The responses of the HadSM3 and NCAR CCM3 general circulation models to a change in solar insolation are compared to their responses to a doubling of CO2 concentration. In both models, it is found that the important difference is a rapid adjustment of the troposphere in the solar case that reduces the value of effective radiative forcing by about 25%. Clear-sky warming appears to make the major contribution. Subsequent warming of the coupled troposphere, land-surface, ocean mixed-layer system occurs with a very similar sensitivity to that expected under a CO2 forcing of the reduced value. Because of adjustment, the overall precipitation response to solar forcing is similar to, or less than the response to CO2 forcing, despite being smaller per unit temperature change.