per decade in sampled tropical regions. Future changes in surface ozone concentrations will be primarily driven by changes in precursor emissions rather than climate change (''high confidence''). Stratospheric ozone has declined between 60°S–60°N by 2.2% from 1964–1980 to 2014–2017 (''high confidence''), with the largest declines during 1980–1995. The strongest loss of stratospheric ozone continues to occur in austral spring over Antarctica (ozone hole), with emergent signs of recovery after 2000. The 1750–2019 ERF for total (stratospheric and tropospheric) ozone is 0.47 [0.24 to 0.71] W m <sup>−2</sup>, which is dominated by tropospheric ozone changes. Links to chapters 2.2.5, 6.3.2, 7.3.2, 7.3.5

The global mean abundance of hydroxyl (OH) radical, or ‘oxidizing capacity’, chemically regulates the lifetimes of many SLCFs, and therefore the radiative forcing of CH <sub>4</sub> , ozone, secondary aerosols and many halogenated species. Model estimates suggest no significant change in oxidizing capacity from 1850 to 1980 (''low confidence''). Increases of about 9% over 1980–2014 computed by ESMs and carbon cycle models are not confirmed by observationally constrained inverse models, rendering an overall ''medium confidence'' in stable OH or positive trends since the 1980s, and implying that OH is not the primary driver of recent observed growth in CH <sub>4</sub> . Links to chapters 6.3.6, Cross-Chapter Box 5.2

Land use and land-cover change exert biophysical and biogeochemical effects. There is ''medium confidence'' that the biophysical effects of land-use change since 1750, most notably the increase in global albedo, have had an overall cooling on climate, whereas biogeochemical effects (i.e., changes in GHG and volatile organic compound emissions or sinks) led to net warming. Overall land-use and land-cover ERF is estimated at –0.2 [–0.3 to –0.1] W m <sup>−2</sup>. Links to chapters 2.2.7, 7.3.4, SRCCL [[IPCC:Wg1:Chapter:Chapter-2#2.5|Section 2.5]]

The total anthropogenic ERF in 2019 relative to 1750 was 2.72 [1.96 to 3.48] W m <sup>−2</sup> (Figure TS.9), dominated by GHGs (positive ERF) and partially offset by aerosols (negative ERF). The rate of change of ERF ''likely'' has increased since the 1970s, mainly due to growing CO <sub>2</sub> concentrations and less negative aerosol ERF (Section TS.3.1). Links to chapters 2.2.8, 7.3

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=== TS.2.3 Upper Air Temperatures and Atmospheric Circulation ===

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'''The effects of human-induced climate change have been clearly identified in observations of atmospheric temperature and some aspects of atmospheric circulation, and these effects are likely to intensify in the future. Tropospheric warming and stratospheric cooling are virtually certain to continue with continued net emissions of greenhouse gases. Several aspects of the atmospheric circulation have likely changed since the mid-20th century, and human influence has likely contributed to the observed poleward expansion of the Southern Hemisphere Hadley Cell and very likely contributed to the observed poleward shift of the Southern Hemisphere extratropical jet in summer. It is likely that the mid-latitude jet will shift poleward and strengthen, accompanied by a strengthening of the storm track in the Southern Hemisphere by 2100 under the high CO 2 emissions scenarios. It is likely that the proportion of intense tropical cyclones has increased over the last four decades and that this cannot be explained entirely by natural variability. There is low confidence in observed recent changes in the total number of extratropical cyclones over both hemispheres. The proportion of tropical cyclones that are intense is expected to increase (high confidence), but the total global number of tropical cyclones is expected to decrease or remain unchanged (medium confidence). Links to chapters 2.3, 3.3, 4.3, 4.4, 4.5, 8.3, 8.4, 11.7'''

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'''Figure TS.10 |''' '''Observed and projected upper air temperature and circulation changes.''' ''The intent of this figure is to visualize upper air temperature and circulation changes and the similarity between observed and projected changes.'' Upper panels: (left) Zonal cross-section of temperature trends for 2002–2019 in the upper troposphere region for the ROM SAF radio-occultation dataset. (Middle) Change in the annual and zonal mean atmospheric temperature (°C) in 2081–2100 in SSP1-2.6 relative to 1995–2014 for 36 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. (right) the same in SSP3-7.0 for 32 models. Lower panels: (left) Long-term mean (thin black colour) and linear trend (colour) of zonal mean December–January–February (DJF) zonal winds for ERA5. (Middle) multi-model mean change in annual and zonal mean wind (m s <sup>–1</sup>) in 2081–2100 in SSP1-2.6 relative to 1995–2014 based on 34 CMIP6 models. The 1995–2014 climatology is shown in contours with spacing of 10 m s <sup>–1</sup>. (right) the same for SSP3-7.0 for 31 models. Links to chapters 2.3.1; Figures 2.12 and 2.18; 4.5.1; Figure 4.2.6

The troposphere has warmed since at least the 1950s, and it is ''virtually certain'' that the stratosphere has cooled. It is ''very likely'' that human-induced increases in GHGs were the main driver of tropospheric warming since 1979. It is ''extremely likely'' that anthropogenic forcing, both from increases in GHG concentrations and depletion of stratospheric ozone due to ozone-depleting substances, was the main driver of upper stratospheric cooling since 1979. It is ''very likely'' that global mean stratospheric cooling will be larger for scenarios with higher atmospheric CO <sub>2</sub> concentrations. In the tropics, since at least 2001 (when new techniques permit more robust quantification), the upper troposphere has warmed faster than the near-surface (''medium confidence'') (Figure TS.10). There is ''medium confidence'' that most CMIP5 and CMIP6 models overestimate the observed warming in the upper tropical troposphere over the period 1979–2014, in part because they overestimate tropical SST warming. It is ''likely'' that future tropical upper tropospheric warming will be larger than at the tropical surface. Links to chapters 2.3.1, 3.3.1, 4.5.1

The Hadley Circulation has ''likely'' widened since at least the 1980s, predominantly in the Northern Hemisphere, although there is only ''medium confidence'' in the extent of the changes. This has been accompanied by a strengthening of the Hadley Circulation in the Northern Hemisphere (''medium confidence''). 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, which is projected to further expand with global warming (''high confidence''). There is ''medium confidence'' that the observed poleward expansion in the Northern Hemisphere is within the range of internal variability. Links to chapters 2.3.1, 3.3.3, 8.4.3

Since the 1970s, near-surface average winds have ''likely'' weakened over land. Over the ocean, near-surface average winds ''likely'' strengthened over 1980–2000, but divergent estimates lead to ''low confidence'' thereafter. Extratropical storm tracks have ''likely'' shifted poleward since the 1980s. There is ''low confidence'' in projected poleward shifts of the Northern Hemisphere mid-latitude jet and storm tracks due to large internal variability and structural uncertainty in model simulations. There is ''medium confidence'' in a projected decrease in the frequency of atmospheric blocking over Greenland and the North Pacific in boreal winter in 2081–2100 under the SSP3-7.0 and SSP5-8.5 scenarios. There is ''high confidence'' that Southern Hemisphere storm tracks and associated precipitation have migrated polewards over recent decades, especially in the austral summer and autumn, associated with a trend towards more positive phases of the Southern Annular Mode (SAM) (Section TS.4.2.2) and the strengthening and southward shift of the Southern Hemisphere extratropical jet in austral summer. In the long term (2081–2100), the Southern Hemisphere mid-latitude jet is ''likely'' to shift poleward and strengthen under the SSP5-8.5 scenario relative to 1995–2014, accompanied by an increase in the SAM (Section TS.4.2.2). It is ''likely'' that wind speeds associated with extratropical cyclones will strengthen in the Southern Hemisphere storm track for SSP5-8.5. There is ''low confidence'' in the potential role of Arctic warming and sea ice loss on historical or projected mid-latitude atmospheric variability. Links to chapters 2.3.1, 3.3.3, 3.7.2, 4.3.3, 4.4.3, 4.5.1, 4.5.3, 8.2.2, 8.3.2, Cross-Chapter Box 10.1

It is ''likely'' that the proportionof major (Category 3–5) tropical cyclones (TCs) and the frequency of rapid TC intensification events have increased over the past four decades. The average location of peak TC wind-intensity has ''very likely'' migrated poleward in the western North Pacific Ocean since the 1940s, and TC forward translation speed has ''likely'' slowed over the contiguous USA since 1900. It is ''likely'' that the poleward migration of TCs in the western North Pacific and the global increase in TC intensity rates cannot be explained entirely by natural variability '''.''' There is ''high confidence'' that average peak TC wind speeds and the proportion of Category 4–5 TCs will increase with warming and that peak winds of the most intense TCs will increase. There is ''medium confidence'' that the average location where TCs reach their maximum wind-intensity will migrate poleward in the western North Pacific Ocean, while the total global frequency of TC formation will decrease or remain unchanged with increasing global warming. Links to chapters 11.7.1

There is ''low confidence'' in observed recent changes in the total number of extratropical cyclones over both hemispheres. There is also ''low confidence'' in past-century trends in the number and intensity of the strongest extratropical cyclones over the Northern Hemisphere due to the large interannual-to-decadal variability and temporal and spatial heterogeneities in the volume and type of assimilated data in atmospheric reanalyses, particularly before the satellite era. Over the Southern Hemisphere, it is ''likely'' that the number of extratropical cyclones with low central pressures (&lt;980 hPa) has increased since 1979. The frequency of intense extratropical cyclones is projected to decrease (''medium confidence''). Projected changes in the intensity depend on the resolution of climate models (''medium confidence''). There is ''medium confidence'' that wind speeds associated with extratropical cyclones will change following changes in the storm tracks. Links to chapters 2.3.1, 3.3.3, 4.5.1, 4.5.3, 8.3.2, 8.4.2, 11.7.2

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