A recent study conducted by the Alfred Wegener Institute (AWI) sheds light on why the Southern Ocean continues to absorb carbon dioxide, despite climate model forecasts suggesting otherwise. Historically, projections indicated that global warming would diminish the Southern Ocean”s ability to sequester CO2, yet long-term data indicates this capability has not significantly changed in recent decades.
The research, published in Nature Climate Change, highlights the role of low-salinity surface water in trapping carbon in the depths of the Southern Ocean, thereby preventing its release into the atmosphere. However, climate change is beginning to disrupt this process, impacting the ocean”s efficiency as a carbon sink.
The Southern Ocean is critical in mitigating climate change, as it absorbs approximately 40 percent of the CO2 captured by the world”s oceans, which collectively account for about one-quarter of human-generated emissions. The ocean”s unique circulation patterns facilitate the exchange of gases with the atmosphere, allowing deep waters to rise and absorb CO2 while releasing naturally occurring carbon.
Historically, deep waters in the Southern Ocean, which have remained submerged for centuries, contain substantial amounts of CO2. Climate models predict that increasing westerly winds around Antarctica will drive more of this CO2-rich water to the surface, potentially limiting the ocean”s capacity to absorb additional human-generated CO2. Contrary to these predictions, recent observational data indicates that the Southern Ocean”s role as a carbon sink remains intact.
Dr. Léa Olivier, an oceanographer at AWI and the lead author of the study, explains, “Deep water in the Southern Ocean is normally found below 200 meters. It is salty, nutrient-rich, and relatively warm compared to near-surface water.” This stratification between deep and surface waters effectively keeps CO2 from rising to the atmosphere, as the less salty and colder surface water acts as a barrier.
Despite the anticipated effects of climate change and stronger winds, there has been no observed decline in the Southern Ocean”s CO2 absorption capacity. The study utilized biogeochemical data gathered from numerous marine expeditions in the Southern Ocean from 1972 to 2021 to analyze long-term changes in water mass properties and circulation.
Since the 1990s, the research indicates that surface and deep water masses have become more distinct due to a decrease in surface water salinity from increased freshwater input, attributed to rainfall and melting glaciers. This “freshening” reinforces the density stratification, trapping CO2-rich deep water below the surface.
Dr. Olivier notes, “Our study shows that this fresher surface water has temporarily offset the weakening of the carbon sink in the Southern Ocean, as model simulations predicted. However, this situation could reverse if the stratification were to weaken.” The potential for such a shift exists, as the deeper waters are being pushed closer to the surface by strengthening winds.
The upper boundary of the deep water mass has moved closer to the surface, where it increasingly interacts with CO2-rich water. This could lead to a scenario where CO2 from deeper layers is released, diminishing the Southern Ocean”s capacity to absorb additional anthropogenic CO2.
Dr. Olivier emphasizes the importance of examining below the ocean”s surface to understand these dynamics fully. “To confirm whether more CO2 has been released from the deep ocean in recent years, we need additional data, particularly from the winter months when water masses are more likely to mix,” says Prof. Alexander Haumann, a co-author of the study.
In the coming years, AWI plans to investigate these processes in greater detail as part of the international Antarctica InSync program, aiming to enhance understanding of climate change impacts on the Southern Ocean and the intricate interactions at play.
