A factorial experiment with a new Earth system model of intermediate complexity is used to assess the sensitivity of atmospheric CO2 to organic, carbonate and solubility pumps, ocean circulation state, and climate feedback. An analysis of variance of the results reveals that the organic, carbonate, and solubility pumps act multiplicatively and account for 94% of the variance of atmospheric CO2. The organic pump explains 63% (89 ppm), the solubility pump 24% (55 ppm), the carbonate pump 6% (28 ppm), and ocean circulation 0.3% (12 ppm) of the variance. Removing all pumps increases atmospheric CO2 from 278 to 525 ppm. Including interactions with all the pumps increases the effects of ocean circulation from 12 to 56 ppm. However, the ocean circulation states used are unlikely to span the full range of possible states. Changes in Pacific circulation have more effect on atmospheric CO2 than Atlantic circulation.