Greening-induced increase in evapotranspiration over Eurasia offset by CO₂-induced vegetational stomatal closure

Abstract

Evapotranspiration (ET), as a key exchanging component of the land energy, water and carbon cycles, is expected to increase in response to greening land under a warming climate. However, the relative importance of major drivers (e.g. leaf area index (LAI), climate forcing, atmospheric CO2, etc) to long-term ET change remain largely unclear. Focusing on the Eurasia which experienced the strong vegetational greening, we aim to estimate the long-term ET trend and its drivers’ relative contributions by applying a remote sensing-based water-carbon coupling model— Penman–Monteith–Leuning version 2 (PML-V2) driven by observational climate forcing and CO2 records, and satellite-based LAI, albedo and emissivity. The PML-V2 estimated an increasing ET trend (6.20 ± 1.13 mm year−1 decade−1, p < 0.01) over Eurasia during 1982–2014, which is close to the ensemble mean (6.51 ± 3.10 mm year−1 decade−1) from other three ET products (GLEAMv3.3a, ERA5 and CRv1.0). The PML-based ET overall agrees well with water-balance derived ET in detecting the trend directions. We find that the Eurasian ET increasing trend was mostly from vegetated regions over central and eastern Europe, Indian and southeast China where ET trends were larger than 20 mm year−1 decade−1. Modeling sensitivity experiments indicate that the Eurasian ET trend was mainly dominated by positive contributions from climate forcing change (40%) and increased LAI (22%), but largely offset by a negative contribution of increased CO2 (26%). Our results highlight the importance of the suppression effect of increasing CO2-induced stomatal closure on transpiration. This effect was rarely considered in diagnostic ET products but plays a key role to ensure that the long-term ET trend should not be overestimated by only accounting for greening-induced increases in transpiration and rainfall interception.

CO2 fertilization is spatially distinct from stomatal conductance reduction in controlling ecosystem water-use efficiency increase

“…The PML-V2 model has been applied to successfully produce the MODIS LAI-based global GPP and ET products at a 500 m and 8 d resolution since 2002, which were noticeably better than most widely used GPP and ET products (Zhang et al 2019). The PML-V2 was also used to explore the long-term impacts from increased CO 2 and strong greening on ET trends over Eurasia since 1980s (Zhang et al 2021).…”

“…The most remarkable aspect is that the greenhouse effect-induced global warming and related environmental changes (e.g. geographical patterns of global precipitation) are expected to increase drought (Dai 2012) and evapotranspiration (ET) (Jung et al 2010, Zhang et al 2012, 2021, Ma et al 2020, accelerate global water cycle (Gedney et al 2006, Huntington 2006, Betts et al 2007, Mankin et al 2019, and significantly influence vegetation water-use efficiency (e.g. Franks et al 2013, Keenan et al 2013), gross primary productivity (GPP) and land carbon sink (e.g.…”

“…GPP) at an ecosystem scale (Ainsworth and Long 2005, Schimel et al 2015, Wang et al 2020. On the other hand, increased CO 2 also induced the suppression effect on plant/canopy transpiration (E c ) by reducing leaf/canopy stomatal conductance which regulates exchanges of both CO 2 and H 2 O molecules between the plant and the atmosphere (Field et al 1995, Katul et al 2009, Damour et al 2010, Medlyn et al 2011, Swann et al 2016, Zhang et al 2021. Yang et al (2018) attributed ET changes using CMIP6 models and showed that an increase in ET caused by a warming-induced vapor pressure deficit increase is almost entirely offset by a decrease in ET caused by stomatal conductance reduction (or resistance increase) driven by rising CO 2 concentration.…”

“…The LAI, due to the combined effect from increased CO 2 fertilization effect, climate change, land use change, and nitrogen deposition, has increased globally and was quite evident by satellitebased observations since 1980s (Norby and Zak 2011, Zhu et al 2016, Chen et al 2019. The LAI is expected to increase E c and precipitation interception by vegetation (E i ), thereby boosting the rate of terrestrial ET (total ET is defined as the sum of soil evaporation E s , E c and E i ) (Zhang et al 2015, 2021, Lemordant et al 2018, Zeng et al 2018, which further reduces PWUE and EWUE by the increased E c and ET.…”

Estimating Vegetation Greening Influences on Runoff Signatures Using a Log‐Based Weighted Ensemble Method

“…From 2000 to 2017, one‐third of the global vegetated area showed an increasing in greening, which was especially obvious in China and India (Chen et al., 2019). The increase in vegetation greenness has brought substantial impacts on climate and the water cycle, including land surface temperature (Chen et al., 2020; Zeng et al., 2017), evapotranspiration (Ma et al., 2021; Meng et al., 2020; Zeng et al., 2018; Zhang et al., 2021), soil water (Liu et al., 2022; Zhang, Wang, et al., 2020) and runoff (Liu et al., 2016, Liu, Liang, et al., 2020; Ukkola et al., 2016). Moreover, several studies projected an increased LAI across the majority of China due to warming and elevated CO 2 in the future (Liu, Wang, et al., 2020; Ma, Zhao, et al., 2019; Yuan et al., 2021; Zhao et al., 2020).…”