The loss of sea ice over the past 1000 years is linked to the loss of Antarctica

At the beginning of the 21st century, the UK recorded a huge area of sea ice that hadn’t existed in its modern form for 10,000 years. This was a discovery that stunned scientists, but wasn’t tied directly to climate change, and it remained under the spotlight for years.

Then a new study published this week came out and outed the source: a different frozen continent. Scientists have long known that the Arctic Ocean had undergone surface ice-enriching drift before, but this new study pinpoints where: the newly-discovered northern edge of Antarctica, which IceBridge and NASA have mapped as they’ve scanned the continent more than 20 times since 2010.

There’s a rare time lapse of an IceBridge observation on Antarctica in 2012. Image: NASA

The period that led to the widespread disappearance of sea ice is known as the southern extension of the Little Ice Age, which occurred roughly 250-700 years ago. This side of Antarctica has some of the ocean’s largest islands, which formed in a cooling of the planet, caused by the confluence of a wind pattern known as the Younger Dryas and an increase in the interaction between the ocean and atmosphere, known as the South Atlantic meridional overturning circulation. The resulting ice sheets and polar winds meant that a lot of the Atlantic Ocean, including the Antarctic Peninsula, was relatively ice-free.

Not only that, but the ice sheets were destabilised by wind changes, namely the change in their gradient and the results of sea level rise. When the air cools and dries, ice is less mobile than ice that melts and solidifies in cold temperatures. The amount of ice floating on the surface of the continent did not remain frozen, which left the prevailing winds and circulation in place with one significant effect: A lack of sea ice.

The sudden speed at which the sea ice disappeared is what makes it so surprising. In the 1995 Climate Change Synthesis Report, scientists estimated that by about the 1850s Antarctica lost around 20 per cent of its sea ice a year. The winds and circulation of the Southern Ocean did change significantly, but how quickly Antarctica lost sea ice was the question.

And now it has been answered.

“This is the icing on the cake. When I published my original paper, I looked forward to writing this follow-up study,” lead author William Connolly, the Pelham Professor of Oceanography at the University of Southampton, told Gizmodo in an email.

The shift in ice-dwelling circulation allowed for an offshore circulation that came from above, to drop huge amounts of ice-cap-carrying water near the Antarctic Peninsula. A cut-off from the continent pushed the ice stream to make more rapid and extensive spread downstream, dumping tons of snow and ice into the ocean. The result is what Connolly calls a “massive iceberg of a floating sea ice shelf.”

There were two primary causes to the reduction in sea ice, he said. One was wind variability, but the other was a shift from an older, more dense, ice cap to a different one with more simple waters beneath. He linked the east to west nature of the shift, “stronger winds in the east from Antarctica, and weaker winds in the west from Greenland,” leading to a broader change.

This latter finding is great news for people who worry about the coming climate change storm season.

“This explains why the current warming of the Arctic Ocean is so abrupt compared to the gradual transformation of the southern ocean a half millennium ago,” Connolly said. “But even more than that, it says that you can give up worrying about the status of the Antarctic ice cap at the moment, because it’s mostly gone.”

He cited sea ice extent “becoming in a large measure almost as thin as the ice trapped in Antarctica,” or “a 55-degree drop in a fortnight,” as two causes for the rapid decline.

During periods of rapid ice loss, the cloud of sunlight that blocked out the Sun and prevented snow from collecting is less extensive, which “strengthens the older ice and blocks more snow from melting in the months ahead,” Connolly said.

“That’s the message I want to bring home,” he said. “Antarctica, the little brother, is the giant brother in our system.”

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