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

==== 11.6.1.2 Atmospheric Evaporative Demand ====

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Atmospheric evaporative demand (AED) quantifies the maximum amount of actual evapotranspiration (ET) that can happen from land surfaces if they are not limited by water availability (Table 11.A.1). AED is affected by radiative and aerodynamic components. For this reason, the atmospheric dryness, often quantified with the relative humidity or the vapour pressure deficit (VPD), is not equivalent to the AED, as other variables are also highly relevant, including solar radiation and wind speed ( [[#Hobbins--2012|Hobbins et al., 2012]] ; [[#McVicar--2012a|McVicar et al., 2012a]] ; [[#Sheffield--2012|Sheffield et al., 2012]] ). AED can be estimated using different methods ( [[#McMahon--2013|McMahon et al., 2013]] ), and those solely based on air temperature (e.g., Hargreaves, Thornthwaite) usually overestimate it in terms of magnitude and temporal trends ( [[#Sheffield--2012|Sheffield et al., 2012]] ), in particular, in the context of substantial background warming. Physically-based combination methods such as the Penman-Monteith equation are more adequate and recommended since 1998 by the United Nations Food and Agriculture Oganization ( [[#Pereira--2015|Pereira et al., 2015]] ). For this reason, the assessment of this Chapter, when considering atmospheric-based drought indices, only includes AED estimates using the latter (see also [[#11.9|Section 11.9]] ). AED is generally higher than ET, since AED represents an upper bound for ET. Hence, an AED increase does not necessarily lead to increased ET ( [[#Milly--2016|Milly and Dunne, 2016]] ), in particular under drought conditions given soil moisture limitation ( [[#Bonan--2014|Bonan et al., 2014]] ; [[#Berg--2016|Berg et al., 2016]] ; [[#Konings--2017|Konings et al., 2017]] ; [[#Stocker--2018|Stocker et al., 2018]] ). In general, AED is highest in regions where ET is lowest (e.g., desert areas), further illustrating the decoupling between the two variables under limited soil moisture.

The influence of AED on drought depends on the drought type, background climate, the environmental conditions and the moisture availability ( [[#Hobbins--2016|Hobbins et al., 2016]] , 2017; [[#Vicente-Serrano--2020a|Vicente-Serrano et al., 2020a]] ). This influence also includes effects not related to increased ET. Under low soil moisture conditions, increased AED increases plant stress, enhancing the severity of agricultural and ecological droughts ( [[#Williams--2013|Williams et al., 2013]] ; [[#Allen--2015|Allen et al., 2015]] ; [[#McDowell--2016|McDowell et al., 2016]] ; [[#Grossiord--2020|Grossiord et al., 2020]] ). Moreover, high VPD impacts overall plant physiology; it affects the leaf and xylem safety margins, and decreases the sap velocity and plant hydraulic conductance ( [[#Fontes--2018|Fontes et al., 2018]] ). VPD also affects the plant metabolism of carbon and, if prolonged, it may cause plant mortality via carbon starvation ( [[#Breshears--2013|Breshears et al., 2013]] ; [[#Hartmann--2015|Hartmann, 2015]] ). Drought projections based exclusively on AED metrics overestimate changes in soil moisture and runoff deficits. Nevertheless, AED also directly impacts hydrological drought, as ET from surface waters is not limited ( [[#Wurbs--2014|Wurbs and Ayala, 2014]] ; [[#Friedrich--2018|Friedrich et al., 2018]] ; [[#Hogeboom--2018|Hogeboom et al., 2018]] ; K. [[#Xiao--2018|]] [[#Xiao--2018|Xiao et al., 2018]] ), and this effect increases under climate change projections (W. [[#Wang--2018|]] [[#Wang--2018|Wang et al., 2018]] ; [[#Althoff--2020|Althoff et al., 2020]] ). In addition, high AED increases crop water consumptions in irrigated lands ( [[#García-Garizábal--2014|García-Garizábal et al., 2014]] ), contributing to intensifying hydrological droughts downstream ( [[#Fazel--2017|Fazel et al., 2017]] ; [[#Vicente-Serrano--2017|Vicente-Serrano et al., 2017]] ).

On subseasonal to decadal scales, temporal variations in AED are strongly controlled by circulation variability ( [[#Williams--2014|Williams et al., 2014]] ; [[#Chai--2018|Chai et al., 2018]] ; [[#Martens--2018|Martens et al., 2018]] ), but thermodynamic processes also play a fundamental role and, under human-induced climate change, dominate the changes in AED. Atmospheric warming due to increased atmospheric CO <sub>2</sub> concentrations increases AED by means of enhanced VPD in the absence of other influences ( [[#Scheff--2015|Scheff and Frierson, 2015]] ). Because of the greater warming over land than over oceans (Sections 2.3.1.1 and 11.3), the saturation pressure of water vapour increases more over land than over oceans; oceanic air masses advected over land thus contain insufficient water vapour to keep pace with the greater increase in saturation vapour pressure over land ( [[#Sherwood--2014|Sherwood and Fu, 2014]] ; [[#Byrne--2018|Byrne and O’Gorman, 2018]] ; [[#Findell--2019|Findell et al., 2019]] ). Land–atmosphere feedbacks are also important in affecting atmospheric moisture content and temperature, with resulting effects on relative humidity and VPD (Box 11.1; [[#Berg--2016|Berg et al., 2016]] ; [[#Haslinger--2019|Haslinger et al., 2019]] ; S. [[#Zhou--2019|]] [[#Zhou--2019|Zhou et al., 2019]] ).

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