An examination of the results of climate models testing the Milankovitch theory suggests that this theory may be closely linked to the broader and as yet unresolved question of the evaluation of climate sensitivity. With this point in mind, a climate model intended to test climate sensitivity in general and the Milankovitch theory in particular is described, and its equilibrium solutions for the present surface boundary conditions are analyzed. The statistical-dynamical hemispheric climate model is zonally averaged, with expanded longitudinal separation at the surface level, and includes topography, and detailed sea ice/snow and land snow schemes capable of simulating nonlinear effects of heat transfer through the layers of ice and snow, a modified long-wave scheme, a provision for a variable cloud top temperature (VCT) model, and an eddy exchange parameterization connecting the eddy coefficients to the n th power of the meridional temperature gradient.

The value of n in the latter relation is varied between the traditional value of 2 associated with the Green theory and a value of 4, which we believe to be more suitable for seasonal climate models of the present type. Four climate model versions are implemented with the aim of investigating three basic feedback mechanisms associated with the ice/snow albedos, the long-wave radiation, and the meridional eddy transport, and estimating the upper bound of climate sensitivity due to their effect. The differences between the results of the four versions are rather small for the simulation of the present climate but are more than tripled for a 1% decrease in the solar constant. It is therefore concluded that an agreement with the observed fields does not automatically guarantee a correct simulation of climate sensitivity. The approach suggested by the present study is to explore a range of variability of some tunable parameters, rather than to tune these parameters to a single optimal set of values. The resulting sensitivity to a 1% decrease in the solar constant varies between 1¿ and 4¿C in there four versions. The equilibrium response to a change between extreme high and low summer insolation orbits is a shift in the region of positive annual snow budgets, varying between 6¿ and 10¿ of latitude according to the sensitivity of the particular model version.

The experiments presented in Part 1 of this study offer an estimate of the range of response of the equilibrium climate solution, leaving the investigation of the feedback effects that the building of ice sheets has on the climate system to Part 2. ¿ American Geophysical Union 1988