Little Change in Apparent Hydrological Sensitivity at Large CO₂ Forcing

Apparent hydrological sensitivity (?(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 ?(a )depends on the forcing amplitude we analyze idealized experiments over a broad range of abrupt CO2 forcing, from 2x to 8x preindustrial values, with two distinct climate models. We find little change in ?(a) between 2x and 4xCO(2), and almost no change beyond 5xCO(2). We validate this finding under transient CO2 forcing at 1%-per-year, up to 8xCO(2). 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 ?(a). This wealth of results demonstrates that ?(a) is a very weak function of CO2 forcing.