The Global-Mean Precipitation Response to CO₂-Induced Warming in CMIP6 Models

We examine the response of globally averaged precipitation to global warming—the hydrologic sensitivity (HS)—in the Coupled Model Intercomparison Project phase 6 (CMIP6) multi-model ensemble. Multi-model mean HS is 2.5% K⁻¹ (ranging from 2.1–3.1% K⁻¹ across models), a modest decrease compared to CMIP5 (where it was 2.6% K⁻¹). This new set of simulations is used as an out-of-sample test for observational constraints on HS proposed based on CMIP5. The constraint based on clear-sky shortwave absorption sensitivity to water vapor has weakened, and it is argued that a proposed constraint based on surface low cloud longwave radiative effects does not apply to HS. Finally, while a previously proposed mechanism connecting HS and climate sensitivity via low clouds is present in the CMIP6 ensemble, it is not an important factor for variations in HS. This explains why HS is uncorrelated with climate sensitivity across the CMIP5 and CMIP6 ensembles.