Potential evapotranspiration and continental drying

Abstract

By various measures (drought area and intensity , climatic aridity index , and climatic water deficits ), 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 evaporation . ‘Offline ’ analyses of climate-model outputs from anthropogenic climate change (ACC) experiments portend continuation of putative drying through the twenty-first century , despite an expected increase in global land precipitation . A ubiquitous increase in estimates of potential evapotranspiration (PET), driven by atmospheric warming , underlies the drying trends , but may be a methodological artefact . Here we show that the PET estimator commonly used (the Penman–Monteith PET for either an open-water surface or a reference crop ) 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 CO 2 concentrations in climate models . 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.