Antarctic Ocean Layer Turns Saltier as Sea Ice Loss Reshapes Circulation, Scientists Warn

Between 2,300 and 4,000 feet below the surface of the Southern Ocean, a band of water that rings Antarctica has been getting saltier. That reverses a freshening trend that lasted for decades, according to a paper published Wednesday. The researchers tie the change to the rapid loss of Antarctic sea ice since 2016.

The layer is called Antarctic Intermediate Water. It does not stay near the pole. It spreads north into the Indian, Pacific, and Atlantic oceans and carries heat, carbon, oxygen, and nutrients toward the tropics, and even across the equator. That is according to co-author Bishakhdatta Gayen, a researcher and professor at the University of Melbourne and at the Centre for Atmospheric and Oceanic Sciences in Bengaluru, India. The authors say the shift resembles ocean changes usually associated with major global climate transitions over millennia, but this one has played out in a few years.

Gayen said the team combined six independent datasets covering 2004 to 2024. The data came largely from Argo floats, instruments that drift through the ocean, descend to different depths to measure properties such as density and salinity, then surface to transmit what they recorded. He added that a 2025 study also found rising surface salinity linked to declining sea ice.

The ice record explains the timing. Antarctic sea ice hit its highest extent in the satellite record in 2014, then fell to a record low just two years later. It stayed well below the 2014 level and reached another low in 2023, when it stood about 1 million square miles under the 1981 to 2010 average. That is an area of ice roughly four times the size of Texas. With less ice, less freshwater enters the ocean during the melt season, and Antarctic Intermediate Water becomes saltier.

Gayen said changes in the layer's density, thickness, and salinity affect how heat moves between the surface and deeper water, and could alter how the ocean stores and redistributes heat. He said more monitoring is needed because impacts could be "knocking on your door" within five years. He pointed to possible effects including more frequent storms and ocean heat loss, which a 2024 study identified. A wide-ranging scientific review earlier this year concluded that Antarctic sea ice changes influence the global distribution of atmospheric heat and moisture, and that the speed and persistence of the decline suggest the ocean around Antarctica may be moving into a new state.

Michael Meredith, an oceanographer with the British Antarctic Survey who led the BAS Polar Ocean for years, said the research shows sea-ice loss since 2016 is already changing the properties and circulation of deep ocean waters. "The Southern Ocean is the key juncture for all the world's oceans," he said, calling it the main place where old waters are reprocessed into new. He added that the region plays a "massive role" in absorbing human carbon dioxide emissions, which moderates the pace of climate change.

Meredith cautioned that it is not yet clear whether the shift is permanent, since the observational record is short. He also said the global climate models used to guide policy still struggle to reproduce recent sea ice losses. "Our climate models need to catch up," he said. If the low sea-ice state persists, he said, the changes will keep spreading through the world's oceans.

He placed the finding alongside earlier research showing that deep Southern Ocean circulation has slowed, reducing how fast new water floods and cools the deepest areas. The new study shows the upper part changing quickly in response to sea ice. That matters, Meredith said, because upper circulation can affect climate within years to decades, not centuries. What to watch is whether continued monitoring shows the salinity rise holding, and whether models improve at capturing it.