This winter, weather reports have been full of extremes. California, for instance, has had the driest year in 119 years of recordkeeping, with the Sierra snowpack less than a third of normal.
The January cold snap across the eastern two-thirds of the United States sent temperatures plummeting 20 to 40 degrees Fahrenheit (11 to 22 degrees Celsius) below average. Across the Atlantic, England and Wales experienced one of the wettest periods in at least 248 years.
Drought. Frigid cold. Devastating rain and floods. On the surface, they don't seem to have much in common beyond the unpleasantness of bad weather. But some researchers argue that these recent events can be tied to a single weather pattern—one that may be caused by climate change.
Jennifer Francis, an atmospheric scientist at Rutgers University in New Jersey, suggests that more persistent weather patterns—for example, cold snaps, drought, and flooding—are connected to Arctic amplification, the accelerated rise in temperature in the far north, which has warmed up faster than anywhere else on the planet. As the Arctic warms, the difference in temperature between it and lower latitudes weakens the westerly winds of the polar jet stream. Instead of blowing strong and straight from west to east, the jet stream now takes a wavier path around the Northern Hemisphere.
When this happens, "weather patterns tend to change more slowly," says Francis. In 2012 she and Stephen Vavrus published an influential paper arguing that a slower, wavier polar jet might lead to prolonged extreme weather, such as drought, flooding, cold spells, and heat waves.
"This winter is a great example of that," Francis says. "A huge northward swing in the jet stream made it very warm in Alaska. It blocked Pacific storms from coming into California and contributed to dry conditions. And then on its way south over the Eastern two-thirds of the United States, it brought cold Arctic air to hang out for a while."
In the Atlantic, the polar jet's wavier path brought it farther south and closer than usual to the subtropical jet, which flows at higher altitudes and carries a lot of moisture. "When this happens," says Francis, "big storms usually result."
Great Britain bore the brunt, with heavy rains, strong winds, and high waves which led to extensive flooding. In a report issued this month, the UK Met Office found that "these extreme weather events ... were linked to a persistent pattern of perturbations to the jet stream over the Pacific Ocean and North America." Those same perturbations also brought warm weather to the Winter Olympics at Sochi.
"We can't say this is clearly linked to the hypothesis of Arctic amplification," says Francis, "but we do see very tantalizing pieces of evidence lining up in the same direction." She received a boost from the White House when John Holdren, President Obama's science advisor, made a popular short video explaining the polar vortex; the video relied heavily on her theory.
Despite that stamp of approval, many scientists remain skeptical. "The wavy jet stream is very much associated with the large anomalous weather," says Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research. The California drought, polar vortex cold snap, and British storms are all associated with it, he says. But he dismisses Francis's hypothesis that Arctic amplification is the cause of the jet's anomalous path. "I don't think it has any scientific basis," he says. "The fluctuations in how wavy the atmosphere is don't require any Arctic influences in order to happen."
Indeed, in a strongly worded letter to Science magazine, Trenberth and four fellow climate scientists described the attention that Francis's hypothesis has been getting, especially as an explanation for the southern cold snaps, as "inappropriate and a distraction."
Trenberth himself, however, has long pointed to the role of climate change in producing extreme weather, and he sees it influencing the recent extremes in other ways. Warming sea temperatures to the south of England have led to more evaporation, more moisture in the air, and thus heavier rains. The effects of the drought in California are worsened because the additional heat from global warming evaporates more soil moisture that isn't replaced by precipitation.
There is wide scientific consensus that global warming promotes weather extremes in those ways, even if the jury is still out on Francis's hypothesis of an Arctic connection. "We used to say you can't attribute any single event to climate change," says Thomas Peterson of NOAA's National Climatic Data Center. Now, he says, scientists emphasize how climate change is tilting the odds toward extreme events, the way steroids pump up a baseball player: "You know he's hitting 20 percent more home runs, but you don't know if a specific home run is a result of the steroids."
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