Freshwater from melting ice and increased precipitation is temporarily trapping carbon dioxide in the deep Southern Ocean, countering predictions of a weakening carbon sink. Scientists from the Alfred Wegener Institute explain this stabilizing effect in a new study. However, intensifying winds may soon reverse this protection, potentially releasing stored CO2 into the atmosphere.
The Southern Ocean plays a vital role in mitigating global warming by absorbing about 40 percent of the CO2 captured by the world's oceans, which together take in roughly one quarter of human-produced emissions. This carbon sink relies on a circulation system where deep water rises to the surface, exchanges gases with the atmosphere, and sinks again, carrying absorbed CO2 into the depths.
Climate models have predicted that global warming would reduce this capacity through stronger westerly winds bringing more ancient, CO2-rich deep water to the surface. Yet, decades of measurements from 1972 to 2021 show the Southern Ocean has remained a strong carbon sink. Researchers from the Alfred Wegener Institute (AWI), led by Dr. Léa Olivier, analyzed biogeochemical data from marine expeditions, focusing on circulation, mixing, and water mass properties while excluding biological processes.
Their findings, published in Nature Climate Change in 2025, reveal that since the 1990s, surface waters have freshened due to increased precipitation, melting glaciers, and sea ice. This freshening has reinforced density stratification, creating a stronger barrier that keeps CO2-rich deep water—normally below 200 meters and characterized as salty, nutrient-rich, and relatively warm—trapped below.
"Deep water in the Southern Ocean is normally found below 200 meters," says Dr. Olivier. "It is salty, nutrient-rich and relatively warm compared to water nearer the surface."
However, this effect appears temporary. Strengthening winds, linked to human-driven climate change, have raised the upper boundary of the deep water layer by about 40 meters since the 1990s, pushing carbon-rich water closer to the surface and making the barrier more vulnerable to mixing. "Our study shows that this fresher surface water has temporarily offset the weakening of the carbon sink in the Southern Ocean, as model simulations predicted," Olivier summarizes. "However, this situation could reverse if the stratification were to weaken."
Prof. Alexander Haumann, a co-author, calls for more winter data to confirm if deep CO2 release has begun. The AWI plans further investigation through the international Antarctica InSync program to understand these changes and their global climate implications.
"What surprised me most was that we actually found the answer to our question beneath the surface," Olivier notes. "We need to look beyond just the ocean's surface, otherwise we run the risk of missing a key part of the story."