The snowstorm that battered the South this weekend, leaving thousands without power, was likely exacerbated by climate change, according to leading climate scientists.
It’s a counterintuitive suggestion, because greenhouse gases are trapping heat and causing higher average temperatures. Sometimes winter weather is milder as a result. But in North America, especially the East Coast and the South, colder winds are blowing in with greater frequency because of how Arctic warming is distorting two phenomena: the jet stream, a band of air flowing west to east, and the polar vortex, a wintertime area of cold air near the North Pole.
“When we get a snowstorm hitting the Southeast like this, it’s always because there’s a big southward dip in the jet stream, basically over the Mississippi Valley, that’s the underlying cause of the snowstorm in that region, and kind of all along the East Coast,” Jennifer Francis, senior scientist at Woodwell Climate Research Center in Massachusetts, told Yahoo News. “When it dips southward like that, the Arctic air, the very cold air, can also penetrate very far south.”
This may be happening more often because the Arctic is warming faster than the rest of the Earth. The temperature differential between the Arctic and other regions powers the jet stream. With a decreasing difference in temperature, a weakened jet stream is more easily diverted.
“Climate change is causing the jet stream to take more of these southward dips and northward swings,” Francis said. Each southward dip causes a rebound with an adjacent northward swing. So extreme weather events like cold snaps and storms in one location can be accompanied by dry spells and heat waves somewhere else.
“Out West right now, they’re dealing with, actually, a very dry situation, warm conditions, and these two things always go hand in hand,” Francis said. “We see these bigger swings in the jet stream linked to climate change, and when they happen we always get unusual weather conditions. We can think back to the Texas cold spell last February, we can think back to the Pacific Northwest heat wave this past summer. It’s always one of these convoluted patterns in the jet stream that causes these things.”
The other climate-change-related reason for colder winter weather is that the polar vortex can get stretched out or split, making cold spells in North America and Eurasia more intense and longer lasting. “Right now we’re seeing the polar vortex being stretched a bit, and a lobe of it is coming down over eastern North America, which is strengthening this cold spell,” Francis said.
Recent research suggests that these disruptions of the polar vortex are happening more frequently now and that the cause is loss of Arctic sea ice, Francis said. She likened the polar vortex to a spinning top. If you poke the top, it tilts. Due to climate change, an emerging heat bubble over the Barents Sea near Norway and Russia “acts just like poking that top,” she explained. “It can cause the top to not just wobble, but collapse completely.”
But whereas the causal connection between global warming and extreme summer weather, such as heat waves, is universally accepted among scientists, the effect of climate change on winter weather remains controversial.
“Everybody agrees that when the polar vortex is disrupted, you get an increase in extreme winter weather. The big [question] is: Can changes in the Arctic affect the polar vortex?” Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research (AER), a firm that helps businesses and governments manage climate risks, told Yahoo News. “I’m in the camp that says, yes, absolutely.”
A paper published in Science last year, with Cohen as the lead author, found that Arctic warming is leading to more extreme winter weather in the United States. “The United States and other regions of the Northern Hemisphere have experienced a conspicuous and increasingly frequent number of episodes of extremely cold winter weather over the past four decades,” Cohen and his colleagues wrote.
Southerners have now seen that firsthand. Snowfalls of 13 inches were reported in Boone, N.C., and 8 inches in Spartanburg, S.C. On Sunday, North Carolina state troopers responded to more than 600 car crashes. The energy utility Duke Energy reported that over 67,000 customers suffered power outages in the Carolinas.
“We show [in that paper] that there’s been a very strong, increasing trend in these episodes where the polar vortex stretches, or it kind of acts like a rubber band, or taffy, where it gets pulled apart, so the polar vortex then extends,” Cohen told Yahoo News. “Then the winds around the polar vortex go to much further southern latitudes. It heads towards the equator, and cold air and snowstorms occur when that happens.”
Last February, when a series of unusually severe winter storms left millions of Texans without power or running water, scientists noted that it stemmed from the effects of Arctic warming on the polar vortex and jet stream. “It’s the same mechanism as the Texas freeze,” Cohen said Tuesday about the recent snowstorm.
Climate change also can lead to stronger snowstorms because of increased precipitation and ocean warming. Warmer weather leads to more evaporation of water and heavier rainfall, and warmer ocean temperatures lead to more intense storms. Just as that can mean more heavy rains and hurricanes in the summer, it can lead to strong winter storms, including snowstorms.
Americans should expect these trends to worsen, especially if greenhouse gas emissions continue unabated.
Meanwhile, the storm that crippled the South has now made its way north, dumping up to a foot of snow in parts of the Northeast. That region has experienced increased snowfall in recent years due to Arctic warming, according to research by Francis, Cohen and Karl Pfeiffer, a colleague of Cohen’s at AER. Atlanta, Boston, Des Moines, Detroit, New York, Salt Lake City, Seattle and Washington, D.C., are among the cities receiving significantly more snowfall, according to a 2018 paper published by the trio in Nature.