Accurate water scarcity projections are essential for effective adaptation strategies. Most existing studies rely on hydrology models that often neglect the effects of plant physiological responses to rising CO₂ on the water cycle , such as reduced stomatal opening, which can decrease transpiration and enhance water availability over large scales. Using a land surface model driven by an Earth system model under a high-emission climate scenario, we evaluate how physiological and structural plant responses to rising CO₂ and subsequent climate change affect the Water Scarcity Index (WSI). Our simulations suggest that the combined effects of these plant responses partially alleviate WSI for most regions, largely due to CO₂-induced stomatal closure. However, CO₂- and climate-induced vegetation changes do exacerbate water scarcity in some places, particularly arid regions. By 2076–2095, when incorporating all plant responses in our projections, global median WSI is approximately 12 % lower, and among 291 global river basins, median WSI is between 10 and 70 % lower in 138 basins, home to 80 % of the global population, and between 10 % and 60 % higher in 11 basins, home to 0.2 % of the population. These model results highlight the potential for plant responses to CO₂ to somewhat alleviate water scarcity, noting water scarcity is still projected to worsen in many regions, including highly populated areas. There is an urgent need to gather empirical evidence on the strength of plant responses to CO₂ at large scales to address modelling uncertainties.