4
Models and Their Validity

The independent studies of the CO2/climate problem that we have examined range from calculations with simple radiative-convective models to zonally and vertically averaged heat-balance models with horizontally diffusive heat exchange and snow-ice albedo feedbacks to full-fledged three-dimensional general circulation models (GCM’s) involving most of the relevant physical processes. Our confidence in our conclusion that a doubling of CO2 will eventually result in significant temperature increases and other climate changes is based on the fact that the results of the radiative-convective and heat-balance model studies can be understood in purely physical terms and are verified by the more complex GCM’s. The last give more information on geographical variations in heating, precipitation, and snow and ice cover, but they agree reasonably well with the simpler models on the magnitudes of the overall heating effects.

The radiative-convective models have been reviewed by Ramanathan and Coakley (1978). The latitudinally varying energy-balance models were originally developed by Budyko (1969) and Sellers (1969) for studies of climatic change. More recently they have been employed by many authors, including Ramanathan et al. (1979) and MacDonald et al. (1979), for CO2/climate-change determinations. These models prescribe the infrared feedback but calculate the snow-ice albedo feedback by coupling to a simple parameterized horizontal heat transport; the snow and ice occur poleward of the latitude at which the temperature has an empirically prescribed value. The principal value of these models lies in their inclusion of the snow-ice albedo feedback. However, they do not deal with real geography or explicit dynamics and therefore can yield only crude approximations to the latitudinal variations of the CO2-induced temperature changes.



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