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

==== 4.4.1.2 Projected Changes in Evapotranspiration ====

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AR5 ( [[#Collins--2013)|Collins et al., 2013)]] found that the CMIP5 model projections of ET increases or decreases followed the same pattern over land as precipitation projections, with additional impacts of reduced transpiration due to plant stomatal closure in response to rising CO 2 concentrations. AR6 WGI ( [[#Douville--2021|Douville et al., 2021]] ) assessed that it is ''very likely'' that ET will increase over land, with regional exceptions in drying areas.

In most CMIP5 and CMIP6 models, projected ET changes are driven not just by meteorological conditions and soil moisture but also by plant physiological responses to elevated CO 2 , which themselves influence meteorology and soil moisture through surface fluxes ( [[#Halladay--2017|Halladay and Good, 2017]] ; [[#Lemordant--2019|Lemordant and Gentine, 2019]] ). Elevated CO 2 causes stomatal closure which decreases ET, but also increases leaf area index (LAI) which in turn increases ET, but these do not necessarily compensate ( [[#Skinner--2017|Skinner et al., 2017]] ). Higher LAI increases transpiration, depleting soil moisture but increasing shading, thus reducing soil evaporation ( [[#Skinner--2017|Skinner et al., 2017]] ), but LAI may not increase in areas where it is already high ( [[#Lemordant--2018|Lemordant et al., 2018]] ). Projected ET decreases from physiological effects alone are widespread but greatest in tropical forests ( [[#Swann--2016|Swann et al., 2016]] ; [[#Kooperman--2018|Kooperman et al., 2018]] ).

Future changes in regional ET are therefore highly uncertain. The CMIP6 multi-model ensemble projects changes in ET varying both in magnitude and sign across the ensemble members (Figure 4.14). At 4°C global warming, the ensemble median projection shows increased ET of approximately 25% in mid/high latitudes but decreases of up to 10% across most of tropical South America, southern Africa and Australia. These CMIP6 ensemble projections resemble ET changes projected by the CMIP5 ensemble, except over central Africa and Southeast Asia ( [[#Berg--2019|Berg and Sheffield, 2019]] ). However, the ensemble ranges are wide and include both increases and decreases in projected ET in many locations, with mid-latitude ET increases being up to approximately 50% and ET decreases in southern Africa being up to approximately 30%. Projected changes are proportionally smaller at lower levels of global warming, while patterns of change remain similar.

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'''Figure 4.14 |''' '''Projected percentage changes in annual mean ET at global warming levels (GWLs) of 4°C (top), 2°C (middle) and 1''' '''.''' '''5°C (bottom) for the CMIP6 multi-model ensemble of GCMs driven by SSP5-8.5 concentrations.''' The distribution of outcomes is shown at local scales with the 5th, 50th and 95th percentile ET changes in individual grid boxes. Note that these are uncertainties at the individual point and are not spatially coherent, that is, they do not represent plausible global patterns of change. Results for 1.5°C, 2°C and 4°C global warming are defined as 20-year means relative to 1850–1900 and use 40, 40 and 31 ensemble members, respectively, due to some members not reaching 4°C global warming.

The relative importance of the physiological and radiative effects of CO 2 on future ET is a crucial knowledge gap, partly because many ESM land surface schemes still use representations of this process based on older experimental studies. Furthermore, large-scale experimental studies using free-air CO 2 enrichment (FACE) techniques to constrain the models have not yet been performed in certain critical ecosystems, such as tropical forests. Finally, uncertainties in equilibrium climate sensitivity (ECS) imply uncertainties in the CO 2 concentration accompanying any given level of warming (Betts and McNeall, 2018).

In summary, the sign of projected ET change depends on region, but there is ''medium confidence'' that ET will increase in the global mean and mid/high latitudes and decrease in northern South America and southern Africa. In addition, the impacts of rising CO 2 concentrations on plant stomata and leaf area play a role in model projections of ET change ( ''high confidence'' ), but there is ''low confidence'' in their overall contribution to global ET change.

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