The Southern Ocean is Capturing More CO₂ Than Anticipated, and That’s Bad News

Among the major climate regulators, certain actors remain invisible from the surface. Far from view, immense volumes of cold water sculpt the thermal balance of the planet while absorbing part of the carbon emitted. This buffering role, long underestimated, is today at the heart of new concerns. Because the Southern Ocean, which we believed to be docile and protective, could well hide an instability with global consequences.

Little-known mechanics of the Southern Ocean revealed by data

Despite alarmist projections, the Southern Ocean continues to absorb nearly 40% of the CO₂ released by human activities into the oceans. This figure, established by the Alfred Wegener Institute, seems to contradict climate models which predicted a decline in this role. Data collected between 1972 and 2021 during several oceanographic campaigns reveal a more nuanced reality. If the carbon sink still works, it is because an invisible mechanism prevents carbon dioxide buried in deep waters from rising to the surface.

Below 200 meters, the deep southern waters are rich in CO₂. These bodies of water, which have remained isolated for centuries, contain the dissolved residue of ancient carbon cycles. Logic would dictate that the strengthening of westerly winds, accelerated by global warming, encourages their rise to the surface. However, this expected degassing did not occur. A barrier is temporarily interposed between CO₂ and the atmosphere.

Stratification, deep sea upwelling and carbon trapping

This lock is explained by a phenomenon called “stratification”. In summary, the surface layer, less salty and colder, forms a waterproof cover which prevents the deeper layers, rich in carbon, from mixing with the upper waters. Since the 1990s, fresh water inputs linked to melting glaciers and increased precipitation have accentuated this vertical separation. The analysis published in Nature Climate Change indicates that this difference in density helped keep the deep waters under control, thus slowing the release of the CO₂ they contain.

But this balance is becoming more and more unstable. On average, the upper limit of deep water has moved 40 meters closer to the surface over the past three decades. These waters, warmer and saltier, gradually eat away at the protective layer. Their slow but regular rise increases the risk of mixing, and therefore of degassing. According to researchers, the current situation is only a reprieve.

Satellites confirm this development. Since 2015, they have recorded a widespread increase in surface salinity, a sudden reversal after decades of desalination. This crucial data coincides with a record decline in the extent of Antarctic sea ice. The study relayed by PNAS highlights that the loss of stratification makes the surface layers more vulnerable to vertical mixing, particularly during southern winters.

Why do researchers fear a reversal of the phenomenon?

The gradual return of carbon-rich waters near the surface could trigger a tipping point. If the surface lid gives way, a large amount of CO₂ could escape, reversing the role of the Southern Ocean. From a carbon sink, it would become a source. The effect would be all the more brutal as these waters contain concentrations of carbon dioxide much higher than those in the current atmosphere.

According to observations compiled in the Eurekalert article, scientists fear that climate models underestimate the resistance of this stratification. However, poor anticipation of this parameter would distort predictions about the speed of warming. It is therefore no longer enough to monitor the surface. The key to climate change now lies in the detailed understanding of invisible exchanges between ocean layers.

International missions like Antarctica InSync aim to better track these changes, particularly during winter. Because during this dark season, water movements become more intense and deeper. Scientists are trying to find out if the shift has already begun. So, even when everything seems stable on the surface, the Southern Ocean hides complex dynamics. This alone can slow down or amplify global warming.