Antarctica’s waters are getting saltier and driving a collapse in its sea ice — and scientists aren’t sure why.

Antarctic sea ice has been declining since 2015, defying model predictions to hit a record 0.6 million square miles (1.55 million square kilometres) below its expected average extent in 2023. At winter’s peak in July of that year, the region was missing a chunk of ice bigger than Western Europe, and it’s showing no signs of recovery.

This decline — the largest environmental shift seen anywhere on Earth in recent decades — will have ripple effects on the world’s climate.

Now, scientists have linked the decline to an unexpected and yet-to-be-explained rise in the saltiness of the waters surrounding the continent. The researchers published their findings June 30 in the journal PNAS .

“We were surprised. It’s a counterintuitive result because we usually associate melting ice with freshening of the ocean,” study lead-author Alessandro Silvano , a senior scientist at the University of Southampton, told Live Science. “This points to a more profound structural shift in the Southern Ocean — not just sea ice, but also the ocean beneath.”

Troubled waters

The sea ice surrounding Earth’s poles melts in the summer and freezes in the winter, fluctuating between minimums and maximums. In Antarctica, this ice acts as a moat that protects the continent’s increasingly precarious land ice from warming ocean waters, while also reflecting some of the sun’s energy back into space and trapping carbon dioxide underneath the ocean’s surface.

Related: ‘We were in disbelief’: Antarctica is behaving in a way we’ve never seen before. Can it recover?

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Since satellites began monitoring sea ice extent in 1979, Arctic ice coverage has plunged by more than 12% each decade . Yet the Antarctic’s ice continued to steadily grow, hitting an all-time high in 2014. But this trend reversed into a precipitous, worsening fall in 2016, marking a fundamental shift.

Scientists agree that the underlying driver of this switch is climate change . Yet sea ice forms at the boundary between the ocean and the air, which are both surprisingly complex systems. This, alongside the Southern Ocean’s remoteness, makes predicting how intricate warming mechanisms will play out upon the ice difficult.

To investigate, the researchers behind the new study turned to the European Space Agency’s Soil Moisture and Ocean Salinity satellite , which measures the subtle changes extra salinity makes to the brightness of microwaves bouncing off the ocean’s surface. This signal is messy, requiring cutting-edge algorithms to untangle, meaning that analyzing it only recently became possible.

After examining daily readings from 2011 to 2023, the researchers found that the sea ice’s decline and reopening of giant holes in its cover (such as the Weddell Sea’s Maud Rise polynya) coincided with a sharp increase in salinity. They were shocked by their result, doubting it until it was confirmed by data from floating buoys.

Conventional wisdom suggests that as temperatures increase, melting ice spilling from Antarctic ice shelves should increase the ocean surface’s freshwater content. Instead, some unknown process is making the water more salty.

“The precise drivers remain unclear,” Silvano said. “One possibility is that salt stored in deeper layers of the ocean were brought up to the surface — a process potentially triggered by changes in ocean circulation or atmospheric forcing. We’re actively investigating how and why this structural change began.”

A group of Adélie penguins (Pygoscelis adeliae) hop from one sea ice chunk to another. The penguins depend upon the ice for breeding and hunting. (Image credit: nicholas_dale via Getty Images)

The increasing salt content will likely bring greater warming to the surface waters and ever faster melting of the ice. Salt water is denser than fresh water, so a freshwater layer at the top of the water column acts as a lid, preventing the warmer water that usually circulates in deeper layers from reaching the surface.

Sea ice is also responsible for bringing fresh water from the coast, where the ice forms, out into the ocean. With the freshwater lid broken, more warm water can rise, shrink the sea ice’s extent, and in turn ensure that there will be less fresh water in future, Silvano said.

Other experts agree with the paper’s findings, describing it as a missing puzzle piece that could explain discrepancies between climate models and real-world data.

“The fact that this change in salinity is counter to what we expect under climate change suggests there are processes at play that we don’t fully understand, and that might not be included or well represented in our climate models,” Ariaan Purich , an Antarctic climate researcher at Monash University in Australia, told Live Science. Purich suggests that investigating the gaps between the predictions of models and readings like these could provide new insights.

Another missing element is “the kind of observations that provide the information to build the right processes into models,” Caroline Holmes , a polar researcher at the British Antarctic Survey, told Live Science. “The Southern Ocean below the surface is chronically underobserved, and there is momentum at the moment building towards two major initiatives, Antarctica InSync and the International Polar Year , that would do just that.”

Meanwhile, Silvano and his colleagues will look into what triggered the salinity spike in 2015 and whether this could be considered a tipping point, alongside how the process may influence the world’s climate through impacts on the sea ice, ocean circulation and carbon cycling.