Saltier, Hotter Sea Springs a Leak in Ice Sheets

The waters around Antarctica have suddenly and unexpectedly started getting saltier. This flips a decades-long trend of falling ocean salinity, which had protected southern sea ice against melting. 

In the mid-1970s, early satellite imaging showed that massive holes were periodically forming in ice sheets, known as polynyas. One such recurring void—in the Weddell Sea’s ice sheet—got so big that it received a name, the Maud Rise polynya (named after an underwater mountain that rises beneath it). Sometimes this hole was larger than the entire state of California. 

But in the intervening decades, the top layers of the Southern Ocean encircling Antarctica have been freshening—becoming less salty. This stratification has helped keep the upper levels of the water cooler, protecting the sea ice like a sort of aqueous shield from warmer, saltier waters below. Tremendous holes in the sea ice became smaller and less common, despite the warming planet. The Maud Rise polynya wasn’t seen again for nearly 40 years. 

The findings flip certain climate assumptions on their heads. Most prior models anticipated that the strongly layered Southern Ocean pattern would continue, helping to protect the store of sea ice in the southern ocean, which is crucial for maintaining Earth’s climate as we know it. “Instead, a rapid reduction in sea ice—an important reflector of solar radiation—has occurred, potentially accelerating global warming,” said Aditya Narayanan, a co-author of the paper and postdoctoral research fellow also at the University of Southampton, in a statement. 

It’s still unclear why this protective trend has reversed. 

The work, funded in large part by the European Space Agency, demonstrates how little we know about the complex workings of our planet’s dynamic, life-as-we-know-it-sustaining systems. 

“If this salty, low-ice state continues,” Silvano said, “it could permanently reshape the Southern Ocean—and with it, the planet.”

Lead photo courtesy of the University of Southampton

• Katherine Harmon Courage

  Posted on July 1, 2025

  Katherine Harmon Courage is the executive editor at Nautilus.