Antarctica’s waters are getting saltier - and it could transform Earth's climate

Antarctica’s surrounding ocean has lost sea ice equal to the size of Greenland since 2015. That’s not just a startling statistic – it marks one of the fastest environmental shifts happening anywhere on Earth today.

For decades, the belief was simple: melting ice made the ocean surface fresher, which helped new ice form. But recent findings show the ocean is behaving differently – and that shift could have far-reaching consequences.

A puzzling change in ocean salinity

A team of researchers from the University of Southampton, working with collaborators in Spain, analyzed ocean salinity data using instruments aboard the European Space Agency’s SMOS satellite.

The analysis revealed a clear shift: surface waters south of 50° S latitude are becoming saltier. This shift marks a break from a long-standing trend.

Since the 1980s, surface waters in the Southern Ocean had been cooling and freshening – conditions that helped sea ice expand. That trend has now reversed.

“The discovery was unexpected because melting ice should freshen the ocean, not make it saltier,” said Dr. Alessandro Silvano, lead author of the study published this week. “Yet SMOS satellite data reveal the opposite is occurring, and this is deeply troubling.”

Saltier waters, less ice

When the ocean’s surface is fresh and cold, it floats on top of warmer, saltier water deep below. This layering keeps the heat trapped underneath, allowing ice to grow and stay intact.

“But now the saltier surface waters are enabling heat from deeper down to rise more readily,” Dr. Silvano explained. “This upward flow of warmer water melts sea ice from beneath, making it significantly harder for ice to reform.”

The consequences are already visible. Antarctic sea ice has dropped sharply. Even more concerning, a large patch of open water – the Maud Rise polynya – has reappeared in the Weddell Sea. It hadn’t been seen since the 1970s.

“The return of the Maud Rise polynya highlights just how abnormal the current situation is,” said Dr. Silvano. “If this trend of higher salinity and reduced ice persists, it could lead to lasting changes in the Southern Ocean, with consequences for the rest of the world.”

Monitoring the ocean from space

Studying the Southern Ocean isn’t easy. It’s isolated, violent, and dark for much of the year. That’s why satellite technology is so essential.

To overcome the challenges, scientists at the University of Southampton and the Barcelona Expert Centre built new algorithms to measure changes in surface salinity more accurately from space. The effort was part of ESA’s Earth Observation FutureEO program.

“This is yet another example of how crucial sea-surface salinity measurements from space are,” said Roberto Sabia, ESA Earth Observation Ocean Scientist.

“This novel regional product has been devised to overcome the conventional limitations of salinity retrieval at high latitudes, an area where seawater density is largely controlled by salinity.”

By combining satellite data with instruments in the ocean, the team built a 15-year dataset tracking changes in salinity, temperature, and ice cover in the Southern Ocean.

A dangerous feedback loop

The findings challenge previous assumptions about how the ocean and climate interact. For years, climate models predicted that warming would lead to more ice melt and freshening at the surface, which would help balance things out. That’s no longer the case.

“Our new study has revealed that the Southern Ocean is changing, but in a different way to what we expected,” said Dr. Silvano.

“We might be closer to passing a tipping point than expected and we have potentially entered a new state defined by persistent sea ice decline, sustained by a newly discovered feedback loop.”

The feedback loop works like this: warmer, saltier surface water melts ice from below. Less ice means more exposed ocean. That exposed water absorbs more heat, driving even more melting.

The global ripple effect

The loss of Antarctic sea ice isn’t just a local problem. It’s linked to more heat escaping from the ocean into the air. That fuels stronger storms and speeds up global warming. The result is more extreme weather, more melting of the Antarctic ice sheet, and faster sea level rise.

There’s also a direct threat to wildlife. Penguins, seals, and other species that rely on sea ice for survival are losing their habitat.

These shifts are happening faster – and differently – than climate models predicted. Scientists are now questioning long-held assumptions about how warming should affect ocean layering and ice stability.

“These findings show that we still have gaps in our understanding of ocean and climate dynamics,” said co-author Alberto Naveira Garabato. “Continuous satellite and in-situ monitoring is essential if we want to track and predict these critical changes.”

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