News•February 24, 2014

Arctic Warming, Extreme Weather Link Far From Certain

The much ballyhooed polar vortex that brought frigid weather and snow to much of the U.S. in early January, and made headlines a few weeks later, had plausible links to global warming — according to some scientists, anyway. The idea is that rapid climate change in the Arctic has disrupted the jet stream, leading to spates of extreme weather in more temperate latitudes.

It makes sense to John Holdren, the White House science advisor and Harvard physicist: “A growing body of evidence suggests that the kind of extreme cold being experienced by much of the United States as we speak is a pattern that we can expect to see with increasing frequency as global warming continues,” he said in a video released in January. “Some people ask, ‘how can changes in the Arctic have such huge effects?’” said Rutgers atmospheric scientist Jennifer Francis, who along with a colleague proposed the theory back in 2012 in an interview. “My response is ‘how can you lose 75 percent of sea ice in the Arctic [in summer] and not have a large-scale impact?”

Visualization of the record low Arctic ice melt in 2012 which reached its minimal extent on September 16. Credit: NOAA

But the nature of that impact is far from clear, say other scientists, arguing that the theory is tentative, at best. In August of 2013, another study argued that the Arctic is not affecting weather further south. And in the Feb. 14, 2014 issue of Science, five prominent climate researchers wrote that “it’s an interesting idea, but . . . the links between Arctic warming and mid-latitude weather are not supported by other observational studies.”

James Screen, an atmospheric scientist at the University of Exeter, in England, agrees. “It’s perfectly reasonable to put out this hypothesis,” he said in an interview. “But the idea has gotten a high profile in the media, which is perhaps not warranted. I’m not saying it’s definitely wrong, but it as a lot of uncertainties and unanswered questions.”RELATEDPolar Vortex in U.S. May be Example of Global Warming
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No one argues with the basic premise that the Arctic is warming. In recent decades, the region around the North Pole has been heating up twice as rapidly as the rest of the globe, in a process known as arctic amplification. Like the rest of the planet, the Arctic is being warmed by heat-trapping greenhouse gases. But the effect is boosted by several factors. The increasing loss of sea ice in summer is one: it exposes relatively dark, heat-absorbing water to the rays of the sun, which in turn warms the air. Another is the fact that cold Arctic air warms up more easily than warmer tropical air.

This rapid warming means the temperature difference between the Arctic and more temperate regions is less than it used to be — and temperature differences, Francis said, are one major driver of the high-altitude jet stream, which in turn steers weather systems across the globe. “When the temperature difference is large, the jet stream blows straight and fast,” she said. “When the difference is smaller, the jet stream weakens, and it’s easily deflected.” A wobbly jet stream in turn can trap weather systems — including both heat waves and cold snaps — in place for an unusually long time.

“What we’ve been trying to measure is whether the jet stream is in fact getting wavier, but it’s not easy to measure,” Francis said. The main problem is that the rate of warming in the Arctic only started taking off about 15 years ago. That’s too short a time, she explained, to find a statistically significant trend in the wobbliness of the jet stream. “This is why people are not convinced. We haven’t been able to demonstrate that the effect is happening. But there is no evidence saying that it’s wrong, only that it’ not proven,” she said.

It’s not the only reason other scientists aren’t convinced, however. “The factors that influence weather in mid-latitudes are so many and so varied that it’s questionable whether changes in Arctic alone can make a significant difference,” Screen said.

Francis acknowledges the criticism. “I totally recognize that many other things affect the jet stream. This is very new area. Some people are on board and think (our theory) is plausible, others don’t.”

In a way, the situation resembles the science of climate change itself, circa the late 1980s. At the time, the proposition that increasing concentrations of greenhouse gases would warm the planet was still largely theoretical. Nobody had yet demonstrated that it was actually happening — although the concrete evidence would start to emerge just a year or two later.

The difference: back then, while the case wasn’t proven, most climate scientists were confident that it was only a matter of time. In the case of the Arctic warming-extreme weather link, that sort of confidence is conspicuously absent.