For decades, sea ice in Antarctica has increased while that in the Arctic has declined drastically. But in a puzzling turn of events, Antarctic sea ice has been decreasing rapidly since 2014, a new study has found. Whether the changes are because of climate change or something else remains to be seen, study author Claire Parkinson of NASA’s Goddard Space Flight Center says.

Parkinson, who analyzed satellite measurements of Antarctic sea ice over a 40-year period from 1979 to 2018, found that the yearly average sea ice extent peaked in 2014. But over the next three years, from 2014 to 2017, the sea ice extent hit its lowest average annual levels. Where the yearly average sea ice extent was a record-high at 12.8 million square kilometers (5 million square miles) in 2014, it reached 10.75 million square kilometers (4 million square miles) in 2017, with a record-low average monthly sea ice extent of 2.29 million square kilometers (0.88 million square miles) in February 2017.

In fact, between 2014 and 2017, Antarctica suffered a precipitous decline, Parkinson writes, losing more yearly average sea ice in just three years than that observed in the Arctic over a period of 33 years.

Is this downward trend going to continue? Researchers aren’t sure.

“But it raises the question of why, and are we going to see some huge acceleration in the rate of decrease in the Arctic? Only the continued record will let us know,” Parkinson told the Guardian.

Despite multiple hypotheses, researchers are yet to figure out why Antarctica’s sea ice extent has generally increased since 1979. The cause of the recent decline, too, is a mystery.

The satellite measurements, for example, showed a small increase in the yearly average sea ice in Antarctica from 2017 to 2018, but there has been a decline in 2019 again. Whether the small uptick in 2018 is a blip in an otherwise long-term downward trend of Antarctic sea ice extent or the start of a rebound, is difficult to say, Parkinson writes in the paper. Moreover, even during the decades of overall increase in Antarctic sea ice, there have been periods of declines followed by an increase.

“There was a period in the 1970s when the Antarctic also had a huge decrease in sea ice and then increased,” Parkinson told New Scientist. “So it could be this huge decrease over a few years [2014 to 2017] is going to reverse.”

Parkinson had previously shown that the increases in Antarctic sea ice through 2014 did not compensate for the rapid loss of sea ice in the Arctic. This was because “the decreases in Arctic sea ice far exceed the increases in Antarctic sea ice,” she told NOAA Climate.gov in March this year.

The Antarctic represents a complex system, and Parkinson says she hopes the 40-year satellite data will spur more research.

“I hope that the 40-y record discussed in this paper will encourage further studies into the atmospheric and oceanic conditions that could have led to the extremely rapid 2014-2017 decline of the Antarctic sea ice cover, the comparably rapid decline in the mid-1970s, and the uneven but overall gradual increases in Antarctic sea ice coverage in the intervening decades,” she writes in the paper.