Editing IPCC:AR6/WGI/TS (section)

=== 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

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'''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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