News Release

Global ecosystem water use efficiency has stalled since 2001

Peer-Reviewed Publication

American Association for the Advancement of Science (AAAS)

Increases in global ecosystem water use efficiency – the ratio between carbon assimilation to water evapotranspiration – have stalled since 2001 due to a rising vapor pressure deficit, according to a new study. The findings highlight one way that the adverse effects of our warming climate may undermine human reliance on nature-based climate solutions to achieve carbon neutrality. The rapid rise of atmospheric carbon dioxide (CO2) has led to substantial changes in global terrestrial carbon and water cycles. One of these impacts has been a generalized increase in ecosystem water use efficiency (WUEeco). On a global scale, WUEeco plays a crucial role in the terrestrial sequestration of atmospheric carbon. However, the multifaceted dynamics involved in the complex trade-off between carbon gain and water loss under increasing atmospheric CO2 concentrations and rising temperatures are poorly understood, and, to date, few studies have evaluated WUEeco trends in response to climate change at the global scale. Some recent studies have suggested that WUEeco could be reaching its limit. To examine long-term trends in WUEeco of global terrestrial ecosystems, Fei Li and colleagues used 24 machine learning approaches to upscale and analyze global FLUXNET in situ observations of CO2 and water vapor fluxes, satellite-derived observations, and climate reanalysis data spanning 1982-2016. Li et al. found that while WUEeco increased from 1982 to 2000, it has not risen since 2001, likely due to the asymmetric effects of an increased vapor pressure deficit (VPD) – the difference between the amount of moisture in the air versus how much moisture the air can hold when saturated – and intensifying evapotranspiration. As a result of temperature rise-induced increases in VPD, global ecosystem photosynthesis has become suppressed and, thus, so has the ability of global ecosystems to assimilate carbon.

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