Southern Ocean may flip from carbon sink to carbon source under mitigation scenarios

Plans to limit the global temperature rise to 1.5°C above preindustrial levels depend on reaching net-zero emissions and, in many cases, actively removing carbon dioxide (CO₂) from the air. While this helps limit extreme weather, sea level rise, ecosystem loss and the risk of triggering irreversible climate tipping points, a new study, published in Science Advances, finds that reducing CO₂ levels may have an unexpected effect. Climate simulations used in the study suggest that the Southern Ocean around Antarctica may switch from being an important carbon sink to being a prolific source of carbon.

The Southern Ocean as a carbon sink

The world’s oceans are estimated to have absorbed roughly 30% of human-caused carbon dioxide emissions, with the Southern Ocean accounting for about 40% to 50% of that absorption. This is accomplished as colder water dissolves CO₂, which then gets mixed and transported into deeper waters. Phytoplankton that live in sunlit surface waters help the process by taking up CO₂ for photosynthesis and converting the carbon into organic matter.

However, scientists aren’t sure whether the Southern Ocean will remain a reliable carbon sink once emissions decline. While climate models generally project continued Southern Ocean carbon uptake as emissions keep rising, some studies have suggested it could switch from absorbing CO₂ to releasing it under carbon-removal pathways.

The authors of the new study write, “Previous studies have shown that the SO [Southern Ocean] physical system responds nonlinearly to CO₂ removal pathways, with this behavior becoming increasingly pronounced over long-term timescales. Such a phenomenon could eventually generate delayed and multifaceted impacts on oceanic carbon dynamics, introducing substantial uncertainty into projections of future CO₂ flux and the overall impact of climate mitigation.”

Southern Ocean releases CO₂ in simulations

To evaluate the Southern Ocean’s response to CO₂ reduction scenarios, the team ran two idealized, long-term emissions scenarios: one reaching net-zero emissions (ZEC) and one including sustained negative emissions (NEG) through direct air capture. They compared 10 simulations for each scenario and checked the model’s recent ocean CO₂ uptake against observation-based estimates.

The simulations showed that the Southern Ocean changes from a modest CO₂ absorber into a substantial CO₂ source in both emissions scenarios. The shift persists even as atmospheric CO₂ declines, indicating a delayed response rather than a quick recovery. The researchers say that once CO₂ emissions are reduced, CO₂ uptake in the Southern Ocean declines rapidly, then gradually turns into a net CO₂ source.

The simulations show eventual CO₂ emissions of around 8.6 grams of carbon per square meter per year in the ZEC simulation and 8.5 grams of carbon per square meter per year in the NEG simulation. For reference, the Southern Ocean absorbed around 2.8 grams of carbon per square meter per year as of 2001.

The team writes, “The temporal evolution of the globally averaged air-sea CO₂ flux is characterized by enhanced oceanic CO₂ uptake until the emission peak is reached (approximately year 2050), followed by weakened uptake as the emissions decline. In the NEG experiment, this weakening continues, and in the year 2154, the ocean transitions from a net sink to a net source of CO₂. Following the cessation of NEGs in the year 2196, when atmospheric CO₂ returns to initial levels, the simulation enters a ZEC phase.”

Mechanisms behind the switch

The team wanted to understand what causes this flip from sink to source. They found that persistent warming of Southern Ocean surface waters seems to be the main trigger, while the ocean’s changing alkalinity serves as a second trigger. Warmer water holds less CO₂, and the broad decline in seawater alkalinity also weakens the ocean’s chemical ability to absorb CO₂. Both changes also affect the partial pressure of CO₂ in the water.

As the oceanic partial pressure of CO₂ increases, the air-sea pressure gradient reverses (partial pressure of CO₂ in the ocean becomes greater than that in the atmosphere). At this point, the switch is flipped, and the ocean can no longer take in CO₂. Instead, it starts releasing it back into the atmosphere.

There are still many unknowns, and these simulations often lack details that might affect the outcome. In this case, projections of Southern Ocean carbon chemistry differ across models, and the study uses only one. The model also leaves out the Antarctic ice sheet, which could affect regional freshwater and ocean processes. Furthermore, we don’t yet know exactly how future emissions will play out in real life.

Still, the study raises awareness that certain carbon sinks may not always function as expected and may even work against carbon reduction measures. It also reinforces that cutting emissions remains essential and that the climate system can respond with long delays that complicate recovery.

The study authors write, “Given the impact of surface ALK on CO₂ fluxes identified in our results, it is essential to validate these model-driven findings through observational evidence. We therefore emphasize the necessity for sustained, multidecadal monitoring of the SO carbonate system, including ALK, to reduce current model uncertainties and confirm the enduring effectiveness of climate mitigation strategies.”

Who’s behind this story?

Krystal Kasal

Freelance science writer with Master’s in physics. Five years clinical research and physics education experience. Science communicator.

Full profile →

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

Full profile →

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

Full profile →

© 2026 Science X Network