Atmospheric Vapor Pressure Deficit Outweighs Soil Moisture Deficit in Controlling Global Ecosystem Water Use Efficiency

Abstract High vapor pressure deficit (VPD) and low soil moisture (SM) lead to soil and atmospheric droughts, which can stress carbon‐water coupling in terrestrial ecosystems. However, the strong collinearity between VPD and SM, particularly under certain climatic conditions, makes it challenging to disentangle their independent contributions to carbon and water dynamics in land‐atmosphere interactions. This study aimed to clarify the long‐term independent response of global vegetation carbon‐water coupling, based on ecosystem water‐use efficiency (WUE E ) and plant canopy water‐use efficiency (WUE Et ), to decoupled VPD and SM from 1982 to 2100. WUE E is defined as the ratio of ecosystem gross primary productivity to evapotranspiration, while WUE Et is defined as the ratio of ecosystem gross primary productivity to vegetation transpiration. The results indicate that from 1982 to 2018, both before and after the decoupling of VPD and SM, over 64% of global vegetation zones experienced stronger atmospheric moisture stress from VPD than soil drought stress from SM, consistently impacting WUE E and WUE Et . The influence of VPD on WUE E and WUE Et gradually declined, while the influence of SM presented a tendency to increase. The small difference in the responses of WUE E and WUE Et to VPD and SM is attributed to the strong collinearity between WUE E and WUE Et . The effects of VPD and SM on WUE E and WUE Et varied across vegetation cover gradients, biomes, and climatic zones. As atmospheric and soil drought intensifies in the coming decades, the effects of VPD on WUE E and WUE Et stress are stronger than those of SM across all four socio‐economic shared pathway (SSP) scenarios. In the high SSP scenarios (SSP5‐8.5 for WUE E and SSP3‐7.0 for WUE Et ), the dominant influence of VPD is expected to expand.