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

==== 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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