Hydrological feedback from projected Earth greening in the 21st century

Earth satellites have observed continuous increasing vegetation growth during the past four decades, a phenomenon called Earth greening. Nearly all Earth System Models (ESMs) participating in Coupled Model Intercomparison Project (CMIP) for Intergovernmental Panel on Climate Change (IPCC) project a continuous greening of the planet during the 21st century. To investigate the hydrological feedback from the projected Earth greening, we prescribed the increase in leaf area index (LAI) in the 21st century as projected by CMIP5 ESMs into a state-of-the-art ESM (IPCLCM), and simulated equilibrium climates for current CO2 and LAI, an increase of CO2 alone, an increase of LAI alone, and increases of both CO2 and LAI, respectively. We find that the greening simultaneously intensifies evapotranspiration and precipitation over land. In terms of soil moisture content, the spatial difference between the responses of evapotranspiration and precipitation causes a hydrological response of the "dry gets drier, wet gets wetter" (DDWW) paradigm. Increasing LAI significantly decreases soil moisture content over dry regions, including the Western North America, Southern South America, East Siberia, Central Asia, South Asia, Northern China, Sahel, Southern Africa, and Australia. Over wet regions, such as the Amazon and Congo rainforests, the greening-induced increase of terrestrial evapotranspiration favors more convective precipitation, so that the new equilibrium does not decrease soil moisture content. The DDWW paradigm in terms of P-ET response does not hold over wet areas. To mitigate climate change with forestry, policymakers should prevent degradation of existing forests, support afforestation over wet regions, and avoid planting trees in dry regions.