Asymmetric impact of the physiological effect of carbon dioxide on hydrological responses to instantaneous negative and positive CO₂ forcing

We conducted sensitivity experiments using a coupled atmosphere–ocean general circulation model to examine the asymmetry between the hydrological responses to instantaneous positive and negative CO₂ forcing and the impact of the CO₂ physiological effects (CDPEs) on these responses. This study focuses on the fast response occurring on time scales shorter than 1 year after imposing CO₂ forcing. Experiments investigating the CO₂ physiological effect show that the fast response of precipitation to positive CO₂ forcing is a decrease in the global and annual mean, whereas that of negative forcing is an increase the global and annual mean precipitation. The fast global precipitation response to negative forcing is stronger than the response to positive forcing. In contrast, the experiments without the CDPE reveal similar magnitudes of the fast global precipitation responses to negative and positive CO₂ forcing. Significant differences in the magnitudes of the fast precipitation response due to the CDPE are found in tropical regions such as the Amazon Basin, the Maritime Continents, and tropical Africa, where C3-type plants are common. The stomatal conductance of plant leaves is decreased by both positive and negative CO₂ forcing, which suppress the transpiration from the leaves. Consequently, the CDPE enhances the asymmetry of the fast precipitation responses to positive and negative CO₂ forcing. The asymmetric impact of CDPE requires a careful evaluation of future hydrological changes which is constrained by paleoclimate evidence.