New computer simulations suggest that the Southern Ocean, which currently absorbs a significant amount of human-caused carbon dioxide (CO₂), might shift from acting as a carbon sink to becoming a source of CO₂, even if global emissions are reduced. This potential change could result in the ocean releasing more CO₂ into the atmosphere over long periods. The Southern Ocean is estimated to absorb between 40 and 50 percent of global CO₂ emissions, mainly because cold ocean water can dissolve more CO₂ and mix it with deep water moving southward through ocean currents. Surface phytoplankton also help absorb CO₂ by using it in photosynthesis. However, researchers are unsure if the Southern Ocean will remain a reliable carbon sink if emissions are reduced. Some models suggest the ocean will continue to absorb CO₂ even as emissions rise, while others predict it could turn into a CO₂ source. Scenarios that aim to limit global warming to 1.5°C depend on achieving carbon neutrality and even net negative emissions, where more CO₂ is removed from the atmosphere than emitted. A recent study published in Science Advances explored possible processes that could weaken the Southern Ocean’s ability to absorb CO₂ and the conditions under which this shift might happen. The study found that the Southern Ocean’s physical system reacts in a nonlinear way to CO₂ absorption, meaning its response becomes more complex over time. This could lead to delayed and varied effects on how the ocean handles carbon, making future projections uncertain. Researchers simulated two long-term emission scenarios: one aiming for carbon neutrality (ZEC) and another including sustainable negative emissions (NEG) through direct CO₂ capture from the atmosphere. Comparing ten simulations for each scenario showed that the Southern Ocean transitions from a modest CO₂ sink to a significant CO₂ source in both cases. This change continues even as atmospheric CO₂ levels decline, indicating a slow, delayed response rather than a sudden shift. The simulations suggest that the Southern Ocean will keep absorbing CO₂ until emissions peak around 2050, after which its absorption will weaken as emissions decrease. In the NEG scenario, this weakening would continue until 2154, when the ocean would become a net carbon source. Once NEG efforts stop around 2196, and atmospheric CO₂ levels return to their original state, the Southern Ocean would enter a phase of zero carbon compensation (ZEC). This shift is linked to warming surface waters, which can dissolve less CO₂ than cold water. A decrease in seawater alkalinity could also reduce the ocean’s ability to absorb CO₂, affecting the balance of CO₂ between the ocean and atmosphere. When the ocean’s surface has more CO₂ than the atmosphere, it starts releasing CO₂ into the air. The study highlights that carbon sinks like the Southern Ocean are not static and can change over time. Researchers stress the importance of continuing efforts to reduce carbon emissions to maintain this balance. They also emphasize the need for long-term monitoring of ocean systems, including alkalinity, to reduce uncertainties in models and ensure the effectiveness of climate change mitigation strategies.