Southern Ocean losing ice fast as decades-long trend reverses

Since 2015, Antarctica has lost sea ice equal to the size of Greenland — the largest environmental shift seen anywhere on Earth in the last decades. Now, research led by the University of Southampton shows that the Southern Ocean is getting saltier — reversing a longstanding trend in which the ocean’s surface freshened, helping sea ice grow — and this is making the problem worse.

In the Southern Ocean’s polar waters, cold, fresh surface water overlays warmer, saltier waters from the deep. In the winter, as the surface cools and sea ice forms, the density difference (stratification) between water layers weakens, allowing these layers to mix and heat to be transported upward, melting the sea ice from below and limiting its growth.

Since the early 1980s, the surface of the Southern Ocean had been freshening and stratification had been strengthening, trapping heat below and sustaining more sea ice coverage. Now new satellite technology, combined with information from floating robotic devices which travel up and down the water column, shows this trend has reversed; surface salinity is increasing south of 50° latitude, stratification is weakening, and sea ice has reached multiple record lows — with large openings of open ocean in the sea ice (polynyas) returning.

Published in the journal PNAS, the research marks the first time scientists have been able to monitor these changes in the Southern Ocean in real time. It also coincides with a dramatic loss of sea ice around Antarctica and the re-emergence of the Maud Rise polynya in the Weddell Sea — a huge hole in the sea ice nearly four times the size of Wales, which hadn’t occurred since the 1970s.

“Saltier surface water allows deep ocean heat to rise more easily, melting sea ice from below,” said research leader Dr Alessandro Silvano. “It’s a dangerous feedback loop: less ice leads to more heat, which leads to even less ice.

“The return of the Maud Rise polynya signals just how unusual the current conditions are. If this salty, low-ice state continues, it could permanently reshape the Southern Ocean — and, with it, the planet. The effects are already global: stronger storms, warmer oceans, and shrinking habitats for penguins and other iconic Antarctic wildlife.”

Contrary to the new findings, manmade climate change was generally expected to sustain Antarctic Sea ice cover over the coming years. As noted by study co-author Aditya Narayanan, “Previous projections emphasised enhanced surface freshening and stronger ocean stratification, which could have supported sustained sea ice cover. Instead, a rapid reduction in sea ice — an important reflector of solar radiation — has occurred, potentially accelerating global warming.”

Study co-author Professor Alberto Naveira Garabato concluded, “The new findings suggest that our current understanding may be insufficient to accurately predict future changes.

“It makes the need for continuous satellite and in situ monitoring all the more pressing, so we can better understand the drivers of recent and future shifts in the ice–ocean system.”

Image credit: iStock.com/Oleksandr Matsibura