When it comes to climate science, good news can be a little unsettling. When scientists said that the Southern Ocean could still absorb carbon dioxide even though it had been warming up and winds getting stronger for decades, it seemed like a relief. Scientists felt better. Climate modelers didn’t understand. The real story was just starting to come to the surface somewhere in the cold water around Antarctica.
An investigation from the Alfred Wegener Institute has begun to explain what is happening down there. The explanation is both clever and unsettling. This turns out to be the Southern Ocean’s way of protecting itself. It’s not because it’s biologically strong, but because of a thin layer of cold freshwater that sits on top of it. That layer gets water from melting glaciers, less thick sea ice, and heavier rain and snow. It is less dense than the saltier water below. And that difference in density is doing something amazing: it’s keeping huge amounts of CO2-rich deep water away from the atmosphere like a lid.
When you see how the mechanism is put together, it makes sense. Water from depths below 200 meters that is denser and higher in carbon can’t easily mix with the freshwater above it. So the ocean doesn’t release the carbon it has stored into the air. Instead, it stays trapped below, and the ocean’s surface keeps taking in new carbon dioxide from the air as if nothing has changed. This has made the Southern Ocean look healthier than climate models said it would for decades. Things are more complicated than that.
Biogeochemical data from marine expeditions that took place from 1972 to 2021 were looked through by researchers. What they found was that the difference in depth between the surface and deep water has gotten stronger since the 1990s. The surface water has become less salty, which is called “freshening” in science, and this has made the barrier stronger. Things are going well so far. But there is a warning in the study’s results: the body of deep water below that barrier has moved about 40 meters closer to the surface since the 1990s. As a result of climate change, westerly winds are getting stronger, which is pushing it up. For now, the lid is on. It’s not clear how long that will last.
The thing that worries researchers the most is what will happen if this stratification breaks down. About 40% of all the carbon stored in the oceans is in the Southern Ocean alone. That’s a huge amount of carbon for a single stretch of water. If the freshwater barrier gets weak enough for the winds to mix deep and surface water, carbon that has been stored for hundreds of years could quickly escape into the air. That release would not be on a small scale.
There are examples of this kind of collapse in the past. Chengfei He, a climatologist at Northeastern University, has been looking into what happened in the Southern Ocean at the end of the last ice age, which was about 15,000 to 17,000 years ago. When the sea ice went away, the current of dense, cold, carbon-trapping Antarctic Bottom Water started to get weaker. This is the same current that works today. What happened next caused about half of the total rise in CO2 in the atmosphere over an 8,000-year period of melting ice. Two huge carbon stores in the oceans—one in the North Atlantic and one in Antarctica—seemed to fail almost at the same time, which is not what scientists thought would happen. “We’re seeing similar patterns today,” he said, referring to new data that shows Antarctic Bottom Water is getting weaker as the Southern Ocean warms. The parallel doesn’t feel right.
It’s still not clear when the freshwater barrier might break down or how quickly the effects would spread. The Southern Ocean’s current behavior is not really a sign of health. Instead, it’s a delay caused by a structural quirk that gives the climate system extra time that it might not be able to use. As part of the international Antarctica InSync program, scientists at the AWI plan to collect more data in the winter months, when its most likely that the Southern Ocean will mix.
As more and more research comes out, it seems like science is catching up to something that has been going on for years. There was always good news about the carbon sink. It wasn’t the whole story.
