This paper evaluates the hypothesis that historical CO2 levels could have been governed by releases of geologic methane. Methane clathrates and natural gas could have contributed to CO2 increases at the ends of the last two glacial periods. Computations indicate that oxidation of CH4 to CO2 could have rapidly provided enough CO2 to cause historical CO2 changes. A simple dynamic model, the hydrocarbon pump, for methane as the source term for changes in CO2 over the glacial cycles matches several features of the historical record. If methane clathrates were the source of CO2, then CO2 at the interglacial transitions should be depleted in 14C because of the low 14C levels in C from this source. Evidence is presented that this is the case. 13C data are also evaluated. Overall, CH4 is a credible source for CO2, but definitive conclusions await further data. ¿ American Geophysical Union 1993