Potential evapotranspiration and continental drying

Abstract

By various measures (drought area¹ and intensity², climatic aridity index³, and climatic water deficits⁴), some observational analyses have suggested that much of the Earth’s land has been drying during recent decades, but such drying seems inconsistent with observations of dryland greening and decreasing pan evaporation⁵. ‘Offline’ analyses of climate-model outputs from anthropogenic climate change (ACC) experiments portend continuation of putative drying through the twenty-first century^3,6,7,8,9,10, despite an expected increase in global land precipitation⁹. A ubiquitous increase in estimates of potential evapotranspiration (PET), driven by atmospheric warming¹¹, underlies the drying trends^4,8,9,12, but may be a methodological artefact⁵. Here we show that the PET estimator commonly used (the Penman–Monteith PET¹³ for either an open-water surface^1,2,6,7,12 or a reference crop^3,4,8,9,11) severely overpredicts the changes in non-water-stressed evapotranspiration computed in the climate models themselves in ACC experiments. This overprediction is partially due to neglect of stomatal conductance reductions commonly induced by increasing atmospheric CO₂ concentrations in climate models⁵. Our findings imply that historical and future tendencies towards continental drying, as characterized by offline-computed runoff, as well as other PET-dependent metrics, may be considerably weaker and less extensive than previously thought.