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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'''Box TS.3 | Low-likelihood, High-warming Storylines'''

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'''Future global warming exceeding the assessed ''very likely'' range cannot be ruled out and is potentially associated with the highest risks for society and ecosystems. Such low-likelihood, high-warming storylines tend to exhibit substantially greater changes in the intensity of regional drying and wetting than the multi-model mean. Even at levels of warming within the ''very likely'' range, global and regional low-likelihood outcomes might occur, such as large precipitation changes, additional sea level rise associated with collapsing ice sheets (see Box TS.4), or abrupt ocean circulation changes. While there is ''medium confidence'' that the Atlantic Meridional Overturning Circulation (AMOC) will not experience an abrupt collapse before 2100, if it were to occur, it would ''very likely'' cause abrupt shifts in regional weather patterns and water cycle. The probability of these low-likelihood outcomes increases with higher global warming levels. If the real-world climate sensitivity lies at the high end of the assessed range, then global and regional changes substantially outside the ''very likely'' range projections occur for a given emissions scenario. With increasing global warming, some very rare extremes and some compound events (multivariate or concurrent extremes) with low likelihood in past and current climate will become more frequent, and there is a higher chance that events unprecedented in the observational record occur ( ''high confidence'' ). Finally, low-likelihood, high-impact outcomes may also arise from a series of very large volcanic eruptions that could substantially alter the 21st century climate trajectory compared to SSP-based Earth system model (ESM) projections. Links to chapters Cross-Chapter Box 4.1, 4.3, 4.4, 4.8, 7.3, 7.4, 7.5, 8.6, 9.2, 9.6, Box 9.4, Box 11.2, Cross-Chapter Box 12.1'''
Previous IPCC reports largely focused their assessment on the projected ''very likely'' range of future surface warming and associated climate change. However, a comprehensive risk assessment also requires considering the potentially larger changes in the physical climate system that are ''unlikely'' or ''very unlikely'' but possible and potentially associated with the highest risks for society and ecosystems (Figure TS.6). Since AR5, the development of physical climate storylines of high warming has emerged as a useful approach for exploring the future risk space that lies outside of the IPCC ''very likely'' range projections. Links to chapters 4.8

Uncertainty in the true values of equilibrium climate sensitivity (ECS) and transient climate response (TCR) dominate uncertainty in projections of future warming under moderate to strong emissions scenarios (Section TS.3.2). A real-world ECS higher than the assessed ''very likely'' range (2°C–5°C) would require a strong historical aerosol cooling and/or a trend towards stronger warming from positive feedbacks linked to changes in SST patterns (pattern effects), combined with a strong positive cloud feedback and substantial biases in paleoclimate reconstructions – each of which is assessed as either ''unlikely'' or ''very unlikely'' , but not ruled out. Since CMIP6 contains several ESMs that exceed the upper bound of the assessed ''very likely'' range in future surface warming, these models can be used to develop low-likelihood, high warming storylines to explore risks and vulnerabilities, even in the absence of a quantitative assessment of likelihood. Links to chapters 4.3.4, 4.8, 7.3.2, 7.4.4, 7.5.2, 7.5.5, 7.5.7

CMIP6 models with surface warming outside, or close to, the upper bound of the ''very likely'' range exhibit patterns of large widespread temperature and precipitation changes that differ substantially from the multi-model mean in all scenarios. For SSP5-8.5, the high-warming models exhibit widespread warming of more than 6°C over most extratropical land regions and parts of the Amazon. In the Arctic, annual mean temperatures increase by more than 10°C relative to present-day, corresponding to about 30% more than the best estimate of warming. Even for SSP1-2.6, high-warming models show on average 2°C–3°C warming relative to present-day conditions over much of Eurasia and North America (about 40% more than the best estimate of warming) and more than 4°C warming relative to the present over the Arctic in 2081–2100 (Box TS.3, Figure 1). Such a high global warming storyline would imply that the remaining carbon budget consistent with a 2°C warming is smaller than the assessed ''very likely'' range. Put another way, even if a carbon budget that ''likely'' limits warming to 2°C is met, a low-likelihood, high-warming storyline would result in warming of 2.5°C or more. Links to chapters 4.8

CMIP6 models with global warming close to the upper bound of the assessed ''very likely'' warming range tend to exhibit greater changes in the intensity of regional drying and wetting than the multi-model mean. Furthermore, these model projections show a larger area of drying and tend to show a larger fraction of strong precipitation increases than the multi-model mean. However, regional precipitation changes arise from both thermodynamic and dynamic processes so that the most pronounced global warming levels are not necessarily associated with the strongest precipitation response. Abrupt human-caused changes to the water cycle cannot be ruled out. Positive land surface feedbacks, involving vegetation and dust, can contribute to abrupt changes in aridity, but there is only ''low confidence'' that such changes will occur during the 21st century. Continued Amazon deforestation, combined with a warming climate, raises the probability that this ecosystem will cross a tipping point into a dry state during the 21st century ( ''low confidence'' ). (See also Box TS.9). Links to chapters 4.8, 8.6.2

While there is ''medium confidence'' that the projected decline in the AMOC (Section TS.2.4) will not involve an abrupt collapse before 2100, such a collapse might be triggered by an unexpected meltwater influx from the Greenland Ice Sheet. If an AMOC collapse were to occur, it would ''very likely'' cause abrupt shifts in the regional weather patterns and water cycle, such as a southward shift in the tropical rain belt, and could result in weakening of the African and Asian monsoons, strengthening of Southern Hemisphere monsoons, and drying in Europe. (See also Boxes TS.9 and TS.13). Links to chapters 4.7.2, 8.6.1, 9.2.3

Very rare extremes and compound or concurrent events, such as the 2018 concurrent heatwaves across the Northern Hemisphere, are often associated with large impacts. The changing climate state is already altering the likelihood of extreme events, such as decadal droughts and extreme sea levels, and will continue to do so under future warming. Compound events and concurrent extremes contribute to increasing probability of low-likelihood, high-impact outcomes and will become more frequent with increasing global warming ( ''high confidence'' ). Higher warming levels increase the likelihood of events unprecedented in the observational record. Links to chapters 9.6.4, Box 11.2

Finally, low likelihood storylines need not necessarily relate solely to the human-induced changes in climate. A low-likelihood, high-impact outcome, consistent with historical precedent in the past 2500 years, would be to see several large volcanic eruptions that could greatly alter the 21st century climate trajectory compared to SSP-based Earth system model projections. Links to chapters Cross-Chapter Box 4.1

[[File:7e63d3fd8262a24f1090c1c7176ff70b IPCC_AR6_WGI_TS_Box_3_Figure_1.png]]

'''Box TS.3, Figure 1 |'''

'''High-warming storylines.''' ''The intent of this figure is to illustrate high warming storylines compared to the CMIP6 multi-model-mean.'' '''(a)''' Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model mean linearly scaled to the assessed best global surface temperature estimate for SSP1-2.6 in 2081–2100 relative to 1995–2014, '''(b)''' mean across five high-warming models with global surface temperature changes nearest to the upper bound of the assessed very likely range, and '''(c)''' mean across five very high-warming models with global surface temperature changes higher than the assessed ''very likely'' . '''(d–f)''' Same as (a–c) but for SSP5-8.5. Note the different colour bars in (a–c) and (d–f). Links to chapters 4.7, Figure 4.41

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<span id="ts.2.4-the-ocean"></span>
=== TS.2.4 The Ocean ===

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'''Observations, models and paleo-evidence indicate that recently observed changes in the ocean are unprecedented for centuries to millennia ( ''high confidence'' ). Over the past four to six decades, it is ''virtually certain'' that the global ocean has warmed, with human influence ''extremely likely'' the main driver since the 1970s, making climate change irreversible over centuries to millennia ( ''medium confidence'' ). It is ''virtually certain'' that upper ocean salinity contrasts have increased since the 1950s and ''extremely likely'' that human influence has contributed. It is ''virtually certain'' that upper ocean stratification has increased since 1970 and that sea water pH has declined globally over the last 40 years, with human influence being the main driver of the observed surface open ocean acidification ( ''virtually certain'' ). A long-term increase in surface open ocean pH occurred over the past 50 million years ( ''high confidence'' ), and surface ocean pH as low as recent times is uncommon in the last 2 million years ( ''medium confidence'' ). There is ''high confidence'' that marine heatwaves have become more frequent in the 20th century, and most of those since 2006 have been attributed to anthropogenic warming ( ''very likely'' ) . There is ''high confidence'' that oxygen levels have dropped in many regions since the mid 20th century and that the geographic range of many marine organisms has changed over the last two decades.''' '''The amount of ocean warming observed since 1971 will ''likely'' at least double by 2100 under a low warming scenario (SSP1-2.6) and will increase by 4–8 times under a high warming scenario (SSP5-8.5). Stratification ( ''virtually certain'' ), acidification ( ''virtually certain'' ), deoxygenation ( ''high confidence'' ) and marine heatwave frequency ( ''high confidence'' ) will continue to increase in the 21st century. While there is ''low confidence'' in 20th century AMOC change, it is ''very likely'' that AMOC will decline over the 21st century (Figure TS.11). Links to chapters 2.3, 3.5, 3.6, 4.3.2, 5.3, 7.2, 9.2, Box 9.2, 12.4'''
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[[File:6647f473cbdb47481186bde6a877fdad IPCC_AR6_WGI_TS_Figure_11.png]]

'''Figure TS.11 |''' '''Past and future ocean and ice-sheet changes.''' ''The intent of this figure is to show that observed and projected time series of many ocean and cryosphere indicators are consistent.'' Observed and simulated historical changes and projected future changes under varying greenhouse gas emissions scenarios. Simulated and projected ocean changes are shown as Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble mean, and 5–95% range (shading) is provided for scenarios SSP1-2.6 and SSP3-7.0 (except in panel a where the range is provided for scenario SSP1-2.6 and SSP5-8.5). Mean and 5–95% range in 2100 are shown as vertical bars on the right-hand side of each panel. (a) Change in multiplication factor in surface ocean marine heatwave days relative to 1995–2014 (defined as days exceeding the 99th percentile in sea surface temperature (SST) from 1995–2014 distribution). Assessed observational change span 1982–2019 from AVHRR satellite SST. (b) Atlantic Meridional Overturning Circulation (AMOC) transport relative to 1995–2014 (defined as maximum transport at 26°N). Assessed observational change spans 2004–2018 from the RAPID array smoothed with a 12-month running mean (shading around the mean shows the 12-month running standard deviation around the mean). (c) Global mean percent change in ocean oxygen (100–600 m depth), relative to 1995–2014. Assessed observational trends and ''very likely'' range are from the SROCC assessment, and span 1970–2010 centred on 2005. (d) Global mean surface pH. Assessed observational change spans 1985–2019, from the CMEMS SOCAT-based reconstruction (shading around the global mean shows the 90% confidence interval). (e), (f) : Ice sheet mass changes. Projected ice-sheet changes are shown as median, 5–95% range (light shading), and 17–83% range (dark shading) of cumulative mass loss and sea level equivalent from ISMIP6 emulation under SSP1-2.6 and SSP5-8.5 (shading and bold line), with individual emulated projections as thin lines. Median (dot), 17–83% range (thick vertical bar), and 5–95% range (thin vertical bar) in 2100 are shown as vertical bars on the right-hand side of each panel, from ISMIP6, ISMIP6 emulation, and LARMIP-2. Observation-based estimates: For Greenland (e), for 1972–2018 (Mouginot), for 1992–2016 (Bamber), for 1992–2020 (IMBIE) and total estimated mass loss range for 1840–1972 (Box). For Antarctica (f), estimates based on satellite data combined with simulated surface mass balance and glacial isostatic adjustment for 1992–2020 (IMBIE), 1992–2016 (Bamber), and 1979–2017 (Rignot). Left inset maps: mean Greenland elevation changes 2010–2017 derived from CryoSat-2 radar altimetry (e) and mean Antarctica elevation changes 1978–2017 derived from restored analogue radar records (f). Right inset maps: ISMIP6 model mean (2093–2100) projected changes under the MIROC5 climate model for the RCP8.5 scenario. Links to chapters 2.3.3; 2.3.4; 3.5.4; 4.3.2; 5.3.2; 5.3.3; 5.6.3; 9.2.3; 9.4.1; 9.4.2; Box 9.2; Box 9.2, Figure 1; Figures 9.10, 9.17 and 9.18

It is ''virtually certain'' that the global ocean has warmed since at least 1971, representing about 90% of the increase in the global energy inventory (Section TS.3.1). The ocean is currently warming faster than at any other time since at least the last deglacial transition ( ''medium confidence'' ), with warming extending to depths well below 2000 m ( ''very high confidence'' ). It is ''extremely likely'' that human influence was the main driver of this recent ocean warming. Ocean warming will continue over the 21st century ( ''virtually certain'' ), and will ''likely'' continue until at least to 2300 even for low CO <sub>2</sub> emissions scenarios. Ocean warming is irreversible over centuries to millennia ( ''medium confidence'' ), but the magnitude of warming is scenario-dependent from about the mid-21st century ( ''medium confidence'' ). The warming will not be globally uniform, with heat primarily stored in Southern Ocean water-masses and weaker warming in the subpolar North Atlantic ( ''high confidence'' ). Limitations in the understanding of feedback mechanisms limit our confidence in future ocean warming close to Antarctica and how this will affect sea ice and ice shelves. Links to chapters 2.3.3, 3.5.1, 4.7.2, 7.2.2, 9.2.2, 9.2.3, 9.2.4, 9.3.2, 9.6.1, Cross-Chapter Box 9.1

Global mean SST has increased since the beginning of the 20th century by 0.88 [0.68 to 1.01] °C, and it is ''virtually certain'' it will continue to increase throughout the 21st century, with increasing hazards to marine ecosystems ( ''medium confidence'' ). Marine heatwaves have become more frequent over the 20th century ( ''high confidence'' ), approximately doubling in frequency ( ''high confidence'' ) and becoming more intense and longer since the 1980s ( ''medium confidence'' ). Most of the marine heatwaves over 2006–2015 have been attributed to anthropogenic warming ( ''very likely'' ) . Marine heatwaves will continue to increase in frequency, with a ''likely'' global increase of 2–9 times in 2081–2100 compared to 1995–2014 under SSP1-2.6, and 3–15 times under SSP5-8.5 (Figure TS.11a), with the largest changes in the tropical and Arctic ocean. Links to chapters 2.3.1, Cross-Chapter Box 2.3, 9.2.1, Box 9.2, 12.4.8

Observed upper-ocean stratification (0–200 m) has increased globally since at least 1970 ''('' ''virtually certain'' '')'' . Based on recent refined analyses of the available observations, there is ''high confidence'' that it increased by 4.9 ± 1.5% from 1970–2018, which is about twice as much as assessed in SROCC, and will continue to increase throughout the 21st century at a rate depending on the emissions scenario ( ''virtually certain'' ). Links to chapters 2.3.3, 9.2.1

It is ''virtually certain'' that since 1950 near-surface high-salinity regions have become more saline, while low-salinity regions have become fresher, with ''medium confidence'' that this is linked to an intensification of the hydrological cycle (Box TS.6). It is ''extremely likely'' that human influence has contributed to this salinity change and that the large-scale pattern will grow in amplitude over the 21st century ( ''medium confidence'' ). Links to chapters 2.3.3, 3.5.2, 9.2.2, 12.4.8

The AMOC was relatively stable during the past 8000 years ( ''medium confidence'' ). There is ''low confidence'' in the quantification of AMOC changes in the 20th century because of ''low agreement'' in quantitative reconstructed and simulated trends, missing key processes in both models and measurements used for formulating proxies, and new model evaluations. Direct observational records since the mid-2000s are too short to determine the relative contributions of internal variability, natural forcing and anthropogenic forcing to AMOC change ( ''high confidence'' ). An AMOC decline over the 21st century is ''very likely'' for all SSP scenarios (Figure TS.11b); a possible abrupt decline is assessed further in Box TS.3. Links to chapters 2.3.3, 3.5.4, 4.3.2, 8.6.1, 9.2.3, Cross-Chapter Box 12.3

There is ''high confidence'' that many ocean currents will change in the 21st century in response to changes in wind stress. There is ''low confidence'' in 21st century change of Southern Ocean circulation, despite ''high confidence'' that it is sensitive to changes in wind patterns and increased ice-shelf melt. Western boundary currents and subtropical gyres have shifted poleward since 1993 ( ''medium confidence'' ). Subtropical gyres, the East Australian Current Extension, the Agulhas Current, and the Brazil Current are projected to intensify in the 21st century in response to changes in wind stress, while the Gulf Stream and the Indonesian Throughflow are projected to weaken ( ''medium confidence'' ). All of the four main eastern boundary upwelling systems are projected to weaken at low latitudes and intensify at high latitudes in the 21st century ( ''high confidence'' ). Links to chapters 2.3.3, 9.2.3

It is ''virtually certain'' that surface pH has declined globally over the last 40 years and that the main driver is uptake of anthropogenic CO <sub>2</sub> . Ocean acidification and associated reductions in the saturation state of calcium carbonate – a constituent of skeletons or shells of a variety of marine organisms – is expected to increase in the 21st century under all emissions scenarios ( ''high confidence'' ). A long-term increase in surface open ocean pH occurred over the past 50 million years ( ''high confidence'' ), and surface ocean pH as low as recent times is uncommon in the last 2 million years ( ''medium confidence'' ). There is ''very high confidence'' that present-day surface pH values are unprecedented for at least 26,000 years and current rates of pH change are unprecedented since at least that time. Over the past 2–3 decades, a pH decline in the ocean interior has been observed in all ocean basins ( ''high confidence'' ) (Figure TS.11d). Links to chapters 2.3.3, 2.3.4, 3.6.2, 4.3.2, 5.3.2, 5.3.3, 5.6.3, 12.4.8

Open-ocean deoxygenation and expansion of oxygen minimum zones have been observed in many areas of the global ocean since the mid 20th century ( ''high confidence'' ), in part due to human influence ( ''medium confidence'' ). Deoxygenation is projected to continue to increase with ocean warming ( ''high confidence'' ) (Figure TS.11c). Higher climate sensitivity and reduced ocean ventilation in CMIP6 compared to CMIP5 results in substantially greater projections of subsurface (100–600 m) oxygen decline than reported in SROCC for the period 2080–2099. Links to chapters 2.3.3, 2.3.4, Cross-Chapter Box 2.4, 3.6.2, 5.3.3, 12.4.8

Over at least the last two decades, the geographic range of many marine organisms has shifted towards the poles and towards greater depths ( ''high confidence'' ), indicative of shifts towards cooler waters. The range of a smaller subset of organisms has shifted equatorward and to shallower depths ( ''high confidence'' ). Phenological metrics associated with the life cycles of many organisms have also changed over the last two decades or longer ( ''high confidence'' ). Since the changes in the geographical range of organisms and their phenological metrics have been observed to differ with species and location, there is the possibility of disruption to major marine ecosystems. Links to chapters 2.3.4

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<span id="ts.2.5-the-cryosphere"></span>
=== TS.2.5 The Cryosphere ===

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'''Over recent decades, widespread loss of snow and ice has been observed, and several elements of the cryosphere are now in states unseen in centuries ( ''high confidence'' ). Human influence was ''very likely'' the main driver of observed reductions in Arctic sea ice since the late 1970s (with late-summer sea ice loss ''likely'' unprecedented for at least 1000 years) and the widespread retreat of glaciers (unprecedented in at least the last 2,000 years, ''medium confidence'' ). Furthermore, human influence ''very likely'' contributed to the observed Northern Hemisphere spring snow cover decrease since 1950.''' '''By contrast, Antarctic sea ice area experienced no significant net change since 1979, and there is only ''low confidence'' in its projected changes. The Arctic Ocean is projected to become practically sea ice-free in late summer under high CO <sub>2</sub> emissions scenarios by the end of the 21st century ( ''high confidence'' ). It is ''virtually certain'' that further warming will lead to further reductions of Northern Hemisphere snow cover, and there is ''high confidence'' that this is also the case for near-surface permafrost volume.''' '''Glaciers will continue to lose mass at least for several decades even if global temperature is stabilized ( ''very high confidence'' ), and mass loss over the 21st century is ''virtually certain'' for the Greenland Ice Sheet and ''likely'' for the Antarctic Ice Sheet. Deep uncertainty persists with respect to the possible evolution of the Antarctic Ice Sheet within the 21st century and beyond, in particular due to the potential instability of the West Antarctic Ice Sheet. Links to chapters 2.3, 3.4, 4.3, 8.3, 9.3–9.6, Box 9.4, 12.4'''
Current Arctic sea ice coverage levels (both annual and late summer) are at their lowest since at least 1850 ( ''high confidence'' ), and for late summer for the past 1000 years ( ''medium confidence'' ). Since the late 1970s, Arctic sea ice area and thickness have decreased in both summer and winter, with sea ice becoming younger, thinner and more dynamic ( ''very high confidence'' ). It is ''very likely'' that anthropogenic forcing, mainly due to greenhouse gas increases, was the main driver of this loss, although new evidence suggests that anthropogenic aerosol forcing has offset part of the greenhouse gas-induced losses since the 1950s ( ''medium confidence'' ). The annual Arctic sea ice area minimum will ''likely'' fall below 1 million km <sup>2</sup> at least once before 2050 under all assessed SSP scenarios. This practically sea ice-free state will become the norm for late summer by the end of the 21st century in high CO <sub>2</sub> emissions scenarios ( ''high confidence'' ). Arctic summer sea ice varies approximately linearly with global surface temperature, implying that there is no tipping point and observed/projected losses are potentially reversible ( ''high'' ''confidence'' ). Links to chapters 2.3.2, 3.4.1, 4.3.2, 9.3.1, 12.4.9

For Antarctic sea ice, there is no significant trend in satellite-observed sea ice area from 1979 to 2020 in both winter and summer, due to regionally opposing trends and large internal variability. Due to mismatches between model simulations and observations, combined with a lack of understanding of reasons for substantial inter-model spread, there is ''low confidence'' in model projections of future Antarctic sea ice changes, particularly at the regional level. Links to chapters 2.3.2, 3.4.1, 9.3.2

In permafrost regions, increases in ground temperatures in the upper 30 m over the past three to four decades have been widespread ( ''high confidence'' ). For each additional 1°C of warming (up to 4°C above the 1850–1900 level), the global volume of perennially frozen ground to 3 m below the surface is projected to decrease by about 25% relative to the present volume ( ''medium confidence'' ). However, these decreases may be underestimated due to an incomplete representation of relevant physical processes in ESMs ( ''low confidence'' ). Seasonal snow cover is treated in Section TS.2.6. Links to chapters 2.3.2, 9.5.2, 12.4.9

There is ''very high confidence'' that, with few exceptions, glaciers have retreated since the second half of the 19th century; this behaviour is unprecedented in at least the last 2000 years ( ''medium confidence'' ). Mountain glaciers ''very likely'' contributed 67.2 [41.8 to 92.6] mm to the observed GMSL change between 1901 and 2018. This retreat has occurred at increased rates since the 1990s, with human influence ''very likely'' being the main driver. Under RCP2.6 and RCP8.5, respectively, glaciers are projected to lose 18% ± 13% and 36% ± 20% of their current mass over the 21st century ( ''medium confidence'' ). Links to chapters 2.3.2, 3.4.3, 9.5.1, 9.6.1

The Greenland Ice Sheet was smaller than at present during the Last Interglacial period (roughly 125,000 years ago) and the mid-Holocene (roughly 6,000 years ago) ( ''high confidence'' ). After reaching a recent maximum ice mass at some point between 1450 and 1850, the ice sheet retreated overall, with some decades ''likely'' close to equilibrium (i.e., mass loss approximately equalling mass gained). It is ''virtually certain'' that the Greenland Ice Sheet has lost mass since the 1990s, with human influence a contributing factor ( ''medium confidence'' ). There is ''high confidence'' that annual mass changes have been consistently negative since the early 2000s. Over the period 1992–2020, Greenland ''likely'' lost 4890 ± 460 Gt of ice, contributing 13.5 ± 1.3 mm to GMSL rise. There is ''high confidence'' that Greenland ice mass losses are increasingly dominated by surface melting and runoff, with large interannual variability arising from changes in surface mass balance. Projections of future Greenland ice-mass loss (Box TS.4, Table 1; Figure TS.11e) are dominated by increased surface melt under all emissions scenarios ( ''high confidence'' ). Potential irreversible long-term loss of the Greenland Ice Sheet, and of parts of the Antarctic Ice Sheet, is assessed in Box TS.9. Links to chapters 2.3.2, 3.4.3, 9.4.1, 9.4.2, 9.6.3, Atlas.11.2

It is ''likely'' that the Antarctic Ice Sheet has lost 2670 ± 530 Gt, contributing 7.4 ± 1.5 mm to GMSL rise over 1992–2020. The total Antarctic ice mass losses were dominated by the West Antarctic Ice Sheet, with combined West Antarctic and Peninsula annual loss rates increasing since about 2000 ( ''very high confidence'' ). Furthermore, it is ''very likely'' that parts of the East Antarctic Ice Sheet have lost mass since 1979. Since the 1970s, snowfall has ''likely'' increased over the western Antarctic Peninsula and eastern West Antarctica, with large spatial and interannual variability over the rest of Antarctica. Mass losses from West Antarctic outlet glaciers, mainly induced by ice shelf basal melt ( ''high confidence'' ), outpace mass gain from increased snow accumulation on the continent ( ''very high confidence'' ). However, there is only ''limited evidence'' , with ''medium agreement'' , of anthropogenic forcing of the observed Antarctic mass loss since 1992 (with ''low confidence'' in process attribution). Increasing mass loss from ice shelves and inland discharge will ''likely'' continue to outpace increasing snowfall over the 21st century (Figure TS.11f). Deep uncertainty persists with respect to the possible evolution of the Antarctic Ice Sheet along high-end mass-loss storylines within the 21st century and beyond, primarily related to the abrupt and widespread onset of marine ice sheet instability and marine ice cliff instability. (See also Boxes TS.3 and TS.4). Links to chapters 2.3.2, 3.4.3, 9.4.2, 9.6.3, Box 9.4, Atlas.11.1

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'''Box TS.4 | Sea Level'''

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'''Global mean sea level (GMSL) increased by 0.20 [0.15 to 0.25] m over the period 1901 to 2018, with a rate of rise that has accelerated since the 1960s to 3.7 [3.2 to 4.2] mm yr <sup>–1</sup> for the period 2006–2018 <sup></sup> ( ''high confidence'' ). Human activities were ''very likely'' the main driver of observed GMSL rise since 1971, and new observational evidence leads to an assessed sea level rise over the period 1901 to 2018 that is consistent with the sum of individual components contributing to sea level rise, including expansion due to ocean warming and melting of glaciers and ice sheets ( ''high confidence'' ). It is ''virtually certain'' that GMSL will continue to rise over the 21st century in response to continued warming of the climate system (Box TS.4, Figure 1). Sea level responds to greenhouse gas (GHG) emissions more slowly than global surface temperature, leading to weaker scenario dependence over the 21st century than for global surface temperature ( ''high confidence'' ). This slow response also leads to long-term committed sea level rise, associated with ongoing ocean heat uptake and the slow adjustment of the ice sheets, that will continue over the centuries and millennia following cessation of emissions ( ''high confidence'' ) (Box TS.9). By 2100, GMSL is projected to rise by 0.28–0.55 m ( ''likely'' range) under SSP1-1.9 and 0.63–1.01 m ( ''likely'' range) under SSP5-8.5 relative to the 1995–2014 average ( ''medium confidence'' ). Under the higher CO <sub>2</sub> emissions scenarios, there is deep uncertainty in sea level projections for 2100 and beyond associated with the ice-sheet responses to warming. In a low-likelihood, high-impact storyline and a high CO <sub>2</sub> emissions scenario, ice-sheet processes characterized by deep uncertainty could drive GMSL rise up to about 5 m by 2150. Given the long-term commitment, uncertainty in the timing of reaching different GMSL rise levels is an important consideration for adaptation planning. Links to chapters 2.3, 3.4, 3.5, 9.6, Box 9.4, Cross-Chapter Box 9.1, Table 9.5'''
GMSL change is driven by warming or cooling of the ocean (and the associated expansion/contraction) and changes in the amount of ice and water stored on land. Paleo-evidence shows that GMSL has been about 70 m higher and 130 m lower than present within the past 55 million years and was ''likely'' 5 to 10 m higher during the Last Interglacial (Box TS.2, Figure 1). Sea level observations show that GMSL rose by 0.20 [0.15 to 0.25] m over the period 1901–2018 at an average rate of 1.7 [1.3 to 2.2] mm yr <sup>–1</sup> . New analyses and paleo-evidence since AR5 show this rate is ''very likely'' faster than during any century over at least the last three millennia ( ''high confidence'' ). Since AR5, there is strengthened evidence for an increase in the rate of GMSL rise since the mid-20th century, with an average rate of 2.3 [1.6–3.1] mm yr <sup>–1</sup> over the period 1971–2018 increasing to 3.7 [3.2–4.2] mm yr <sup>–1</sup> for the period 2006–2018 ( ''high confidence'' ). Links to chapters 2.3.3, 9.6.1, 9.6.2

[[File:716d64a7d5c79cf79e3eb0d85c55b6b9 IPCC_AR6_WGI_TS_Box_4_Figure_1.png]]

'''Box TS.4, Figure 1 |'''

'''Global mean sea level (GMSL) change on different time scales and under different scenarios.''' ''The intent of this figure is to (i) show the century-scale GMSL projections in the context of the 20th century observations, (ii) illustrate ‘deep uncertainty’ in projections by considering the timing of GMSL rise milestones, and (iii) show the long-term commitment associated with different warming levels, including the paleo evidence to support this.'' '''(a)''' GMSL change from 1900 to 2150, observed (1900–2018) and projected under the SSP scenarios (2000–2150), relative to a 1995–2014 baseline. Solid lines show median projections. Shaded regions show ''likely'' ranges for SSP1-2.6 and SSP3-7.0. Dotted and dashed lines show respectively the 83rd and 95th percentile ''low confidence'' projections for SSP5-8.5. Bars at right show ''likely'' ranges for SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5 in 2150. Lightly shaded thick/thin bars show 17th–83rd/5th–95th percentile ''low-confidence'' ranges in 2150 for SSP1-2.6 and SSP5-8.5, based upon projection methods incorporating structured expert judgement and marine ice cliff instability. ''Low confidence'' range for SSP5-8.5 in 2150 extends to 4.8/5.4 m at the 83rd/95th percentile. '''(b)''' GMSL change on 100- (blue), 2000- (green) and 10,000-year (magenta) time scales as a function of global surface temperature, relative to 1850–1900. For 100-year projections, GMSL is projected for the year 2100, relative to a 1995–2014 baseline, and temperature anomalies are average values over 2081–2100. For longer-term commitments, warming is indexed by peak warming above 1850–1900 reached after cessation of emissions. Shaded regions show paleo-constraints on global surface temperature and GMSL for the Last Interglacial and mid-Pliocene Warm Period. Lightly shaded thick/thin blue bars show 17th–83rd/5th–95th percentile ''low confidence'' ranges for SSP1-2.6 and SSP5-8.5 in 2100, plotted at 2°C and 5°C. '''(c)''' Timing of exceedance of GMSL thresholds of 0.5, 1.0, 1.5 and 2.0 m, under different SSPs. Lightly shaded thick/thin bars show 17th–83rd/5th–95th percentile ''low-confidence'' ranges for SSP1-2.6 and SSP5-8.5. Links to chapters 4.3.2, 9.6.1, 9.6.2, 9.6.3, Box 9.4
GMSL will continue to rise throughout the 21st century (Box TS.4, Figure 1a). Considering only those processes in whose projections we have at least ''medium confidence'' , relative to the period 1995–2014, GMSL is projected to rise between 0.18 m (0.15–0.23 m, ''likely range'' ; SSP1-1.9) and 0.23 m (0.20–0.30 m, ''likely range'' ; SSP5-8.5) by 2050. By 2100, the projected rise is between 0.38 m (0.28–0.55 m, ''likely range'' ; SSP1-1.9) and 0.77 m (0.63–1.01 m, ''likely range'' ; SSP5-8.5) Links to chapters Table 9.9 . The methods, models and scenarios used for sea level projections in the AR6 are updated from those employed by SROCC, with contributions informed by the latest model projections described in the ocean and cryosphere Sections (Sections TS.2.4 and TS.2.5). Despite these differences, the sea level projections are broadly consistent with those of SROCC. Links to chapters 4.3.2, 9.6.3

Importantly, ''likely'' range projections do not include those ice-sheet-related processes whose quantification is highly uncertain or that are characterized by deep uncertainty. Higher amounts of GMSL rise before 2100 could be caused by earlier-than-projected disintegration of marine ice shelves, the abrupt, widespread onset of marine ice sheet instability (MISI) and marine ice cliff instability (MICI) around Antarctica, and faster-than-projected changes in the surface mass balance and dynamical ice loss from Greenland (Box TS.4, Figure 1). In a low-likelihood, high-impact storyline and a high CO <sub>2</sub> emissions scenario, such processes could in combination contribute more than one additional meter of sea level rise by 2100 (Box TS.3). Links to chapters 4.3.2, 9.6.3, Box 9.4

Beyond 2100, GMSL will continue to rise for centuries to millennia due to continuing deep ocean heat uptake and mass loss from ice sheets, and will remain elevated for thousands of years ( ''high confidence'' ). By 2150, considering only those processes in whose projections we have at least ''medium confidence'' and assuming no acceleration in ice-mass flux after 2100, GMSL is projected to rise between 0.6 m (0.4–0.9 m, ''likely'' range, SSP1-1.9) and 1.3 m (1.0–1.9 m, ''likely'' range) (SSP5-8.5), relative to the period 1995–2014 based on the SSP scenario extensions. Under high CO <sub>2</sub> emissions, processes in which there is ''low confidence'' , such as MICI, could drive GMSL rise up to about 5 m by 2150 (Box TS.4, Figure 1a). By 2300, GMSL will rise 0.3–3.1 m under low CO <sub>2</sub> emissions (SSP1-2.6) ( ''low confidence'' ). Under high CO <sub>2</sub> emissions (SSP5-8.5), projected GMSL rise is between 1.7 and 6.8 m by 2300 in the absence of MICI and by up to 16 m considering MICI ( ''low confidence'' ). Over 2000 years, there is ''medium agreemen'' t and ''limited evidence'' that committed GMSL rise is projected to be about 2–3 m with 1.5°C peak warming, 2–6 m with 2°C of peak warming, 4–10 m with 3°C of peak warming, 12–16 m with 4°C of peak warming, and 19–22 m with 5°C of peak warming. Links to chapters 9.6.3

Looking at uncertainty in time provides an alternative perspective on uncertainty in future sea level rise (Box TS.4, Figure 1c). For example, considering only ''medium confidence'' processes, GMSL rise is likely to exceed 0.5 m between about 2080 and 2170 under SSP1-2.6 and between about 2070 and 2090 under SSP5-8.5. Given the long-term commitment, uncertainty in the timing of reaching different levels of GMSL rise is an important consideration for adaptation planning. Links to chapters 9.6.3

At regional scales, additional processes come into play that modify the local sea level change relative to GMSL, including vertical land motion, ocean circulation and density changes, and gravitational, rotational, and deformational effects arising from the redistribution of water and ice mass between land and the ocean. These processes give rise to a spatial pattern that tends to increase sea level rise at the low latitudes and reduce sea level rise at high latitudes. However, over the 21st century, the majority of coastal locations have a median projected regional sea level rise within ±20% of the projected GMSL change ( ''medium confidence'' ). Further details on regional sea level change and extremes are provided in Section TS.4. Links to chapters 9.6.3

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'''Box TS.5 | The Carbon Cycle'''

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'''The continued growthof atmospheric CO <sub>2</sub> concentrations over the industrial era is unequivocally due to emissions from human activities. Ocean and land carbon sinks slow the rise of CO <sub>2</sub> in the atmosphere. Projections show that while land and ocean sinks absorb more CO <sub>2</sub> under high emissions scenarios than low emissions scenarios, the fraction of emissions removed from the atmosphere by natural sinks decreases with higher concentrations ( ''high confidence'' ). Projected ocean and land sinks show similar responses for a given scenario, but the land sink has a much higher interannual variability and wider model spread. The slowed growth rates of the carbon sinks projected for the second half of this century are linked to strengthening carbon–climate feedbacks and stabilization of atmospheric CO <sub>2</sub> under medium-to-no-mitigation and high-mitigation scenarios, respectively (see FAQ 5.1). Links to chapters 5.2, 5.4'''
Carbon sinks for anthropogenic CO <sub>2</sub> are associated with mainly physical ocean and biospheric land processes that drive the exchange of carbon between multiple land, ocean and atmospheric reservoirs. These exchanges are driven by increasing atmospheric CO <sub>2</sub> , but are modulated by changes in climate (Box TS.5, Figure 1c,d). The Northern and Southern Hemispheres dominate the land and ocean sinks, respectively (Box TS.5, Figure 1). Ocean circulation and thermodynamic processes also play a critical role in coupling the global carbon and energy (heat) cycles. There is ''high confidence'' that this ocean carbon–heat nexus is an important basis for one of the most important carbon–climate metrics, the transient climate response to cumulative CO <sub>2</sub> emissions (TCRE; Section TS.3.2.1) used to determine the remaining carbon budget. Links to chapters 5.1, 5.2, 5.5, 9.2, Cross-Chapter Box 5.3

Based on multiple lines of evidence using interhemispheric gradients of CO <sub>2</sub> concentrations, isotopes, and inventory data, it is unequivocal that the growth in CO <sub>2</sub> in the atmosphere since 1750 (see Section TS.2.2) is due to the direct emissions from human activities. The combustion of fossil fuels and land-use change for the period 1750–2019 resulted in the release of 700 ± 75 PgC ( ''likely'' range, 1 PgC = 10 <sup>15</sup> g of carbon) to the atmosphere, of which about 41% ± 11% remains in the atmosphere today ( ''high confidence'' ). Of the total anthropogenic CO <sub>2</sub> emissions, the combustion of fossil fuels was responsible for about 64% ± 15%, growing to an 86% ± 14% contribution over the past 10 years. The remainder resulted from land-use change. During the last decade (2010–2019), average annual anthropogenic CO <sub>2</sub> emissions reached the highest levels in human history at 10.9 ± 0.9 PgC yr <sup>–1</sup> ( ''high confidence'' ). Of these emissions, 46% accumulated in the atmosphere (5.1 ± 0.02 PgC yr <sup>–1</sup> ), 23% (2.5 ± 0.6 PgC yr <sup>–1</sup> ) was taken up by the ocean and 31% (3.4 ± 0.9 PgC yr <sup>–1</sup> ) was removed by terrestrial ecosystems ( ''high confidence'' ). Links to chapters 5.2.1, 5.2.2, 5.2.3

The ocean ( ''high confidence'' ) and land ( ''medium confidence'' ) sinks of CO <sub>2</sub> have increased with anthropogenic emissions over the past six decades (Box TS.5, Figure 1). This coherence between emissions and the growth in ocean and land sinks has resulted in the airborne fraction of anthropogenic CO <sub>2</sub> remaining at 44 ± 10% over the past 60 years ( ''high confidence'' ). Interannual and decadal variability of the ocean and land sinks indicate that they are sensitive to changes in the growth rate of emissions as well as climate variability and are therefore also sensitive to climate change ( ''high confidence'' ). Links to chapters 5.2.1

The land CO <sub>2</sub> sink is driven by carbon uptake by vegetation, with large interannual variability, for example, linked to the El Niño–Southern Oscillation (ENSO). Since the 1980s, carbon fertilization from rising atmospheric CO <sub>2</sub> has increased the strength of the net land CO <sub>2</sub> sink ( ''medium confidence'' ). During the historical period, the growth of the ocean sink has been primarily determined by the growth rate of atmospheric CO <sub>2</sub> . However, there is ''medium confidence'' that changes to physical and chemical processes in the ocean and in the land biosphere, which govern carbon feedbacks, are already modifying the characteristics of variability, particularly the seasonal cycle of CO <sub>2</sub> , in both the ocean and land. However, changes to the multi-decadal trends in the sinks have not yet been observed. Links to chapters 2.3.4, 3.6.1, 5.2.1

In AR6, ESM projections are assessedwith CO <sub>2</sub> concentrations by 2100 from about 400 ppm (SSP1-1.9) to above 1100 ppm (SSP5-8.5). Most simulations are performed with prescribed atmospheric CO <sub>2</sub> concentrations, which already account for a central estimate of climate–carbon feedback effects. Carbon dioxide emissions-driven simulations account for uncertainty in these feedbacks, but do not significantly change the projected global surface temperature changes ( ''high confidence'' ). Although land and ocean sinks absorb more CO <sub>2</sub> under high emissions than low emissions scenarios, the fraction of emissions removed from the atmosphere decreases ( ''high confidence'' ). This means that the more CO <sub>2</sub> that is emitted, the less efficient the ocean and land sinks become ( ''high confidence'' ), an effect which compensates for the logarithmic relationship between CO <sub>2</sub> and its radiative forcing, which means that for each unit increase in additional atmospheric CO <sub>2</sub> the effect on global temperature decreases. (Box TS.5, Figure 1f,g). Links to chapters 4.3.1, 5.4.5, 5.5.1.2

Ocean and land sinks show similar responses for a given scenario, but the land sink has a much higher interannual variability and wider model spread. Under SSP3-7.0 and SSP5-8.5, the initial growth of both sinks in response to increasing atmospheric concentrations of CO <sub>2</sub> is subsequently limited by emerging carbon–climate feedbacks ( ''high confidence'' ) (Box TS.5, Figure 1f). Projections show that the ocean and land sinks will stop growing from the second part of the 21st century under all emissions scenarios, but with different drivers for different emissions scenarios. Under SSP3-7.0 and SSP5-8.5, the weakening growth rate of the ocean CO <sub>2</sub> sink in the second half of the century is primarily linked to the strengthening positive feedback from reduced carbonate buffering capacity, ocean warming and altered ocean circulation (e.g., AMOC changes). In contrast, for SSP1-1.9, SSP1-2.6 and SSP2-4.5, the weakening growth rate of the ocean carbon sink is a response to the stabilizing or declining atmospheric CO <sub>2</sub> concentrations. Under SSP1-1.9, models project that combined land and ocean sinks will turn into a weak source by 2100 ( ''medium confidence'' ). Under high CO <sub>2</sub> emissions scenarios, it is ''very likely'' that the land carbon sink will grow more slowly due to warming and drying from the mid-21st century, but it is ''very unlikely'' that it will switch from being a sink to a source before 2100.

[[File:d76380f25c73dae46566738fb31df1a2 IPCC_AR6_WGI_TS_Box_5_Figure_1.png]]

'''Box TS.5, Figure 1 |'''

'''Carbon cycle processes and projections.''' ''The intent of this figure is to show the response of the carbon cycle to carbon dioxide (CO'' 2 '') emissions and climate and its role in determining future CO'' 2 ''levels through projected changes to sinks and sink fractions.'' The figure shows changes in carbon storage in response to elevated CO <sub>2</sub> '''(a, b)''' and the response to climate warming '''(c, d)''' . Maps show spatial patterns of changes in carbon uptake during simulations with 1% per year increase in CO <sub>2</sub> ( [[IPCC:Wg1:Chapter:Chapter-5#5.4.5.5|Section 5.4.5.5]] ), and zonal mean plots show distribution of carbon changes is dominated by the land (green lines) in the tropics and Northern Hemisphere and ocean (blue lines) in the Southern Hemisphere. Hatching indicates regions where fewer than 80% of models agree on the sign of response. '''(e)''' Future CO <sub>2</sub> projections: projected CO <sub>2</sub> concentrations in the Shared Socio-economic Pathway (SSP) scenarios in response to anthropogenic emissions, results from coupled Earth system models for SSP5-8.5 and from the MAGICC7 emulator for other scenarios ( [[IPCC:Wg1:Chapter:Chapter-4#4.3.1|Section 4.3.1]] ). '''(f)''' Future carbon fluxes: projected combined land and ocean fluxes (positive downward) up to 2100 for the SSP scenarios, and extended to 2300 for available scenarios, 5–95% uncertainty plumes shown for SSP1-2.6 and SSP3-7.0 (Sections 4.3.2.4, 5.4.5.4 and 5.4.10). The numbers near the top show the number of model simulations used. '''(g)''' Sink fraction: the fraction of cumulative emissions of CO <sub>2</sub> removed by land and ocean sinks. The sink fraction is smaller under conditions of higher emissions. Links to chapters Figure 4.3; 5.4.5; Figures 5.25, 5.27 and 5.30
Climate change alone is expected to increase land carbon accumulation in the high latitudes (not including permafrost, which is assessed in Sections TS.2.5 and TS.3.2.2), but also to lead to a counteracting loss of land carbon in the tropics ( ''medium confidence'' ). Earth system model projections show that the overall uncertainty of atmospheric CO <sub>2</sub> by 2100 is still dominated by the emissions pathway, but carbon–climate feedbacks (see Section TS.3.3.2) are important, with increasing uncertainties in high emissions pathways (Box TS.5, Figure 1e). Links to chapters 4.3.2, 5.4.1, 5.4.2, 5.4.4, 5.4.5, 11.6, 11.9, Cross-Chapter Box 5.1, Cross-Chapter Box 5.3

Under three SSP scenarios with long-term extensions until 2300 (SSP5-8.5, SSP5-3.4-OS, SSP1-2.6), ESMs project a change of the land from a sink to a source ( ''medium confidence'' ). The scenarios make simplified assumptions about emissions reductions, with SSP1-2.6 and SSP5-3.4-OS reaching about 400 ppm by 2300, while SSP5-8.5 exceeds 2000 ppm. Under high emissions, the transition is warming-driven, whereas it is linked to the decline in atmospheric CO <sub>2</sub> under net negative CO <sub>2</sub> emissions. The ocean remains a sink throughout the period to 2300 except under very large net negative emissions. The response of the natural aspects of the carbon cycle to carbon dioxide removal is further developed in Section TS.3.3.2. Links to chapters 5.4.9

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=== TS.2.6 Land Climate, Including Biosphere and Extremes ===

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'''Land surface air temperatures have risen faster than the global surface temperature since the 1850s, and it is ''virtually certain'' that this differential warming will persist into the future. It is ''virtually certain'' that the frequency and intensity of hot extremes and the intensity and duration of heatwaves have increased since 1950 and will further increase in the future even if global warming is stabilized at 1.5°C. The frequency and intensity of heavy precipitation events have increased over a majority of those land regions with good observational coverage ( ''high confidence'' ) and will ''extremely likely'' increase over most land regions with additional global warming.''' '''Over the past half century, key aspects of the biosphere have changed in ways that are consistent with large-scale warming: climate zones have shifted poleward, and the growing season length in the Northern Hemisphere extratropics has increased ( ''high confidence'' ). The amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> poleward of 45°N has increased since the 1960s ( ''very high confidence'' ), with increasing productivity of the land biosphere due to the increasing atmospheric CO <sub>2</sub> concentration as the main driver ( ''medium confidence'' ). Global-scale vegetation greenness has increased since the 1980s ( ''high confidence'' ). Links to chapters 2.3, 3.6, 4.3, 4.5, 5.2, 11.3, 11.4, 11.9, 12.4'''
Observed temperatures over land have increased by 1.59 [1.34–1.83] °C between the period 1850–1900 and 2011–2020. Warming of the land is about 45% larger than for global surface temperature and about 80% larger than warming of the ocean surface. Warming of the land surface during the period 1971–2018 contributed about 5% of the increase in the global energy inventory (Section TS.3.1), nearly twice the estimate in AR5 ( ''high confidence'' ). It is ''virtually certain'' that the average surface warming over land will continue to be higher than over the ocean throughout the 21st century. The warming pattern will ''likely'' vary seasonally, with northern high latitudes warming more during winter than summer ( ''medium confidence'' ). Links to chapters 2.3.1, 4.3.1, 4.5.1, 7.2.2, Box 7.2, Cross-Chapter Box 9.1, 11.3, Atlas 11.2

The frequency and intensity of hot extremes (warm days and nights) and the intensity and duration of heatwaves have increased globally and in most regions since 1950, while the frequency and intensity of cold extremes have decreased ( ''virtually certain'' ). There is ''high confidence'' that the increases in frequency and severity of hot extremes are due to human-induced climate change. Some recent extreme events would have been ''extremely unlikely'' to occur without human influence on the climate system. It is ''virtually certain'' that further changes in hot and cold extremes will occur throughout the 21st century in nearly all inhabited regions, even if global warming is stabilized at 1.5°C (Table TS.2, Figure TS.12a). Links to chapters 1.3, Cross-Chapter Box 3.2, 11.1.4, 11.3.2, 11.3.4, 11.3.5, 11.9, 12.4

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[[File:9dd167a3c0d644133e472f8dd404196f IPCC_AR6_WGI_TS_Figure_12.png]]

'''Figure TS.12 |''' '''Land-related changes relative to the 1850-1900 as a function of global warming levels.''' ''The intent of this figure is to show that extremes and mean land variables change consistently with warming levels and to show the changes with global warming levels of water cycle indicators (i.e., precipitation and runoff) over tropical and extratropical land in terms of mean and interannual variability (interannual variability increases at a faster rate than the mean)'' ''.'' (a) Changes in the frequency (left scale) and intensity (in °C, right scale) of daily hot extremes occurring every 10 and 50 years. (b) as (a), but for daily heavy precipitation extremes, with intensity change in %. (c) Changes in 10-year droughts aggregated over drought-prone regions (WNA, CNA, NCA, SCA, NSA, NES, SAM, SWS, SSA, WCE, MED, WSAF, ESAF, MDG, SAU, and EAU; for definitions of these regions, see Figure Atlas.2), with drought intensity (right scale) represented by the change of annual mean soil moisture, normalized with respect to interannual variability. Limits of the 5% − 95% confidence interval are shown in panels (a–c). (d) Changes in Northern Hemisphere spring (March–April–May) snow cover extent relative to 1850–1900; (e,f) Relative change (%) in annual mean of total precipitable water (grey line), precipitation (red solid lines), runoff (blue solid lines) and in standard deviation (i.e., variability) of precipitation (red dashed lines) and runoff (blue dashed lines) averaged over (e) tropical and (f) extratropical land as function of global warming levels. Coupled Model Intercomparison Project Phase 6 (CMIP6) models that reached a 5°C warming level above the 1850–1900 average in the 21st century in SSP5-8.5 have been used. Precipitation and runoff variability are estimated by respective standard deviation after removing linear trends. Error bars show the 17–83% confidence interval for the warmest +5°C global warming level. Links to chapters Figures 8.16, 9.24, 11.6, 11.7, 11.12, 11.15, 11.18 and Atlas.2

Greater warming over land alters key water cycle characteristics (Box TS.6). The rates of change in mean precipitation and runoff, and their variability, increase with global warming (Figure TS.12e,f). Human-induced climate change has contributed to increases in agricultural and ecological droughts in some regions due to increases in evapotranspiration ( ''medium confidence'' ). More regions are affected by increases in agricultural and ecological droughts with increasing global warming ( ''high confidence'' ; see also Figure TS.12c).

There is ''low confidence'' that the increase of plant water-use efficiency due to higher atmospheric CO <sub>2</sub> concentration alleviates extreme agricultural and ecological droughts in conditions characterized by limited soil moisture and increased atmospheric evaporative demand. Links to chapters 2.3.1, Cross-Chapter Box 5.1, 8.2.3, 8.4.1, 11.2.4, 11.4, 11.6, Box 11.1

Northern Hemisphere spring snow cover has decreased since at least 1978 ( ''very high confidence'' ), and there is ''high confidence'' that trends in snow cover loss extend back to 1950. It is ''very likely'' that human influence contributed to these reductions. Earlier onset of snowmelt has contributed to seasonally dependent changes in streamflow ( ''high confidence'' ). A further decrease of Northern Hemisphere seasonal snow cover extent is ''virtually certain'' under further global warming (Figure TS.12d). Links to chapters 2.3.2, 3.4.2, 8.3.2. 9.5.3, 12.4, 9.2, 11.2, [[IPCC:Wg1:Chapter:Atlas|Atlas]] 8.2

The frequency and intensity of heavy precipitation events have increased over a majority of land regions with good observational coverage since 1950 ( ''high confidence,'' Box TS.6, Table TS.2). Human influence is ''likely'' the main driver of this change (Table TS.2). It is ''extremely likely'' that on most land regions heavy precipitation will become more frequent and more intense with additional global warming (Table TS.2, Figure TS.12b). The projected increase in heavy precipitation extremes translates to an increase in the frequency and magnitude of pluvial floods ( ''high confidence'' ) (Table TS.2). Links to chapters Cross-Chapter Box 3.2, 8.4.1, 11.4.2, 11.4.4, 11.5.5, 12.4

Theprobability of compound extreme events has ''likely'' increased due to human-induced climate change. Concurrent heatwaves and droughts have become more frequent over the last century, and this trend will continue with higher global warming ( ''high confidence'' ). The probability of compound flooding (storm surge, extreme rainfall and/or river flow) has increased in some locations and will continue to increase due to both sea level rise and increases in heavy precipitation, including changes in precipitation intensity associated with tropical cyclones ( ''high confidence'' ). Links to chapters 11.8.1, 11.8.2, 11.8.3

Changes in key aspects of the terrestrial biosphere, such as an increase of the growing season length in much of the Northern Hemisphere extratropics since the mid-20th century ( ''high confidence'' ), are consistent with large-scale warming. At the same time an increase in the amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> poleward of 45°N since the early 1960s ( ''high confidence'' ) and a global-scale increase in vegetation greenness of the terrestrial surface since the early 1980s ( ''high confidence'' ) have been observed. Increasing atmospheric CO <sub>2</sub> , warming at high latitudes, and land management interventions have contributed to the observed greening trend, but there is ''low confidence'' in their relative roles. There is ''medium confidence'' that increased plant growth associated with CO <sub>2</sub> fertilization is the main driver of the observed increase in amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> in the Northern Hemisphere. Reactive nitrogen, ozone and aerosols affect terrestrial vegetation and carbon cycle through deposition and effects on large-scale radiation ( ''high confidence'' ), but the magnitude of these effects on the land carbon sink, ecosystem productivity and indirect CO <sub>2</sub> forcing remains uncertain. Links to chapters 2.3.4, 3.6.1, 5.2.1, 6.4.5, 12.3.7, 12.4

Over the last century, there has been a poleward and upslope shift in the distribution of many land species ( ''very high confidence'' ) as well as increases in species turnover within many ecosystems ( ''high confidence'' ). There is ''high confidence'' that the geographical distribution of climate zones has shifted in many parts of the world in the last half century. The SRCCL concluded that continued warming will exacerbate desertification processes ( ''medium confidence'' ) and that ecosystems will become increasingly exposed to climates beyond those that they are currently adapted to ( ''high confidence'' ). There is ''medium confidence'' that climate change will increase disturbance by, for example, fire and tree mortality, across several ecosystems. Increases are projected in drought, aridity and fire weather in some regions (Section TS.4.3; ''high confidence'' ). There is ''low confidence'' in the magnitude of these changes, but the probability of crossing uncertain regional thresholds (e.g., fires, forest dieback) increases with further warming ( ''high confidence'' ). The response of biogeochemical cycles to the anthropogenic perturbation can be abrupt at regional scales, and irreversible on decadal to century time scales ( ''high confidence'' ). Links to chapters 2.3.4, 5.4.3, 5.4.9, 11.6, 11.8, 12.5, SRCCL 2.2, SRCCL 2.5, SR1.5 3.4

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'''Box TS.6 | Water Cycle'''

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'''Human-caused climate change has driven detectable changes in the global water cycle since the mid-20th century ( ''high confidence'' ), and it is projected to cause substantial further changes at both global and regional scales ( ''high confidence'' ).''' '''Global land precipitation has ''likely'' increased since 1950, with a faster increase since the 1980s ( ''medium confidence'' ). Atmospheric water vapour has increased throughout the troposphere since at least the 1980s ( ''likely'' ) . Annual global land precipitation will increase over the 21st century as global surface temperature increases ( ''high confidence'' ). Human influence has been detected in amplified surface salinity and precipitation minus evaporation (P–E) patterns over the ocean ( ''high confidence'' ).''' '''The severity of very wet and very dry events increase in a warming climate ( ''high confidence'' ), but changes in atmospheric circulation patterns affect where and how often these extremes occur. Water cycle variability and related extremes are projected to increase faster than mean changes in most regions of the world and under all emissions scenarios ( ''high confidence'' ).''' '''Over the 21st century, the total land area subject to drought will increase and droughts will become more frequent and severe ( ''high confidence'' ). Near-term projected changes in precipitation are uncertain mainly because of internal variability, model uncertainty and uncertainty in forcings from natural and anthropogenic aerosols ( ''medium confidence'' ).''' '''Over the 21st century and beyond, abrupt human-caused changes to the water cycle cannot be excluded ( ''medium confidence'' ). Links to chapters 2.3, 3.3, 4.3, 4.4, 4.5, 4.6, 8.2, 8.3, 8.4, 8.5, 8.6, 11.4, 11.6, 11.9'''
There is ''high confidence'' that the global water cycle has intensified since at least 1980 expressed by, for example, increased atmospheric moisture fluxes and amplified precipitation minus evaporation patterns. Global land precipitation has ''likely'' increased since 1950, with a faster increase since the 1980s ( ''medium confidence'' ), and a ''likely'' human contribution to patterns of change, particularly for increases in high-latitude precipitation over the Northern Hemisphere. Increases in global mean precipitation are determined by a robust response to global surface temperature ( ''very likely'' 2–3% per °C) that is partly offset by fast atmospheric adjustments to atmospheric heating by greenhouse gases (GHGs) and aerosols (Section TS.3.2.2). The overall effect of anthropogenic aerosols is to reduce global precipitation through surface radiative cooling effects ( ''high confidence'' ). Over much of the 20th century, opposing effects of GHGs and aerosols on precipitation have been observed for some regional monsoons ( ''high confidence'' ) (Box TS.13). Global annual precipitation over land is projected to increase on average by 2.4% (–0.2% to +4.7% ''likely'' range) under SSP1-1.9, 4.6% (1.5% to 8.3% ''likely'' range) under SSP2-4.5, and 8.3% (0.9% to 12.9% ''likely'' range) under SSP5-8.5 by 2081–2100 relative to 1995–2014 (Box TS.6, Figure 1). Inter-model differences and internal variability contribute to a substantial range in projections of large-scale and regional water cycle changes ( ''high confidence'' ). The occurrence of volcanic eruptions can alter the water cycle for several years ( ''high confidence'' ). Projected patterns of precipitation change exhibit substantial regional differences and seasonal contrast as global surface temperature increases over the 21st century (Box TS.6, Figure 1). Links to chapters 2.3.1, 3.3.2, 3.3.3, 3.5.2, 4.3.1, 4.4.1, 4.5.1, 4.6.1, Cross-Chapter Box 4.1, 8.2.1, 8.2.2, 8.2.3, Box 8.1, 8.3.2.4, 8.4.1, 8.5.2, 10.4.2

Global total column water vapour content has ''very likely'' increased since the 1980s, and it is ''likely'' that human influence has contributed to tropical upper tropospheric moistening. Near-surface specific humidity has increased over the ocean ( ''likely'' ) and land ( ''very likely'' ) since at least the 1970s, with a detectable human influence ( ''medium confidence'' ). Human influence has been detected in amplified surface salinity and precipitation minus evaporation (P–E) patterns over the ocean ( ''high confidence'' ). It is ''virtually certain'' that evaporation will increase over the ocean and ''very likely'' that evapotranspiration will increase over land, with regional variations under future surface warming (Box TS.6, Figure 1). There is ''high confidence'' that projected increases in precipitation amount and intensity will be associated with increased runoff in northern high latitudes (Box TS.6, Figure 1). In response to cryosphere changes (Section TS.2.5), there have been changes in streamflow seasonality, including an earlier occurrence of peak streamflow in high-latitude and mountain catchments ( ''high confidence'' ). Projected runoff (Box TS.6, Figure 1c) is typically decreased by contributions from small glaciers because of glacier mass loss, while runoff from larger glaciers will generally increase with increasing global warming levels until their mass becomes depleted ( ''high confidence'' ). Links to chapters 2.3.1, 3.3.2, 3.3.3, 3.5.2, 8.2.3, 8.4.1, 11.5

Warming over land drives an increase in atmospheric evaporative demand and in the severity of drought events ( ''high confidence'' ). Greater warming over land than over the ocean alters atmospheric circulation patterns and reduces continental near-surface relative humidity, which contributes to regional drying ( ''high confidence'' ). A ''very likely'' decrease in relative humidity has occurred over much of the global land area since 2000. Projected increases in evapotranspiration due to growing atmospheric water demand will decrease soil moisture over the Mediterranean region, south-western North America, South Africa, South-Western South America and south-western Australia ( ''high confidence'' ) (Box TS.6, Figure 1). Some tropical regions are also projected to experience enhanced aridity, including the Amazon basin and Central America ( ''high confidence'' ). The total land area subject to increasing drought frequency and severity will expand ( ''high confidence'' ), and in the Mediterranean, South-Western South America, and Western North America, future aridification will far exceed the magnitude of change seen in the last millennium ( ''high confidence'' ). Links to chapters 4.5.1, 8.2.2, 8.2.3, 8.4.1, Box 8.2, 11.6, 11.9

Land-use change and water extraction for irrigation have influenced local and regional responses in the water cycle ( ''high confidence'' ). Large-scale deforestation ''likely'' decreases evapotranspiration and precipitation and increases runoff over the deforested regions relative to the regional effects of climate change ( ''medium confidence'' ). Urbanization increases local precipitation ( ''medium confidence'' ) and runoff intensity ( ''high confidence'' ) (Box TS.14). Increased precipitation intensities have enhanced groundwater recharge, most notably in tropical regions ( ''medium confidence'' ). There is ''high confidence'' that groundwater depletion has occurred since at least the start of the 21st century, as a consequence of groundwater withdrawals for irrigation in agricultural areas in drylands. Links to chapters 8.2.3, 8.3.1, 11.1.6, 11.4, 11.6, FAQ 8.1

Water cycle variability and related extremes are projected to increase faster than mean changes in most regions of the world and under all emissions scenarios ( ''high confidence'' ). A warmer climate increases moisture transport into weather systems, which intensifies wet seasons and events ( ''high confidence'' ). The magnitudes of projected precipitation increases and related extreme events depend on model resolution and the representation of convective processes ( ''high confidence'' ). Increases in near-surface atmospheric moisture capacity of about 7% per 1ºC of warming lead to a similar response in the intensification of heavy precipitation from sub-daily up to seasonal time scales, increasing the severity of flood hazards ( ''high confidence'' ). The average and maximum rain-rates associated with tropical and extratropical cyclones, atmospheric rivers and severe convective storms will therefore also increase with future warming ( ''high confidence'' ). For some regions, there is ''medium confidence'' that peak tropical cyclone rain-rates will increase by more than 7% per 1°C of warming due to increased low-level moisture convergence caused by increases in wind intensity. In the tropics year-round and in the summer season elsewhere, interannual variability of precipitation and runoff over land is projected to increase at a faster rate than changes in seasonal mean precipitation (Figure TS.12e,f) ( ''medium confidence'' ). Sub-seasonal precipitation variability is also projected to increase, with fewer rainy days but increased daily mean precipitation intensity over many land regions ( ''high confidence'' ). Links to chapters 4.5.3, 8.2.3, 8.4.1, 8.4.2, 8.5.1, 8.5.2, 11.4, 11.5, 11.7, 11.9

[[File:dc131ab5f093e225937615a4c3afd167 IPCC_AR6_WGI_TS_Box_6_Figure_1.png]]

'''Box TS.6, Figure 1 |'''

'''Projected water cycle changes.''' ''The intent of this figure is to give a geographical overview of changes in multiple components of the global water cycle using an intermediate emissions scenario. Important key message: without drastic reductions in greenhouse gas emissions, human-induced global warming will be associated with widespread changes in all components of the water cycle.'' Long-term (2081–2100) projected annual mean changes (%) relative to present-day (1995–2014) in the SSP2-4.5 emissions scenario for '''(a)''' precipitation, '''(b)''' surface evapotranspiration, '''(c)''' total runoff and '''(d)''' surface soil moisture. Numbers in top right of each panel indicate indicate the number of Coupled Model Intercomparison Project Phase 6 (CMIP6) models used for estimating the ensemble mean. For other scenarios, please refer to relevant figures in Chapter 8. Uncertainty is represented using the simple approach: No overlay indicates regions with high model agreement, where ≥80% of models agree on sign of change; diagonal lines indicate regions with low model agreement, where &lt;80% of models agree on sign of change. For more information on the simple approach, please refer to the Cross-Chapter Box Atlas.1. Links to chapters 8.4.1; Figures 8.14, 8.17, 8.18, and 8.19

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'''Infographic TS.1 |''' '''Climate Futures'''

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'''Infographic TS.1 |''' '''Climate Futures.''' ''The intent of this figure is to show possible climate futures: The climate change that people will experience this century and beyond depends on our greenhouse gas emissions, how much global warming this will cause and the response of the climate system to this warming.''

'''(top left)''' Annual emissions of CO <sub>2</sub> for the five core Shared Socio-economic Pathway (SSP) scenarios (very low: SSP1-1.9, low: SSP1-2.6, intermediate: SSP2-4.5, high: SSP3-7.0, very high: SSP5-8.5). '''(bottom left)''' Projected warming for each of these emissions scenarios. '''(top right)''' Response of some selected climate variables to four levels of global warming (°C). Changes in the ‘Today’ column are based on a global warming level of 1°C. '''(bottom right)''' The long-term effect of each global warming level on sea level. See Section TS.1.3.1 for more detail on the SSP climate change scenarios. This infographic builds from several figures in the Technical Summary: Figure TS.4 (for top left panel), Figure TS.6 (bottom left), Figure TS.12 (top right) and Box TS.4, Figure 1b (bottom right).

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