To characterize the response of Earth's climate system to increases in atmospheric CO2, climate modelers define climate sensitivity as the change in global mean temperature in response to prescribed forcing. Here we turn this approach around and use estimates of ocean temperature change to investigate the mechanisms driving CO2 variations over the last glacial. New records provide evidence of a link between deep ocean temperature and atmospheric CO2 over the last glacial cycle. Two mechanisms simultaneously couple pCO2 and deep ocean temperature: the temperature-dependent solubility of CO2 in seawater and the atmospheric CO2-dependent radiative forcing of temperature. Each of these forcing mechanisms leaves a unique slope of covariation between CO2 and deep ocean temperature, which we estimate using numerical models of climate and the carbon cycle. The pCO2/T slopes derived from paleoclimate data differ between the deglaciation and shorter 5-kyr duration events in marine isotope stage 3 (MIS 3), revealing different mechanisms driving atmospheric CO2 variability. The amplitude of changes over the deglaciation coincides with estimates for CO2 forcing of temperature; however, CO2 changes during MIS 3 can be explained solely by temperature-dependent solubility driving variations in atmospheric pCO2. The deep water temperature changes during MIS 3 may reflect changes in the temperature or relative contribution of Antarctic Bottom Water and play a role in the bipolar seesaw.