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

==== 8.3.1.4 Evapotranspiration ====

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The AR5 assessed that there was ''medium confidence'' that pan evaporation declined in most regions over the last 50 years, yet ''medium confidence'' that evapotranspiration increased from the early 1980s to the late 1990s. Since AR5, these conflicting observations have been attributed to internal variability and by the fact that evapotranspiration is less sensitive to trends in wind speed and is partly controlled by vegetation greening ( [[#Zhang--2015|K. Zhang et al., 2015]] ; [[#Zhang--2016|Y. Zhang et al., 2016]] ; [[#Zeng--2018b|Z. Zeng et al., 2018b]] ). Observation-based estimates show a robust positive trend in global terrestrial evapotranspiration between the early 1980s and the early 2010s ( [[#Miralles--2014b|Miralles et al., 2014b]] ; [[#Zeng--2014|Z. Zeng et al., 2014]] , [[#Zeng--2018b|2018b]] ; [[#Zhang--2015|K. Zhang et al., 2015]] ; [[#Zhang--2016|Y. Zhang et al., 2016]] ). The rate of increase varies among datasets, with an ensemble mean terrestrial average rate of 7.6 ± 1.3 mm yr <sup>–</sup> <sup>1</sup> per decade for 1882–2011 (Z. [[#Zeng--2018|Zeng et al., 2018]] a). In addition, a decreasing trend in pan evaporation plateaued or reversed after the mid-1990s (C.M. [[#Stephens--2018|]] [[#Stephens--2018|Stephens et al., 2018]] ) has been reported as due to a shift from a dominant influence of wind speed to a dominant effect of water vapour pressure deficit, which has increased sharply since the 1990s ( [[#Yuan--2019|Yuan et al., 2019]] ). The absence of a trend in evapotranspiration in the decade following 1998 was shown to be at least partly an episodic phenomenon associated with ENSO variability (Miralles et al. , 2014b; K. Zhang et al. , 2015; Martens et al. , 2018). Thus, there is ''medium confidence'' that the apparent pause in the increase in global evapotranspiration from 1998 to 2008 is mostly due to internal variability. In contrast to AR5, there are now consistent trends in pan evaporation and evapotranspiration at the global scale, given the recent increase in both variables since the mid-1990s ( ''medium confidence'' ). Given the growing number of quantitative studies, there is ''high confidence'' that global terrestrial annual evapotranspiration has increased since the early 1980s.

Since AR5, the predominant contribution of transpiration to the observed trends in terrestrial evapotranspiration has been revisited and confirmed ( [[#Good--2015|Good et al., 2015]] ; [[#Wei--2017|Wei et al., 2017]] ). Using satellite and ecosystem models, [[#Zhu--2016|Zhu et al. (2016)]] found a positive trend in leaf area index during 1982 – 2009, indicating that greening could contribute to the observed positive trend of evapotranspiration, in line with similar studies that focused on the 1981–2012 (Y. Zhang et al. , 2016) and 1982–2013 (K. Zhang et al. , 2015) periods . [[#Zeng--2018|Zeng et al. (2018)]] determined that the 8% global increase in satellite-observed leaf area index between the 1980s and the 2010s may explain an increase in evapotranspiration of 12.0 ± 2.4 mm yr <sup>–1</sup> (about 55 ± 25% of the total observed increase). [[#Forzieri--2020|Forzieri et al. (2020)]] estimated that the recent increase in leaf area index led to 3.66 ± 0.45 W m <sup>–2</sup> in latent heat flux (about 51 ± 6 mm yr <sup>–1</sup> ) and that the sensitivity of energy fluxes to leaf area index increased by about 20% over the 1982–2016 period. Overall, there is ''medium confidence'' that greening has contributed to the global increase in evapotranspiration since the 1980s.

Plant water use efficiency (WUE) is expected to rise with CO <sub>2</sub> levels ( ''high confidence'' ) ( [[#8.2.3.3|Section 8.2.3.3]] and Box 5.2), and can in theory counteract rising evapotranspiration in a warmer atmosphere ( [[#8.2.3.3|Section 8.2.3.3]] ). However, observational studies suggest that this may not be the case in some ecosystems. For example, [[#Frank--2015|Frank et al. (2015)]] found that while the WUE increased in European forests across the 20th century, transpiration also increased due to more plant growth, a lengthened growing season, and increased evaporative demand. Likewise [[#Guerrieri--2019|Guerrieri et al. (2019)]] observed that while WUE and photosynthesis increased in North American forests, stomatal conductance experienced only modest declines that were restricted to moisture-limited forests. Other studies further suggest that in many ecosystems increased WUE will not compensate for increased plant growth, amplifying declines in surface water availability (De Kauwe et al. , 2013; Ukkola et al. , 2016b; A. Singh et al. , 2020) , while drought conditions can also offset the CO <sub>2</sub> fertilization effect and lead to a decline in WUE (N. [[#Liu--2020|]] [[#Liu--2020|]] [[#Liu--2020|Liu et al., 2020]] ). There is ''low confidence'' regarding the impact of plant physiological effects on observed trends in evapotranspiration.

An increasing number of studies have identified signals of attribution in the recent observed trends in evapotranspiration. [[#Douville--2013|Douville et al. (2013)]] found that the post-1960 rise in evapotranspiration in both the mid-latitudes and northern high latitudes was related to anthropogenic radiative forcing. An analysis of CMIP5 simulations suggests that anthropogenic forcing accounts for a large fraction of the global mean evapotranspiration trend from 1982 to 2010 ( [[#Dong--2017|Dong and Dai, 2017]] ) . [[#Padrón--2020|Padrón et al. (2020)]] determined that increases in evapotranspiration were responsible for the majority of the anthropogenic pattern in dry-season water availability that dominates global trends since 1984. These findings are further supported by CMIP6 model results (Figure 8.8) that show that the recent summer increase in evapotranspiration in the northern mid- and high latitudes is due to GHG forcing and decreasing anthropogenic aerosol emissions over Europe.

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[[File:215fc21844454b8df7280e4cb1c97939 IPCC_AR6_WGI_Figure_8_8.png]]

'''Figure 8.8 |''' '''Linear trends in annual mean evapotranspiration (mm day''' <sup>–1</sup> '''per decade) for''' '''1901–1984''' '''(left) and''' '''1985–2014''' '''(right):''' '''(a, e) Land Model Intercomparison Project''' '''(LMIP) and observational dataset, and the CMIP6 multi-model ensemble mean historical simulations driven by (b, f) all radiative forcings, (c, g) GHG-only radiative forcings, (d, h) aerosol-only radiative forcings experiment.''' Colour shade without grey cross correspond to the regions exceeding 10% significant level. Grey crosses correspond to the regions not reaching the 10% statistically significant level. Nine CMIP6-DAMIP models have been used having at least three members. The ensemble mean is weighted per each model on the available and used members. The Global Land Data Assimilation System (GLDAS) was not available over the early 20th century so was replaced by a multi-model off-line reconstruction, LMIP, which is consistent with GLDAS over the recent period but may be less reliable over the early 20th century given larger uncertainties in the atmospheric forcings. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

In summary, there is ''high confidence'' that terrestrial evapotranspiration has increased since the 1980s. There is ''medium confidence'' that this trend is driven by both increasing atmospheric water demand and vegetation greening, and ''high confidence'' that it can be partly attributed to anthropogenic forcing. There is ''low confidence'' about the extent to which increases in plant water use efficiency have influenced observed changes in evapotranspiration.

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