Scientists say the sharp decline of Antarctic sea ice since 2016 has set off a major change in a key part of Earth's climate system. A paper published Wednesday describes a rapid rise in salinity in a vast layer of the Southern Ocean.
The affected layer lies between roughly 2,300 and 4,000 feet below the surface and encircles Antarctica. The increase in saltiness reverses a trend of freshening that had lasted for decades. According to the researchers, the shift resembles ocean changes normally linked to major global climate transitions over thousands of years, yet it has unfolded within only a few years.
The layer is known as Antarctic Intermediate Water. It moves northward into the Indian, Pacific and Atlantic oceans, carrying heat, carbon, oxygen and nutrients toward the tropics and even across the equator. Co-author Bishakhdatta Gayen, a professor at the University of Melbourne and at the Centre for Atmospheric and Oceanic Sciences in Bengaluru, India, said this means the consequences could reach well beyond the Antarctic region.
Gayen said the team combined six independent datasets to follow changes in the Southern Ocean from 2004 to 2024. The measurements came from Argo floats, instruments that drift through the ocean and periodically dive to different depths to record properties such as density and salinity. They then return to the surface to transmit their readings. He added that a 2025 study also found rising surface salinity connected to shrinking sea ice.
Sea ice around Antarctica reached its greatest extent in the satellite record in 2014. Just two years later, it fell to a record low. It stayed far below the 2014 level and hit another low in 2023, when it was about 1 million square miles under the 1981 to 2010 average. That represented a loss of ice covering an area roughly four times the size of Texas.
The link to salinity comes down to freshwater. With less sea ice, less freshwater is released during the melt season, and that leaves the Antarctic Intermediate Water saltier.
Changes in the layer's density, thickness and salinity affect how heat travels between the ocean surface and deeper waters. That, Gayen said, "could affect how the ocean stores and redistributes heat."
He called for more monitoring to follow the changes, noting that the effects are potentially "knocking on your door" within five years.
