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

==== 11.6.5.1 Precipitation Deficits ====

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Studies based on CMIP5, CMIP6 and Coordinated Regional Climate Downscaling Experiment (CORDEX) projections show a consistent signal in the sign and spatial pattern of projections of precipitation deficits. Global studies based on these multi-model ensemble projections ( [[#Orlowsky--2013|Orlowsky and Seneviratne, 2013]] ; [[#Martin--2018|Martin, 2018]] ; [[#Spinoni--2020|Spinoni et al., 2020]] ; [[#Ukkola--2020|Ukkola et al., 2020]] ; [[#Coppola--2021b|Coppola et al., 2021b]] ) show particularly strong signal-to-noise ratios for increasing meteorological droughts in the following AR6 regions: MED, ESAF, WSAF, SAU, CAU, NCA, SCA, NSA and NES ( [[#11.9|Section 11.9]] ). There is also substantial evidence of changes in meteorological droughts at 1.5°C versus 2°C of global warming from global studies ( [[#Wartenburger--2017|Wartenburger et al., 2017]] ; L. [[#Xu--2019|]] [[#Xu--2019|Xu et al., 2019]] ). The patterns of projected changes in mean precipitation are consistent with the changes in the drought duration, but they are not consistent with the changes in drought intensity ( [[#Ukkola--2020|Ukkola et al., 2020]] ). In general, CMIP6 projections suggest a stronger increase of the probability of precipitation deficits than CMIP5 projections ( [[#Cook--2020|Cook et al., 2020]] ; [[#Ukkola--2020|Ukkola et al., 2020]] ). Projections for the number of CDDs in CMIP6 (Figure 11.19) for different levels of global warming relative to 1850–1900 show similar spatial patterns as projected precipitation deficits. The robustness of the patterns in projected precipitation deficits identified in the global studies is also consistent with results from regional studies ( [[#Giorgi--2014|Giorgi et al., 2014]] ; [[#Marengo--2016|Marengo and Espinoza, 2016]] ; [[#Pinto--2016|Pinto et al., 2016]] ; J. [[#Huang--2018|]] [[#Huang--2018|Huang et al., 2018]] ; [[#Maúre--2018|Maúre et al., 2018]] ; [[#Nangombe--2018|Nangombe et al., 2018]] ; [[#Tabari--2018|Tabari and Willems, 2018]] ; [[#Abiodun--2019|Abiodun et al., 2019]] ; [[#Dosio--2019|Dosio et al., 2019]] ).

In Africa, a strong increase in the length of dry spells (CDD) is projected for 4°C of global warming over most of the continent, with the exception of central and eastern Africa ( [[#11.9|Section 11.9]] ; [[#Sillmann--2013a|Sillmann et al., 2013a]] ; [[#Giorgi--2014|Giorgi et al., 2014]] ; [[#Han--2019|Han et al., 2019]] ). In West Africa, a strong reduction of precipitation is projected ( [[#Sillmann--2013a|Sillmann et al., 2013a]] ; [[#Diallo--2016|Diallo et al., 2016]] ; [[#Akinsanola--2019|Akinsanola and Zhou, 2019]] ; [[#Han--2019|Han et al., 2019]] ; [[#Todzo--2020|Todzo et al., 2020]] ) at 4°C of global warming, and CDD would increase with stronger global warming levels ( [[#Klutse--2018|Klutse et al., 2018]] ). The regions most strongly affected are southern Africa (ESAF, WSAF) ( [[#Nangombe--2018|Nangombe et al., 2018]] ; [[#Abiodun--2019|Abiodun et al., 2019]] ) and northern Africa (part of the MED region), with increases in meteorological droughts already at 1.5°C of global warming, and further increases with increasing global warming ( [[#11.9|Section 11.9]] ). CDD is projected to increase more in the southern Mediterranean (northern Africa) than in the northern part of the Mediterranean region ( [[#Lionello--2020|Lionello and Scarascia, 2020]] ).

In Asia, most AR6 regions show ''low confidence'' in projected changes in meteorological droughts at 1.5°C and 2°C of global warming, with a few regions displaying a decrease in meteorological droughts at 4°C of global warming (RAR, ESB, RFE, ECA; ''medium confidence'' ), although there is a projected increase in meteorological droughts in South East Asia at 4°C ( ''medium confidence'' ) ( [[#11.9|Section 11.9]] ). In South East Asia, an increasing frequency of precipitation deficits is projected as a consequence of an increasing frequency of extreme El Niño ( [[#Cai--2014b|Cai et al., 2014b]] , 2015, 2018).

In Central America, projections suggest an increase in mid-summer meteorological drought ( [[#Imbach--2018|Imbach et al., 2018]] ) and increased CDD ( [[#Chou--2014a|Chou et al., 2014a]] ; [[#Giorgi--2014|Giorgi et al., 2014]] ; [[#Nakaegawa--2014|Nakaegawa et al., 2014]] ). In the Amazon, there is also a projected increase in dryness ( [[#Marengo--2016|Marengo and Espinoza, 2016]] ), which is the combination of a projected increase in the frequency and geographic extent of meteorological drought in the eastern Amazon, and an opposite trend in the west ( [[#Duffy--2015|Duffy et al., 2015]] ). In South-Western South America, there is a projected increase of CDD ( [[#Chou--2014a|Chou et al., 2014a]] ; [[#Giorgi--2014|Giorgi et al., 2014]] ) and in Chile, drying is projected to prevail ( [[#Boisier--2018|Boisier et al., 2018]] ). In the South America monsoon region, an increase in CDD is projected ( [[#Chou--2014a|Chou et al., 2014a]] ; [[#Giorgi--2014|Giorgi et al., 2014]] ), but a decrease is projected in South-Eastern and Southern South America ( [[#Giorgi--2014|Giorgi et al., 2014]] ). In Central America, mid-summer meteorological drought is projected to intensify during 2071–2095 for the RCP8.5 scenario ( [[#Corrales-Suastegui--2020|Corrales‐Suastegui et al., 2020]] ).

An increase in the frequency, duration and intensity of meteorological droughts is projected in south-west, south and east Australia ( [[#Kirono--2020|Kirono et al., 2020]] ; [[#Shi--2020|Shi et al., 2020]] ). In Canada and most of the USA, based on the SPI, [[#Swain--2015|Swain and Hayhoe (2015)]] identified drier summer conditions in projections over most of the region, and there is a consistent signal toward an increase in duration and intensity of droughts in southern North America ( [[#Pascale--2016|Pascale et al., 2016]] ; [[#Escalante-Sandoval--2017|Escalante-Sandoval and Nuñez-Garcia, 2017]] ). In California, more precipitation variability is projected, characterized by increased frequency of consecutive drought and humid periods ( [[#Swain--2018|Swain et al., 2018]] ).

Substantial increases in meteorological drought are projected in Europe, in particular in the Mediterranean region, already at 1.5°C of global warming ( [[#11.9|Section 11.9]] ). In southern Europe, model projections display a consistent drying among models ( [[#Russo--2013|Russo et al., 2013]] ; [[#Hertig--2017|Hertig and Tramblay, 2017]] ; [[#Guerreiro--2018a|Guerreiro et al., 2018a]] ; [[#Raymond--2019|Raymond et al., 2019]] ). In Western and Central Europe there is some spread in CMIP5 projections, with some models projecting very strong drying, and others close to no trend ( [[#Vogel--2018|Vogel et al., 2018]] ), although CDD is projected to increase in CMIP5 projections under the RCP 8.5 scenario ( [[#Hari--2020|Hari et al., 2020]] ). The overall evidence suggests an increase in meteorological drought at 4°C in the WCE region ( ''medium confidence'' ) ( [[#11.9|Section 11.9]] ).

Overall, based on global and regional studies, several hot spot regions are identified, displaying more frequent and severe meteorological droughts with increasing global warming, including several AR6 regions at 1.5°C (WSAF, ESAF, SAU, MED, NES) and 2°C of global warming (WSAF, ESAF, EAU, SAU, MED, NCA, SCA, NSA, NES) ( [[#11.9|Section 11.9]] ). At 4°C of global warming, there is also ''confidence'' in increases in meteorological droughts in further regions (WAF, WCE, ENA, CAR, NWS, SAM, SWS, SSA; [[#11.9|Section 11.9]] ), showing a geographical expansion of meteorological drought with increasing global warming. Only few regions are projected to have less intense or frequent meteorological droughts ( [[#11.9|Section 11.9]] ).

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