The image caption does say “could double”, but the PNAS article doesn’t mention that. As far as I understand, the role of the Southern Ocean as a whole as a carbon sink is big (two-digit percentage of human-caused emissions). But the effect subdivides into biological (phytoplankton) and physical (currents downwelling CO2-rich water and upwelling CO2-poor water). And I’m not aware or capable of pointing out a balance sheet of how much each component does.