Southern Ocean freshening stalls deep ocean CO2 release in a changing climate

The Southern Ocean mitigates global surface warming by taking up a large portion of the carbon released by human activities. While models suggest this carbon sink should weaken as climate change increases upwelling of carbon-rich deep water, such a decline has not been observed over the past decades. Here, using circumpolar hydrographic observations, we reveal that Southern Ocean freshening since the 1990s has enhanced density stratification, which prevents these CO ₂ -enriched waters from reaching the surface. Meanwhile, the surface layer has become thinner, allowing the CO ₂ -rich circumpolar deep water to get closer to the surface, replacing winter water between 100 m and 200 m. In this layer, the CO ₂ fugacity increased by ~10 µatm because of changing ocean dynamics since the 1990s. Our findings imply that the observed surface freshening temporarily buffered the model-predicted weakening of the Southern Ocean carbon sink, but that such a signal could potentially emerge if stratification weakened.