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

==== 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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