Last week's arctic temperatures may have shocked millennials, but Minnesotans of a certain age swear they can remember such frigid spells as fairly regular events.
They're right, and climatologist Kenny Blumenfeld can explain why.
Five or six decades ago, the polar vortex — the thick mass of cold low-pressure air that swirls counterclockwise around the North Pole — would spill bone-chilling air down to Minnesota every two to three years.
"This one is as bad as we've had in three decades," said Blumenfeld, senior climatologist in the Minnesota State Climatology Office.
And for anyone who developed doubts about global warming in last week's deep freeze, Blumenfeld says not so. In his mind, the data demonstrate that climate change is real: Decades have passed since Minnesota trudged through its last extreme deep freeze.
"These used to be much more regular occurrences," he said.
Even the dangerous windchills recorded last week — they hit 63 below in Alexandria on Tuesday — did not set an absolute record, Blumenfeld said. "We think there was probably a negative-71 degrees in 1982 in northwest Minnesota," he said. "Our cold events are less intense."
Blumenfeld thinks that, as the Earth grows warmer, the severe deep freezes will become more rare.
Other researchers, however, say climate science is so complicated that it's difficult to tease firm predictions from the masses of data.
Arctic weather plunges into North America
Desperately cold weather is now gripping the Midwest and Northern Plains of the United States, as well as interior Canada. The culprit is a familiar one: the polar vortex. A large area of low pressure and extremely cold air usually swirls over the Arctic, with strong counter-clockwise winds that trap the cold around the Pole. But disturbances in the jet stream and the intrusion of warmer mid-latitude air masses can disturb this polar vortex and make it unstable, sending Arctic air south into middle latitudes.
Source: Earth Observatory; NASA Goddard Space Flight Center.
Some current research, for example, suggests that global warming may be changing the jet stream, causing it to disturb the polar vortex in such a way that the arctic blasts could again become more common, despite generally warming winters.
Exactly how climate change affects the jet stream, a narrow band of strong wind in the upper atmosphere, is a topic of intense research, said Tracy Twine, who teaches atmospheric science at the University of Minnesota.
The jet stream helps contain the frigid Arctic air over the North Pole.
But it has always been wavy — meandering off course from time to time and allowing subzero Arctic air to bulge south.
Scientists are studying whether the Earth's rising temperatures might be causing the jet stream to wander even more, Twine said, leading to more frequent leaks of severe cold to more southerly latitudes.
"We don't know enough about the science to be able to predict whether this is going to happen more or less in the future," said Twine.
"We're pretty confident that overall, the winters here are averaging warmer. We just don't know how these colder outbreaks are going to change."
One group of researchers suggests that rapid loss of ice in the Barents and Kara seas off northwest Russia, and the heat rising from the open waters, amplifies a naturally occurring wave in the jet stream.
That disrupts the polar vortex in just such a way, they suspect, that certain parts of the globe are getting clobbered by arctic air more frequently.
Last week the polar vortex actually split from one spinning mass of cold air into two separate pools, said Jennifer Francis, senior scientist at the Woods Hole Research Center in Massachusetts.
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One of the pools parked over middle North America. The two parts have since merged again.
Francis said she can't say the ice loss caused last week's arctic outbreak in the Midwest, but said "it's certainly consistent with the new research, this new hypothesis." The climate-change link to the polar vortex, she said, is supported by "a handful of studies by a few dozen scientists." But it's gaining traction, she added.
Judah Cohen, a climate researcher at Atmospheric and Environmental Research, a consulting firm in Massachusetts, and a visiting scientist at MIT, said polar vortex disruptions are becoming more frequent.
The severe winter weather typically occurs after a split vortex reassembles, with effects felt mostly in North America east of the Rockies, in Europe and in East Asia.
In fact, a second wave of arctic temperatures is expected this week, Cohen said, but it is related to a polar vortex split that occurred earlier in January.
These disruptions don't always lead to severe cold outbreaks.
But Cohen said he thinks they may be preventing Minnesota's winters from warming as fast as they otherwise would.
It may be cold comfort, but he added: "Our winters would be warmer if we weren't getting these increased polar vortex disruptions."