The remains of plankton rain down from the surface ocean to the deep ocean, acting to store CO₂ in the deep ocean. We used a model of biology and ocean circulation to explore the importance of this process in different regions of the ocean. The amount of CO₂ stored in the deep ocean is most sensitive to changes in the Southern Ocean. As plankton in the Southern Ocean are likely those most impacted by future climate change, the amount of CO₂ they store in the deep ocean could also be affected.