Future global water scarcity partially moderated by vegetation responses to rising CO2

Most studies of future water scarcity rely on hydrological models that often neglect the effects of plant physiological responses to rising CO2, such as reduced stomatal opening, which can decrease transpiration and enhance water availability over large scales. To evaluate how such vegetation responses to rising CO2 and subsequent climate change affect water scarcity in typical impact studies, we replicate their experimental design by driving an offline land surface model with Earth system model output. Underahighemission climate scenario, our simulations suggest that water scarcity is projected to worsen in many regions. However, combined vegetation stomatal and structural responses increase overall water supply and partially alleviate water scarcity in most regions, mainly because of CO2-induced reduced stomatal opening. In contrast, CO2-driven increases in vegetation cover and leaf area exacerbate water scarcity in certain regions, particularly arid and already water-scarce areas. For the period 2076–2095, incorporating all vegetation structural and stomatal responses to CO2 and climate change reduces global median Water Scarcity Index (WSI) by 12%. Across 291 river basins, substantially more show reductions than increases in median WSI; decreases of 10%–70% occur in 139 basins, representing 80% of the global population, and increases of 10%–60% occur in 11 basins, representing 0.2 % of the population. Overall, these findings highlight the importance of considering vegetation stomatal and structural responses to rising CO2 in water scarcity assessments.