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

==== 4.2.1.2 Observed and Reconstructed Changes in Evapotranspiration ====

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WGI ( [[#Douville--2021|Douville et al., 2021]] ) conclude with ''high confidence'' that global terrestrial annual ET has increased since the early 1980s, driven by both increasing atmospheric water demand and vegetation greening ( ''medium confidence'' ), and can be partly attributed to anthropogenic forcing ( ''high confidence)'' .

Regional changes in ET depend on changes in both the climate and the properties of the land surface and ecosystems. The latter also responds to changes in climate and atmospheric composition. For example, a warming climate increases evaporative demand (Huang M et al., 2015; [[#Berg--2016|Berg et al., 2016]] ), although seasonal rainfall totals ( [[#Hovenden--2014|Hovenden et al., 2014]] ) affect the amount of soil moisture available for evaporation. Since transpiration accounts for much of the land-atmosphere water flux ( [[#Good--2015|Good et al., 2015]] ), vegetation changes also play a significant role in overall changes in ET.

With higher CO 2 , the increase in evaporative demand can, to some extent, be counteracted by reduced stomatal conductance (‘physiological effect’), which reduces transpiration and increases leaf-level water use efficiency (WUE), but is highly species-specific. There is evidence for recent increases in leaf-scale WUE from tree rings (14 ± 10%, broadleaf to 22 ± 6%, evergreen over the 20th century: ( [[#Frank--2015|Frank et al., 2015]] )), carbon isotopes (30 to 35% increase in 150 years: ( [[#van%20der%20Sleen--2014|van der Sleen et al., 2014]] )), and satellite-based measurements (1982–2008) combined with data-driven models (Huang M et al., 2015). WUE is also affected by aerodynamic conductance ( [[#Knauer--2017|Knauer et al., 2017]] ), nutrient limitation ( [[#Medlyn--2015|Medlyn et al., 2015]] ; [[#Donohue--2017|Donohue et al., 2017]] ), soil moisture availability ( [[#Bernacchi--2015|Bernacchi and VanLoocke, 2015]] ; [[#Medlyn--2015|Medlyn et al., 2015]] ), and ozone pollution ( [[#King--2013|King et al., 2013]] ; [[#Frank--2015|Frank et al., 2015]] ).

Higher CO 2 also increases photosynthesis rates, though this may not be maintained in the longer term ( [[#Warren--2015|Warren et al., 2015]] ; [[#Adams--2020|Adams et al., 2020]] ), particularly where temperatures exceed the thermal maxima for photosynthesis (Duffy et al., 2021). Higher photosynthesis increases leaf area index (LAI) (‘structural effect’) and therefore transpiration; 55 ± 25% of observed increases in ET (1980–2011) have been attributed to LAI change (Zeng Z. et al., 2018). Increases in ET driven by increased LAI (from satellite observations 1982–2012) are estimated at 0.32 ± 0.07 mm month –1 per decade, generating a climate forcing of −0.31 Wm– 2 per decade ( [[#Zeng--2017|Zeng et al., 2017]] ).

Overall regional transpiration change depends on the balance between the physiological and structural effects (e.g., [[#Tor-ngern--2015|Tor-ngern et al., 2015]] ; [[#Ukkola--2015|Ukkola et al., 2015]] ). In dry regions, ET may increase due to increasing LAI (Huang M et al., 2015), but in some densely vegetated regions, the stomatal effect dominates ( [[#Mao--2015|Mao et al., 2015]] ). Reductions in transpiration due to rising CO 2 concentrations may also be offset by a longer growing season ( [[#Frank--2015|Frank et al., 2015]] ; [[#Mankin--2019|Mankin et al., 2019]] ). Other factors modulate the transpiration effect both temporally and spatially, for example, additional vegetation structural changes ( [[#Kim--2015|Kim et al., 2015]] ; [[#Domec--2017|Domec et al., 2017]] ), vegetation disturbance and age ( [[#Donohue--2017|Donohue et al., 2017]] ) and species ( [[#Bernacchi--2015|Bernacchi and VanLoocke, 2015]] ).

Recent studies report global ET increases from the early 1980s to 2009 and 2013 (Table 4.1). Calculations informed by observations suggest that ET has increased in most regions, with statistically significant (p&lt;0.05) trends of up to 10 mm yr -2 observed in large parts of North America and northern Eurasia. Larger increases in ET are also observed in several regions, including northeast Brazil, western central Africa, southern Africa, southern India, southern China, and northern Australia. Decreases of around 10 mm yr -2 are reported for western Amazonia and central Africa ( [[#Miralles--2014|Miralles et al., 2014]] ), although not across all data sets ( [[#Zeng--2018|Zeng et al., 2018]] ). In estimates of past changes in long-term drying or wetting of the land surface driven by climate, uncertainties in ET observations or reconstructions make a more substantial contribution to the overall uncertainty than observed changes in precipitation ( [[#Greve--2014|Greve et al., 2014]] ). Other changes in ET are also driven strongly by land cover changes and irrigation ( [[#Bosmans--2017|Bosmans et al., 2017]] ).

'''Table 4.1 |''' Trends in global evapotranspiration for different periods between 1981–1982 and 2009–2013.

{| class="wikitable"
|-
! Trend (mm yr -2 )

! Period

! Data source

! Author(s)

|-
| +0.54

| 1981 to 2012

| Observations

| (Zhang Y. et al., 2016)

|-
| +1.18

| 1982 to 2010

| Observations

| ( [[#Mao--2015|Mao et al., 2015]] )

|-
| +0.93 ± 0.31

| 1982 to 2010

| LSMs

| ( [[#Mao--2015|Mao et al., 2015]] )

|-
| +0.88

| 1982 to 2013

| Remote-sensing data

| (Zhang K. et al., 2015)

|-
| +1.5

| 1982 to 2009

| Remote-sensing and surface observations

| ( [[#Zeng--2014|Zeng et al., 2014]] )

|}

The contribution of changes in WUE to observed changes in ET is a key knowledge gap. WGI assigned ''low confidence'' to this contribution. Estimating large-scale transpiration response to increased CO 2 based on leaf-level responses of WUE is not straightforward ( [[#Bernacchi--2015|Bernacchi and VanLoocke, 2015]] ; [[#Medlyn--2015|Medlyn et al., 2015]] ; [[#Tor-ngern--2015|Tor-ngern et al., 2015]] ; [[#Walker--2015|Walker et al., 2015]] ; [[#Kala--2016|Kala et al., 2016]] ) and new methodological approaches are needed.

In summary, there is ''high confidence'' that ET increased by between approximately 0.5 and 1.5 mm yr -2 between the 1980s and early 2010s due to warming-induced increased atmospheric demand worldwide and greening of vegetation in many regions. Increases in many areas are 10 mm yr –2 or more, but in some tropical land areas, ET has decreased by 10 mm yr –2 . Plant stomatal responses to rising CO 2 concentrations may play a role, but there is ''low confidence'' in quantifying this. Changes in land cover and irrigation have also changed regional ET ( ''medium confidence'' ).

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