The move to a warmer Arctic in the winter months is simply explained, scientists believe—by a matching growth in stormy weather. Warm spells when the thermometer goes above minus 10°C always happened—but now, according to new research, these mellow moments happen more often, and for longer.
Norwegian, US and German glaciologists report in the journal Geophysical Research Letters that for the last three winters polar temperatures have risen to almost freezing. That is, one of the most extreme places on the planet, condemned to a night that lasts for many weeks, has from time to time been warmer than New York or London in winter.
The Arctic region is, notoriously, the fastest warming place on the planet: As global average temperatures rise, in response to ever greater levels of greenhouse gases in the atmosphere, the thermometer rises even faster in the far north.
Towards the end of 2016, scientists reported temperatures an estimated 20°C above what used to be considered normal, and researchers have predicted that at present rates of fossil fuel combustion, the Arctic ocean could be more or less ice-free in summer by 2050.
But the latest study takes the long view: Scientists report that they analyzed field readings, and data from drifting weather stations across the Arctic Ocean from 1893 to 2017.
Although Arctic winter temperatures are typically often as low as minus 30°C, at the end of December 2015 temperatures in the central Arctic rose to 2.2°C, the warmest ever recorded in the winter months from December to March.
And though warm spells have always happened, as hot moist air is driven northward from the temperate zones, they now happen more often: an additional six episodes in recent years, compared to 1980.
Such sudden warm moments used to last less than two days: Now typically they last nearly two and a half days. They have been linked to storms.
“The warming events and storms are in effect one and the same," said Robert Graham, a climate scientist at the Norwegian Polar Institute in Tromsø, Norway, and lead author. “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.”
“This new study provides the long-term context we were missing.
It shows that these warm events have occurred in the past,
but they were not as long-lasting or frequent as we're seeing now”
Storms bring warm air that prevents ice from freezing, breaks up existing ice cover, and deposits snow that then insulates the ice below it from the returning cold atmosphere.
Two of the authors, Alek Petty and Linette Boisvert of NASA, tracked such a storm in the winter of 2015–2016.
“That particular cyclone, which lasted several days and raised temperatures in the region close to the melting point, hindered sea ice growth while its associated strong winds pushed the sea ice edge back, leading to a record low spring sea ice pack in 2016,” they said in a statement.
“This new study provides the long-term context we were missing, using direct observations going back to the end of the 19th century,” the researchers continued. “It shows that these warm events have occurred in the past, but they were not as long-lasting or frequent as we're seeing now. 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.”