A new study published in Science Advances suggests that the Southern Ocean surrounding Antarctica may shift from being a major carbon sink to a significant carbon source under certain climate mitigation scenarios. This finding is important because global efforts to limit temperature rise to 1.5°C above preindustrial levels rely on reducing emissions and actively removing carbon from the atmosphere. However, the study shows that these actions might have unexpected consequences on the Southern Ocean’s role in the global carbon cycle. The Southern Ocean is currently responsible for absorbing 40% to 50% of the carbon dioxide taken up by the world's oceans. This process occurs because colder water can dissolve more CO2, which is then mixed into deeper waters. Additionally, phytoplankton in the surface waters take in CO2 during photosynthesis, converting it into organic matter. However, scientists are unsure if the Southern Ocean will continue to act as a reliable carbon sink once global emissions decline. Climate simulations used in the study suggest that the Southern Ocean could shift from absorbing CO2 to releasing it under scenarios involving carbon removal. The researchers examined two idealized, long-term emissions scenarios: one reaching net-zero emissions and another involving sustained negative emissions through technologies like direct air capture. In both cases, the simulations showed that the Southern Ocean would transition from a modest CO2 absorber into a significant CO2 source, even as atmospheric CO2 levels begin to decline. The simulations indicate that once CO2 emissions are reduced, the Southern Ocean’s ability to absorb CO2 declines rapidly and then gradually turns into a net CO2 source. The study found that persistent warming of the Southern Ocean’s surface waters and a decline in seawater alkalinity are key factors behind this shift. Warmer water holds less CO2, and lower alkalinity reduces the ocean’s ability to absorb CO2. These changes affect the partial pressure of CO2 in the water, reversing the natural flow of CO2 from the atmosphere to the ocean and causing the ocean to release CO2 back into the air. The study highlights the need for long-term monitoring of the Southern Ocean’s carbonate system, including changes in alkalinity, to better understand and model these processes. It also underscores that some carbon sinks may not behave as expected and could potentially undermine carbon reduction efforts. The research emphasizes that the climate system can respond with delays, making it difficult to predict and manage the outcomes of mitigation strategies.