The Role of CO2 Physiological Forcing in Driving Future Precipitation Variability and Precipitation Extremes

Transpired water contributes roughly 25% to total precipitation over the Earth's land surface. In addition to transpiration's impact on climatological mean precipitation, recent work suggests that transpiration reduces daily and intraseasonal precipitation variability in tropical forest regions. Projected increases in the concentration of CO2 are expected to reduce transpiration through changes in plant physiology (termed the CO2 physiological effect). Here, we use an ensemble of climate model experiments to assess the potential contribution of the CO2 physiological effect to future changes in precipitation variability and extreme precipitation events. Within our model simulations, precipitation responses to the physiological effects of increased CO2 concentrations are greatest throughout the tropics. In most tropical forest regions CO2 physiological forcing increases the annual number of dry (less than 0.1 mm/day) and extremely wet (rainfall exceeds 95th percentile) days. Changes in precipitation are primarily driven by an increase in surface temperature and subsequent changes in atmospheric stability and moisture convergence over vegetated tropical land regions. Our results suggest that the plant physiological response to CO2 forcing may serve as an important contributor to future precipitation variability in the tropics, and that future work should aim to reduce uncertainty in the response of plant physiology to changes in climate.