The polar vortex came swirling back into the U.S. this week, bringing with it record cold temperatures not seen in 20-plus years, and wind chills capable of causing frostbite in mere minutes.
Why it matters: It was a reminder that it's possible to have extreme cold in a warming world. That's partly because there will always be weather variability. But there's also some evidence that, paradoxically, global warming may be leading to more frequent disruptions of the polar vortex — which can cause extreme cold and high-impact winter storms across the U.S., Europe and Asia.
The details: The main questions revolve around how Arctic climate change is altering the major circulation patterns in the Earth's atmosphere.
Not only are melting sea ice and warming oceans causing more heat and moisture to escape into the atmosphere in the Far North, but increasingly, mild and wet air masses from lower latitudes are intruding into the Arctic as well.
- These invasions are made possible by disappearing sea ice, since open ocean allows air masses to have higher temperatures and humidity than they may have had in the past.
- And Arctic climate change is happening more than twice as fast as climate change in the rest of the world, thanks to a process known as Arctic amplification, which refers to positive feedbacks in the Arctic climate system.
- For example, when sea ice melts, it exposes darker ocean water to incoming sunlight, which warms the water and then the air, melting more ice, and so on.
Following the Las Vegas-like adage of "What happens in the Arctic doesn't stay in the Arctic," both of these dynamics may be changing weather patterns far outside the Arctic.
- There are many studies showing statistical correlations between long-term climate change, sea ice loss in parts of the Arctic, and changes to the polar vortex.
- Scientists are now trying to nail down the physical cause-and-effect mechanisms behind these links.
- Some researchers are more sold on this than others.
What they're saying: Judah Cohen, director of seasonal forecasting at AER, a Verisk company, argues that less sea ice in the Barents and Kara Seas — a region between Finland and Russia — tends to strengthen a high-pressure area across northwestern Eurasia.
Another part of the chain reaction, of sorts, is increasing Siberian snow cover in the fall, which tends to favor low pressure in East Asia and the North Pacific. Both of these trends can alter the airflow in the upper atmosphere and knock the polar vortex off kilter, Cohen says.
- Cohen's studies show that polar vortex disruptions may be happening more frequently, and are behind an observed increase in winter cold spells in Eurasia. They're also tied to increased late winter storminess in the Northeast U.S., he's found.
Michael Mann, director of the Earth System Science Center at Penn State University, says climate models don't properly capture some of the key jet stream processes driving extreme weather events such as polar vortex disruptions.
"I would say that this particular debate is about where the debate about detectability of global warming was back in the early 1990's — honest scientists can legitimately differ based on reasonable interpretations of the evidence to date."— Michael Mann, Penn State University
Dim Comou, a climate scientist at the Potsdam Institute for Climate Impact Research, says computer models do show a link between low sea ice in the Barents and Kara Seas and more frequent weak polar vortex states, but they underestimate the strength of this tie-in.
The bottom line: Jennifer Francis, a senior scientist at the Woods Hole Research Center and an early proponent of the idea that rapid Arctic warming is leading to more extreme weather events, calls the link between warming and polar vortex disruptions "complicated and still controversial."
Still, she says, "Evidence is piling up that disruptions of the stratospheric polar vortex are happening more often and global warming is a player."