This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 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." 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.
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