Less Ice Makes the Deep Ocean Saltier, Not Fresher

A vast Southern Hemisphere ocean layer has become saltier, denser and deeper, reversing decades of freshening and changing a water mass that helps carry carbon and oxygen into the ocean. ^[1][2]

The reversal followed a sharp decline in freshwater supplied by melting Antarctic sea ice after 2015, according to a study published on October 28, 2026, in Nature Climate Change. From 2016 to 2024, Antarctic Intermediate Water changed direction across all three measures. ^[1][2]

This water mass connects conditions near the ocean surface with the interior and forms part of the global overturning circulation, the connected movement of water between surface and deeper layers. A change across such a broad layer gives the Antarctic sea-ice retreat consequences well beyond the ice edge. ^[1]

The peer-reviewed study, by Kaushik Mishra, Bishakhdatta Gayen and Alberto C. Naveira Garabato, combined temperature and salinity observations derived from Argo ocean floats with satellite measurements and reanalysis datasets that reconstruct past conditions. ^[1][2]

Gayen described the Antarctic region as the oceans' "freshwater engine", saying its seasonal summer meltwater delivery had fallen from roughly three times the Amazon's annual flow to roughly twice that flow after 2016. ^[2]

The finding establishes a shift in the water itself. How much it will change heat and carbon storage, or circulation farther north, is uncertain, as are the timing of those effects and the relative roles of human-driven climate change and natural variability in the recent sea-ice losses. ^[2][3]

Less ice means less summer freshwater

Sea ice has a seasonal job that is easy to overlook: it supplies freshwater when it melts. From 1995 to 2015, increasing summer meltwater helped strengthen the separation between the upper polar ocean's layers and made Antarctic Intermediate Water fresher. After 2015, repeated sea-ice losses reversed that pattern. ^[1][2]

The apparent puzzle is why ice loss should mean less meltwater. The study concerns the seasonal supply from sea ice. With less ice available to melt in summer, that freshwater contribution falls. The researchers link the reduced supply to the subsequent rise in the water mass's salinity and density. ^[1][2]

Their estimates show a sharp change of direction. Before 2016, sea-ice meltwater input was increasing. After 2015, it declined at more than twice the rate of the earlier increase. ^[1]

The independent record of Antarctic sea ice shows how unusual the recent conditions have been. Summer ice extent reached its lowest level in the satellite record in 2023. The 2024 summer minimum was statistically tied for the second lowest, according to NOAA. ^[3]

That record also contains large swings from year to year. NOAA says scientists are still investigating how much natural variability and climate change contributed to the recent lows. ^[3]

The new study follows the change below the surface. It identifies a hemispheric-scale shift in an intermediate water mass, extending the story from the area covered by ice to the properties of the ocean beneath and beyond it. ^[1]

A small salt change across a huge layer

At the water mass's core, salinity rose at an estimated rate of 0.0048 grams per kilogram per decade, with an uncertainty of 0.0017 grams per kilogram per decade. That is 4.8 milligrams of additional salt per kilogram of seawater per decade. ^[1]

The number is small because ocean salinity changes are measured finely. Its importance here lies in the scale of the water mass and the accompanying changes: the core also became denser and moved deeper. The University of Melbourne described the findings as the first direct observational evidence of this change in Antarctic Intermediate Water's properties. ^[1][2]

Density helps determine how ocean waters arrange themselves vertically. Antarctic Intermediate Water contributes to the transition between waters of different density, a structure scientists call the pycnocline. Changing that structure can alter the routes by which surface conditions are carried into the interior. ^[1][2]

This is why a salinity measurement belongs in a climate story. Water carries heat, dissolved carbon and oxygen with it. The researchers say changes in its movement into deeper layers could affect how much carbon the ocean stores. ^[2]

The useful result is the observed reversal itself. It gives scientists a physical change to track as they investigate how the ocean's storage and transport systems respond to Antarctic sea-ice decline.

The next question is where the change travels

Antarctic Intermediate Water forms part of the upper limb of global overturning circulation. That places it within a system whose connections extend far beyond the Southern Ocean. ^[1][2]

Naveira Garabato said a sustained change could feed into the Atlantic Meridional Overturning Circulation, the Atlantic's large-scale circulation system, over years to decades. He also said it could ultimately influence Southern Hemisphere atmospheric circulation, weather and storm tracks. These are possible consequences identified by the researchers. ^[2]

Gayen put the speed in a much longer perspective. He said the previous transition he was referring to occurred around 130,000 years ago and unfolded over thousands of years, compared with roughly two decades for the change described in this study. ^[2]

The immediate scientific task is closer to hand: follow the water. Further observations will show whether its salinity, density and depth continue along their new paths, stabilize or turn again. Measuring how those changes travel through the ocean will be essential to determining what the reversal means for heat and carbon storage. ^[1][2]