Atmospheric CO2 increased by 90 ppm during the last deglaciation, but the sequence of events leading to this increase remains poorly constrained and a combination of mechanisms has been invoked to explain its full amplitude. In this presentation, I will first combine numerical simulations and paleoproxy records to show that the formation of deep and bottom water at the Last Glacial Maximum was weaker than during pre-industrial times. This globally weaker ocean circulation led to a greater carbon content in the ocean, thus contributing to the low glacial atmospheric CO2 . Then, the role of changes in oceanic circulation in driving the atmospheric CO2 increase during the early part of the deglaciation, corresponding to Heinrich stadial 1 (HS1), will be discussed. I will show that an increase in Southern Hemisphere westerlies played a crucial role in leading to Southern Ocean CO2 outgassing thus inducing an abrupt atmospheric CO2 rise during HS1.