A FLEXIBLE CLIMATE MODEL FOR USE IN INTEGRATED ASSESSMENTS

Because of significant uncertainty in the behavior of the climate syst em, evaluations of the possible impact of an increase in greenhouse ga s concentrations in the atmosphere require a large number of long-term climate simulations. Studies of this kind are impossible to carry out with coupled atmosphere ocean general circulation models (AOGCMs) bec ause of their tremendous computer resource requirements. Here we descr ibe a two dimensional (zonally averaged) atmospheric model coupled wit h a diffusive ocean model developed for use in the integrated framewor k of the Massachusetts Institute of Technology (MIT) Joint Program on the Science and Policy of Global Change. The 2-D model has been develo ped from the Goddard Institute for Space Studies (GISS) GCM and includ es parametrizations of all the main physical processes. This allows it to reproduce many of the nonlinear interactions occurring in simulati ons with GCMs. Comparisons of the results of present-day climate simul ations with observations show that the model reasonably reproduces the main features of the zonally averaged atmospheric structure and circu lation. The model's sensitivity can be varied by changing the magnitud e of an inserted additional cloud feedback. Equilibrium responses of d ifferent versions of the 2-D model to an instantaneous doubling of atm ospheric CO2 are compared with results of similar simulations with dif ferent AGCMs. It is shown that the additional cloud feedback does not lead to any physically inconsistent results. On the contrary, changes in climate variables such as precipitation and evaporation, and their dependencies on surface warming produced by different versions of the MIT 2-D model are similar to those shown by GCMs. By choosing appropri ate values of the deep ocean diffusion coefficients, the transient beh avior of different AOGCMs can be matched in simulations with the 2-D m odel, with a unique choice of diffusion coefficients allowing one to m atch the performance of a given AOGCM for a variety of transient forci ng scenarios. Both surface warming and sea level rise due to thermal e xpansion of the deep ocean in response to a gradually increasing forci ng are reasonably reproduced on time scales of 100-150 y. However a wi de range of diffusion coefficients is needed to match the behavior of different AOGCMs. We use results of simulations with the 2-D model to show that the impact on climate change of the implied uncertainty in t he rate of heat penetration into the deep ocean is comparable with tha t of other significant uncertainties.