Editing IPCC:AR6/WGII/Chapter-4 (section)

=== 4.4.5 Projected Changes in Droughts ===

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AR6 WGI ( [[#Douville--2021|Douville et al., 2021]] ) concluded that the total land area subject to increasing drought frequency and severity would expand ( ''high confidence'' ), and in the Mediterranean, southwestern South America and western North America, future aridification will far exceed the magnitude of change seen in the last millennium ( ''high confidence'' ). WGI ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ) also find many consistencies among projections of climate change effects on different forms of drought (meteorological, agricultural/ecological and hydrological drought, 4.2.5), but also significant differences in some regions, particularly in the levels of confidence in projected changes.

Many studies focus on precipitation-based drought indices ( [[#Carrão--2018|Carrão et al., 2018]] ), but higher evaporative demands and changes in snow cover are additional drivers of hydrological, agricultural and ecological drought ( ''medium confidence'' ) in many regions of the world ( [[#Koirala--2014|Koirala et al., 2014]] ; [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#Touma--2015|Touma et al., 2015]] ; [[#Wanders--2015|Wanders et al., 2015]] ; [[#Zhao--2015|Zhao and Dai, 2015]] ; [[#Naumann--2018|Naumann et al., 2018]] ; [[#Cook--2020a|Cook et al., 2020a]] ). Furthermore, these droughts (hydrological, agricultural and ecological) are often modulated by prevailing soil and hydro-morphological characteristics. Therefore, the choice of drought definition can affect the magnitude and even the sign of the projected drought change.

In a study with multiple climate models, global water models and scenarios, the choice of drought definition was the dominant source of uncertainty in the sign of projected change in drought frequency in over 17% of global land by 2070–2099, including several major wheat- and maize-growing areas where agricultural (soil moisture) drought is of high importance ( [[#Satoh--2021|Satoh et al., 2021]] ). [[#Cook--2020a|Cook et al. (2020a)]] noted that in the CMIP6 projections, soil moisture and runoff drying are more robust, spatially extensive and severe than precipitation, resulting in the frequency of agricultural drought increasing over wider areas than for meteorological drought. At 1.5°C global warming, the likelihood of extreme agricultural (soil moisture) drought is projected to at least double (100% increase) over large areas of northern South America, the Mediterranean, western China and high latitudes in North America and Eurasia (Figure 4.18, left column). The likelihood is projected to increase by 150–200% in these regions at 2°C global warming, with an expansion of the affected areas, and increase by over 200% at 4°C global warming. Agricultural drought likelihood also increases by 100–250% at 4°C global warming in southwestern North America, southwest Africa, southern Asia and Australia. The likelihood of extreme drought is projected to decrease in central North America, the Sahel, the Horn of Africa, the eastern Indian sub-continent and parts of western and eastern Asia. Using eight global hydrological models driven by a subset of four of the CMIP5 climate models, [[#Lange--2020|Lange et al. (2020)]] projected a 370% (30–790%) increase of the global population annually exposed to agricultural (soil moisture) droughts in response to 2°C global warming. Therefore, it is essential to consider the drought type when applying drought projections to impact and risk in decision-making, especially for informing adaptation. For example, if responses are explicitly tailored to agricultural (soil moisture) drought changes, projected changes in a meteorological (precipitation) drought metric may not provide accurate information.

Compared to CMIP5, the CMIP6 ensemble projects more consistent drying in the Amazon basin ( [[#Parsons--2020|Parsons, 2020]] ), more extensive declines in total soil moisture in Siberia ( [[#Cook--2020a|Cook et al., 2020a]] ) and stronger declines in westernmost North Africa and southwestern Australia. Projected declines in soil moisture in these geographies would cause a significant risk of agricultural drought. Also, importantly, projected changes in drought in many regions depend on the season and may not be evident in annual mean changes. For example, in northwestern Asia, hydrological (runoff) drought frequency is projected to decrease by 50–100% in autumn and winter but increase by up to 250% in spring and summer ( [[#Cook--2020a|Cook et al., 2020a]] ). In contrast, meteorological (precipitation) drought frequency is projected to increase by up to 350% throughout the year.

Drought projections are subject to uncertainties due to limits of predictability and understanding of the relevant biophysical processes. Uncertainties in regional climate changes are significant in many regions (see Figure 4.10, Figure 4.13, Figure 4.15), and in climate model ensembles, the range of regional outcomes generally increases with global warming. This widening of the range of outcomes can contribute to the increased likelihood of extreme droughts across the ensemble as a whole (Figure 4.18, right column). The response of transpiration to elevated CO 2 is also a significant uncertainty. The inclusion of CO 2 physiological effects leads to smaller projected increases in agricultural, ecological or hydrological drought ( [[#Milly--2016|Milly and Dunne, 2016]] ; [[#Yang--2020|Yang et al., 2020]] ). However, the level of uncertainties in representing the effects of CO 2 is still very high, precluding conclusive results in a global analysis ( [[#de%20Kauwe--2013|de Kauwe et al., 2013]] ; [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#Yang--2016|Yang et al., 2016]] ). Most CMIP6 climate models include CO 2 physiological effects, but many hydrological models used for impacts studies do not.

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[[File:18d4a266e2c8ef8940e5dc8cdd2619dd IPCC_AR6_WGII_Figure_4_018.png]]

'''Figure 4.18 |''' '''Projected changes in the likelihood of an extreme single-year agricultural (soil moisture) drought event, with extreme drought defined as the driest 10% of years from 1995 to 2014, using total soil moisture projections pooled from the CMIP6 ensemble following Cook et al.''' '''(2020a).''' All ensemble members are treated as equally likely potential outcomes, and likelihoods are calculated using the whole ensemble. Left: Percentage change in the likelihood of extreme drought at GWLs of 4°C (top), 2°C (middle) and 1.5°C (bottom), with ‘extreme drought’ defined locally as the 10th percentile in individual grid boxes. Right: probability distribution functions of regional mean soil moisture anomalies for the climatic regions Mediterranean (MED), South American Monsoon (SAM) and West Southern Africa (WSAF) ( [[#Iturbide--2020|Iturbide et al., 2020]] ), at 1.5°C, 2°C and 4°C GWLs. The solid vertical line shows the baseline, that is, the 50th percentile in 1995–2014. The dashed vertical line shows the 10th percentile for 1995–2014, defining ‘extreme drought’ at the regional scale. Projections used the SSP5-8.5 scenario to maximise the number of ensemble members at higher GWLs, but global patterns of change are very similar for all scenarios ( [[#Cook--2020a|Cook et al., 2020a]] ), and for any given GWL, similar results can be expected with other scenarios ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ).

Terrestrial water storage (TWS) is the sum of continental water stored in canopies, snow and ice, rivers, lakes and reservoirs, wetlands, soil and groundwater ( [[#Pokhrel--2021|Pokhrel et al., 2021]] ). TWS drought can therefore be considered to be a combination of agricultural, ecological and hydrological drought. The proportion of the global population exposed to TWS drought is projected to increase with ongoing climate change (Figure 4.19). By the late 21st century, under RCP6.0, the global land area in extreme-to-exceptional TWS drought is projected to increase from 3% to 7% ( [[#Pokhrel--2021|Pokhrel et al., 2021]] ), with increasing uncertainty over time. Combined with a medium population growth scenario (SSP2), this leads to the global population in this level of drought increasing from 3% to 8%, again with increasing uncertainty over time. Hydrological droughts can also be driven by direct human impact via water abstraction ( [[#Javadinejad--2019|Javadinejad et al., 2019]] ).

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'''Figure 4.19 |''' '''Projected changes in the area under drought and population affected, defined with changes in the Terrestrial Water Storage–Drought Severity Index (TWS-DSI) projected with seven terrestrial hydrology models driven by four CMIP5 climate models using RCP6.''' 0.

'''(a)''' Fractional global land area under moderate-to-severe drought (top), defined as −0.8 ≤ TWS-DSI &lt; −1.6, and extreme-to-exceptional drought (bottom), defined as TWS-DSI &lt; −1.6.

'''(b)''' Fraction of global population exposed to moderate-to-severe (top) and extreme-to-exceptional (bottom) drought, using the SSP2 population projection. Dark lines show the ensemble means; shaded areas indicate uncertainty as ± 1 standard deviation. Reproduced from [[#Pokhrel--2021|Pokhrel et al. (2021)]] .

Critical knowledge gaps include uncertainties in regional drought due to regional climate change uncertainties, challenges in constraining plant physiological responses to atmospheric CO 2, and the uncertainties in modelling the role of different population projections in projecting regional drought risk.

In summary, the likelihood of drought is projected to increase in many regions over the 21st century ( ''high confidence'' ) even with strong climate change mitigation, and more severely in the absence of this. Different forms of drought broadly show similar patterns of projected change in many regions ( ''high confidence'' ), but the frequency of agricultural drought is projected to increase over wider areas than for meteorological drought ( ''medium confidence'' ). Clarity on the definition of drought is therefore important for informing decision-making. With the RCP6.0 and SSP2 scenarios, the global population exposed to extreme-to-exceptional terrestrial water storage drought is projected to increase from 3% to 8% over the 21st century.

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