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Abstract

The effect of the Southern Ocean on global climate change is assessed using Earth system model CIMP6 projections. For an idealised 1% annual rise in atmospheric CO2 , the Southern Ocean plays a broadly comparable role in the global uptake of heat and anthropogenic carbon, accounting for 40+/-5% of heat uptake (based on 12 CMIP6 models) and 42+/-2% of anthropogenic carbon uptake over the global ocean (based on 11 CMIP6 models). In comparison in historical scenarios, the Southern Ocean plays a similar role in the global uptake of anthropogenic carbon, but plays a much more important role in the global uptake of heat. The reason for these differing responses is that in historical scenarios there is a marked reduction in radiative forcing over the northern hemisphere due to aerosol forcing. For the idealised scenario, there are still significant inter-model differences in global and Southern Ocean heat uptake, which are not primarily controlled by ocean overturning, but instead by differences in physical feedbacks, especially cloud feedbacks over the globe and surface albedo feedbacks from sea-ice loss in high latitudes. The ocean carbon response is broadly similar in most models with carbon storage increasing from rising atmospheric CO2, but weakly decreasing from climate change. In summary, while there have been contrasts in the relative importance of the Southern Ocean for the global uptake of heat and anthropogenic carbon, these contrast in heat and carbon uptake are likely to decline as greenhouse gas forcing dominates in the future.