A flexible climate model for use in integrated assessments |
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Authors: | A P Sokolov P H Stone |
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Institution: | (1) Center for Global Change Science, Massachusetts Institute of Technology, 77 Massachusetts Ave., Room 54-1312, Cambridge, MA 02139, USA E-mail: sokolov@mit.edu, US |
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Abstract: | Because of significant uncertainty in the behavior of the climate system, evaluations of the possible impact of an increase
in greenhouse gas 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) because of their tremendous
computer resource requirements. Here we describe a two dimensional (zonally averaged) atmospheric model coupled with a diffusive
ocean model developed for use in the integrated framework of the Massachusetts Institute of Technology (MIT) Joint Program
on the Science and Policy of Global Change. The 2-D model has been developed from the Goddard Institute for Space Studies
(GISS) GCM and includes parametrizations of all the main physical processes. This allows it to reproduce many of the nonlinear
interactions occurring in simulations with GCMs. Comparisons of the results of present-day climate simulations with observations
show that the model reasonably reproduces the main features of the zonally averaged atmospheric structure and circulation.
The model’s sensitivity can be varied by changing the magnitude of an inserted additional cloud feedback. Equilibrium responses
of different versions of the 2-D model to an instantaneous doubling of atmospheric CO2 are compared with results of similar simulations with different 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 appropriate values of the deep ocean diffusion coefficients, the transient behavior of different AOGCMs
can be matched in simulations with the 2-D model, with a unique choice of diffusion coefficients allowing one to match the
performance of a given AOGCM for a variety of transient forcing scenarios. Both surface warming and sea level rise due to
thermal expansion of the deep ocean in response to a gradually increasing forcing are reasonably reproduced on time scales
of 100–150 y. However a wide 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 the rate of heat
penetration into the deep ocean is comparable with that of other significant uncertainties.
Received: 10 March 1997 / Accepted: 20 October 1997 |
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