Potential evapotranspiration and continental drying

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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 century3,6,7,8,9,10, despite an expected increase in global land precipitation9. A ubiquitous increase in estimates of potential evapotranspiration (PET), driven by atmospheric warming11, underlies the drying trends4,8,9,12, but may be a methodological artefact5. Here we show that the PET estimator commonly used (the Penman–Monteith PET13 for either an open-water surface1,2,6,7,12 or a reference crop3,4,8,9,11) 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.

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Figure 1: Changes (future − historical; mm d−1) of ET.
Figure 2: Future versus historical stomatal conductance (m s−1), for the GFDL-ESM2M climate model.
Figure 3: Scatter plot of change in non-water-stressed ET from the GFDL-ESM2M climate model (dNWSET) against change in PET (dPET).
Figure 4: Multi-model median of the relative change (%) of the annual-mean runoff from the historical to the future time period.


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The World Climate Research Programme’s Working Group on Coupled Modelling is responsible for CMIP; the climate modelling groups listed in Supplementary Table 1 produced, and made available, their model output. For CMIP, the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. A. Berg, S. Kapnick, M. Roderick, J. Scheff and G. Wang gave helpful reviews of our manuscript.

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P.C.D.M. conceived and led the study, interpreted the data and prepared the manuscript. K.A.D. carried out all computations, prepared all figures and assisted with manuscript preparation.

Correspondence to P. C. D. Milly.

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The authors declare no competing financial interests.

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Milly, P., Dunne, K. Potential evapotranspiration and continental drying. Nature Clim Change 6, 946–949 (2016) doi:10.1038/nclimate3046

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