Editing IPCC:AR6/WGI/TS (section)

==== TS.4.3.1 Common Regional Changes in Climatic Impact-Drivers ====

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'''Heat and cold:''' Changes in temperature-related CIDs such as mean temperatures, growing season length, and extreme heat and frost have already occurred ( ''high confidence'' ), and many of these changes have been attributed to human activities ( ''medium confidence'' ). Over all land regions with sufficient data (i.e., all except Antarctica), observed changes in temperature have already clearly emerged outside the range of internal variability, relative to 1850–1900 (Figure TS.23). In tropical regions, recent past temperature distributions have already shifted to a range different to that of the early 20th century ( ''high confidence'' ) (Section TS.1.2.4). Most land areas have ''very likely'' warmed by at least 0.1°C per decade since 1960, and faster in recent decades. On regional-to-continental scales, trends of increased frequency of hot extremes and decreased frequency of cold extremes are generally consistent with the global-scale trends in mean temperature ( ''high confidence'' ). In a few regions, trends are difficult to assess due to limited data availability. Links to chapters 2.3.1.1, 11.3, 11.9, 12.4, Atlas.3.1

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'''Figure TS.23 |''' '''Time period during which the signals of temperature change in observed data aggregated over the reference regions emerged from the noise of annual variability in the respective aggregated data, using a signal-to-noise ratio of two as the threshold for emergence.''' ''The intent of this figure is to show, for the AR6 WGI reference regions, when a signal of annual mean surface temperature change emerged from the noise of annual variability in two global datasets and thus also provide some information on observational uncertainty.'' Emergence time is calculated for two global observational datasets: (a) Berkeley Earth and (b) CRUTEM5. Regions in the CRUTEM5 map are shaded grey when data are available over less than 50% of the area of the region. (Section TS.1.2.4) Links to chapters Figure Atlas.11

Warming trends observed in recent decades are projected to continue over the 21st century and over most land regions at a rate higher than the global average ( ''high confidence'' ). For given global warming levels, model projections from CMIP6 show future regional warming changes that are similar to those projected by CMIP5. However, projected regional warming in CMIP6 for given time periods and emissions scenarios has a wider range with a higher upper limit compared to CMIP5 because of the higher climate sensitivity in some CMIP6 models and differences in the forcings. Links to chapters Atlas.3–Atlas.11

Under RCP8.5/SSP5-8.5, it is ''likely'' that most land areas will experience further warming of at least 4°C compared to a 1995–2014 baseline by the end of the 21st century, and in some areas significantly more. At increasing warming levels, extreme heat will exceed critical thresholds for health, agriculture and other sectors more frequently ( ''high confidence'' ), and it is ''likely'' that cold spells will become less frequent towards the end of the century. For example, by the end of the 21st century, dangerous humid heat thresholds, such as the National Oceanic and Atmospheric Administration (NOAA) heat index (HI) threshold of 41°C, will be exceeded much more frequently under the SSP5-8.5 scenario than under SSP1-2.6 and will affect many regions ( ''high confidence'' ). In many tropical regions, the number of days per year where a heat index of 41°C is exceeded would increase by more than 100 days relative to the recent past under SSP5-8.5, while this increase will be limited to less than 50 days under SSP1-2.6 ( ''high confidence'' ) (Figure TS.6). The number of days per year where temperature exceeds 35°C would increase by more than 150 days in many tropical areas, such as the Amazon basin and South East Asia, by the end of century for the SSP5-8.5 scenario, while it is expected to increase by less than 60 days in these areas under SSP1-2.6 (except for the Amazon Basin) ( ''high confidence'' ) (Figure TS.24). Links to chapters 4.6.1, 11.3, 11.9, 12.4, 12.5.2, Atlas

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'''Figure TS.24 |''' '''Projected change in the mean number of days per year with maximum temperature exceeding 35°C for Coupled Model Intercomparison Project Phase 5 (CMIP5; first column), Phase 6 (CMIP6; second column) and Coordinated Regional Climate Downscaling Experiment (CORDEX; third column) ensembles.''' ''The intent of this figure is to show that there is a consistent message about the patterns of projected change in extreme daily temperatures from the CMIP5, CMIP6 and CORDEX ensembles.'' The map shows the median change in the number of days per year between the mid-century (2041–2060) or end-century (2081–2100) and historical (1995–2014) periods for the CMIP5 and CORDEX RCP8.5 and RCP2.6 scenario ensembles and the CMIP6 SSP5-8.5 and SSP1-2.6 scenario ensembles. Hatching indicates areas where less than 80% of the models agree on the sign of change. Links to chapters Interactive Atlas

'''Wet and dry:''' Compared to the global scale, precipitation internal variability is stronger at the regional scale while uncertainties in observations, models and external forcing are all larger. However, GHG forcing has driven increased contrasts in precipitation amounts between wet and dry seasons and weather regimes over tropical land areas ( ''medium confidence'' ), with a detectable precipitation increase in the northern high latitudes ( ''high confidence'' ) (Box TS.6). The frequency and intensity of heavy precipitation events have increased over a majority of land regions with good observational coverage ( ''high confidence'' ). A majority of land areas have experienced decreases in available water in dry seasons due to human-induced climate change associated with changes in evapotranspiration ( ''medium confidence'' ). Global hydrological models project a larger fraction of land areas to be affected by an increase rather than by a decrease in river floods ( ''medium confidence'' ). Extreme precipitation and pluvial flooding will increase in many regions around the world on almost all continents ( ''high confidence'' ), but regional changes in river floods are more uncertain than changes in pluvial floods because complex hydrological processes, including land cover and human water management, are involved. Links to chapters 8.2.2.1, 8.3.1, Box 8.2, 10.4.1, 11.5, 11.6, 11.9, 12.4, 12.5.1, Atlas.3.1

'''Wind:''' Mean wind speed has decreased over most land areas with good observational coverage ( ''medium confidence'' ). It is ''likely'' that the global proportion of major tropical cyclone (TC) intensities (Categories 3–5) over the past four decades has increased. The proportion of intense TCs, average peak TC wind speeds, and peak wind speeds of the most intense TCs will increase on the global scale with increasing global warming ( ''high confidence'' ). Links to chapters 11.7.1

'''Snow and ice:''' Many aspects of the cryosphere either have seen significant changes in the recent past or will see them during the 21st century ( ''high confidence'' ). Glaciers will continue to shrink and permafrost to thaw in all regions where they are present ( ''high confidence'' ). Also, it is ''virtually certain'' that snow cover will experience a decline over most land regions during the 21st century, in terms of water equivalent, extent and annual duration. There is ''high confidence'' that the global warming-induced earlier onset of spring snowmelt and increased melting of glaciers have already contributed to seasonal changes in streamflow in high-latitude and low-elevation mountain catchments. Nevertheless, it is ''very likely'' that some high-latitude regions will experience an increase in winter snow water equivalent due to the effect of increased snowfall prevailing over warming-induced increased snowmelt. (Section TS.2.5) Links to chapters 8.2.2.1, 8.3.1, Box 8.2, 9.4, 9.5.1, 9.5.2, 12.4, Atlas.4–Atlas.9, Atlas.11

'''Coastal and oceanic:''' There is ''high confidence'' that SST will increase in all oceanic regions except the North Atlantic. Regional sea level change has been the main driver of changes in extreme sea levels across the quasi-global tide gauge network over the 20th century ( ''high confidence'' ). With the exception of a few regions with substantial land uplift, relative sea level rise is ''very likely to virtually certain'' (depending on the region) to continue during the 21st century, contributing to increased coastal flooding in low-lying areas ( ''high confidence'' ) and coastal erosion along most sandy coasts ( ''high confidence'' ) over the 21st century. In the open ocean, acidification, changes in sea ice, and deoxygenation have already emerged in many areas ( ''high confidence'' ). Marine heatwaves are also expected to increase around the globe over the 21st century ( ''high confidence'' ). (Section TS.2.4) Links to chapters Box 9.2, 9.2.1.1, 9.6, 9.6.4, 9.6.4.2, 12.4

'''Other variables and concurrent CID changes:''' It is ''virtually certain'' that atmospheric CO <sub>2</sub> and oceanic pH will increase in all climate scenarios, until net zero CO <sub>2</sub> emissions are achieved (Section TS.2.2). In nearly all regions, there is ''low confidence'' in changes in hail, ice storms, severe storms, dust storms, heavy snowfall, and avalanches, although this does not indicate that these CIDs will not be affected by climate change. For such CIDs, observations are often short-term or lack homogeneity, and models often do not have sufficient resolution or accurate parametrizations to adequately simulate them over climate change time scales. The probability of compound events has increased in the past due to human-induced climate change and will ''likely'' continue to increase with further global warming, including for concurrent heatwaves and droughts, compound flooding, and the possibility of connected sectors experiencing multiple regional extreme events at the same time (for example, in multiple breadbaskets) ( ''high confidence'' ). Links to chapters 5.3.4.2, 11.8, Box 11.3, Box 11.4, 12.4

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