Extreme winter weather in the US linked to a warming Arctic

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When the Arctic is unusually warm, extreme winter weather is two to four times more likely in the eastern United States, according to new research. It’s too early to tell whether the warming Arctic is causing this severe cold spells and if so, how exactly. But the study shows how global climate change can have ripple effects at the local level, close to home.

Researchers analyzed a variety of atmospheric data in the Arctic, as well as how severe winter weather was in 12 cities across the US from 1950 to 2016. Since 1990, as the Arctic has been warming up and losing ice, extreme cold snaps and heavy snow in the winter have been two to four times more frequent in the eastern US and the Midwest, while in the western US, their frequency has decreased, according to a study published today in Nature Communications. The study, however, only shows there might be a correlation — not a direct causal link — between the warming Arctic and severe winters in the US. And it doesn’t show how exactly the two are connected, so it doesn’t really add much to what scientists already knew, according to several experts.

The Arctic is warming up at unprecedented rates, and the sea ice is melting. At the same time, extreme cold snaps and heavy snowfalls have increased in North America, Europe, and Asia. So there is a forceful debate in the climate science community about how, if at all, the changing Arctic may be driving these weather extremes in the Northern Hemisphere. It’s also unclear if the increase in extreme winter weather is just happening naturally, or because of climate change. Today’s paper doesn’t show that the Arctic is responsible, so it doesn’t put the debate to rest, some experts say.

“It’s not the first paper and it won’t be last to link the warming Arctic to cold winters, but I remain skeptical of that link,” says James Screen, an associate professor in climate science at the University of Exeter, who was not involved in the study. The mechanisms at play are still a mystery, and climate models don’t really support this hypothesis, he tells The Verge. “This is solely based on observations. In the real world, it’s really hard to untangle cause and effect.”

Ted Shepherd, a professor of climate science at the University of Reading, agrees. Observations alone aren’t enough to link extreme weather events to climate change, especially if they’ve been occurring over a regional area for a relatively short period of time. For that, you need models. “I don’t think this paper is really helping add new evidence to the table,” Shepherd tells The Verge.

This year alone, the eastern US has seen record-breaking freezing temperatures, a “bomb cyclone,” and three nor’easters in just 11 days — one of which brought serious flooding to Massachusetts. While the exceptional cold prompted some — including US President Donald Trump — to say it disproves global warming, scientists say it’s exactly the type of weather you’d expect in a warming world. And there are several mechanisms at play. For instance, the increased snowfalls in the northeastern US and mid-Atlantic are in part due to warmer ocean temperatures and stronger coastal storms, which “produce stronger nor’easters like we’ve seen this season, with larger snowfall totals,” says Michael Mann, a climatologist and director of the Earth System Science Center at Pennsylvania State University, in an email to The Verge.

Today’s study focuses on the Arctic as the main culprit for the extreme winter weather. Previous research has suggested that the warming Arctic may disrupt the polar vortex, a ring of swirling cold air circling the North Pole. Think of the polar vortex as a river, says study co-author Judah Cohen, a climatologist and director of seasonal forecasting at Atmospheric and Environmental Research. The fast flow of this river locks up the cold air over the Arctic. But as the Arctic warms — especially in some areas like the Barents-Kara seas north of Europe and Russia — a boulder springs up in this river, disrupting the polar vortex and allowing the freezing Arctic air to flow south, Cohen says. (These cold blasts, for instance, swept Europe last month, bringing snow to Rome for the first time in six years.)

This same mechanism is what’s causing extreme winter weather in the eastern US, according to Cohen. Today’s study, however, only shows there’s a link between the changing Arctic and the severe cold spells in the US, it doesn’t show that one causes the other. That link is “an obvious one,” says Kevin Trenberth, a distinguished senior scientist at the National Center for Atmospheric Research, who was not involved in the research. “The cold air has to go somewhere. The question is where and what is the cause.” Climate models don’t confirm that the warming Arctic is, in fact, driving these winter extremes in the US, so there might be some other mechanisms at play, says Screen, at the University of Exeter. “Either the models are wrong, which is possible, or the interpretation of the observed correlation is wrong,” he says.

Cohen agrees that the research is only showing correlation, not causation, and the paper acknowledges that as well. As for the models, they’re not very good at predicting winter weather in the mid-latitudes. “Just as the observations are flawed so are the models,” Cohen tells The Verge.

The whole debate shows just how much we still don’t know about the complicated mechanisms by which climate change could wreak havoc on our planet. “It’s unequivocal that the Arctic is warming and it’s losing its sea ice, but people may ask, ‘Why should I be bothered about that?’” says Screen. The point of today’s study is to show that climate systems are interlinked, so changes in the Arctic might mean knockoff effects elsewhere. “What happens in the Arctic does not stay in the Arctic,” he says.

2018-03-13 16:00:31

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