Southern Ocean May Flip From Carbon Sink to Carbon Source, Study Suggests

Efforts to limit global warming to 1.5 degrees Celsius above preindustrial levels rely on reaching net-zero emissions and removing carbon dioxide from the air. A new study in Science Advances finds that climate simulations suggest an unexpected side effect: the Southern Ocean around Antarctica may flip from a carbon sink into a carbon source.

The world's oceans are estimated to absorb roughly 30% of human-caused carbon dioxide emissions, with the Southern Ocean responsible for 40% to 50% of that. Colder water dissolves carbon dioxide and carries it into deeper waters, while phytoplankton take up the gas for photosynthesis. Researchers were unsure whether this uptake would continue once emissions decline.

Running two idealized, long-term scenarios -- net-zero emissions (ZEC) and sustained negative emissions via direct air capture (NEG) -- with 10 simulations each checked against observations, the team found the Southern Ocean shifted from a modest absorber into a substantial source in both cases, persisting even as atmospheric carbon dioxide fell. The region absorbed about 2.8 grams of carbon per square meter yearly as of 2001, but eventually emits around 8.6 grams (ZEC) and 8.5 grams (NEG) per square meter yearly, with emissions peaking near 2050.

The flip traces to two triggers: persistent warming of Southern Ocean surface waters, which hold less dissolved carbon dioxide, and a broad decline in seawater alkalinity, weakening the water's chemical capacity to absorb the gas. Both raise the ocean's internal carbon dioxide pressure until it exceeds the atmosphere's, reversing the flow so the ocean releases carbon dioxide instead of absorbing it. In the NEG scenario, the ocean turns from a net sink to a net source by 2154, before easing back after negative-emissions efforts stop in 2196.

The study relies on a single climate model and excludes the Antarctic ice sheet. Its authors say sustained, multidecade monitoring of the Southern Ocean's carbonate chemistry, including alkalinity, is needed to confirm how effective climate mitigation strategies remain.