The rapid decline of Antarctic sea ice since 2016 has triggered a dramatic shift in a crucial part of Earth’s climate system, scientists say.

In a paper published Wednesday, researchers documented a sharp salinity increase in a vast layer of the Southern Ocean, extending from 2,300 to 4,000 feet deep and encircling Antarctica. The shift reverses decades of freshening and echoes ocean changes usually associated with major global climate shifts over millennia but is now happening in just a few years.

The switch from a decades-long trend of freshening water to increasing salinity has implications that could extend beyond Antarctica. The layer, known as Antarctic Intermediate Water, spreads northward into the Indian, Pacific, and Atlantic oceans, transporting heat, carbon, oxygen, and nutrients toward the tropics, and even across the equator, said 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.

Gayen said the study combined six independent datasets to track changes in the Southern Ocean from 2004 to 2024. The data came from Argo floats, which drift through the ocean and periodically descend to different depths to measure water properties such as density and salinity, then resurface to transmit the readings. A 2025 study also detected increasing surface salinity linked to declining sea ice, he added.

Antarctic sea ice reached its highest extent in the satellite record in 2014 and suddenly dropped to a record low just two years later. The sea ice remained well below the 2014 extent and reached another low point in 2023, when it measured about 1 million square miles below the 1981 to 2010 average, with an area of ice about four times the size of Texas gone. As the ice dwindles, there’s less freshwater available during the melt season, making the Antarctic Intermediate Water saltier.

Changes to the density, thickness and salinity of the layer influence how heat moves between the surface of the ocean and deeper waters, which “could affect how the ocean stores and redistributes heat,” Gayen said. More monitoring is needed to track the changes, he added, because impacts are potentially “knocking on your door” within five years.

Those impacts, he said, could include increased storm frequency and ocean heat loss identified by a 2024 study. Earlier this year, a far-reaching scientific review concluded that changes in Antarctic sea ice cover “influence the global distribution of atmospheric heat and moisture, as well as atmospheric variability.” The “speed and persistence” of the sea ice decline suggests the ocean around Antarctica may be moving into a new state that is likely to affect not only Antarctic ecosystems, but the entire global climate system, the review found.

Michael Meredith, an oceanographer with the British Antarctic Survey who led the BAS Polar Ocean for years, said the new research shows that dramatic Antarctic sea-ice loss since 2016 is already affecting the properties and circulation of deep ocean waters.

“The Southern Ocean is the key juncture for all the world’s oceans,” he said. “It’s the major site where old waters are reprocessed into new, so this could have global consequences.”

The Southern Ocean also plays a “massive role” in drawing down human carbon dioxide emissions, which moderates the rate of climate change where people live, he said. Anything that disrupts key ocean cycles demands our attention, he added.

Meredith said it’s not yet clear whether the shift is permanent because it’s based on a relatively short observational record. But, he said, global climate models used to guide policy still struggle to accurately reflect recent sea ice losses, which shows that more research and monitoring is needed.

“Our climate models need to catch up,” he said. “But if the low sea-ice state persists, these changes will keep spreading through the world’s oceans, and the effects will ripple out across our climate and ecosystems.”

Related research in recent years found that deep circulation in the Southern Ocean has slowed, reducing the rate at which new waters flood and cool the deepest areas. The new study makes it clear that the upper part is rapidly changing in response to changes in sea ice, he said. “That matters,” he added, “because the upper circulation can affect our climate within years to decades, not centuries.”

Bob Berwyn is an Austria-based reporter who has covered climate science and international climate policy for more than a decade. Previously, he reported on the environment, endangered species and public lands for several Colorado newspapers, and also worked as editor and assistant editor at community newspapers in the Colorado Rockies.

This story originally appeared on Inside Climate News.