How stomatal function shapes evapotranspiration in a rising CO2 world

Evapotranspiration (ET) is a key process in the land water cycle, with plant transpiration accounting for ~60% of land ET. Transpiration is regulated through both stomatal functioning and total leaf area. Stomata control the diffusion of water vapor from leaves to the atmosphere, while leaf area determines the total surface over which transpiration occurs. Both processes are expected to change under elevated CO₂ (eCO₂), with increased CO₂ availability allowing plants to optimize carbon gain to water loss by closing their stomata and decreasing transpiration per leaf. At the same time, CO₂ fertilization increases leaf area, which can contribute to increasing total transpiration, as well as increasing rain water interception and reevaporation. The combined influence of these opposite physiological responses creates uncertainty in the total plant-driven ET response to eCO₂. Observations also reveal a range of stomatal function across and within plant types in varying environments, much of which is not represented in Earth system models, contributing to uncertainty in the magnitude of stomatal closure under eCO₂ and its impact on future ET. We quantify how uncertainty in stomatal functioning propagates into ET responses under eCO₂ using Community Earth System Model (CESM2) simulations, where we perturb stomatal function across the observed range for each plant type at preindustrial and doubled preindustrial CO₂. We also compare ET responses driven by stomatal uncertainty with those from leaf area growth and identify regions where ET is most sensitive to stomatal function assumptions. The total plant-driven ET response to eCO₂ is a combination of the opposing contributions from stomatal closure and leaf area growth. Of the two contributors, leaf area growth tends to have a larger ET response to eCO₂ compared with stomatal closure in CESM2. However, we find that stomatal uncertainty drives ET changes of comparable magnitude to the total combined plant-driven ET response to eCO₂. Further, about 32% of land has greater ET sensitivity to stomatal uncertainty than the ET response to eCO₂ driven leaf area growth. This occurs particularly in wet regions where stomata can strongly regulate transpiration yet remain sensitive to water availability. These results improve understanding of how uncertainty in plant physiological processes propagates into future water cycle responses and climate projections, and identify where uncertainties may be most influential.