The physiological effect of CO2 on the hydrological cycle in summer over Europe and land-atmosphere interactions

Past studies have shown that the physiological effect of CO2 may be important for future climate change, yet many regional climate models and hydrological models used in impact studies do not simulate it. Here, the role the physiological effect of CO2 on the changes in the continental hydrological cycle over Europe in summer – when severe drying is projected – is assessed using a large ensemble of idealized climate simulations, and compared to the radiative impacts of CO2. As expected, the physiological effect of CO2 leads to a decrease in evapotranspiration. However, it generally does not lead to an increase in river flows or soil moisture as often expected, because it is also associated with a large decrease in precipitation. This decrease in precipitation is likely due to small warming and decrease in specific humidity caused by the physiologically driven decrease in evapotranspiration, which together lead to a large decrease in relative humidity. Important inter-model uncertainties, however, exist regarding the physiological impact of CO2 on the summer hydrological cycle over Europe.