Quantifying global water cycle—CO2 feedbacks from Earth system models

Large uncertainty in predicting surface water availability (precipitation minus evaporation) in a CO₂-enriched climate is associated to rising CO₂-related hydrological feedbacks to the global water cycle, primarily including hydrological sensitivity to CO₂-physiological and -radiative effects. Using the 1pctCO₂ experiments of twelve CMIP6 models, we first decoupled and quantified the magnitudes of these sensitivities at global and regional scales. Results show that under a 140-year transient 4×CO₂ scenario, the global hydrological sensitivity (precipitation or evaporation) for CO₂-physiological effect feedback is -0.09 ±0.07 % (100 ppm) ^‒1 and for CO₂ radiative effect feedback is 1.54 ±0.24 % K^‒1. The latter is about 10% larger than the global apparent hydrological sensitivity ( = 1.39 ±0.22 % K^‒1), as estimated from the fully coupled simulations. These hydrological sensitivities are relatively stable over transient 2× to 4×CO₂ scenarios. The CMIP6 models project that global precipitation or evaporation increases at 4×CO₂ are dominated by the CO₂ radiative effect feedback (79 ±12%) and positively contributed by the interaction between the two feedbacks (6 ±12%) but are reduced by the CO₂ physiological effect feedback (-10 ±8%). This underlines the importance of CO₂ vegetation physiology in global water cycle projections under a CO₂-enriched and warming climate.