Global water use efficiency saturation due to increased vapor pressure deficit

The ratio of carbon assimilation to water evapotranspiration (ET) of an ecosystem, referred to as ecosystem water use efficiency (WUE_eco), is widely expected to increase because of the rising atmospheric carbon dioxide concentration (C_a). However, little is known about the interactive effects of rising C_a and climate change on WUE_eco. On the basis of upscaled estimates from machine learning methods and global FLUXNET observations, we show that global WUE_eco has not risen since 2001 because of the asymmetric effects of an increased vapor pressure deficit (VPD), which depressed photosynthesis and enhanced ET. An undiminished ET trend indicates that rising temperature and VPD may play a more important role in regulating ET than declining stomatal conductance. Projected increases in VPD are predicted to affect the future coupling of the terrestrial carbon and water cycles.