TRANSITIVITY PROPERTIES OF SURFACE-TEMPERATURE AND ICE COVER IN THE CCM1

A frequently made assumption in simple models of long-term climatic be havior(e.g. 'energy balance' models) is that, owing to instability eng endered by the ice-albedo feedback, the climatic system (surface tempe rature, in particular) can exhibit multiple steady states within the p aleoclimatologically observed range of temperature. Here we show that, for a more comprehensive model than an energy balance model, the CCM1 general circulation model, such a bimodality does not exist for prese nt values of the solar constant and atmospheric CO2, even if one exclu des the existence of a seasonal cycle of radiative forcing. Thus, no e vidence is found to support the whole class of ice-age theories tuned to present CO2 levels that depend on this bimodality. As a corollary, support is found for the idea that surficial temperature and snow-sea- ice cover are essentially 'slaved', fast-response, climatic variables that equilibrate uniquely with the prescribed external forcing and the slow-response climatic variables (e.g. the ice sheets and deep ocean state). It is also implied that, although care should be exercised in the choice of initial conditions to minimize computer time, the select ion of initial conditions is not likely to affect the final outcome of general circulation model studies of climate for fixed (near-present) values of the solar constant and CO2 forcing. (C) 1997 Elsevier Scien ce B.V.