Even After Carbon Is Reduced, Oceans Remember the Heat... Southern Ocean Could Become a CO₂ Source [Reading Science]

Seoul National University Research Team Identifies the Mechanism of Carbon “Outgassing” in the Southern Ocean
Ocean Warming and Strong Westerlies Amplify CO₂ Release
"Urgent Action Needed for Carbon Reduction"

Does reducing atmospheric carbon dioxide (CO₂) allow the oceans to absorb carbon as they once did? According to a recent study, this may not be the case. Even with reduced carbon emissions and the removal of CO₂ from the atmosphere, the Southern Ocean, which has already absorbed heat, could remain warm for an extended period. Combined with strong winds, this could actually cause the stored CO₂ to be released back into the atmosphere.

Seoul National University announced on September 11 that Professor Kuk Jongseong's research team from the School of Earth and Environmental Sciences analyzed Earth system models and multiple climate models to identify the mechanisms by which the Southern Ocean could shift from being a "carbon sink" that absorbs CO₂ to a "carbon source" that emits CO₂ into the atmosphere after carbon reduction. The study, with Lee Heeji as the lead author and Professor Kuk as the corresponding author, was published in the international journal Science Advances.

Changes in temperature and alkalinity of the Southern Ocean according to carbon reduction scenarios. (A) The phenomenon where warming in the Southern Ocean relatively persists despite a decrease in atmospheric carbon dioxide concentration, (B-C) Changes in surface alkalinity of the Southern Ocean at the beginning of the experiment and during the recovery phase. Provided by the research team

The Southern Ocean plays a crucial role in buffering global climate change by absorbing CO₂ emitted by human activities. The problem, however, is that even if humans stop emitting carbon or remove existing CO₂ from the atmosphere, the ocean does not instantly revert to its previous state.

Even After CO₂ Decreases, Oceans Remain Warm

The research team used the Earth system model CESM2 to simulate scenarios in which carbon emissions are reduced and atmospheric CO₂ is removed. The results showed that even when atmospheric CO₂ concentration dropped to initial levels or below, the sea surface temperature continued to rise in the Southern Ocean.

At the same time, alkalinity in the surface ocean—which relates to its ability to absorb CO₂—decreased. This was attributed to delayed release of heat stored in the ocean interior, ocean stratification, and the phenomenon known as "alkalinity trapping" by phytoplankton.

The simultaneous rise in temperature and decrease in alkalinity led to a reversal where the partial pressure of CO₂ (pCO₂) in seawater exceeded that of the atmosphere. Typically, CO₂ moves from areas of higher partial pressure to lower, so when the ocean's pCO₂ is higher than the atmosphere's, CO₂ stored in the ocean can be released back into the air.

This effect was even more pronounced in the Southern Ocean. The region's characteristic strong westerly winds and high solubility of CO₂ promote active gas exchange between the ocean and atmosphere. The research team recalculated by lowering the gas exchange coefficient of the Southern Ocean to the global average. They found that the cumulative carbon emission sharply declined, or the Southern Ocean maintained its status as a carbon sink.

This signifies that the Southern Ocean’s strong winds and vigorous gas exchange are key factors amplifying the CO₂ outgassing caused by warming and chemical changes in the ocean.

The team also validated these results using multiple climate models from the Sixth Coupled Model Intercomparison Project (CMIP6). All models showed a common trend where, as time passed after halting carbon emissions, carbon release from the Southern Ocean intensified. However, there were differences among models in terms of the timing and scale of this carbon outgassing.

This study demonstrates the existence of a "legacy effect" in climate change. Even after lowering atmospheric CO₂ concentrations, the heat and chemical changes accumulated in the ocean during previous warming events do not disappear instantly. Attempting to compensate for delayed reductions with large-scale carbon removal in the future may only partially offset the delayed response of the ocean, limiting climate mitigation efforts.

Based on these findings, the researchers emphasized the need to prioritize earlier emission reductions rather than abandoning carbon removal altogether, and to reflect long-term changes in the Southern Ocean's carbon cycle in carbon removal strategies. They also stressed the importance of long-term monitoring of the Southern Ocean’s gas exchange, alkalinity, and other components of the marine carbonate system, and the need to incorporate these observations precisely into climate models.

This content was produced with the assistance of AI translation services.