Divergent Impacts of Evapotranspiration by Plant CO2 Physiological Forcing on the Mean and Variability of Water Availability

Vegetation responses to rising atmospheric CO2 levels can significantly affect water availability (defined as precipitation minus evapotranspiration (ET)). While this effect has long been recognized and assessed for the mean state, its influence on interannual variability, which is more closely associated with extreme events, has yet to be comprehensively quantified. In this study, our primary focus is to evaluate the impacts of ET by plant physiology (denoted as ETPhy) on the mean and interannual variability of water availability under elevated CO2 using multiple CO2 sensitivity experiments from the coupled model intercomparison project phase 6. We show that the contribution of vegetation physiological effects to the mean water availability varies among regions, while it consistently contributes to variability by about 33%. Considering CO2 physiological effects alone, ETPhy exerts a more significant influence on the mean state than on variability, particularly in humid regions. Consequently, ETPhy contributes less than 5% to the variability of water availability in humid regions under rising CO2, whereas it accounts for about 20% of the mean state. This distinction could be attributed to the different mechanisms governing the mean and variability of ETPhy. Specifically, evaporation from CO2 physiological forcing is the most critical contributor to the variability of ETPhy in most regions while showing minimal impacts on the mean state. Our findings identify the divergent effects of ETPhy on the mean state and interannual variability of water availability under elevated CO2, as important in future climate projections.