Global Ecosystem's Water Use Efficiency Stalls Since 2001

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 (WUE_eco). On a global scale, WUE_eco 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 WUE_eco trends in response to climate change at the global scale. Some recent studies have suggested that WUE_eco could be reaching its limit. To examine long-term trends in WUE_eco 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 WUE_eco 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.