Critical impact of vegetation physiology on the continental hydrologic cycle in response to increasing CO2

Predicting how increasing atmospheric CO 2 will affect the hydrologic cycle is of utmost importance for a wide range of applications. It is typically thought that future dryness will depend on precipitation changes, i.e., change in water supply, and changes in evaporative demand due to either increased radiation or temperature. Opposite to this viewpoint, using Earth system models, we show that changes in key water-stress variables will be strongly modified by vegetation physiological effects in response to increased [CO 2 ] at the leaf level. These results emphasize that the continental carbon and water cycles have to be studied as an interconnected system. Predicting how increasing atmospheric CO 2 will affect the hydrologic cycle is of utmost importance for a range of applications ranging from ecological services to human life and activities. A typical perspective is that hydrologic change is driven by precipitation and radiation changes due to climate change, and that the land surface will adjust. Using Earth system models with decoupled surface (vegetation physiology) and atmospheric (radiative) CO 2 responses, we here show that the CO 2 physiological response has a dominant role in evapotranspiration and evaporative fraction changes and has a major effect on long-term runoff compared with radiative or precipitation changes due to increased atmospheric CO 2 . This major effect is true for most hydrological stress variables over the largest fraction of the globe, except for soil moisture, which exhibits a more nonlinear response. This highlights the key role of vegetation in controlling future terrestrial hydrologic response and emphasizes that the carbon and water cycles are intimately coupled over land.