Responses of land evapotranspiration to Earth’s greening in CMIP5 Earth System Models

Satellite-observed Earth’s greening has been reproduced by the latest generation of Earth System Models (ESMs) participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). Land evapotranspiration (ET) is expected to rise with increasing leaf area index (LAI, Earth’s greening). The responses of ET play a key role in the land–climate interaction, but they have not been evaluated previously. Here, we assessed the responses of ET to Earth’s greening in these CMIP5 ESMs. We verified a significant and positive response of ET to the modeled greening in each model. However, the responses were not comparable across the ESMs because of an inherent bias in the sensitivity of ET to LAI $(\partial {\rm{E}}{\rm{T}}/\partial {\rm{L}}{\rm{A}}{\rm{I}})$ in the models: $\partial {\rm{E}}{\rm{T}}/\partial {\rm{L}}{\rm{A}}{\rm{I}}$ is precisely and inversely proportional to the trend of LAI $(\partial {\rm{L}}{\rm{A}}{\rm{I}}/\partial t)$ across the ESMs. Constrained by this inversely proportional relationship with the satellite-observed $\partial {\rm{L}}{\rm{A}}{\rm{I}}/\partial t,$ the Earth’s $\partial {\rm{E}}{\rm{T}}/\partial {\rm{\text{LAI}}}$ is 0.26 (0.21–0.34) mm d ^−1 per m ^2 m ^−2 , equaling the independent estimates from satellite-derived reconstructions of ET and LAI. Thus, the Earth’s greening-induced acceleration of ET is about 11.4 mm yr ^−1 , accounting for more than 50% of the observed increase in land ET over the last 30 years. To better model the land–climate interaction, $\partial {\rm{E}}{\rm{T}}/\partial {\rm{L}}{\rm{A}}{\rm{I}}$ in these ESMs should be calibrated. A feasible means is to improve the representation of the magnitude of LAI in these CMIP5 ESMs.