CO2 physiological forcing mitigates climate-driven water limitation on vegetation growth and carbon uptake.

Climate-driven increases in water limitation are expected to threaten ecosystem functioning and services. However, the extent to which ecosystems are water-limited remains uncertain due to the interplay of external CO2 physiological forcing and internal adaptation mechanisms. Here, we develop a unified ecosystem limitation index that quantifies the relative limitation of water and energy on key ecosystem functions and show that climate-driven water limitation on vegetation growth and carbon uptake is effectively mitigated by CO2 physiological effects. Consequently, projected expansion of water-limited areas is moderate (4-5% of assessed land area) compared to estimates based on evapotranspiration (10%) during 1980-2099. While climate change drives widespread shifts from energy to water limitation across all ecosystem functions, CO2 physiological effects alleviate constraints on photosynthesis and growth but not on evapotranspiration. Previous sole reliance on evapotranspiration has therefore systematically overestimated ecosystem water limitation. Our findings reconcile the aridity paradox-widespread greening despite intensifying climate aridity-and underscore the importance of multifunctional assessments for robust projections of ecosystem dynamics under climate change.