As global climate change intensifies, scientists continue to monitor its effects on ecosystems, particularly the world’s oceans. The Southern Ocean plays a vital role in absorbing carbon dioxide (CO₂) from the atmosphere, a process essential to mitigating the impacts of climate change. However, recent studies, including those from the Alfred Wegener Institute, suggest that despite predictions to the contrary, the Southern Ocean has maintained its capacity to absorb CO₂ in recent decades. But what’s keeping this process intact? And how long will it last? New research is shedding light on the complex mechanisms at play beneath the ocean’s surface.
The Role of the Southern Ocean in CO₂ Absorption
The Southern Ocean is responsible for storing about 40% of the CO₂ absorbed by oceans worldwide. This process is crucial in slowing down the effects of climate change. The ocean’s ability to trap carbon lies in its circulation system, where deep water masses rise to the surface, bringing with them carbon-rich water. This natural CO₂ is released into the atmosphere and exchanged for the human-made CO₂ from the air. However, studies have shown that climate change, particularly stronger westerly winds, could disrupt this delicate balance, pushing more carbon-rich deep water to the surface, reducing the ocean’s ability to act as a carbon sink.
Dr. Léa Olivier, an oceanographer at the Alfred Wegener Institute (AWI) and lead author of a recent study published in Nature Climate Change, explains that deep water in the Southern Ocean typically resides below 200 meters, where it is
“salty, nutrient-rich, and relatively warm compared to water nearer the surface.”
This stratification between the surface and deep waters has historically helped to keep the carbon-rich water contained, slowing its release to the atmosphere. However, changes in this stratification, driven by climate factors, could eventually release more of this stored carbon, amplifying the climate crisis.
Southern Ocean circulation, stratification and subsurface CO2 anomalies. (Nature Climate Change)
Why the Southern Ocean Hasn’t Yet Lost Its CO₂ Absorption Capacity
Despite predictions, observational data has shown that the Southern Ocean’s CO₂ absorption capacity has not significantly decreased in recent decades. This unexpected resilience can be attributed to the continued stratification between deep and surface waters. Dr. Olivier’s study, which analyzed biogeochemical data from Southern Ocean expeditions conducted between 1972 and 2021, uncovered that since the 1990s, the water masses have become increasingly distinct.
“We were able to determine that, since the 1990s, the two water masses have become more distinct from one another,” she notes.
This distinct separation means that the warmer, CO₂-rich deep water has not been able to rise to the surface as easily.
The salinity of surface waters, influenced by increased freshwater input from melting glaciers and sea ice, has decreased, creating a barrier that prevents carbon-rich deep waters from mixing with the surface. This phenomenon, often referred to as “freshening,” has temporarily strengthened the stratification, keeping CO₂ trapped at lower ocean layers.
The Changing Dynamics of Ocean Stratification
However, the long-term stability of this process remains uncertain. As the westerly winds continue to strengthen due to climate change, they could push the boundary between deep and surface waters upward, bringing more CO₂-rich water closer to the surface. This could undermine the Southern Ocean’s capacity to store CO₂. Dr. Olivier cautions,
“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.”
This highlights the critical importance of monitoring changes in ocean circulation and water properties. The layering of surface and deep waters plays a crucial role in determining the ocean’s ability to sequester carbon. If the stratification weakens, mixing between the layers could release large amounts of CO₂, further accelerating the climate crisis.
What’s Beneath the Surface: A Hidden Solution?
In their research, the AWI team focused on the exchange processes between surface and deep water masses. Dr. Olivier points out that by focusing on circulation and mixing, rather than biological processes, they were able to uncover essential patterns that help explain why the Southern Ocean is still absorbing CO₂.
“What surprised me most was that we actually found the answer to our question beneath the surface. We need to look beyond just the ocean’s surface, otherwise we run the risk of missing a key part of the story,” says Olivier.
To fully understand how climate change is affecting the Southern Ocean’s carbon sink, it’s essential to look at deeper layers and examine how they interact with surface waters. The AWI team intends to continue studying these processes, especially during the winter months when water masses tend to mix more frequently.
“To confirm whether more CO₂ has been released from the deep ocean in recent years, we need additional data, particularly from the winter months,” says Professor Alexander Haumann, a co-author of the study.
The AWI’s forthcoming international Antarctica InSync program will focus on these exact interactions, helping to shed light on the ocean’s changing role in climate regulation.