Deep Sea Currents may be Slowed by Climate Change
Deep ocean currents may be slowing down due to climate change, according to a new study.
Ocean currents act as conveyor belts that channel heat, carbon, oxygen and other nutrients around the world's seas, and the slowdown of deep sea currents is worrisome, scientists say.
"Our observations are showing us that there is less formation of these deep waters near Antarctica," said Irina Marinov, a climate scientist at the University of Pennsylvania. "This is worrisome because, if this is the case, we're likely going to see less uptake of human produced, or anthropogenic, heat and carbon dioxide by the ocean, making this a positive feedback loop for climate change."
Writing in the journal Nature Climate Change, Marinov and her collaborators, including UPenn's Raffaele Bernardello and colleagues from McGill University.
In deep Antarctic waters, researchers have observed the mass of cold, salty, dense water that makes up the deep ocean current to be shrinking. This is concerning, the scientists say, because the currents are able to "hide" heat and carbon from the atmosphere.
"The Southern Ocean is emerging as being very, very important for regulating climate," Marinov said, noting that Antarctic waters take up approximately 60 percent of the anthropogenic heat produced on Earth and 40 to 50 percent of the anthropogenic carbon dioxide.
Climate change, the scientists report, is responsible for the shrinking of the deep sea currents. The observation hinges on an ocean phenomenon known as polynyas, which are openings in sea ice that form when warm water is pushed up to the surface of the cold sea. This merger of warm and cold water results in a process called open-sea convection, which is rarely observed. The last time polynyas were observed in Antarctica's Wendell Sea was in the late 1970s.
In their study, the researchers suggest that polynyas were likely more common before the industrial era.
"The reason has to do with the fact that climate change has led to more precipitation around the Antarctic continent, which leads to greater levels of fresh water at the surface," UPenn said in a news release. "Fresh water is more buoyant than saltwater and thus doesn't sink through the layers of the ocean as saltier water does, leading to fewer polynyas and less open-sea convection in the Southern Ocean."
Marinov noted the importance of this "because this process of deep convection that happens in polynyas is a big contribution to the Antarctic Bottom Waters, these deep currents that feed the rest of the ocean."
"We see that the convective process is shutting down as the water gets fresher and fresher," Marinov said.
The absence of polynyas in recent decades could be an indication that heat is being trapped in the deeper ocean, and possibly contributing to the recent "slowdown" in global warming and the increase of Antarctic sea ice in recent years, the researchers report.
However, "the slow down of polynyas will likely be a positive feedback on warming, as the convective process is shutting down and reducing the amount of new, anthropogenic carbon and heat being taken out of the atmosphere," Marinov said. "We are pursuing these implications in our current work."