Plant Physiological Responses to Rising CO 2 Modify Simulated Daily Runoff Intensity With Implications for Global‐Scale Flood Risk Assessment

Climate change is expected to increase the frequency of flooding events and, thus, the risks of flood‐related mortality and infrastructure damage. Global‐scale assessments of future flooding from Earth system models based only on precipitation changes neglect important processes that occur within the land surface, particularly plant physiological responses to rising CO 2 . Higher CO 2 can reduce stomatal conductance and transpiration, which may lead to increased soil moisture and runoff in some regions, promoting flooding even without changes in precipitation. Here we assess the relative impacts of plant physiological and radiative greenhouse effects on changes in daily runoff intensity over tropical continents using the Community Earth System Model. We find that extreme percentile rates increase significantly more than mean runoff in response to higher CO 2 . Plant physiological effects have a small impact on precipitation intensity but are a dominant driver of runoff intensification, contributing to one half of the 99th and one third of the 99.9th percentile runoff intensity changes. Floods are one of the most devastating natural disasters in the world, contributing to thousands of deaths and billions of dollars in damages annually. Climate change is expected to increase flood exposure considerably through the 21st century. However, recent studies assessing future flood risk on global scales by downscaling precipitation from Earth system models often neglect important plant physiological responses to rising CO 2 . In particular, higher CO 2 concentrations may lower stomatal conductance and, in the absence of significant plant growth, reduce water loss through transpiration, increasing soil moisture in many regions. For a given precipitation rate, higher soil moisture can decrease the amount of rain that infiltrates the soil and increase runoff. Here we apply a simulation design that isolates the independent effects of higher CO 2 on radiatively driven precipitation intensification from plant‐driven soil moisture changes. We show that plant‐physiological responses to increasing CO 2 are major drivers of the runoff intensity change in the tropics. Land surface changes contribute to one half of the 99th percentile runoff change and one third of the 99.9th percentile change. Our results suggest that comprehensive flood assessments should account for plant physiology as well as radiative impacts of higher CO 2 in order to better inform flood prediction and mi