WASHINGTON • After mysteriously expanding for decades, Antarctica's sea ice cover melted by an area four times greater than France in just a few years and now stands at a record low, according to a study.
Scientists already knew Antarctica was thawing at an increasing rate, like the Arctic, because of accelerating discharge from glaciers - the rivers of ice that push up slowly against the shore.
But between 1979 and 2014, they observed a phenomenon that was both intriguing and reassuring: the sea ice cover was expanding.
From 2014 to 2017, however, "the Antarctic lost almost as much as the Arctic" over almost 40 years, Nasa climatologist Claire Parkinson told Agence France-Presse, and the trend has continued ever since.
From a peak area of 12.8 million sq km, the sea ice cover receded 2 million sq km for reasons that remain unknown.
"It went from its 40-year high in 2014, all the way down in 2017 to its 40-year low," said Dr Parkinson, whose findings were published on Monday in the Proceedings of the National Academy of Sciences.
The team had analysed microwave measurements from US space agency Nasa and military satellites over the period to build up the most precise picture to date of the historic sea ice cover, measuring only its area but not thickness.
Neither the reason for the earlier expansion nor the current decline is well understood.
Competing hypotheses exist, pinning the changes on everything from the hole in the ozone layer to shifting winds and ocean currents, but it is far from clear cut.
"None of the hypotheses are good in my opinion," said Dr Douglas Martinson, an oceanographer from Columbia University, one of the paper's peer reviewers.
But he cautioned against trying to apply findings from the Arctic to the Antarctic, saying it would be "like comparing apples to army trucks".
The Arctic is an ocean surrounded by land, while the Antarctica is a continent surrounded by oceans, where icebergs are less constrained.
While parts of western Antarctica are warming, the continent remains the coldest place on Earth, as well as its largest source of freshwater.
Its ice-covered mountains are capable of raising the level of the oceans by 57m, according to a 2013 study.
Professor Chris Rapley, a climate scientist from the University College of London, said the previous gains did not in any way undermine the thesis of global warming.
He said: "It simply demonstrates that in a complex, interconnected system, counter-intuitive outcomes can occur - at least for a while.
"We have a tendency to seek simplistic explanations of cause and effect, when in reality the situation is much more complicated and nuanced."