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

Abstract

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.