Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.4.1.4 Evapotranspiration ====

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Since AR5, there is a growing body of evidence suggesting that future projections in evapotranspiration are driven by changes in temperature and relative humidity (Laîné et al. , 2014; Pan et al. , 2015; Ukkola et al. , 2016a) , as well as precipitation patterns, as found in AR5 .

Analysis of CMIP5 models suggests that atmospheric evaporative demand will increase over most areas of the world in high-emissions scenarios ( ''virtually certain'' ), mostly as a consequence of an increase in vapour pressure deficit ( [[#Scheff--2014|Scheff and Frierson, 2014]] , 2015; [[#Greve--2015|Greve and Seneviratne, 2015]] ; [[#Vicente-Serrano--2020|Vicente-Serrano et al., 2020]] ). CMIP5 models also project an increase in evapotranspiration over most land areas ( ''medium confidence'' ) ( [[#Laîné--2014|Laîné et al., 2014]] ). However, regional changes in evapotranspiration can also be influenced by changes in soil moisture and vegetation, which modulate the moisture flux from the land to the atmosphere. Several studies of CMIP5 projections suggest that increases in plant water use efficiency will limit or counteract rising evapotranspiration ( [[#Milly--2016|Milly and Dunne, 2016]] ; Swann et al. , 2016; Lemordant et al. , 2018; Y. Yang et al. , 2018) . However, other studies have found that transpiration increases due to the impact of climate change on growing season length, leaf area, and evaporative demand ( [[#8.2.3.3|Section 8.2.3.3]] ; Frank et al. , 2015; Mankin et al. , 2017, 2018, 2019; Guerrieri et al. , 2019; S. Zhou et al. , 2019; Vicente-Serrano et al. , 2020) . The parametrizations accounting for these complex physiological processes in global climate models may also be insufficient ( [[#Franks--2017|Franks et al., 2017]] ; [[#Peters--2018|Peters et al., 2018]] ; [[#Peano--2019|Peano et al., 2019]] ). Thus, there is currently ''low confidence'' in the role of vegetation physiology in modulating future projections of evapotranspiration.

CMIP6 models project a geographical pattern of changes in evapotranspiration similar to previous generation models (Figure 8.17), although the magnitude is generally larger than found for CMIP5 projections (X. [[#Liu--2020|]] [[#Liu--2020|]] [[#Liu--2020|Liu et al., 2020]] ). There is, however, a strong seasonality in many regions, with a larger relative increase in the winter season of the Northern Hemisphere (NH) and smaller relative changes in the summer (Figure 8.17). Evapotranspiration increases in most land regions, except in areas that are projected to become moisture-limited (due to reduced precipitation and increased evaporative demand), such as the Mediterranean, South Africa, and the Amazonian basin ( ''medium confidence'' ). The patterns of change increase in magnitude from low to high-emissions SSP scenarios ( ''medium co'' ''nfidence'' ).

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[[File:5413af4c229e74f01b1a4990457645c8 IPCC_AR6_WGI_Figure_8_17.png]]

'''Figure 8.17 |''' '''Projected long-term relative changes in seasonal mean evapotranspiration.''' Global maps of projected relative changes (%) in seasonal mean of surface evapotranspiration for December–January–February (DJF; left panels) and June–July–August (JJA; right panels) averaged across available CMIP6 models (number provided at the top right of each panel) for SSP1.2-6 '''(a, b)''' SSP2-4.5 '''(c, d)''' and SSP5-8.5 '''(e, f)''' scenario respectively. All changes are estimated in 2081–2100 relative to 1995–2014. Uncertainty is represented using the simple approach. No overlay indicates regions with high model agreement, where ≥80% of models agree on sign of change; diagonal lines indicate regions with low model agreement, where &lt;80% of models agree on sign of change. For more information on the simple approach, please refer to the Cross-Chapter Box Atlas.1. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

In summary, future projections indicate that anthropogenic forcings will drive an increase in global mean evaporation over most oceanic areas ( ''high confidence'' ) (Figure 8.17), an increase in global atmospheric demand ( ''virtually certain'' ) and an increase in evapotranspiration over most land areas, with the exception of moisture-limited regions ( ''medium confidence'' ). However, substantial uncertainties in projections of evapotranspiration, especially at seasonal and regional scales, remain (see also [[#8.2.3.3|Section 8.2.3.3]] and Cross-Chapter Box 5.1).

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