Climate models and the astronomical theory of the ice ages

Attempts at determining the climatic response to perturbations in the Earth's orbital parameters are reviewed. The relationship between equilibrium and nonequilibrium responses and its implications for climatic sensitivity are discussed in the context of an empirical model due to Imbrie and Imbrie. Some counterintuitive features of the linear equilibrium response to the perihelion cycle in a simple energy balance model are then described in detail. The results of North and Coakley and of Pollard are examined as examples of results that are, respectively, discouraging and mildly encouraging for proponents of the astronomical theory of the ice ages. The attempt by Suarez and Held to address some of the deficiencies in the simplest energy balance models is reviewed, followed by a brief preview of some calculations of relevance to the ice age problem performed by Manabe and co-workers with an atmospheric general circulation model.