Editing IPCC:AR6/WGI/Chapter-11 (section)

=== 11.3.5 Projections ===

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The AR5 (Chapter12, [[#Collins--2013|Collins et al., 2013]] ) concluded that it is ''virtually certain'' there will be more frequent hot extremes and fewer cold extremes at the global scale and over most land areas in a future warmer climate, and it is ''very likely'' that heatwaves will occur with a higher frequency and longer duration.The SR1.5 (Chapter 3, [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) assessment on projected changes in hot extremes at 1.5°C and 2°C global warming is consistent with the AR5 assessment, concluding that it is ''very likely'' a global warming of 2°C, when compared with a 1.5°C warming, would lead to more frequent and more intense hot extremes on land, as well as to longer warm spells, affecting many densely inhabited regions. The SR1.5 also assessed it is ''very likely'' that the strongest increases in the frequency of hot extremes are projected for the rarest events, while cold extremes will become less intense and less frequent, and cold spells will be shorter.

New studies since AR5 and SR1.5 confirm these assessments. New literature since AR5 includes projections of temperature-related extremes in relation to changes in mean temperatures, projections based on CMIP6 simulations, projections based on stabilized global warming levels, and the use of new metrics. Constraints for the projected changes in hot extremes were also provided ( [[#Borodina--2017b|Borodina et al., 2017b]] ; [[#Sippel--2017b|Sippel et al., 2017b]] ; [[#Vogel--2017|Vogel et al., 2017]] ). Overall, projected changes in the magnitude of extreme temperatures over land are larger than changes in global mean temperature, over mid-latitude land regions in particular (Figures 11.3, 11.11; [[#Fischer--2014|Fischer et al., 2014]] ; [[#Seneviratne--2016|Seneviratne et al., 2016]] ; B.M. [[#Sanderson--2017|]] [[#Sanderson--2017|Sanderson et al., 2017]] ; [[#Wehner--2018b|Wehner et al., 2018b]] ; [[#Di%20Luca--2020b|Di Luca et al., 2020b]] ). Large warming in hot and cold extremes will occur, even at the 1.5°C GWL (Figure 11.11). At this level, widespread significant changes at the grid-box level occur for different temperature indices ( [[#Aerenson--2018|Aerenson et al., 2018]] ). In agreement with CMIP5 projections, CMIP6 simulations show that a 0.5°C increment in global warming will significantly increase the intensity and frequency of hot extremes, and decrease the intensity and frequency of cold extremes on the global scale (Figures 11.6, 11.8 and 11.12). It takes less than half of a degree for the changes in TXx to emerge above the level of natural variability (Figure 11.8) and the 66% ranges of the land medians of the 10-year or 50-year TXx events do not overlap between 1.0°C and 1.5°C in the CMIP6 multi-model ensemble simulations(Figure 11.6, [[#Li--2021|Li et al., 2021]] ).

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[[File:f76a530dedf85907da4303bd377c6445 IPCC_AR6_WGI_Figure_11_11.png]]

'''Figure 11.11 |''' '''Projected changes in (a–c) annual maximum temperature (TXx) and (d–f) annual minimum temperature (TNn) at 1.5°C, 2°C, and 4°C of global warming compared to the 1850–1900 baseline.''' Results are based on simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensemble under the Shared Socio-economic Pathways (SSPs) SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios. The numbers in the top right indicate the number of simulations included. Uncertainty is represented using the simple approach: no overlay indicates regions with high model agreement, where ≥80% of models agree on the sign of change; diagonal lines indicate regions with low model agreement, where &lt;80% of models agree on the sign of change. For more information on the simple approach, please refer to the Cross-Chapter Box ( [[IPCC:Wg1:Chapter:Atlas|Atlas]] 1. For details on the methods see Supplementary Material 11.SM.2. Changes in TXx and TNn are also displayed in the Interactive Atlas. Further details on data sources and processing are available in the chapter data table (Table 11.SM.9).

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[[File:edd08452914b028e39967547a032c0ea IPCC_AR6_WGI_Figure_11_12.png]]

'''Figure 11.12 |''' '''Projected changes in the intensity of extreme temperature events under 1°C, 1.5°C, 2°C, 3°C, and 4°C global warming levels relative to the 185''' ''0–1900 baseline.'' Extreme temperature events are defined as the daily maximum temperatures (TXx) that were exceeded on average once during a 10-year period (10-year event, blue) and once during a 50-year period (50-year event, orange) during the 1850–1900 base period. Results are shown for the global land. For each box plot, the horizontal line and the box represent the median and central 66% uncertainty range, respectively, of the intensity changes across the multi-model ensemble, and the ‘whiskers’ extend to the 90% uncertainty range. The results are based on the multi-model ensemble from simulations of global climate models contributing to the Coupled Model Intercomparison Project Phase 6 (CMIP6) under different Shared Socio-economic Pathway forcing scenarios. Adapted from [[#Li--2021|Li et al. (2021)]] . Further details on data sources and processing are available in the chapter data table (Table 11.SM.9).

Projected warming is larger for TNn and exhibits strong equator-to-pole amplification, similar to the warming of boreal winter mean temperatures. The warming of TXx is more uniform over land and does not exhibit this behaviour (Figure 11.11). The warming of temperature extremes on global and regional scales tends to scale linearly with global warming ( [[#11.1.4|Section 11.1.4]] ; Fischer et al., 2014; [[#Seneviratne--2016|Seneviratne et al., 2016]] ; [[#Wartenburger--2017|Wartenburger et al., 2017]] ; [[#Li--2021|Li et al., 2021]] ; see also SR1.5, Chapter 3). In the mid-latitudes, the rate of warming of hot extremes can be as large as twice the rate of global warming (Figure 11.11). In the Arctic winter, the rate of warming of the temperature of the coldest nights is about three times the rate of global warming (Appendix, Figure 11.A.1). Projected changes in temperature extremes can deviate from projected changes in annual mean warming in the same regions (Figures 11.3, 11.A.1 and 11.A.2; [[#Di%20Luca--2020b|Di Luca et al., 2020b]] ; [[#Wehner--2020|Wehner, 2020]] ) due to the additional processes that control the response of regional extremes, including, in particular, soil moisture–evapotranspiration–temperature feedbacks for hot extremes in the mid-latitudes and subtropical regions, and snow/ice–albedo–temperature feedbacks in high-latitude regions.

The probability of exceeding a certain hot extreme threshold will increase, while those for cold extreme will decrease with global warming ( [[#Mueller--2016|]] [[#Mueller--2016|B. Mueller et al., 2016]] ; [[#Lewis--2017b|Lewis et al., 2017b]] ; [[#Suarez-Gutierrez--2020b|Suarez-Gutierrez et al., 2020b]] ). The changes tend to scale nonlinearly with the level of global warming, with larger changes for more rare events ( [[#11.2.4|Section 11.2.4]] ; Cross-Chapter Box 11.11; Figures 11.6 and 11.12; [[#Fischer--2015|Fischer and Knutti, 2015]] ; [[#Kharin--2018|Kharin et al., 2018]] ; [[#Li--2021|Li et al., 2021]] ). For example, the CMIP5 ensemble projects the frequency of the present-day climate 20-year hottest daily temperature to increase by 80% at the 1.5°C GWL and by 180% at the 2.0°C GWL, and the frequency of the present-day climate 100-year hottest daily temperature to increase by 200% and more than 700% at the 1.5°C and 2.0°C warming levels, respectively ( [[#Kharin--2018|Kharin et al., 2018]] ). CMIP6 simulations project similar changes ( [[#Li--2021|Li et al., 2021]] ).

[[#Tebaldi--2018|Tebaldi and Wehner (2018)]] showed that, at the middle of the 21st century, 66% of the land surface area would experience the present-day 20-year return values of TXx and the running three-day average of the daily maximum temperature every other year, on average, under the Representative Concentration Pathway 8.5 (RCP8.5) scenario, as opposed to only 34% under RCP4.5. By the end of the century, these area fractions increase to 92% and 62%, respectively. Such nonlinearities in the characteristics of future regional extremes are shown, for instance, for Europe ( [[#Dosio--2018|Dosio and Fischer, 2018]] ; [[#Spinoni--2018b|Spinoni et al., 2018b]] ; [[#Lionello--2020|Lionello and Scarascia, 2020]] ), Asia ( [[#Guo--2017|Guo et al., 2017]] ; [[#Harrington--2018b|Harrington and Otto, 2018b]] ; [[#King--2018|King et al., 2018]] ), and Australia ( [[#Lewis--2017a|Lewis et al., 2017a]] ) under various global warming thresholds. The nonlinear increase in fixed-threshold indices (e.g., based on a percentile for a given reference period, or on an absolute threshold) as a function of global warming is consistent with a linear warming of the absolute temperature of the temperature extremes (e.g., [[#Whan--2015|Whan et al., 2015]] ). Compared to the historical climate, warming will result in strong increases in heatwave area, duration and magnitude ( [[#Vogel--2020b|Vogel et al., 2020b]] ). These changes are mostly due to the increase in mean seasonal temperature, rather than changes in temperature variability, though the latter can have an effect in some regions ( [[#Brown--2020|Brown, 2020]] ; [[#Di%20Luca--2020b|Di Luca et al., 2020b]] ; [[#Suarez-Gutierrez--2020a|Suarez-Gutierrez et al., 2020a]] ).

Projections of temperature-related extremes in RCMs in the CORDEX regions demonstrate robust increases under future scenarios and can provide information on finer spatial scales than GCMs (e.g., [[#Coppola--2021b|Coppola et al., 2021b]] ). Five RCMs in the CORDEX–East Asia region project increases in the 20-year return values of temperature extremes (summer maxima), with models that exhibit warm biases projecting stronger warming ( [[#Park--2019|Park and Min, 2019]] ). Similarly, in the African domain, future increases in TX90p and TN90p are projected ( [[#Dosio--2017|Dosio, 2017]] ; [[#Mostafa--2019|Mostafa et al., 2019]] ). This regional-scale analysis provides fine-scale information, such as distinguishing the increase in TX90p over sub-equatorial Africa (Democratic Republic of the Congo, Angola, and Zambia) with values over the Gulf of Guinea, Central African Republic, South Sudan, and Ethiopia. Empirical statistical downscaling has also been used to produce more robust estimates for future heatwaves compared to RCMs based on large multi-model ensembles ( [[#Furrer--2010|Furrer et al., 2010]] ; [[#Keellings--2014|Keellings and Waylen, 2014]] ; [[#Wang--2015|Wang et al., 2015]] ; [[#Benestad--2018|Benestad et al., 2018]] ).

In all continental regions, including Africa (Table 11.4), Asia (Table 11.7), Australasia (Table 11.10), Central and South America (Table 11.13), Europe (Table 11.16), North America (Table 11.19) and at the continental scale, it is ''very likely'' that the intensity and frequency of hot extremes will increase and the intensity and frequency of cold extremes will decrease compared with the 1995–2014 baseline, even under 1.5°C global warming. Those changes are ''virtually certain'' to occur under 4°C global warming. At the regional scale, and for almost all AR6 regions, it is ''likely'' that the intensity and frequency of hot extremes will increase and the intensity and frequency of cold extremes will decrease compared with the 1995–2014 baseline, even under 1.5°C global warming. Those changes are ''virtually certain'' to occur under 4°C global warming. Exceptions include lower confidence in the projected decrease in the intensity and frequency of cold extremes compared with the 1995–2014 baseline under 1.5°C of global warming ( ''medium confidence'' ) and 4°C of global warming ( ''very likely'' ) in Northern Central America, Central North America, and Western North America.

In Africa (Table 11.4), evidence includes increases in the intensity and frequency of hot extremes, such as warm days, warm nights, and heatwaves, and decreases in the intensity and frequency of cold extremes, such as cold days and cold nights over the continent, as projected by CMIP5, CMIP6, and CORDEX simulations (Giorgi et al., 2014; [[#Engelbrecht--2015|Engelbrecht et al., 2015]] ; [[#Lelieveld--2016|Lelieveld et al., 2016]] ; [[#Russo--2016|Russo et al., 2016]] ; [[#Dosio--2017|Dosio, 2017]] ; [[#Bathiany--2018|Bathiany et al., 2018]] ; [[#Mba--2018|Mba et al., 2018]] ; [[#Nangombe--2018|Nangombe et al., 2018]] ; [[#Weber--2018|Weber et al., 2018]] ; [[#Kruger--2019|Kruger et al., 2019]] ; [[#Coppola--2021b|Coppola et al., 2021b]] ; [[#Li--2021|Li et al., 2021]] ). Cold spells are projected to decrease under all RCPs, and even at low warming levels in Western and Central Africa ( [[#Diedhiou--2018|Diedhiou et al., 2018]] ). The number of cold days is projected to decrease in East Africa ( [[#Ongoma--2018b|Ongoma et al., 2018b]] ).

In Asia (Table 11.7), evidence includes increases in the intensity and frequency of hot extremes, such as warm days, warm nights, and heatwaves, and decreases in the intensity and frequency of cold extremes, such as cold days and cold nights over the continent, as projected by CMIP5, CMIP6, and CORDEX simulations ( [[#Sillmann--2013b|Sillmann et al., 2013b]] ; [[#Zhou--2014|Zhou et al., 2014]] ; R. [[#Zhang--2015|Zhang et al., 2015]] ; [[#Zhao--2015|Zhao et al., 2015]] ; [[#Pal--2016|Pal and Eltahir, 2016]] ; [[#Singh--2016|Singh and Goyal, 2016]] ; [[#Xu--2017|Xu et al., 2017]] ; [[#Gao--2018|Gao et al., 2018]] ; [[#Han--2018|Han et al., 2018]] ; [[#Shin--2018|Shin et al., 2018]] ; [[#Sui--2018|Sui et al., 2018]] ; L. [[#Li--2019|]] [[#Li--2019|]] [[#Li--2019|Li et al., 2019]] ; [[#Zhu--2020|Zhu et al., 2020]] ). More intense heatwaves of longer durations and occurring at a higher frequency are projected over India ( [[#Murari--2015|Murari et al., 2015]] ; [[#Mishra--2017|Mishra et al., 2017]] ) and Pakistan ( [[#Nasim--2018|Nasim et al., 2018]] ). Future mid-latitude warm extremes, similar to those experienced during the 2010 event, are projected to become more extreme, with temperature extremes increasing potentially by 8.4°C (RCP8.5) over north-west Asia ( [[#van%20der%20Schrier--2018|van der Schrier et al., 2018]] ). Over West and East Siberia, and Russian Far East, an increase in extreme heat durations is expected in all scenarios ( [[#Sillmann--2013b|Sillmann et al., 2013b]] ; [[#Kattsov--2017|Kattsov et al., 2017]] ; [[#Reyer--2017|Reyer et al., 2017]] ). In the MENA regions (Arabian Peninsula and Western Central Asia), extreme temperatures could increase by almost 7°C by 2100 under RCP8.5 ( [[#Lelieveld--2016|Lelieveld et al., 2016]] ).

In Australasia (Table 11.10), evidence includes increases in the intensity and frequency of hot extremes, such as warm days, warm nights, and heatwaves, and decreases in the intensity and frequency of cold extremes, such as cold days and cold nights over the continent, as projected by CMIP5, CMIP6, and CORDEX simulations (CSIROand BOM, 2015; [[#Alexander--2017|Alexander and Arblaster, 2017]] ; [[#Lewis--2017a|Lewis et al., 2017a]] ; [[#Herold--2018|Herold et al., 2018]] ; [[#Coppola--2021b|Coppola et al., 2021b]] ; [[#Evans--2021|Evans et al., 2021]] ). Over most of Australia, increases in the intensity and frequency of hot extremes are projected to be predominantly driven by the long-term increase in mean temperatures ( [[#Di%20Luca--2020b|Di Luca et al., 2020b]] ). Future projections indicate a decrease in the number of frost days regardless of the region and season considered ( [[#Alexander--2017|Alexander and Arblaster, 2017]] ; [[#Herold--2018|Herold et al., 2018]] ).

In Central and South America (Table 11.13), evidence includes increases in the intensity and frequency of hot extremes, such as warm days, warm nights, and heatwaves, and decreases in the intensity and frequency of cold extremes, such as cold days and cold nights over the continent, as projected by CMIP5, CMIP6, and CORDEX simulations ( [[#Chou--2014a|Chou et al., 2014a]] ; [[#Cabré--2016|Cabré et al., 2016]] ; [[#López-Franca--2016|López-Franca et al., 2016]] ; [[#Stennett-Brown--2017|Stennett-Brown et al., 2017]] ; [[#Coppola--2021b|Coppola et al., 2021b]] ; [[#Li--2021|Li et al., 2021]] ; [[#Vichot-Llano--2021|Vichot-Llano et al., 2021]] ). Over South-Eastern South America during the austral summer, the increase in the frequency of TN90p is larger than that projected for TX90p, consistent with observed past changes ( [[#López-Franca--2016|López-Franca et al., 2016]] ). Under RCP8.5, the number of heatwave days are projected to increase for the intra-Americas region for the end of the 21st century ( [[#Angeles-Malaspina--2018|Angeles-Malaspina et al., 2018]] ). A general decrease in the frequency of cold spells and frost days is projected, as indicated by several indices based on minimum temperature ( [[#López-Franca--2016|López-Franca et al., 2016]] ).

In Europe (Table 11.16), evidence includes increases in the intensity and frequency of hot extremes, such as warm days, warm nights, and heatwaves, and decreases in the intensity and frequency of cold extremes, such as cold days and cold nights over the continent, as projected by CMIP5, CMIP6, and CORDEX simulations ( [[#Lau--2014|Lau and Nath, 2014]] ; [[#Ozturk--2015|Ozturk et al., 2015]] ; [[#Russo--2015|Russo et al., 2015]] ; [[#Schoetter--2015|Schoetter et al., 2015]] ; [[#Vogel--2017|Vogel et al., 2017]] ; [[#Winter--2017|Winter et al., 2017]] ; [[#Jacob--2018|Jacob et al., 2018]] ; [[#Lhotka--2018|Lhotka et al., 2018]] ; [[#Rasmijn--2018|Rasmijn et al., 2018]] ; [[#Suarez-Gutierrez--2018|Suarez-Gutierrez et al., 2018]] ; [[#Cardoso--2019|Cardoso et al., 2019]] ; [[#Lionello--2020|Lionello and Scarascia, 2020]] ; [[#Molina--2020|Molina et al., 2020]] ; [[#Coppola--2021b|Coppola et al., 2021b]] ; [[#Li--2021|Li et al., 2021]] ). Increases in heatwaves are greater over the southern Mediterranean and Scandinavia ( [[#Forzieri--2016|Forzieri et al., 2016]] ; [[#Abaurrea--2018|Abaurrea et al., 2018]] ; [[#Dosio--2018|Dosio and Fischer, 2018]] ; [[#Rohat--2019|Rohat et al., 2019]] ). Thebiggest increases in the number of heatwave days are expected for southern European cities ( [[#Guerreiro--2018a|Guerreiro et al., 2018a]] ; [[#Junk--2019|Junk et al., 2019]] ), and Central European cities will see the biggest increases in maximum heatwave temperatures ( [[#Guerreiro--2018a|Guerreiro et al., 2018a]] ).

In North America (Table 11.19), evidence includes increases in the intensity and frequency of hot extremes, such as warm days, warm nights, and heatwaves, and decreases in the intensity and frequency of cold extremes, such as cold days and cold nights over the continent, as projected by CMIP5, CMIP6, and CORDEX simulations ( [[#Grotjahn--2016|Grotjahn et al., 2016]] ; [[#Vose--2017|Vose et al., 2017]] ; [[#Alexandru--2018|Alexandru, 2018]] ; [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|C. Li et al., 2018]] , 2021; [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|C. Yang et al., 2018]] ; X. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ; [[#Coppola--2021b|Coppola et al., 2021b]] ). Projections of temperature extremes for the end of the 21st century show that warm days and nights are ''very likely'' to increase, and cold days and nights are ''very likely'' to decrease in all regions. There is ''medium confidence'' in large increases in warm days and warm nights in summer, particularly over the USA, and in large decreases in cold days in Canada in autumn and winter ( [[#Grotjahn--2016|Grotjahn et al., 2016]] ; [[#Vose--2017|Vose et al., 2017]] ; [[#Alexandru--2018|Alexandru, 2018]] ; [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|C. Li et al., 2018]] , 2021; [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|C. Yang et al., 2018]] ; X. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ; [[#Coppola--2021b|Coppola et al., 2021b]] ). Minimum winter temperatures are projected to rise faster than mean winter temperatures ( [[#Underwood--2017|Underwood et al., 2017]] ). Projections for the end of the century under RCP8.5 showed the four-day cold spell that happens on average once every five years is projected to warm by more than 10°C. CMIP5 models do not project current 1-in-20-year annual minimum temperature extremes to recur over much of the continent ( [[#Wuebbles--2014|Wuebbles et al., 2014]] ).

In summary, it is ''virtually certain'' that further increases in the intensity and frequency of hot extremes, and decreases in the intensity and frequency of cold extremes, will occur throughout the 21st century and around the world. It is ''virtually certain'' that the number of hot days and hot nights and the length, frequency, and/or intensity of warm spells or heatwaves compared to 1995–2014 will increase over most land areas. In most regions, changes in the magnitude of temperature extremes are proportional to global warming levels ( ''high confidence'' ). The highest increase of temperature of hottest days is projected in some mid-latitude and semi-arid regions, at about 1.5 times to twice the rate of global warming ( ''high confidence'' ). The highest increase of temperature of coldest days is projected in Arctic regions, at about three times the rate of global warming ( ''high confidence'' ). The probability of temperature extremes generally increases nonlinearly with increasing global warming levels ( ''high confidence'' ). Confidence in assessments depends on the spatial and temporal scales of the extreme in question, with ''high confidence'' in projections of temperature-related extremes at global and continental scales for daily to seasonal scales. There is ''high confidence'' that, on land, the magnitude of temperature extremes increases more strongly than global mean temperature.

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