Antarctic sea ice collapse linked to a sudden surge in ocean salt

For decades, oceanographers assumed the surface waters circling Antarctica were becoming progressively fresher – a shift that helped lock deep heat away and nurture expansive winter sea ice.

However, new observations show the opposite is now happening. An international team led by the University of Southampton reported a sudden rise in surface salinity south of 50° latitude. This shift coincided with the steepest drop in Antarctic sea-ice extent ever recorded.

The analysis combines European satellite data with readings from autonomous floats that profile the water column. The result reveals an unanticipated pivot in the Southern Ocean’s salt balance sometime after 2015.

Excessive salt melts sea ice

Cold, fresh surface water normally overlies warmer, saltier layers at depth, creating strong density “stratification” that traps heat below. When the surface grows saltier, that barrier weakens. Heat then mixes upward, melting sea ice from beneath and preventing extensive winter growth.

“Saltier surface water allows deep ocean heat to rise more easily, melting sea ice from below,” said lead author Alessandro Silvano, an oceanographer at the University of Southampton. “It’s a dangerous feedback loop: less ice leads to more heat, which leads to even less ice.”

Since 2015, the continent has shed sea ice equivalent to the area of Greenland. Researchers call it the largest environmental shift detected anywhere on Earth in recent decades.

Sea ice hole returns after decades

One striking symptom is the re-opening of the Maud Rise polynya – a yawning hole in the Weddell Sea ice cover. It spans nearly four times the size of Wales and last appeared in the late 1970s.

“The return of the Maud Rise polynya signals just how unusual the current conditions are,” Silvano said. “If this salty, low-ice state continues, it could permanently reshape the Southern Ocean – and with it, the planet.”

Reduced Antarctic ice darkens the ocean, absorbing more sunlight and already feeding back into the global climate system. This, as Silvano put it, will give rise to “stronger storms, warmer oceans, and shrinking habitats for Antarctic wildlife.”

Old predictions fall short

Climate models had projected eventual Antarctic sea-ice loss under greenhouse forcing. However, many scientists believed that enhanced snowfall and increased melt from continental ice sheets would keep surface waters fresh for decades, delaying large declines. The new evidence challenges that assumption.

“While scientists expected that human-driven climate change would eventually lead to Antarctic sea ice decline, the timing and nature of this shift remained uncertain,” said co-author Aditya Narayanan, a physical oceanographer at the University of Southampton.

“Previous projections emphasized enhanced surface freshening and stronger ocean stratification, which could have supported sustained sea ice cover. Instead, a rapid reduction in sea ice – an important reflector of solar radiation – has occurred, potentially accelerating global warming.”

Eyes on the Southern Ocean

Because this turnaround was detected only after satellites began tracking sea-surface salinity in detail, researchers worry that other rapid transitions might be missed without constant observation.

“The new findings suggest that our current understanding may be insufficient to accurately predict future changes,” said senior author Alberto Naveira Garabato, a professor of physical oceanography at the University of Southampton.

“It makes the need for continuous satellite and in-situ monitoring all the more pressing, so we can better understand the drivers of recent and future shifts in the ice-ocean system.”

Sea ice loss changes the planet

Antarctic sea ice acts as a reflective shield, bouncing sunlight back to space and modulating heat exchange between ocean and atmosphere.

Its precipitous decline could amplify warming worldwide, disturb wind patterns, and alter the formation of the deep water masses that power global ocean circulation.

Saltier surface layers may also influence how much carbon the Southern Ocean can absorb, potentially weakening one of Earth’s most important climate buffers.

Heat and salt reinforce sea ice loss

The study leaves open questions about what triggered the sudden salinity jump. Possibilities include shifts in wind patterns, changes in precipitation, or increased intrusion of warm Circumpolar Deep Water onto the continental shelf.

Whatever the cause, the new feedback between salt and sea ice could lock the region into a self-reinforcing cycle of warming and ice loss.

Scientists stress that only sustained measurements – combining satellites, autonomous floats, and ship-based surveys – will reveal whether the Southern Ocean snaps back to its former fresher state or continues toward an unfamiliar, saltier future.

For now, the message is clear: Antarctica’s floating fringe, once thought relatively resistant to near-term change, is undergoing a rapid transformation that matters far beyond the frozen continent.

The study is published in the journal Proceedings of the National Academy of Sciences.

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