Antarctic sea ice loss is making a key Southern Ocean water mass saltier, study finds

A denser AAIW can push more excess heat into deeper waters, so existing models may underestimate deep-ocean warming and miscalculate regional sea-level rise. That leaves existing projections open to revision. How large the effect will be, and whether it persists, is not established.

What the floats show

The team analysed data from the global Argo program, a network of about 4,000 robotic floats that sample the upper oceans, and from satellite and atmospheric datasets. The study, published in Nature Climate Change, covers the years 1995 to 2024. From 1995 to 2015, Antarctic summer sea-ice melt helped maintain the AAIW. From 2016, Antarctic sea ice declined to record lows. Between 2016 and 2024, reduced freshwater input led the layer to become saltier, denser and deeper. The shift followed a period in which summer melt had helped hold the layer steady.

The Deccan Herald reported a direct link between the reversal in AAIW salinity and less freshwater from melting sea ice. It also pointed to warmer, saltier water moving up from the ocean's depths.

A 130,000-year comparison

The AAIW last underwent a similar shift around 130,000 years ago, at the end of the last interglacial period. Antarctica then had very little ice.

That transition took thousands of years, rather than two decades. The study's starkest comparison is this: a change that took millennia in the geological record happened in two decades. The modern shift has run its course between 2016 and 2024, inside a single stretch of the Argo record. The published findings do not say whether the water mass is returning to its earlier state. Nor do they put a figure on how much freshwater the missing sea-ice melt represents.

What the models miss

Tech Times reported that climate models missed the shift. ETV Bharat described it as a reshaping of global ocean circulation.

The University of Melbourne researchers said changes in the AAIW could affect the ocean's ability to store carbon and push more excess heat into deeper waters. The university statement says the AAIW helps store carbon, supply oxygen and drive global ocean currents. It says the layer redistributes heat, freshwater and carbon around the planet. The university released the statement on Wednesday, and Xinhua reported it the following day.

What is still unknown

Neither the study nor the university statement gives a figure for how much extra heat the shifted water mass will store. Neither gives a timescale.

The study period ends in 2024, and the published findings do not project the trend forward. Later Argo data would show whether the salinity shift persists or levels off, but the study includes no such analysis. The researchers do not quantify the change's effect on ocean carbon uptake.