The differing response of global mean precipitation to natural versus anthropogenic climate change

In a climate with rising CO2 concentrations, climate models predict that globally averaged precipitation will increase with surface warming at a much lower rate than increases in water vapor. Previous studies have explained that this modeled difference in the response to warming infers that global precipitation increase is dictated by atmospheric radiative cooling increases and not moisture availability. However, results from observational studies using 20 years of satellite measurements disagree with this framework, and show global-mean precipitation and water vapor have increased with surface warming at similar rates. While the modeling results represent change at long-term, anthropogenic time scales, the observational results are based on data over a limited period of record and are therefore dominated by inter-annual changes. We investigate whether the physical constraints on the hydrological cycle fundamentally differ between these time scales. Using simulations from models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) where CO2 is increased 1% per year to a quadrupling of pre-industrial concentrations, we isolate and compare the inter-annual time scale and the decadal-to-century time scale responses of global-mean water vapor, precipitation and radiative cooling to surface warming. We show that while global-mean precipitation is constrained by radiative cooling on both time scales, at the longer time scales the effects of CO2 act to suppress the increase of radiative cooling with warming, resulting in a reduced precipitation increase compared to inter-annual time scales, where the effects of CO2 are not as strong. We further show that the water vapor response to warming does not have a similar dependency on CO2 effects, which may have implications on the magnitude of atmospheric overturning circulation changes at the different time scales. Our findings imply one cannot expect the global hydrological cycle to have the same response to warming at all time scales.