Compound drought slow down the greening of the Earth

Vegetation response to soil and atmospheric drought has raised extensively controversy, however, the relative contributions of soil drought, atmospheric drought and their compound drought on global vegetation growth remain unclear. Combining the changes in soil moisture (SM), vapor pressure deficit (VPD) and vegetation growth (NDVI) during 1982-2015, here we evaluated the trends of these three drought types and quantified their impacts on global NDVI. We found that global VPD has increased 0.22±0.05 kPa·decade-1 during 1982-2015, and this trend was doubled after 1996 (0.32±0.16 kPa·decade-1) than before 1996 (0.16±0.15 kPa·decade-1). Regions with large increase in VPD trend generally accompanied with decreasing trend in SM, leading to a widespread increasing trend in compound drought across 37.62% land areas. We further found compound drought dominated the vegetation browning since late 1990s. Earth system models agree with the dominant role of compound drought on vegetation growth, but their negative magnitudes are considerably underestimated, with half of the observed results (34.48%). Our results provided the evidence of compound drought induced global vegetation browning, highlighting the importance of correctly simulating the ecosystem-scale response to the under-appreciated exposure to compound drought as it will increase with climate change.