• Remote sensing confirms a shift from long-term freshening to increasing surface salinity south of 50° S, beginning around 2015.
• Denser surface waters are enhancing vertical mixing, allowing warmer subsurface water to reach the surface, accelerating Antarctic ice loss.
• The shift in surface salinity and stratification could impact deep ocean water formation and disrupt components of the global thermohaline circulation, including the Atlantic Meridional Overturning Circulation (AMOC).