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

=== 11.6.5 Projections ===

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The SREX (Chapter 3) asssessed with ''medium confidence'' projections of increased drought severity in some regions, including southern Europe and the Mediterranean, central Europe, central America and Mexico, north-east Brazil, and southern Africa, and ''low confidence'' elsewhere given large inter-model spread. The AR5 (Chapters 11 and 12) also assessed large uncertainties in drought projections at the regional and global scales. The assessment of drought mechanisms under future climate change scenarios depends on the model used ( [[#11.6.3|Section 11.6.3]] ). Moreover, uncertainties in drought projections are affected by the consideration of plant physiological responses to increasing atmospheric CO <sub>2</sub> (Cross-Chapter Box 5.1; [[#Milly--2016|Milly and Dunne, 2016]] ; [[#Greve--2019|Greve et al., 2019]] ; [[#Mankin--2019|Mankin et al., 2019]] ; [[#Yang--2020|Yang et al., 2020]] ), the role of soil-moisture–atmosphere feedbacks for changes in water balance and aridity ( [[#Berg--2016|Berg et al., 2016]] ; [[#Zhou--2021|Zhou et al., 2021]] ), and statistical issues related to considered drought time scales ( [[#Vicente-Serrano--2020c|Vicente-Serrano et al., 2020c]] ). Nonetheless, the extensive literature available since AR5 allows a substantially more robust assessment of projected changes in droughts, also subdivided in different drought types (meteorological drought, agricultural and ecological drought, and hydrological drought). This includes assessments of projected changes in droughts, including changes at 1.5°C, 2°C and 4°C of global warming, for all AR6 regions ( [[#11.9|Section 11.9]] ). Projected changes show increases in drought frequency and intensity in several regions as function of global warming ( ''high confidence'' ). There are also substantial increases in drought hazard probability from 1.5°C to 2°C global warming and for further additional increments of global warming ( ''high confidence'' ) (Figures 11.18 and 11.19). These findings are based on both CMIP5 and CMIP6 analyses ( [[#11.9|Section 11.9]] ; [[#Wartenburger--2017|Wartenburger et al., 2017]] ; [[#Greve--2018|Greve et al., 2018]] ; L. [[#Xu--2019|]] [[#Xu--2019|Xu et al., 2019]] ), and strengthen the conclusions of SR1.5 Chapter 3.

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==== 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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==== 11.6.5.2 Atmospheric Evaporative Demand ====

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Effects of AED on droughts in future projections is under debate. The CMIP5 models project an increase in AED over the majority of the world with increasing global warming, mostly as a consequence of strong VPD increases ( [[#Scheff--2015|Scheff and Frierson, 2015]] ; [[#Vicente-Serrano--2020a|Vicente-Serrano et al., 2020a]] ). However, ET is projected to increase less than AED in many regions due to plant physiological responses related to: i) CO <sub>2</sub> effects on plant photosynthesis; and ii) soil moisture control on ET.

Several studies suggest that increasing atmospheric CO <sub>2</sub> could lead to reduced leaf stomatal conductance, which would increase water-use efficiency and reduce plant water needs, thus limiting ET (Cross-Chapter Box 5.1; [[#Roderick--2015|Roderick et al., 2015]] ; [[#Milly--2016|Milly and Dunne, 2016]] ; [[#Swann--2016|Swann et al., 2016]] ; [[#Greve--2017|Greve et al., 2017]] ; [[#Scheff--2017|Scheff et al., 2017]] ; [[#Lemordant--2018|Lemordant et al., 2018]] ; [[#Swann--2018|Swann, 2018]] ). The implemention of a CO <sub>2</sub> -dependent land resistance parameter has been suggested for the estimation of AED ( [[#Yang--2019|Yang et al., 2019]] ). Nevertheless, there are other relevant mechanisms, as soil moisture deficits and VPD also play an important role in the control of the leaf stomatal conductance (Z. [[#Xu--2016|]] [[#Xu--2016|Xu et al., 2016]] ; [[#Menezes-Silva--2019|Menezes-Silva et al., 2019]] ; [[#Grossiord--2020|Grossiord et al., 2020]] ), and a number of ecophysiological and anatomical processes affect the response of plant physiology under higher atmospheric CO <sub>2</sub> concentrations (Cross-Chapter Box 5.1; [[#Mankin--2019|Mankin et al., 2019]] ; [[#Menezes-Silva--2019|Menezes-Silva et al., 2019]] ). The benefits of the atmospheric CO <sub>2</sub> for plant stress and agricultural and ecological droughts would be minimal precisely during dry periods given stomatal closure in response to limited soil moisture ( [[#Allen--2015|Allen et al., 2015]] ; Z. [[#Xu--2016|]] [[#Xu--2016|Xu et al., 2016]] ). In addition, CO <sub>2</sub> effects on plant stomatal conductance could not entirely compensate for the increased demand associated with warming ( [[#Liu--2017|Liu and Sun, 2017]] ); in large tropical and subtropical regions (e.g., southern Africa, the Amazon, the Mediterranean and southern North America), AED is projected to increase, even considering the possible CO <sub>2</sub> effects on land resistance ( [[#Vicente-Serrano--2020a|Vicente-Serrano et al., 2020a]] ). Moreover, these CO <sub>2</sub> effects would not affect the direct evaporation from soil and water bodies, which is very relevant in the reservoirs of warm areas ( [[#Friedrich--2018|Friedrich et al., 2018]] ). Because of these uncertainties, there is ''low confidence'' whether increased CO <sub>2</sub> -induced water-use efficiency in vegetation will substantially reduce global plant transpiration and will diminish the frequency and severity of soil moisture and streamflow deficits associated with the radiative effect of higher CO <sub>2</sub> concentrations (Cross-Chapter Box 5.1).

Another mechanism reducing the ET response to increased AED in projections is the control of soil moisture limitations on ET, which leads to reduced stomatal conductance under water stress ( [[#Berg--2018|Berg and Sheffield, 2018]] ; [[#Stocker--2018|Stocker et al., 2018]] ; [[#Zhou--2021|Zhou et al., 2021]] ). This response may be further amplified through VPD-induced decreases in stomatal conductance ( [[#Anderegg--2020|Anderegg et al., 2020]] ). However, the decreased stomatal conductance in response to soil moisture limitation and enhanced CO <sub>2</sub> would further enhance AED ( [[#Sherwood--2014|Sherwood and Fu, 2014]] ; [[#Berg--2016|Berg et al., 2016]] ; [[#Teuling--2018|Teuling, 2018]] ; [[#Miralles--2019|Miralles et al., 2019]] ), whereby the overall effects on AED in ESMs are found to be of similar magnitude for soil moisture limitation and CO <sub>2</sub> physiological effects on stomatal conductance ( [[#Berg--2016|Berg et al., 2016]] ). Increased AED is thus both a driver and a feedback with respect to changes in ET, complicating the interpretation of its role on drought changes with increasing CO <sub>2</sub> concentrations and global warming.

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==== 11.6.5.3 Soil Moisture Deficits ====

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Areas with projected soil moisture decreases do not fully coincide with areas that have projected precipitation decreases, although there is substantial consistency in the respective patterns ( [[#Dirmeyer--2013|Dirmeyer et al., 2013]] ; [[#Berg--2018|Berg and Sheffield, 2018]] ). However, there are more regions affected by increased soil moisture deficits (Figure 11.19) than precipitation deficits (Figures 2a,b,c and Cross-Chapter Box 11.1) as a consequence of enhanced AED and the associated increased ET, as highlighted by some studies ( [[#Orlowsky--2013|Orlowsky and Seneviratne, 2013]] ; [[#Dai--2018|Dai et al., 2018]] ; [[IPCC:Wg1:Chapter:Chapter-8#8.2.2.1|Section 8.2.2.1]] ). Moisture in the top soil layer is projected to decrease more than precipitation at all warming levels ( [[#Lu--2019|Lu et al., 2019]] ), extending the regions affected by severe soil moisture deficits over most of south and central Europe ( [[#Lehner--2017|Lehner et al., 2017]] ; [[#Ruosteenoja--2018|Ruosteenoja et al., 2018]] ; [[#Samaniego--2018|Samaniego et al., 2018]] ; [[#van%20Der%20Linden--2019|van Der Linden et al., 2019]] ), southern North America ( [[#Cook--2019|Cook et al., 2019]] ), South America ( [[#Orlowsky--2013|Orlowsky and Seneviratne, 2013]] ), southern Africa ( [[#Lu--2019|Lu et al., 2019]] ), East Africa ( [[#Rowell--2015|Rowell et al., 2015]] ), Southern Australia ( [[#Kirono--2020|Kirono et al., 2020]] ), India ( [[#Mishra--2014a|Mishra et al., 2014a]] ) and East Asia (Figure 11.19; [[#Cheng--2015|Cheng et al., 2015]] ). Projected changes in total soil moisture display less widespread drying than those for surface soil moisture ( [[#Berg--2017a|Berg et al., 2017a]] ), but still more than for precipitation (Cross-Chapter Box 11.1, Figures 2a,b,c). The severity of droughts based on surface soil moisture in future projections is stronger than projections based on precipitation and runoff ( [[#Dai--2018|Dai et al., 2018]] ; [[#Vicente-Serrano--2020c|Vicente-Serrano et al., 2020c]] ). Nevertheless, in many parts of the world where soil moisture is projected to decrease, the signal-to-noise ratio among models is low; only the projections in the Mediterranean, Europe, the south-western USA, and southern Africa show a high signal-to-noise ratio in soil moisture projections (Figure 11.19; [[#Lu--2019|Lu et al., 2019]] ). Increases in soil moisture deficits are found to be statistically signicant at regional scale in the Mediterranean region, southern Africa and western South America for changes as small as 0.5°C in global warming, based on differences between +1.5°C and +2°C of global warming ( [[#Wartenburger--2017|Wartenburger et al., 2017]] ). Several other regions are affected when considering changes in droughts for higher changes in global warming ( [[#11.9|Section 11.9]] and Figure 11.19). Seasonal projections of drought frequency for boreal winter (December–January–February) and summer (June–July–August), from CMIP6 multi-model ensemble for 1.5°C, 2°C and 4°C global warming levels, show contrasting trends (Figure 11.19). In the boreal winter in the Northern Hemisphere, the areas affected by drying show ''high agreement'' with those characterized by an increase in meteorological drought projections (Figures 8.14 and 12.4). On the contrary, in the boreal summer, the drought frequency increases worldwide in comparison to meteorological drought projections, with large areas of the Northern Hemisphere displaying a high signal-to-noise ratio (low spead between models). This stresses the dominant influence of ET (as a result of increased AED) in intensifying agricultural and ecological droughts in the warm season in many locations, including mid- to high latitudes.

Increased soil moisture limitation and associated changes in droughts are projected to lead to increased vegetation stress affecting the global land carbon sink in ESM projections ( [[#Green--2019|Green et al., 2019]] ), with implications for projected global warming (Cross-Chapter Box 5). There is ''high confidence'' that the global land sink will become less efficient due to soil moisture limitations and associated agricultural and ecological drought conditions in some regions in higher-emissions scenarios, specially under global warming levels above 4°C; however, there is ''low confidence'' in how these water cycle feedbacks will play out in lower-emissions scenarios (at 2°C global warming or lower; Cross-Chapter Box 5.1).

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[[File:a35dd7dfa52068b8566ee043aeb0b54e IPCC_AR6_WGI_Figure_11_18.png]]

'''Figure 11.18 |''' '''Projected changes in (a) the intensity and (b) the frequency of drought under 1°C, 1.5°C, 2°C, 3°C, and 4°C global warming levels relative to the 1850–1900 baseline. (c)''' Summaries are computed for the AR6 regions in which there is at least medium confidence in an increase in agriculture/ecological drought at the 2°C global warming level (‘drying regions’), including Western North America, Central North America, North Central America, Southern Central America, Northern South America, North-Eastern South America, South American Monsoon, South-Western South America, Southern South America, West and Central Europe, Mediterranean, West Southern Africa, East Southern Africa, Madagascar, Eastern Australia, Southern Australia. Caribbean is not included in the calculation because the number of land grid points was too small. A drought event is defined as a 10-year drought event whose annual mean soil moisture was below its 10th percentile from the 1850–1900 base period. For each box plot, the horizontal line and the box represent the median and central 66% uncertainty range, respectively, of the frequency or the intensity changes across the multi-model ensemble, and the ‘whiskers’ extend to the 90% uncertainty range. The line of zero in (a) indicates no change in intensity, while the line of one in (b) indicates no change in frequency. The results are based on the multi-model ensemble estimated from simulations of global climate models contributing to the Coupled Model Intercomparison Project Phase 6 (CMIP6) under different Shared Socio-economic Pathway (SSP) forcing scenarios. Intensity changes in (a) are expressed as standard deviations of the interannual variability in the period 1850–1900 of the corresponding model. For details on the methods see Supplementary Material 11.SM.2. Further details on data sources and processing are available in the chapter data table (Table 11.SM.9).

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==== 11.6.5.4 Hydrological Deficits ====

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Some studies support wetting tendencies as a response to a warmer climate when considering globally averaged changes in runoff over land ( [[#Roderick--2015|Roderick et al., 2015]] ; [[#Greve--2017|Greve et al., 2017]] ; Y. [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|Yang et al., 2018]] ), and streamflow projections respond to enhanced CO <sub>2</sub> concentrations in CMIP5 models ( [[#Yang--2019|Yang et al., 2019]] ). Nevertheless, when focusing regionally on low-runoff periods, model projections also show an increase of hydrological droughts in large world regions ( [[#Wanders--2015|Wanders and Van Lanen, 2015]] ; [[#Dai--2018|Dai et al., 2018]] ; [[#Vicente-Serrano--2020c|Vicente-Serrano et al., 2020c]] ). In general, the frequency of hydrological deficits is projected to increase over most of the continents, although with regionally and seasonally differentiated effects ( [[#11.9|Section 11.9]] ), with ''medium confidence'' of increase in the following AR6 regions: WCE, MED, SAU, WCA, WNA, SCA, NSA, SAM, SWS, SSA, WSAF, ESAF and MDG ( [[#11.9|Section 11.9]] ; [[#Forzieri--2014|Forzieri et al., 2014]] ; [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#Giuntoli--2015|Giuntoli et al., 2015]] ; [[#Wanders--2015|Wanders and Van Lanen, 2015]] ; [[#Roudier--2016|Roudier et al., 2016]] ; [[#Marx--2018|Marx et al., 2018]] ; [[#Cook--2019|Cook et al., 2019]] ; [[#Zhao--2020|Zhao et al., 2020]] ). However, there are large uncertainties related to the hydrological/impact model used ( [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#Schewe--2014|Schewe et al., 2014]] ; [[#Gosling--2017|Gosling et al., 2017]] ), limited signal-to-noise ratio (due to model spread) in several regions ( [[#Giuntoli--2015|Giuntoli et al., 2015]] ), and also uncertainties in the projection of future human activities, including water demand and land cover changes, which may represent more than 50% of the projected changes in hydrological droughts in some regions ( [[#Wanders--2015|Wanders and Wada, 2015]] ).

Regions dependent on mountainous snowpack as a temporary reservoir may be affected by severe hydrological droughts in a warmer world. In the southern European Alps, both winter and summer low flows are projected to be more severe, with a 25% decrease in the 2050s ( [[#Vidal--2016|Vidal et al., 2016]] ). In western USA, a 22% reduction in winter snow water equivalent is projected at around 2°C of global warming, with a further decrease of a 70% reduction at 4°C global warming ( [[#Rhoades--2018|Rhoades et al., 2018]] ). This decline would cause less predictable hydrological droughts in snowmelt-dominated areas of North America ( [[#Livneh--2020|Livneh and Badger, 2020]] ). The exact magnitude of the influence of higher temperatures on snow-related droughts is, however, difficult to estimate ( [[#Mote--2016|Mote et al., 2016]] ), since the streamflow changes could affect the timing of peak streamflows but not necessarily their magnitude. In addition, projected changes in hydrological droughts downstream of declining glaciers can be very complex to assess (Chapter 9, see also SROCC).

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==== 11.6.5.5 Atmospheric-based Drought Indices ====

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Studies show a stronger drying in projections based on atmospheric-based drought indices compared to ESM projections of changes in soil moisture ( [[#Berg--2018|Berg and Sheffield, 2018]] ) and runoff ( [[#Yang--2019|Yang et al., 2019]] ). It has been suggested that this difference is due to physiological CO <sub>2</sub> effects ( [[#11.6.5.2|Section 11.6.5.2]] ; [[#Roderick--2015|Roderick et al., 2015]] ; [[#Milly--2016|Milly and Dunne, 2016]] ; [[#Swann--2016|Swann et al., 2016]] ; [[#Lemordant--2018|Lemordant et al., 2018]] ; [[#Scheff--2018|Scheff, 2018]] ; [[#Swann--2018|Swann, 2018]] ; [[#Greve--2019|Greve et al., 2019]] ; [[#Yang--2020|Yang et al., 2020]] ). Nonetheless, there is evidence that differences in projections between atmospheric-based drought indices and water-balance metrics from ESMs are not alone due to CO <sub>2</sub> -plant effects ( [[#Berg--2016|Berg et al., 2016]] ; [[#Scheff--2021|Scheff et al., 2021]] ). Differences can also be related to the fact that AED is an upper bound for ET in dry regions and conditions ( [[#11.6.1.2|Section 11.6.1.2]] ) and that soil moisture stress limits increases in ET in projections ( [[#11.6.5.2|Section 11.6.5.2]] ; [[#Berg--2016|Berg et al., 2016]] ; [[#Zhou--2021|Zhou et al., 2021]] ). In general, atmospheric-based indices show more drying than total column soil moisture ( [[#Berg--2018|Berg and Sheffield, 2018]] ; [[#Cook--2020|Cook et al., 2020]] ; [[#Scheff--2021|Scheff et al., 2021]] ), but are more consistent with projected increases in surface soil moisture deficits ( [[#Dirmeyer--2013|Dirmeyer et al., 2013]] ; [[#Dai--2018|Dai et al., 2018]] ; [[#Lu--2019|Lu et al., 2019]] ; [[#Cook--2020|Cook et al., 2020]] ; [[#Vicente-Serrano--2020c|Vicente-Serrano et al., 2020c]] ).

Atmospheric-based drought indices are not metrics of soil moisture or runoff ( [[#11.6.1.5|Section 11.6.1.5]] ) so their projections may not necessarily reflect the same trend of online simulated soil moisture and runoff. Independently of effects on the land water balance, atmospheric-based drought indices will reflect the potential vegetation stress resulting from deficits between available water and enhanced AED, even in conditions with no or low ET. Under dry conditions, the enhanced AED associated with human forcing would increase plant water stress ( [[#Brodribb--2020|Brodribb et al., 2020]] ), with effects on widespread forest dieback and mortality ( [[#Anderegg--2013|Anderegg et al., 2013]] ; [[#Williams--2013|Williams et al., 2013]] ; [[#Allen--2015|Allen et al., 2015]] ; [[#McDowell--2015|McDowell and Allen, 2015]] ; [[#McDowell--2016|McDowell et al., 2016]] , 2020), and stronger risk of megafires ( [[#Flannigan--2016|Flannigan et al., 2016]] ; [[#Podschwit--2018|Podschwit et al., 2018]] ; [[#Clarke--2019|Clarke and Evans, 2019]] ; [[#Varela--2019|Varela et al., 2019]] ). For these reasons, there is ''high confidence'' that the future projections of enhanced drought severity showed by the PDSI-PM and the SPEI-PM are representative of more frequent and severe plant stress episodes and more severe agricultural and ecological drought impacts in some regions.

Global tendencies towards more severe and frequent agricultural and ecological drought conditions are identified in future projections when focusing on atmospheric-based drought indices such as the PDSI-PM or the SPEI-PM. They expand the spatial extent of drought conditions compared to meteorological drought to most of North America, Europe, Africa, Central and East Asia and Southern Australia ( [[#Cook--2014a|Cook et al., 2014a]] ; [[#Chen--2017a|Chen and Sun, 2017a]] , b; [[#Gao--2017b|Gao et al., 2017b]] ; [[#Lehner--2017|Lehner et al., 2017]] ; [[#Zhao--2017|Zhao and Dai, 2017]] ; [[#Dai--2018|Dai et al., 2018]] ; [[#Naumann--2018|Naumann et al., 2018]] ; [[#Potopová--2018|Potopová et al., 2018]] ; [[#Gu--2020|Gu et al., 2020]] ; Vicente-Serrano et al., 2020c; [[#Dai--2021|Dai, 2021]] ). Projections in PDSI-PM and SPEI-PM are used to complement total soil moisture projections in assessing projected changes in agricultural and ecological drought ( [[#11.9|Section 11.9]] ).

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==== 11.6.5.6 Synthesis for Different Drought Types ====

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The tables in [[#11.9|Section 11.9]] provide assessed projected changes in metorological drought, agricultural and ecological drought, and hydrological droughts. The assessment shows that several regions will be affected by more severe agricultural and ecological droughts even if global warming is stabilized at 2°C, including MED, WSAF, SAM and SSA ( ''high confidence'' ), and ESAF, MDG, EAU, SAU, SCA, CAR, NSA, NES, SWS, WCE, NCA, WNA and CNA ( ''medium confidence'' ). Some regions are also projected to be affected by more severe agricultural and ecological droughts at 1.5°C (MED, WSAF, ESAF, SAU, NSA, SAM, SSA, can; ''medium confidence'' ) At 4°C of global warming, even more regions would be affected by agricultural and ecological droughts (WCE, MED, CAU, EAU, SAU, WCA, EAS, SCA, CAR, NSA, NES, SAM, SWS, SSA, NCA, CNA, ENA, WNA, WSAF, ESAF and MDG). NEAF, SAS are also projected to experience less agricultural and ecological drought with global warming ( ''medium confidence'' ). Projected changes in meteorological droughts are, overall, less extended but also affect several AR6 regions, at 1.5°C and 2°C (MED, EAU, SAU, SCA, NSA, NCA, WSAF, ESAF, MDG) and 4°C of global warming (WCE, MED, EAU, SAU, SEA, SCA, CAR, NWS, NSA, NES, SAM, SWS, SSA, NCA, ENA, WAF, WSAF, ESAF, MDG). Several regions are also projected to be affected by more hydrological droughts at 1.5°C and 2°C (WCE, MED, WNA, WSAF, ESAF) and 4°C of global warming (NEU, WCE, EEU, MED, SAU, WCA, SCA, NSA, SAM, SWS, SSA, WNA, WSAF, ESAF, MDG). To illustrate the changes in both intensity and frequency of drought in the regions where strongest changes are projected, Figure 11.18 displays changes in the intensity and frequency of soil moisture drought under different global warming levels (1.5°C, 2°C, 4°C) relative to the 1851-1900 baseline based on CMIP6 simulations under different SSP forcing scenarios averaged over “drying regions”, i.e. AR6 regions for which there is at least ''medium confidence'' in increase in agricultural and ecological drought at 2°C of global warming. The 90% uncertainty ranges for the projected changes in both intensity and frequency are above zero, indicating significant increase in both intensity and frequency of drought in these regions as whole.

In summary, more regions are affected by increases in agricultural and ecological droughts with increasing global warming ( ''high confidence'' ). New evidence strengthens the SR1.5 conclusion that even relatively small incremental increases in global warming (+0.5°C) cause a worsening of droughts in some regions ( ''high confidence'' ) ''.'' Some regions are projected to be affected by more severe agricultural and ecological droughts at 1.5°C of global warming (MED, WSAF, ESAF, SAU, NSA, SAM, SSA, can; ''medium confidence'' ). A larger number of regions are projected to be affected by more severe agricultural and ecological droughts at 2°C of global warming, including MED, WSAF, SAM and SSA ( ''high confidence'' ), and ESAF, MDG, EAU, SAU, SCA, CAR, NSA, NES, SWS, WCE, NCA, WNA and CNA ( ''medium confidence'' ). At 4°C of global warming, even more regions would be affected by agricultural and ecological droughts (WCE, MED, CAU, EAU, SAU, WCA, EAS, SCA, CAR, NSA, NES, SAM, SWS, SSA, NCA, CNA, ENA, WNA, WSAF, ESAF and MDG). Some regions are also projected to experience less agricultural and ecological drought with global warming ( ''medium confidence;'' NEAF, SAS). There is ''high confidence'' that the projected increases in agricultural and ecological droughts are strongly affected by AED increases in a warming climate, although ET increases are projected to be smaller than those in AED due to soil moisture limitations and CO <sub>2</sub> effects on leaf stomatal conductance. Enhanced atmospheric CO <sub>2</sub> concentrations lead to enhanced water-use efficiency in plants ( ''medium confidence'' ), but there is ''low confidence'' that it can alleviate agricultural and ecological droughts, or hydrological droughts, at higher global warming levels characterized by limited soil moisture and enhanced AED.

Projected changes in meteorological droughts are overall less extended than for agricultural and ecological droughts, but also affect several AR6 regions, even at 1.5°C and 2°C of global warming. Several regions are also projected to be more strongly affected by hydrological droughts with increasing global warming (NEU, WCE, EEU, MED, SAU, WCA, SCA, NSA, SAM, SWS, SSA, WNA, WSAF, ESAF, MDG). Increased soil moisture limitation and associated changes in droughts are projected to lead to increased vegetation stress in many regions, with implications for the global land carbon sink (Cross-Chapter Box 5). There is ''high confidence'' that the global land carbon sink will become less efficient due to soil moisture limitations and associated drought conditions in some regions in higher-emissions scenarios, especially under global warming levels above 4°C; however, there is ''low confidence'' on how these water cycle feedbacks will play out in lower-emissions scenarios (at 2°C global warming or lower; Cross-Chapter Box5.1).

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