Super-warm Arctic winter 'extremely unlikely' without climate change, scientists say

The Arctic continues to amaze.

Hit by a second bout of extremely warm winter temperatures in recent days, the seasonal growth of floating sea ice has flattened out, just as it did when hit by similarly dramatic heat in November.

The area of the Arctic Ocean covered by sea ice is currently far smaller than it was in 2012 at this time of year. And while 2012 holds the all-time record for lowest ice extent in September, 2016 has been beating it since mid-October.

The current extent of Arctic ice sea also is far smaller than it was at the same time in 2010, which previously held the record for lowest Arctic sea ice extent in late December, according to records from the National Snow and Ice Data Center that date to 1979.

Clearly this is abnormal. But how abnormal?

The answer is that what has happened this year in the Arctic, and particularly the high Arctic, appears to be not only out of the norm for a stable climate — like the one on Earth before the era of fossil fuels — but also for what you might expect from our supercharged, artificially warmed climate.

Such is the upshot of a recently published "detection and attribution" analysis of November's and December's Arctic warmth by a group of scientists who are not waiting around for the peer review process – they are increasingly releasing these documents in near real time. And they find that "it is extremely unlikely that this event would occur in the absence of human-induced climate change."

The analysis was conducted by a World Weather Attribution, a consortium of researchers who are perfecting the study of how a changing climate affects local weather conditions, or conditions in a specific place.

It came out last week, but the researchers say that in light of another burst of Arctic warmth since, the analysis holds up. If anything, the ensuing temperatures were slightly hotter than they'd expected.

"We probably slightly underestimated the temperature as a whole for November-December, but not by much," said Andrew King, a researcher at the University of Melbourne who worked on the study.

An "attribution" analysis of this type uses a variety of techniques in order to determine the odds of a particular event occurring with or without human interference in the climate. In this case, the scientists used three methods that all came to the same broad conclusion, King said.

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One was simply to look how far outside of past temperatures these Arctic temperatures were. But the researchers also used climate change models, including and excluding human caused greenhouse gas emissions, to try to determine the odds of this event occurring with or without human interference in the climate.

The event under study was the Arctic extreme temperatures of this winter, especially around the North Pole, which the researchers describe, in part, as follows:

Mid-November saw an early winter "heat wave" with the temperature on November 11 reaching -7 ºC (19 ºF) – that is 15 ºC (27 ºF) above normal for the time of year. The monthly mean November temperature was 13 ºC (23 ºF) above normal on the pole. Temperatures in this region declined slightly after that but remained well above normal.

So how unusual is this?

The analysis found that the temperatures above 80 degrees North latitude were "unprecedented in the satellite era from 1979 onwards." It also found that these temperatures would be highly unlikely in a climate that is unperturbed by human influences and that even in our current climate, heavily influenced by humans, they are pretty unexpected.

"We found that in our natural simulations, those without any human influences, we didn't see Arctic winters as warm as this at all," King said. "In our simulations that kind of represent the world of today, including human forcings, it was a roughly one in a 200 year event. So it's a very unusual event even in the current climate model simulations. Even including human forcings, it's right at the tail of the distribution. It's an extreme."

Since this was a near real-time analysis that was not subjected to peer review, I checked with several outside researchers to get their thoughts on its validity.

"I find the study very persuasive because they addressed the same question using three different approaches and got consistent answers," says Phil Duffy, the president of the Woods Hole Research Center.

Noah Diffenbaugh is a Stanford climate scientist who performs similar analyses in the attribution of extreme events to climate change.

"I really admire the World Weather Attribution group for taking the lead on producing these near real-time rapid analyses using techniques and methods that have been peer reviewed," he said, "and I think that's really important."

As for the event itself, Diffenbaugh pointed out that the Arctic is responding extremely rapidly to climate change — more rapidly even than scientists had predicted.

"One interesting thing in looking at the history of the peer reviewed literature over the last decade and a half is that the climate models have, as a whole, have really been less sensitive, in the Arctic, than what's happened in the real world," he said.

But perhaps some of the strongest words came from Mark Serreze, who heads the National Snow and Ice Data Center, a top source for information on the state of the Arctic. Serreze said by email that notwithstanding what happened this November and December, what's going on in the Arctic is well outside of the norm and has been for a while. It's worth quoting in full, because it captures well what scientists see as a new "normal":

This is only the most recent remarkable event that we've seen in the Arctic over the past decade.

Last winter saw another impressive heat wave, when in late December, temperatures at the North Pole almost reached the melting point. The seasonal maximum sea extent of last March was the lowest ever seen. Many people thought that we'd never see as little sea ice in the Arctic as we did in September 2007, then along came 2012 which blew that record out of the water.

There have been rain on snow events in winter, resulting in massive die-offs of reindeer. As some point, one has to admit that the string of remarkable events in the Arctic is more than just a string of unrelated coincidences.