Sensitivity of a coupled atmosphere-dynamic upper ocean GCM to varying CO2, solar constant, and orbital forcing was examined. Response to atmospheric CO2 concentrations ranging from 100–3500 ppm is logarithmic at all latitudes and seasons, with highest sensitivity at high latitudes, during the winter season. Solar constant response is approximately linear over the range of values +2%, but the sensitivity at high latitudes is less than for equivalent CO2 forcing. Sensitivity to “cold northern summer” orbital forcing, which occurred at the start of the last glacial cycle, is strongly affected by CO2. For CO2 at or below the present level, perennial snow cover in the northern hemisphere expands dramatically with “cold summer” orbital forcing, but this effect becomes very small for CO2 levels in the range 410–460 ppm. This result suggests that the Quaternary “ice age” mode of climatic behaviour may have been initiated by an atmospheric CO2 decrease below a critical value, probably around 350–450 ppm.