Little Change in Apparent Hydrological Sensitivity at Large CO₂ Forcing

Apparent hydrological sensitivity (η_a), the change in the global mean precipitation per degree K of global surface warming, is a key aspect of the climate system's response to increasing CO₂ forcing. To determine whether η_a depends on the forcing amplitude we analyze idealized experiments over a broad range of abrupt CO₂ forcing, from 2× to 8× preindustrial values, with two distinct climate models. We find little change in η_a between 2× and 4×CO₂, and almost no change beyond 5×CO₂. We validate this finding under transient CO₂ forcing at 1%-per-year, up to 8×CO₂. We further corroborate this result by analyzing the 1%-per-year output of more than 15 CMIP5/6 models. Lastly, we examine the 1,000-year long LongrunMIP model output, and again find little change in η_a. This wealth of results demonstrates that η_a is a very weak function of CO₂ forcing.