Balance at the tipping point Researchers discover an eerie climate puzzle in the Antarctic

While most of the world's oceans are absorbing less and less CO₂, the Southern Ocean around Antarctica remains a stable climate buffer. Researchers at the Alfred Wegener Institute have now discovered why - and why this protection will not last forever.

The Southern Ocean around Antarctica plays a central role in the global climate system: no other ocean stores as much carbon dioxide as this one. Its waters absorb around 40 percent of the CO₂ emitted by humans - a huge brake on global warming.

But this should no longer be the case. According to current climate models, the ocean should have lost its storage capacity because warmer temperatures and stronger westerly winds should be driving CO₂-rich deep water to the surface. The fact that this has not yet happened was considered one of the great mysteries of climate research.

A new study by the Alfred Wegener Institute (AWI) now provides an explanation: a stabilizing layer of fresh and meltwater is currently preventing the carbon dioxide from rising from the depths.

A kind of protective cover on the sea

Since the 1990s, rain and melting ice have made the upper ocean layer around Antarctica colder and less salty. This lighter layer of water forms a kind of "lid" over the heavier, CO₂-rich deep water.

"The deep water in the Southern Ocean is normally below 200 meters," explains study leader Dr. Léa Olivier from the AWI. "It is salty, nutrient-rich and significantly warmer than the water at the surface."

Using data from more than 1100 ship expeditions between 1972 and 2021, the researchers found that the upper limit of CO₂-rich water is now around 40 meters higher than it was 30 years ago. The CO₂ pressure in these layers rose by an average of ten microatmospheres - in some regions by as much as 17.

This means that the CO₂ is moving closer to the surface, but remains trapped under the freshwater layer.

An equilibrium with an expiry date

This stability only lasts as long as the density differences between the layers persist. If they are broken up by stronger winds or rising temperatures, the stored CO₂ could suddenly escape into the atmosphere.

"We can't just focus on the ocean surface, because otherwise we could overlook an important part of the story," says Olivier.

AWI researcher Prof. Alexander Haumann adds that new winter measurements should show whether CO₂ is already escaping from the depths. If the stratification continues to weaken, the ocean would lose its function as a CO₂ sink - with serious consequences: more greenhouse gases in the air, faster warming, rising sea levels and more frequent extreme weather events.

The AWI now wants to closely monitor developments as part of the Antarctica InSync project.