Drivers and effects of drying terrestrial water storage on ecosystem carbon uptake

Terrestrial water storage (TWS) is a crucial component in regulating global water and energy budgets, exerting significant impacts on the ecosystem carbon cycle. However, the physical mechanisms behind changes in TWS and its causal relationship with terrestrial carbon uptake remain elusive. Here, we explore water-heat-carbon dynamics using a convergent cross mapping method based on eddy-covariance flux measurements. Then, we employ a supervised machine learning model and path analysis to evaluate the effects of drying TWS on vegetation photosynthesis and respiration. Finally, we project future TWS and drought conditions as well as their impacts on ecosystem vegetation productivity at the global scale. We find that temperature, soil moisture, and radiation are dominant factors regulating carbon update. In most regions of the globe, soil moisture influences vegetation photosynthesis, while the Leaf area index (LAI) plays a dominant role in humid and hyper-dry regions. Our cascade model chain projects that future drought events may have severe negative impacts on vegetation productivity. Terrestrial productivity is projected to be constrained over a growing proportion of global land surface, from the historical period (65.36%) to SSP126 (68.5%), SSP370 (67.4%) and SSP585 (70.67%). As drought severity escalates from moderate to severe, gross primary productivity (GPP) anomalies decrease from -3.53 to -8.4 , suggesting that higher water stress lowers the terrestrial carbon sink. Our results highlight the urgency of enhancing ecosystem resilience to increasingly severe drought conditions.