Multi-decadal trends in global terrestrial evapotranspiration and its components

TLDR

Evapotranspiration converts liquid water to vapor, regulating temperatures, and recent studies report declining trends that may reflect climate variability. We applied a validated diagnostic model to estimate daily land evapotranspiration and its components—transpiration, soil evaporation, and intercepted rainfall vaporization—from 1981 to 2012. Land evapotranspiration rose significantly during this period, mainly due to increases in transpiration and intercepted rainfall that offset declining soil evaporation, a pattern linked to greening and poorly simulated by CMIP5 models.

Abstract

Abstract Evapotranspiration (ET) is the process by which liquid water becomes water vapor and energetically this accounts for much of incoming solar radiation. If this ET did not occur temperatures would be higher, so understanding ET trends is crucial to predict future temperatures. Recent studies have reported prolonged declines in ET in recent decades, although these declines may relate to climate variability. Here, we used a well-validated diagnostic model to estimate daily ET during 1981–2012 and its three components: transpiration from vegetation (E t ), direct evaporation from the soil (E s ) and vaporization of intercepted rainfall from vegetation (E i ). During this period, ET over land has increased significantly ( p < 0.01), caused by increases in E t and E i , which are partially counteracted by E s decreasing. These contrasting trends are primarily driven by increases in vegetation leaf area index, dominated by greening. The overall increase in E t over land is about twofold of the decrease in E s . These opposing trends are not simulated by most Coupled Model Intercomparison Project phase 5 (CMIP5) models and highlight the importance of realistically representing vegetation changes in earth system models for predicting future changes in the energy and water cycle.