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 is too early to tell whether warming Arctic is causing these severe cold spells, and if so, how exactly. But the study shows how global climate change can have pervasive effects locally, close to home.
The researchers analyzed a variety of atmospheric data in the Arctic, as well as how severe the winter weather was in 12 cities in the US from 1950 to 2016. Since 1990, when the Arctic warmed and lost ice, extreme cold pops and Heavy snow in the winter has been two to four times more frequent in the eastern United States and in the Midwest, while in the western United States, their frequency has decreased, according to a study published today in Nature Communications. . The study, however, only shows that there could be a correlation, not a direct causal relationship, between Arctic warming and severe winters in the U.S. 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.
"In the real world, it's really hard to unravel cause and effect."
The Arctic is warming at an unprecedented rate, and the sea ice is melting. At the same time, extremely cold pops and heavy snowfall have increased in North America, Europe, and Asia. So there is a vigorous debate in the climate science community about how, if at all, the changing Arctic may be leading to these climate extremes in the Northern Hemisphere. It's also unclear whether the increase in extreme winter weather is simply happening naturally or due to climate change. Today's document does not show that the Arctic is responsible, so it does not put the debate to rest, some experts say.
"It is not the first document and it will not be the last to link the warm Arctic to cold winters, but I remain skeptical of that link," says James Screen, associate professor of 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 based solely on observations. In the real world, it's really difficult to untangle cause and effect. "
Ted Shepherd, professor of climate science at the University of Reading, agrees. Observations alone are not enough to link extreme weather events to climate change, especially if they have been occurring in a regional area for a relatively short period of time. For that, you need models. "I don't think this document really helps add new evidence to the table," Shepherd tells The Verge.
This year alone, the eastern US has seen record freezing temperatures, a "bomb cyclone" and three natural disasters in just 11 days, one of which brought severe flooding to Massachusetts. While the exceptional cold led some, including US President Donald Trump, to say it disproves global warming, scientists say it is exactly the kind of weather to expect in a warming world. And there are several mechanisms at play. For example, the increase in snowfall in the northeastern US and in the mid-Atlantic is due in part to warmer ocean temperatures and stronger coastal storms, which “produce heavier snowfall like we've seen this season. , with huge total snowfall, ”says Michael Mann, a climatologist and director of the Earth System Science Center at Pennsylvania State University, in an email to The Verge.
“Cold air has to go somewhere. The question is where and what is the cause ”.
Today's study focuses on the Arctic as the main culprit for extreme winter weather. Previous research has suggested that warming Arctic may disrupt the polar vortex, a ring of cold air eddies that surrounds the North Pole. Think of the polar vortex as a river, says study co-author Judah Cohen, a climatologist and director of seasonal forecasts at Atmospheric and Environmental Research. The rapid flow of this river blocks cold air over the Arctic. But as the Arctic warms, especially in some areas like the Barents-Kara seas in northern Europe and Russia, a rock emerges in this river, disrupting the polar vortex and allowing frozen Arctic air to flow south, says Cohen. (These cold blasts, for example, swept across Europe last month, bringing snow to Rome for the first time in six years.)
This same mechanism is what is causing extreme winter weather in the eastern U.S., according to Cohen. Today's study, however, only shows that there is a link between the changing Arctic and severe cold spells in the US, but 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. “Cold air has to go somewhere. The question is where and what is the cause. "Climate models don't confirm that Arctic warming is, in fact, driving these winter extremes in the US, so there could 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 only shows correlation, not causality, and the paper acknowledges this as well. As for the models, they are not very good at predicting winter weather in the mid-latitudes.
"Just as observations are flawed, so are models," Cohen tells The Verge.
The whole debate shows 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 losing its sea ice, but people may ask, 'Why should I worry about that?'" Screen says. The goal of today's study is to show that climate systems are interrelated, so changes in the Arctic could mean side effects elsewhere. "What happens in the Arctic does not stay in the Arctic," he says.
By Alessandra Potenza
Original article (in English)