Newly Found Ocean Feedback Loop Threatens the Future of Antarctic Sea Ice

Proceedings of the National Academy of Sciences, 30 June 2025

Satellite observations have revealed a sharp increase in the salinity of ocean surface waters surrounding Antarctica since 2015, with strong implications for sea ice having passed a threshold of continued decline; a surprising finding that models had not predicted. For decades, the surface of the Southern Ocean around Antarctica has become fresher and colder in response to increasing ice melt caused by climate warming. This cold, fresh water stabilized the layers of the water column and helped Antarctic sea ice to form and expand. However, this new satellite data shows that since 2015, surface salinity in the Southern Ocean began abruptly increasing, at the same time that Antarctic sea ice extent began to crash — in the last decade, an area of sea ice the size of Greenland has been lost. The authors suggest that saltier surface water has allowed deep ocean heat to rise more easily, melting sea ice from below in a dangerous new feedback loop that may continue to threaten formation of Antarctic sea ice in the future. Open questions remain about what triggered this sudden salinity jump. Possibilities include shifts in wind patterns, changes in precipitation, or increased intrusion of warm Circumpolar Deep Water onto the continental shelf. These unexpected findings emphasize that our current understanding may be insufficient to accurately predict future changes in the oceans surrounding Antarctica, stressing the need to urgently improve both observations and modeling to better predict and prepare for changes in and around Antarctica, as well as the global impacts of such changes.

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