Justin S Mankin , Noel Siegert, Jason E. Smerdon, Benjamin Cook, Richard Seager, Park Williams, Corey Lesk, Zhiying Li, Harmanveer Singh, Emily Martinez

Will plants consume more or less water in a high carbon dioxide [CO2] world? What will the hydrologic consequences of those changes will be? Some theories and analyses of earlier generations of Earth System Models (ESM) suggest that transpiration will decline with higher atmospheric carbon dioxide concentrations [CO2] due to stomatal closure, thereby enhancing runoff and soil moisture and countering the continental drying predicted by warming alone. We show that the opposite effect prevails in the latest generation of ESMs forced with increasing [CO2]: plants themselves contribute to projected soil drying, with smaller effects on runoff, and this picture emerges by considering the interactions between radiatively driven warming and the physiological effects of high [CO2] on plants. These interactions act to increase plant-based evapotranspiration (ET) by expanding leaf area and lengthening and warming growing seasons beyond what would be predicted by radiative or biogeochemical effects alone. Collectively, these interactions increase ecosystem water use and dry soils, compensating any land water savings from stomatal closure. At the same time, these interactions have grown and become more uncertain across model generations. We also find that the strength of these plant-water interactions scale with the simulated resilience of the land carbon sink to warming—a key feedback in the carbon cycle. Our results emphasize that a linearity assumption underpinning analyses of carbon, plant, and water interactions is not appropriate for the latest generation of ESMs, with implications for model development, as well as the accurate interpretation of projected changes to the carbon cycle and the consequences for future climate, drought, and water availability.