We evaluate the equilibrium response of the atmospheric climate to a wide range of systematic variations in carbon dioxide (100 to 1000 ppm) and solar luminosity (¿5% of today's value) using the NCAR CCM1. The quantitative responses of each set of simulations are compared using a formal sensitivity analysis. Although the forcings on the climate are different, both temporally and spatially, similarities appear in the climatic response. The results of this analysis have broader implications for the use of simulations of past climates as a guide for possible future climate change. ¿ American Geophysical Union 1994