We have analyzed a 340,000-year record of benthic and planktonic oxygen and carbon isotope measurements from an equatorial Pacific deep-sea core. The data provide estimates of both global ice volume and atmospheric carbon dioxide concentration over this period. The frequencies characteristic of changes in the earth-sun orbital geometry dominate all the records. Examination of phase relationships shows that atmospheric carbon dioxide concentration leads ice volume over the orbital bandwidth, and is forced by orbital changes through a mechanism, at present not fully understood, with a short response time. Changes in atmospheric CO₂ are not primarily caused by glacial-interglacial sea level changes, which had been hypothesized to affect atmospheric CO₂ through the effect on ocean chemistry of changing sedimentation on the continental shelves. Instead, variations in atmospheric CO₂ should be regarded as part of the forcing of ice volume changes.