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

Abstract Prediction of surface freshwater flux (precipitation or evaporation) in a CO 2 ‐enriched climate is highly uncertain, primarily depending on the hydrological responses to physiological and radiative forcings of CO 2 increase. Using the 1pctCO 2 (a 1% per year CO 2 increase scenario) experiments of 12 CMIP6 models, we first decouple and quantify the magnitude of global hydrological sensitivity to CO 2 physiological and radiative forcings. Results show that the direct global hydrological sensitivity (for land plus ocean precipitation) to CO 2 increase only is −0.09 ± 0.07% (100 ppm) −1 and to CO 2 ‐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, = 1.39 ± 0.22% K −1 ). These hydrological sensitivities are relatively stable over transient 2× to 4 × CO 2 scenario. The intensification of the global water cycle are dominated by the CO 2 radiative effect (79 ± 12%) with a smaller positive contribution from the interaction between the two effects (6 ± 12%), but are reduced by the CO 2 physiological effect (−10 ± 8%). This finding underlines the importance of CO 2 vegetation physiology in global water cycle projections under a CO 2 ‐enriched and warming climate.