It’s polar bear weather today for much of the Midwest. Temperatures are in the -20sº F (-28º C) and -30sº F (-35º C) in eastern Montana, North Dakota, northeast South Dakota, Minnesota and northern Iowa. With the stiff wind, it’s even worse—wind chills in the -40sº F (-40º C) and -50sº F (-45º C) are common across Minnesota and North Dakota, cold enough for exposed skin to suffer frostbite in just five minutes. By tonight, the freeze will reach the East Coast, where temperatures from Florida to Maine are expected to be 30º F to 40º F (16º C to 22º C) degrees below normal, extremes that haven’t been seen in decades. The National Weather Service isn’t kidding when it calls the cold “life-threatening.”
Unsurprisingly, the extreme cold has brought out the climate change skeptics, who point to the freeze and the recent snowstorms and say, essentially, “nyah-nyah.” Now this is where I would usually point to the fact that the occasional cold snap—even one as extreme as much of the U.S. is experiencing now—doesn’t change the overall trajectory of a warming planet. Weather is what happens in the atmosphere day to day; climate is how the atmosphere behaves over long periods of time. Winters in the U.S. have been warming steadily over the past century, and even faster in recent decades, so it would take more than a few sub-zero days to cancel that out.
But not only does the cold spell not disprove climate change, it may well be that global warming could be making the occasional bout of extreme cold weather in the U.S. even more likely. Right now much of the U.S. is in the grip of a polar vortex, which is pretty much what it sounds like: a whirlwind of extremely cold, extremely dense air that forms near the poles. Usually the fast winds in the vortex—which can top 100 mph (161 k/h)—keep that cold air locked up in the Arctic. But when the winds weaken, the vortex can begin to wobble like a drunk on his fourth martini, and the Arctic air can escape and spill southward, bringing Arctic weather with it. In this case, nearly the entire polar vortex has tumbled southward, leading to record-breaking cold, as you can see in this weatherbell.com graphic:
That disruption to the polar vortex may have been triggered by a sudden stratospheric warming event, a phenomenon Rick Grow explained at the Washington Post a few days ago:
Large atmospheric waves move upward from the troposphere — where most weather occurs — into the stratosphere, which is the layer of air above the troposphere. These waves, which are called Rossby waves, transport energy and momentum from the troposphere to the stratosphere. This energy and momentum transfer generates a circulation in the stratosphere, which features sinking air in the polar latitudes and rising air in the lowest latitudes. As air sinks, it warms. If the stratospheric air warms rapidly in the Arctic, it will throw the circulation off balance. This can cause a major disruption to the polar vortex, stretching it and — sometimes — splitting it apart.
What does that have to do with climate change? Sea ice is vanishing from the Arctic thanks to climate change, which leaves behind dark open ocean water, which absorbs more of the heat from the sun than reflective ice. That in turn is helping to cause the Arctic to warm faster than the rest of the planet, almost twice the global average. The jet stream—the belt of fast-flowing, westerly winds that essentially serves as the boundary between cold northern air and warmer southern air—is driven by temperature difference between the northerly latitudes and the tropical ones. Some scientists theorize that as that temperature difference narrows, it may weaken the jet stream, which in turns makes it more likely that cold Arctic air will escape the polar vortex and flow southward. Right now, an unusually large kink in the jet stream has that Arctic air flowing much further south than it usually would.
Still, this research is fairly preliminary, in part because extreme Arctic sea ice loss is a fairly recent phenomenon, so scientists don’t have the long data sets they need to draw more robust conclusions about the interaction between Arctic warming and cold snaps. In fact, the most recent report from the Intergovernmental Panel on Climate Change concluded that it was likely that the jet stream would shift towards the north as the climate warmed, and that the polar vortex would actually contract, even as a 2009 study found that sudden stratospheric warming events are becoming more frequent, which in turn seems to be driven by the rapid loss in Arctic sea ice.
And while a muddle like that would seem to make the science less rather than more reliable, it’s actually one more bit of proof that climate change is real. Global warming is sometimes thought of more as “global weirding,” with all manner of complex disruptions occurring over time. This week’s events show that climate change is almost certainly screwing with weather patterns ways that go beyond mere increases in temperature—meaning that you’d be smart to hold onto those winter coats for a while longer.