Uh oh. Again. It was the Canadian Arctic, it was the Himalayas, now it’s the Antarctic. All the ice that we thought was so locked up is melting much faster than expected.

The vast expanse of sea ice around Antarctica has suffered a “precipitous” fall since 2014, satellite data shows, and fell at a faster rate than seen in the Arctic.

The plunge in the average annual extent means Antarctica lost as much sea ice in four years as the Arctic lost in 34 years. The cause of the sharp Antarctic losses is as yet unknown and only time will tell whether the ice recovers or continues to decline.

But researchers said it showed ice could disappear much more rapidly than previously thought. Unlike the melting of ice sheets on land, sea ice melting does not raise sea level. But losing bright white sea ice means the sun’s heat is instead absorbed by dark ocean waters, leading to a vicious circle of heating.

The two poles work differently.

The loss of sea ice in the Arctic clearly tracks the rise in global air temperatures resulting from human-caused global heating, but the two poles are very different. The Arctic is an ocean surrounded by continents and is exposed to warming air, while Antarctica is a freezing continent surrounded by oceans and is protected from warming air by a circle of strong winds.

Antarctic sea ice had been slowly increasing during the 40 years of measurements and reached a record maximum in 2014. But since then sea ice extent has nosedived, reaching a record low in 2017.

“There has been a huge decrease,” said Claire Parkinson, at Nasa’s Goddard Space Flight Center in the US. In her study, published in the journal Proceedings of the National Academy of Sciences, she called the decline precipitous and a dramatic reversal.

The scientists don’t know why it’s happening.