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 1pctCO₂ (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, ${\eta }_a$ = 1.39 ± 0.22% K⁻¹). These hydrological sensitivities are relatively stable over transient 2× to 4 × 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.