Time Varying Climate Sensitivity in the Current Generation of GFDL Coupled Models

Abstract:

We report the results of a 3000 year run of GFDL ESM2M forced by a doubling of CO₂. We show that estimating the radiative forcing for ESM2M using both the instantaneous and fixed SST methods results in the need for a time-varying climate sensitivity during the first 1000 years after CO2 is doubled. However, the use of an unrealistically small radiative forcing of 2.1 Wm^-2, estimated by applying the ‘Gregory’ linear fit method to the full 3000 year period results in an almost constant climate sensitivity as a function of time. We found that the primary cause behind such a small forcing estimated by the Gregory method is an increase in the reflection of solar radiation by clouds, mainly over the Southern Ocean. This occurs within the first few decades after CO2 is increased in the coupled model. The fact that such cloud adjustments are not seen in a fixed SST model run suggests that the primary cause is the initial pattern of ocean warming induced by the radiative forcing. We also show that these cloud adjustments can lead to an underestimate of the equilibrium temperature of up to 30 % if the Gregory method is only applied to the first 150 years after an instantaneous increase in forcing. A similar magnitude of cloud adjustments also occurs in GFDL-CM3 in response to both greenhouse gas and aerosol forcings.