Southern Ocean Could Shift From Carbon Sink to Carbon Source, Study Finds

A new climate study suggests the Southern Ocean around Antarctica could eventually shift from absorbing carbon dioxide to releasing it under long-term emissions reduction and carbon removal scenarios.

The Southern Ocean, which currently plays a major role in absorbing human-caused carbon dioxide (CO2), could eventually become a net source of the greenhouse gas if atmospheric CO2 levels decline, according to a new study published in Science Advances.

Researchers reached the conclusion after running long-term climate simulations under two different emissions pathways. The results suggest that the ocean's carbon uptake could weaken after global emissions begin to fall and, over time, reverse direction.

The finding does not mean that the Southern Ocean will definitely become a carbon source in the future. The study relies on climate modeling and includes several limitations, including the use of a single model.

Why the Southern Ocean Matters for Carbon Absorption

The world's oceans absorb an estimated 30% of human-generated CO2 emissions. The Southern Ocean is responsible for roughly 40% to 50% of that oceanic uptake, making it one of the planet's most important natural carbon sinks.

Cold seawater can dissolve CO2 and transport it into deeper parts of the ocean. Phytoplankton also contribute by absorbing CO2 through photosynthesis and converting carbon into organic matter.

Scientists, however, have questioned whether the Southern Ocean will continue to absorb carbon at the same rate if global emissions begin to decline.

Previous research has indicated that the ocean's response to atmospheric CO2 removal may not be straightforward. Changes in the physical and chemical conditions of the Southern Ocean could produce delayed effects on its ability to absorb carbon.

Climate Simulations Show a Possible Reversal

To investigate this possibility, researchers examined two idealized long-term scenarios.

The first scenario reaches net-zero emissions, while the second assumes continued negative emissions through direct removal of CO2 from the atmosphere. The researchers conducted 10 simulations for each scenario and compared the modeled ocean carbon uptake with observation-based estimates.

In both scenarios, the simulations showed the Southern Ocean moving from a modest carbon sink toward becoming a substantial carbon source.

The change was not immediate. Instead, carbon uptake weakened after emissions began declining, followed by a gradual reversal. This suggests that the ocean could respond to changes in atmospheric CO2 with a significant delay.

In the simulations, eventual CO2 emissions from the Southern Ocean reached approximately 8.6 grams of carbon per square meter per year in the net-zero scenario and 8.5 grams in the negative-emissions scenario.

For comparison, the Southern Ocean was estimated to absorb about 2.8 grams of carbon per square meter per year in 2001.

Under the negative-emissions experiment, the model indicated that the ocean could transition from a net carbon sink to a net carbon source around 2154.

What Could Cause the Shift?

The researchers identified persistent warming of the Southern Ocean's surface waters as a major factor behind the simulated reversal.

Warmer seawater generally holds less dissolved CO2. At the same time, changes in seawater alkalinity could reduce the ocean's chemical capacity to absorb carbon dioxide.

As these processes alter the amount of CO2 dissolved in seawater, the difference in CO2 concentration between the ocean and atmosphere can change.

Eventually, the researchers say, the partial pressure of CO2 in the ocean could become higher than that of the surrounding atmosphere. Under those conditions, the direction of CO2 exchange can reverse, causing the ocean to release carbon dioxide rather than absorb it.

Researchers Highlight Important Uncertainties

The study's findings come with several important limitations.

Different climate models produce different projections of the Southern Ocean's carbon chemistry, while the new study relies on only one model. The model also does not include the Antarctic ice sheet, which could influence freshwater input and other regional ocean processes.

Future global emissions and the scale of atmospheric carbon removal are also uncertain. As a result, the simulated transition should not be interpreted as a guaranteed future outcome.

The researchers say sustained, multidecadal monitoring of the Southern Ocean's carbonate system will be necessary to determine whether the modeled changes occur in the real ocean.

The study nevertheless highlights a broader challenge for climate mitigation: natural carbon sinks may not always respond immediately or predictably to falling atmospheric CO2 levels.

The findings do not reduce the importance of cutting greenhouse gas emissions. Instead, they point to the need to better understand delayed responses within the climate system and to account for potential changes in natural carbon sinks when assessing long-term climate strategies.