Melting ice is hiding a massive climate secret beneath Antarctica

Why the Southern Ocean Matters So Much

The world's oceans capture roughly one quarter of all human-produced CO₂ emissions. The Southern Ocean alone accounts for about 40 percent of that total, making it one of the planet's most powerful natural defenses against global warming. This immense carbon sink functions through a complex circulation system: deep water rises to the surface, exchanges gases with the atmosphere, and then sinks again, carrying absorbed CO₂ back into the depths.

The balance depends on how much natural CO₂ from ancient deep waters resurfaces. When more carbon-rich water from below reaches the surface, it limits how much new, human-made CO₂ the ocean can take in. This interplay is governed by the layering, or stratification, of different water masses and by the strength of ocean currents.

Ancient Waters and Strengthening Winds

The deep water that resurfaces in the Southern Ocean has been isolated for centuries or even millennia, accumulating large amounts of CO₂. Climate models predict that stronger westerly winds, a result of human-driven climate change, will bring more of this carbon-rich water to the surface, reducing the ocean's capacity to absorb CO₂ in the long term.

However, despite these stronger winds, data collected over recent decades show that the Southern Ocean remains a strong carbon sink. The new AWI research helps explain why: ocean layering has changed in a way that keeps much of the deep carbon locked away.

The Invisible Barrier Holding Carbon Below

"Deep water in the Southern Ocean is normally found below 200 meters," says Dr. Léa Olivier, AWI oceanographer and lead author of the study. "It is salty, nutrient-rich and relatively warm compared to water nearer the surface."

This deep water contains large stores of dissolved CO₂ that entered the ocean long ago. In contrast, near-surface water is cooler, less salty, and holds less CO₂.

As long as this density layering remains strong, the CO₂-rich deep water stays sealed off. But if the boundary between layers weakens, that trapped carbon could more easily reach the surface and escape into the atmosphere.

Stronger Winds, Rising Risks

"Previous studies suggested that global climate change would strengthen the westerly winds over the Southern Ocean, and with that, the overturning circulation too," says Léa Olivier. "However, that would transport more carbon-rich water from the deep ocean to the surface, which would consequently reduce the Southern Ocean's ability to store CO₂."

Although such wind intensification has been observed and linked to human activity, measurements still show no major decline in the ocean's carbon uptake -- at least not yet.

Freshwater Inputs Strengthen Ocean Layers

Long-term monitoring by AWI and other research institutions shows that climate change is already altering the characteristics of both surface and deep waters. "In our study, we used a dataset comprising biogeochemical data from a large number of marine expeditions in the Southern Ocean between 1972 and 2021. We looked for long-term anomalies, as well as changes in both circulation patterns and the properties of water masses. In doing so, we only considered processes related to the exchange between the two water masses, namely circulation and mixing, and not biological processes, for example," explains Léa Olivier. "We were able to determine that, since the 1990s, the two water masses have become more distinct from one another." The Southern Ocean's surface water salinity has reduced as a result of increased input of freshwater caused by precipitation and melting glaciers and sea ice. This "freshening" reinforces the density stratification between the two water masses, which in turn keeps the CO₂-rich deep water trapped in the lower layer and prevents it from breaking through the barrier between the two layers.

A Temporary Shield Against Climate Change

"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," summarizes Léa Olivier. Strengthening westerly winds are already pushing the deep water closer to the surface. Since the 1990s, the upper boundary of the deep water layer has risen by about 40 meters.

As CO₂-rich water replaces more of the surface layer, the boundary between them becomes more vulnerable to mixing, likely caused by those same winds. Once mixing increases, stored CO₂ could begin to leak upward and escape into the atmosphere.

Warning Signs Beneath the Waves

Recent research suggests that this process may already be starting. If more carbon from the deep ocean reaches the surface, the Southern Ocean's role as a global carbon sink could weaken, accelerating climate change.

"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 Léa Olivier.

"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 the water masses tend to mix," explains Prof. Alexander Haumann, co-author of the study. "In the coming years, the AWI is planning to carefully examine these exact processes as part of the international Antarctica InSync program, and gain a better understanding of the effects of climate change on the Southern Ocean and potential interactions."