Quantifying Global Hydrological Sensitivity to CO2 Physiological and Radiative Forcings Under Large CO2 Increases

Prediction of surface freshwater flux (precipitation or evaporation) in a CO2-enriched climate is highly uncertain, primarily depending on the hydrological responses to physiological and radiative forcings of CO2 increase. Using the 1pctCO(2) (a 1% per year CO2 increase scenario) experiments of 12 CMIP6 models, we first decouple and quantify the magnitude of global hydrological sensitivity to CO2 physiological and radiative forcings. Results show that the direct global hydrological sensitivity (for land plus ocean precipitation) to CO2 increase only is -0.09 +/- 0.07% (100 ppm)(-1) and to CO2-induced warming alone is 1.54 +/- 0.24% K-1. The latter is about 10% larger than the global apparent hydrological sensitivity (i.e., including all effects, not only direct responses to warming, eta a ${\eta }_{a}$ = 1.39 +/- 0.22% K-1). These hydrological sensitivities are relatively stable over transient 2x to 4 x CO2 scenario. The intensification of the global water cycle are dominated by the CO2 radiative effect (79 +/- 12%) with a smaller positive contribution from the interaction between the two effects (6 +/- 12%), but are reduced by the CO2 physiological effect (-10 +/- 8%). This finding underlines the importance of CO2 vegetation physiology in global water cycle projections under a CO2-enriched and warming climate.