Southern Ocean Could Turn From Carbon Sink Into Carbon Source, Study Warns

The vast, cold waters that circle Antarctica have long acted as one of the planet's most important defences against climate change, quietly soaking up a large share of the carbon dioxide humans pump into the air. A new study suggests that role may not last forever, and that the Southern Ocean could one day start giving that carbon back.

A worrying reversal

Research published in the journal Science Advances on 12 September 2026 used climate simulations to project how the Southern Ocean would behave as emissions eventually fall. The results point to a striking reversal, with the ocean shifting from absorbing carbon dioxide to releasing it back into the atmosphere.

The finding challenges a comforting assumption about how the planet will respond to falling emissions. It implies that even as humanity works to cut pollution, one of Earth's great natural buffers could gradually turn from a help into a hindrance over the very long term.

What sink and source mean

In climate science the distinction is simple but crucial. A carbon sink is something that absorbs more carbon dioxide than it releases, while a carbon source does the opposite, adding to the greenhouse gases already in the atmosphere. For decades the Southern Ocean has been firmly in the first category.

According to the study, the region absorbed roughly 2.8 grams of carbon per square metre each year around the start of the century. The simulations suggest that balance could eventually invert, with the same waters releasing on the order of 8.5 to 8.6 grams of carbon per square metre per year in the modelled future.

Why the switch could happen

Two main mechanisms drive the projected change. The first is surface warming, because warmer water is less able to dissolve and hold on to carbon dioxide. As the upper layers of the ocean heat up, their capacity to store carbon steadily weakens.

The second is a decline in the water's alkalinity, which reduces the ocean's chemical ability to lock away carbon. Together these shifts change the balance of pressure between ocean and air, so that carbon dioxide begins to flow out of the sea rather than into it.

Even under net zero

Perhaps the most surprising part of the study is when this reversal appears. The simulations show it emerging not during an era of runaway pollution, but after emissions decline or even turn negative, in scenarios built around aggressive climate mitigation and carbon removal.

In one sustained negative emissions scenario, the model placed the tipping point at around the year 2154. That is far in the future, but the mechanism matters now, because it suggests the ocean's response to our actions may lag behind by many decades.

What it means for policy

The results carry a sobering message for climate strategy. If the ocean eventually releases stored carbon, some carbon removal efforts could be partially offset by these delayed natural responses, making the task of stabilising the climate even harder than current plans assume.

The researchers stress that this is not an argument against cutting emissions. If anything, it reinforces the case, since reducing carbon dioxide at the source remains the most reliable way to limit warming and to ease the pressures that could one day flip the Southern Ocean.

Caveats and uncertainties

As with any long range projection, the study comes with important caveats. It relies on a single climate model, does not include the effects of the Antarctic ice sheet, and acknowledges that projections of ocean carbon chemistry differ from one model to another. Real world emissions pathways also remain deeply uncertain.

Even so, the work adds to a growing body of research warning that the Southern Ocean is more sensitive than once thought. Understanding how and when its behaviour might change could prove vital as the world tries to chart a safe course through the coming century and beyond.