Fast versus slow response in climate change: implications for the global hydrological cycle
G. Bala, K. Caldeira & R. Nemani
Climate responds quickly to changes in CO2 and sunlight and then evolves more slowly as the Earth heats up. We examined how temperatures and precipitation varies with a change in global mean temperature due to changes in solar radiative forcing and CO2 forcing. The fast response of these two forcings is different, but the slow response associated with global temperature increases is very similar. Here we examine the significance of fast response in the interpretation of the total response to a change in climate forcing.
Recent studies have shown that changes in global mean precipitation are larger for solar forcing than for CO2 forcing of similar magnitude. In this paper, we use an atmospheric general circulation model to show that the differences originate from differing fast responses of the climate system. We estimate the adjusted radiative forcing and fast response using Hansen’s “fixed-SST forcing” method. Total climate system response is calculated using mixed layer simulations using the same model. Our analysis shows that the fast response is almost 40% of the total response for few key variables like precipitation and evaporation. We further demonstrate that the hydrologic sensitivity, defined as the change in global mean precipitation per unit warming, is the same for the two forcings when the fast responses are excluded from the definition of hydrologic sensitivity, suggesting that the slow response (feedback) of the hydrological cycle is independent of the forcing mechanism. Based on our results, we recommend that the fast and slow response be compared separately in multi-model intercomparisons to discover and understand robust responses in hydrologic cycle. The significance of this study to geoengineering is discussed.