Energy Surplus and an Atmosphere-Land-Surface “Tug of War” Control Future Evapotranspiration

Abstract

The 21st century evapotranspiration (ET) trends over the continental U.S. are assessed using innovative, energy-based principles. Annual ET is projected to increase with high confidence at the rate of 20 mm for every 1℃ of rise in near-surface air temperature, or 0.45 or 0.98 mm/year/year, depending on the emission scenario. The ET trajectory is dominated (58%) by the increase of land-surface net radiative energy. An enhancement of the fraction of energy taken up by ET becomes a more important controller (53%) in late 21st century, under the high emission scenario. This increase is explained by the “tug of war” between atmospheric vapor demand and land-surface ability to supply water. An assessment of future water availability (precipitation minus ET) shows no significant changes at the continental scale. This outcome nevertheless hides strong spatial variability, emphasizing the role of ET in shaping the distribution of water availability among human populations.