Henry Fountain

The 2016 polynya and the storms were no doubt linked, Dr. Turner said, as the rough conditions served to break up the sea ice.

“But that would just freeze over again,” he said. “You need an ocean element to keep it open.” That came in the form of warmer deep-ocean waters that rose to the surface, keeping the polynya from refreezing.

With less ice in the Weddell in the winter of 2016-17, a feedback mechanism came into play. Ice reflects most of the sun’s rays, while the darker ocean absorbs more of them. As a result the ocean warms, which in turn causes more sea ice to melt, leading to more open water and still more ocean warming and melting ice. It’s the same phenomenon that occurs in the Arctic, and has led to steep declines in sea ice extent there over the last four decades.

In the Weddell, the researchers said, the initial loss of sea ice that winter led to a rise in the sea temperature of about 0.5 degree Celsius, or 0.9 degree Fahrenheit. That led to the near-runaway melting of ice during the rest of the winter, and is why summer ice coverage has only slightly increased in the three summers since, including this year.

But that leads back to the second question: Will this reduced summer coverage in the Weddell persist, or will the sea ice eventually recover?

“It might rebuild,” Dr. Turner said. “We just don’t know.”

The recent history of Antarctic sea ice is much different from that of the Arctic, Dr. Turner said. While Arctic sea ice has been steadily declining, Antarctic sea ice was growing until about 2014, largely because of the presence of a large ozone hole over the region that led to colder conditions.

Since then, sea ice around the continent has been declining overall, with the drop in the Weddell accounting for much of the change. And with the ozone hole slowly healing, most climate models are forecasting that Antarctic sea ice will continue to decline. What’s happened in the Weddell may be a sign of that emerging long-term decline.

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