Southern Ocean losing ice – researchers warn of dangerous development

A dramatic shift is becoming apparent in the Southern Ocean: The ocean has been becoming saltier, hotter, and rapidly losing ice for several years, as a research team reports in the journal " PNAS ." Previously, the surface water in the Southern Ocean was less salty for decades, allowing sea ice to grow. This trend has abruptly reversed south of the 50th parallel since 2015, explains the team led by Alessandro Silvano of the University of Southampton after analyzing satellite data.

A sudden increase in salinity coincided with a dramatic decline in sea ice around Antarctica and the repeated reappearance of the so-called Maud Rise Polynya. This is a vast ice-free area in the Weddell Sea, roughly the size of Bavaria, which hadn't been seen since the 1970s.

"Saltier surface water allows heat in the ocean depths to rise more easily and melt the sea ice from below," explains Silvano. "This is a dangerous feedback loop: Less ice leads to more heat, which in turn leads to even less ice." The return of the Maud Rise polynya shows how unusual the current conditions are.

"If this salty, ice-poor condition persists, it could permanently reshape the Southern Ocean—and thus the planet." Open waters, for example, release significantly more energy into the atmosphere than ice-covered areas, which influences the climate regionally and globally.

In the Southern Ocean, cold, fresh surface water overlies warmer, saltier water from the depths, the study states. In winter, when the surface cools and sea ice forms, the density difference between the layers diminishes, causing them to mix more intensively. Heat is transported upward, causing the sea ice to melt from below.

Since the early 1980s, the salinity of the Southern Ocean's surface waters had decreased and stratification had increased, trapping heat in the ocean's interior and increasing sea ice cover, the team explains. Satellite data, along with measurements from robots that move up and down the water column, now show that this trend has reversed.

“A dangerous level of global warming has been reached”

This is what it's about: At the United Nations Ocean Conference in Nice, representatives from 130 countries are discussing how the oceans can be better protected and sustainably managed.

Why it matters: The oceans are suffering from climate change and increasing pollution. Heatwaves are killing corals and melting sea ice in the polar regions, while millions of tons of plastic waste are releasing toxic chemicals into the water and killing marine animals. Added to this is increasing overfishing of the oceans.

Here's what's next: Experts hope that decisions on ocean protection will be made at the Ocean Conference.

What experts hope to achieve from the conference

Until now, climate researchers assumed that Antarctic sea ice cover would remain largely stable, at least for the time being, or even increase slightly as a result of climate change. "Instead, there has been a rapid decline in sea ice—a key reflector of solar radiation—potentially accelerating global warming," said co-author Aditya Narayanan of the University of Southampton.

The mechanisms underlying the trend reversal are still too unclear to draw conclusions about future developments. Continuous satellite observations will be needed to determine whether the world may be experiencing a permanent transition to reduced sea ice cover in the Southern Ocean.

The Southern Ocean's influence on the global climate is enormous. It is one of the most important buffers against global warming, controls key climate processes, and influences sea level.