Scientists have uncovered one of the biggest mysteries in climate science, why the Southern Ocean continues to absorb large amounts of carbon dioxide (CO2) even as the planet warms. A new study from researchers at the Alfred Wegener Institute (AWI), published in Nature Climate Change, shows that fresh water from melting ice and increased rainfall may have created a hidden "cap" over the ocean's surface. This layer helps keep carbon stored deep beneath the surface, preventing it from returning to the atmosphere. However, this balance is fragile and could change over time.

Why the Southern Ocean Matters

The world's oceans act like a massive sponge, absorbing about a quarter of all the CO2 that humans release into the atmosphere each year. Of that, the Southern Ocean alone takes in around 40%, making it one of the most important natural defences against climate change.

This ocean acts as a carbon sink through a slow-moving process known as the overturning circulation. Cold, dense water sinks deep into the ocean, absorbing CO2 from the air. Meanwhile, older, deeper water rises to the surface, releasing some of the carbon it has stored. This process relies on the ocean's layers staying separate and on the strength of wind-driven mixing.

When more of this carbon-rich deep water rises to the surface, it reduces the ocean's ability to absorb new CO2. That's why scientists have long worried that stronger winds, caused by global warming, might disrupt this balance.

The Mystery of the Missing Carbon

According to computer models, climate change should have weakened the Southern Ocean's ability to store CO2 over time. Stronger westerly winds were expected to mix the ocean's layers and bring up carbon-laden deep water. However, real-world data over the past few decades show little sign of such a decline.

The AWI team thinks they have finally uncovered the reason. Their research found that the top layer of the Southern Ocean has become fresher and less salty in recent years, due to melting ice, glaciers, and increased rainfall. This lighter layer forms a kind of cap that prevents the deeper carbon-rich water from rising to the surface and releasing its CO2 into the air.

Normally, the difference in density between these layers keeps them from mixing. This process is called stratification. As long as this layering remains strong, the deep CO2 stays locked away. But if the layers start to blend, that stored carbon could quickly escape into the atmosphere.

A Delicate Balance

Using data from nearly 50 years of marine expeditions, spanning from 1972 to 2021, the researchers tracked long-term changes in the Southern Ocean's chemistry and circulation. They found that since the 1990s, the boundary between surface and deep waters has become stronger.

Fresh water from ice melt and rainfall has reduced the salinity of the surface layer, making it less dense. This has strengthened the barrier between the two layers, keeping most of the carbon safely stored below.

A Shield That Won't Last

However, this situation may not last forever. The same powerful westerly winds that have become stronger due to global warming are now pushing the deep water closer to the surface. The researchers estimate that since the 1990s, the upper edge of this deep layer has risen by around 40 metres.

If this trend continues, the two layers could mix more frequently. Once that happens, the CO2 stored in the deep ocean could start rising to the surface, reducing the Southern Ocean's capacity to absorb emissions and possibly accelerating global warming.

Warning Signs Beneath the Waves

Some recent studies suggest that this process might already be beginning. If more of the deep, carbon-rich water reaches the surface, the Southern Ocean's role as a global carbon sponge could weaken significantly.

To find out if deep-ocean CO2 is already being released, the researchers say more data are needed, especially from the harsh Antarctic winter, when the ocean layers tend to mix more.

Future Research

Professor Alexander Haumann, a co-author of the study, says the AWI team plans to investigate these changes further through the international Antarctica InSync programme. The goal is to better understand how climate change is altering the Southern Ocean and how this might affect the global carbon cycle in the years to come.

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