Global climate and the atmospheric partial pressure of carbon dioxide (pco2atm) are correlated over recent glacial cycles, with lower pco2atm during ice ages, but the causes of the pco2atm changes are unknown. The modern Southern Ocean releases deeply sequestered CO2 to the atmosphere. Growing evidence suggests that the Southern Ocean CO2 leak was stemmed during ice ages, increasing ocean CO2 storage. Such a change would also have made the global ocean more alkaline, driving additional ocean CO2 uptake. This explanation for lower ice-age pco2atm, if correct, has much to teach us about the controls on current ocean processes. © 2010 Macmillan Publishers Limited. All rights reserved.