Plans to limit global warming to 1.5°C rely on achieving net-zero emissions and removing CO2 from the atmosphere. However, new research indicates that this could have an unexpected effect. At lower CO2 levels, the Southern Ocean surrounding Antarctica might shift from a carbon sink to a carbon source, thereby impacting the climate.
The global ocean absorbs approximately 30% of human-generated CO2 emissions, with the Southern Ocean accounting for a significant share.
Cold waters and phytoplankton help sequester carbon within the ocean. Still, scientists remain uncertain whether the Southern Ocean will continue to absorb CO2 as atmospheric levels decline or if it might begin releasing it back into the atmosphere.
Simulations of the future of the carbon cycle
Researchers from South Korea, Norway and Germany used climate simulations to examine how the Southern Ocean would respond to a reduction in CO2 emissions. Two scenarios were analyzed: achieving net-zero emissions and achieving negative emissions, where CO2 is actively removed from the atmosphere.
In both cases, the models show a similar trend: following an initial decline in CO2 uptake, the Southern Ocean could gradually shift from being a carbon sink to a carbon source. This means that even as atmospheric CO2 levels drop, the ocean could begin releasing some of its stored carbon back into the atmosphere. The process is gradual, demonstrating that the ocean’s response to climate change involves a significant time lag.
According to simulations, future emissions could reach approximately 8.5 – 8.6 g of carbon per square meter annually, whereas in 2001 the Southern Ocean absorbed an average of 2.8 g/m² per year. This demonstrates how significantly its role in the global carbon cycle could change.
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The primary reason for this change is the warming of surface waters, which reduces their capacity to retain CO2. A decline in ocean alkalinity also plays a significant role. Consequently, CO2 levels in the water can exceed those in the atmosphere, causing the ocean to release the gas back out.
Scientists emphasize that uncertainties remain, as the model does not account for certain factors, such as Antarctic ice. However, the study indicates that oceanic carbon sinks may respond with a time lag and might not always retain their ability to absorb CO2.
The study authors write, “Given the impact of surface ALK on CO2 fluxes identified in our results, it is essential to validate these model-driven findings through observational evidence. We therefore emphasize the necessity for sustained, multidecadal monitoring of the SO carbonate system, including ALK, to reduce current model uncertainties and confirm the enduring effectiveness of climate mitigation strategies.”
