Geologic studies indicate that the Carboniferous glaciation on Gondwanaland was approximately as extensive as the ice sheets during the Pleistocene. However, there is one major difference between the climate boundary conditions for the two ice sheets: the Gondwanan ice sheet was located on a supercontinent. Three different levels of sensitivity experiments were conducted to examine the effect of the large landmass on the magnitude of summer warming over the ice sheets: (1) simulations with present solar luminosity and present orbital forcing resulted in summer temperatures over the Gondwanan Ice Sheet 12¿--17 ¿C greater than over the Pleistocene Laurentide Ice Sheet; (2) lowering the solar constant 3% or modifying the seasonal insolation cycle to a ''cold summer orbit'' reduced the warming but still led to significant differences between Gondwanan and Pleistocene simulations; and (3) the combined effect of lowering the solar constant and modifying the seasonal insolation cycle to a cold summer orbit resulted in temperature patterns over the Gondwanan Ice Sheet similar to the Pleistocene. Model simulation also predict tropical sea surface temperatures about 4 ¿C less than at present as a result of reduced solar luminosity. These results suggest that conditions necessary to explain Gondwanan ice sheet stability may be known, but required boundary conditions would be more extreme than in the Pleistocene. Although a number of uncertainties remain in these calculations, they help to better define critical conditions for glaciation for one of the most prolonged periods of continuous glaciation in Earth history. ¿1991 American Geophysical Union