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 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.”

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.

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.