Global warming is putting a “freshwater lid” on the surface of the Antarctic Ocean, a new study has found, trapping heat and carbon and possibly choking off oxygen from the deeper reaches of the sea.
Researchers from McGill University and the University of Pennsylvania studied tens of thousands of measurements collected from the waters around Antarctica over a 60-year period. What they discovered was that since the 1950s, the ocean’s surface there has grown less salty, creating a layer of freshwater that keeps deeper, warmer saltwater from cycling to the surface.
“Deep ocean waters only mix directly to the surface in a few small regions of the global ocean, so this has effectively shut one of the main conduits for deep ocean heat to escape,” Casimir de Lavergne, one of the paper’s lead authors, said in a statement.
Water from around Antarctica makes up the deepest part of the world's ocean and, all told, composes the largest volume of ocean water in the world. It gets its oxygen and releases heat and carbon in a cycle, the researchers described: Warm water from the depths rises to the surface, and cooler water – now laden with oxygen – sinks back to the ocean depths, helping microbes and other deep-sea organisms breathe and stay alive.
This process occurs all over the globe, scientists say, but it occurred most efficiently in the Antarctic, in a giant, ice-free region known as a polynya. While there are actually still plenty of polynyas along coastlines, the one in the Antarctic was the only known polynya on the open water – and it became closed off by ice in the 1970s.
"In coastal polynyas, the dynamics are different – they’re not driven by this deep mixing," explains Jaime Palter, an assistant professor of physical oceanography at McGill University who helped lead the study. With the closing of the open-water polynya, Palter says "the hypothesis is that it’s caused a slowing of that overturning of water."
The research team believes that the open-water polynya, while the only one ever confirmed, was not a solitary phenomenon. Instead, by combining firsthand observations from early oceanographers with modern climate data and modeling, they say that it was more likely the last of what were once many openings above the deep sea, which together helped the ocean "breathe" more effectively.
"Although the observational scope that we have is short, when climate models are run [to simulate] preindustrial conditions, polynyas are very common," Palter says. "Then they disappear in projections of the future, based on rising CO2 levels."
The situation is not likely to get better anytime soon. Just as wet areas are predicted to become even wetter as a result of climate change and dry areas are projected to get drier, salty parts of the ocean will get saltier, and ocean regions with more freshwater will get even fresher, building an ever-thicker "lid" along the top of the ocean surface.
“In the absence of a phenomenon like polynya, the ocean stores more heat and carbon and less oxygen, in reverse of the atmosphere,” Palter says. "It’s like a door to the deep ocean. If that door is open, then the ocean can release that heat and carbon and absorb oxygen. And if that door is closed, then the ocean stores more heat.”
The findings were published Sunday in the journal Nature Climate Change.