Changes in the ocean circulation on millennial timescales can impact the atmospheric CO2 concentration by two distinct mechanisms: either by modifying the non-buffered ocean carbon storage (through changes in the physical and biological oceanic pumps) or by directly varying the surface mean oceanic partial pressure of pCO2 (through changes in mean surface alkalinity, temperature or salinity). This second mechanism has not been explicitly examined in previous studies. The equal importance of the two mechanisms will be illustrated in this talk by introducing a new diagnostic buffered carbon budget on the results of simulations performed with an Earth System Climate Model. The cases of ocean circulation changes due to a modification of the Southern Hemisphere Westerlies or to a shutdown of the North Atlantic Deep Water production will be explicitly presented. Changes in the ocean circulation on millennial timescales can impact the atmospheric CO2 concentration by two distinct mechanisms: either by modifying the non-buffered ocean carbon storage (through  changes in the physical and biological oceanic pumps) or by directly varying the surface mean oceanic partial pressure of pCO2 (through  changes in mean surface alkalinity, temperature or salinity). This second mechanism has not  been explicitly examined in previous studies. The equal importance of the two mechanisms will be illustrated in this  talk by introducing a new diagnostic buffered carbon budget on the results of simulations performed with an Earth System Climate Model. The cases of ocean circulation changes due to a modification of the Southern Hemisphere Westerlies or to a shutdown of the North Atlantic Deep Water production will be explicitly presented.