Arctic winter warming events are becoming far more frequent and long-lasting than they were three decades ago, due to human-driven climate change, a NASA study warns.
Since fall and winter is when Arctic sea ice grows and thickens, warmer winter air temperatures may further impede ice growth and expansion, accelerating the effects of global warming in the Arctic, researchers said.
The team analysed winter air temperatures over the Arctic Ocean from 1893 to 2017 and found that since 1980, an additional six Arctic winter warming events are occurring each winter at the North Pole.
Researchers, including those from NASA's Goddard Space Flight Center in Greenbelt in the US, also noted that these events are lasting about 12 hours longer than usual, on average.
Since 1980, the number of winter warming events in the North Pole each year has more than doubled, from fewer than five events to more than 10 events, on average, and the average length of each event has grown from fewer than two days to nearly two and a half days, researchers said.
In December 2015, scientists recorded a temperature of 2.2 degrees Celsius in the Central Arctic, the warmest temperature ever recorded in this region from December through March.
"These events are not unusual, but they are happening more frequently and with longer durations," said Robert Graham, a scientist at Norwegian Polar Institute in Norway.
Researchers attribute the increase in warming events to an increase in major storms in the Arctic.
In recent studies each warming event was associated with a major storm entering the region. During these storms, strong winds from the south blow warm, moist air from the Atlantic into the Arctic, Graham said.
"The warming events and storms are in effect one and the same. The more storms we have, the more warming events, the more days with temperatures greater than minus 10 degrees Celsius rather than below minus 30 degrees Celsius, and the warmer the mean winter temperature is," he said.
Human-caused climate change could also be driving the increase in warm temperatures, said Julienne Stroeve, professor at University College London in the UK.
"This new study shows that these warm events have occurred in the past, but they were maybe not as long-lasting or frequent as we are seeing now," said Alek Petty from NASA's Goddard Space Flight Center.
"That, combined with the weakened sea ice pack, means that winter storms in the Arctic are having a larger impact on the Arctic climate system," he added.
Researchers gathered data from field campaigns, drifting weather stations and buoys across the Arctic Ocean from 1893- 2017 and analysed the ERA-Interim record, a global atmospheric reanalysis provided by The European Centre for Medium-Range Weather Forecasts (ECMWF), from 1979-2016.
The study was published in the journal Geophysical Research Letters.