CHICAGO—The US is freezing.The UK is flooding. Alaska and Scandinavia are unusually warm. And, most remarkably, all of that has been going on for roughly the entire winter. It's not just unusual weather; it's consistently unusual.

A few years back, researchers suggested that strange weather in the temperate regions of the Northern Hemisphere might be a consequence of changes taking place in the Arctic. Now, with a few years of additional data, some researchers are arguing that we have detected clear signs that Arctic warming is driving our weird weather.

We sat down with Rutgers University's Jennifer Francis at the American Association for the Advancement of Science to find out what's new.

Why the polar vortex is your friend

When the first big chill hit the US' East Coast, media sources were quick to blame it on a polar vortex. But the use of that term is a bit confusing; typically, a strong polar vortex keeps cold air bottled up in the Arctic, rather than letting it spill across the US.

So, what is the polar vortex? Typically in the winter, as the Arctic stops seeing the sun, the atmosphere above it cools down dramatically, creating a large temperature gradient between it and the warmer air to the south. This temperature gradient sets up a pressure gradient, which drives strong winds: the jet stream. As the jet stream circles the Arctic, it forms the polar vortex, which traps the cold air in the Arctic.

What the US saw was a weakening of this polar vortex. As the jet stream winds weaken, they become more prone to wandering much farther south. And, as they do, the cold air follows them. Thus, it wasn't that a polar vortex moved south over the US; instead, an arm of the vortex extended south over the central and eastern parts of the country. And the jet stream's winds have had consequences on either side of the cold portion. Winter storms have been blocked from reaching California, while warm, wet air has been drawn north over the UK and Scandinavia.

This sort of meandering polar vortex has happened in the past, so deep polar chills are a feature of many people's memories. But the new research suggests changes in the Arctic have made odd behaviors both more frequent and more persistent when they do occur.

Vanishing ice and wandering winds

Although much of the globe has become warmer over the last century, the warming has been strongest in the Arctic. And that warming was underscored by the dramatic collapse of sea ice that occurred in 2007. As Dr. Francis explained, the collapse of the sea ice is part of a process that's amplifying the underlying warming trend.

One aspect of the amplification is the loss of the ice itself, which leaves dark ocean water exposed, leading to an increased absorption of sunlight. A similar thing happens on land, where the snow cover is vanishing earlier each spring. The rate of change in snow cover is even larger than that of ice loss—"and that's saying something," Francis told Ars, "because the sea ice is going fast."

Finally, the warmer land and ocean are putting more water vapor into the atmosphere, forming more clouds. Clouds normally have a mixed effect, both insulating the underlying Earth and reflecting some of the incoming sunlight back into space. But, in the Arctic, there is no sunlight for several months, and the Earth is often covered with snow and ice. As a result, the clouds have a larger insulating effect. The condensation of water vapor into clouds also releases heat.

Collectively, these phenomena contribute to what's called the Arctic amplification: the Arctic is warming much faster than the mid-latitudes south of it. And that, Frances has argued, can lead to more extreme weather in these mid-latitudes.

Southerly extremes

The reason for this is that the site of the jet stream—the border of the polar vortex—is set by a sharp boundary between cold Arctic air and the more moderate temperatures of the mid-latitudes. As the Arctic warms, that boundary becomes diffuse, and the jet stream is less likely to be pinned at the edge of the Arctic. Francis made the analogy of a river rushing directly down a steep slope—that's the situation you have when the contrast between the Arctic and mid-latitude temperatures is sharp. When it's much more gradual, the jet stream can meander, much like a river running down a broad, flat valley.

And, as it meanders, its force is weaker; rather than driving the mid-latitude weather, it begins to be pushed around by it—and even gets locked in place. And that's what we seem to be seeing this winter, where the Northern Hemisphere's weather patterns have largely held steady since mid-December. And, at the same time, the Arctic sea ice has failed to regrow to its normal extent for this time of year and is near a record low.

What Francis has done is simply trace out the location of the jet stream. If you take pressure measurements from across the Northern Hemisphere and treat it like a topographical map, the jet stream should track a specific contour line. By measuring this track, you can generate a measure of its "loopiness"—how often it meanders far to the south. It's a simple measure and not everybody's convinced by it, but it suggests that the predictions of jet stream changes are coming to pass—and with them, longer, more extreme winter weather events.

Although not all scientists are convinced, Francis said "nobody's coming out with contrary evidence yet." Beyond that, all she can really do is wait: "it's a matter of time and waiting for the real world to have a few more years of data."

So, as far as Francis' proposal is concerned, the Northern Hemisphere is behaving exactly as predicted. "We can't say that it's directly linked," she said, "but it's the kind of situation we'd expect to see more often." Attribution studies—the ones that link specific weather events to specific aspects of climate change—take lots of time and computing power. "I'd bet that someone does attribution," Francis said, but so far, there hasn't been time.

It's also extremely difficult to determine whether the link between the Arctic Amplification and jet stream meanders is causal. Radical changes in Arctic sea ice only occurred in 2007, so there's only been seven years of data; not enough for a significant trend to rise above the statistical noise. Complicating matters further, while the frequency of large dips in the jet stream may increase, the location of these dips will remain random. While the US is now shivering because of a dip, in the spring of 2012, a similar dip located farther west caused record high temperatures.

You can also study it using climate models—like Francis' research group is—by removing a lot of the sea ice and determining if similar behavior occurs. But this only captures one aspect of the Arctic Amplification, and so it wouldn't be expected to produce the same behavior.

Santa's revenge

In the meantime, those of us stuck with the consequences of the meandering jet stream aren't necessarily feeling so patient. And Francis is sympathetic; "I think this is all bad news," she told Ars. If anything good comes out of it, however, she suggests that it might be a greater recognition that climate change is not only real, but already making its presence felt.

Generally, Francis told Ars, people are thinking of climate change's impacts as being gradual and destined to be felt in the distant future. But the persistent weather of this winter is hard to ignore. "People are noticing that 'oh my God, this has been going on forever,'" Francis said.

And that may make them more likely to accept some things that climate researchers have been saying for a while. "This weird weather has gotten people realizing that climate change won't be some gradual warming," Francis told Ars. "It's happening already and in destructive ways."