In early 2011, the US and Europe froze, even as Greenland and Alaska experienced unusual periods of warmth. This year, the US and Europe were baking as the winter drew to an end, even as cold air hovered over Central Europe and Asia. In the Northern Hemisphere, extreme winter weather tends to be associated with the negative phase of the Arctic Oscillation, a wind pattern that dominates the polar region. And a consensus is building that changes in the Arctic may have permanently placed the Oscillation in the negative mode, leading to stable changes in the winters of the Northern Hemisphere. Cornell professor Charles H. Greene has just published a review of this idea, and we talked with him about what the warming Arctic might mean for the US and Europe.

Greene's paper describes a key determinant of the Northern Hemisphere's winter weather: the Arctic Oscillation. When that is in its positive phase, a strong set of winds called the Polar Vortex forms. These winds help trap Arctic air masses at the pole, keeping the cold out of the mid-latitudes. This also allows the jet stream to take a more direct route around the globe, moderating the weather.

But over the last few years, the Oscillation has been strongly negative; in fact, in 2010, we saw a record for the most strongly negative period we'd ever recorded. During this phase, the winds of the Polar Vortex weaken, allowing the cold Arctic air to intrude or mix into the air at lower latitudes. As a result of this, Greene told Ars two things happen to the jet stream: it gets substantially weaker, and it tends to meander widely from north to south as it traverses the globe. This can lead to the severe chills the US and Europe have experienced over the past several winters, but the meandering jet stream can also draw warmer southern air north, as happened in the US this spring.

Last year, when we covered Europe's deep freeze, Mark Serreze of the National Snow and Ice Data Center suggested that the negative phase of the Arctic Oscillation may be the product of the loss of Arctic sea ice, though he described the idea as "very much in its infancy." According to Greene, the idea has matured quite a bit over the last year, with two papers being published early this year arguing there is a direct connection. Greene's review explains how it works.

Although Arctic sea ice has been trending downward since we started monitoring it with satellites, 2007 saw a sudden and dramatic drop, one that the ice has never really recovered from. All but one year since then has seen ice minima below previous trends, as seen on the graph below.

This has a dramatic effect on the local conditions. Over the summer, the open ocean absorbs far more light than would have been reflected by the ice, allowing it to retain heat. In the autumn, it returns some of that heat to the atmosphere, warming the air above the Arctic. This erases a lot of the difference between the Arctic and more temperate latitudes, which makes it very difficult to form a polar vortex. In short, the missing ice forces us into a negative Arctic Oscillation, and the increased likelihood of cold winters in various places across the Northern Hemisphere. That in turn causes a weaker, meandering jet stream that tends to stay stuck in specific patterns for longer.

Because of the meandering jet stream, there's always the chance that some unusual warmth can leak north. In the case of the March heat wave in the US, Greene said that the weak Arctic Oscillation interacted with the presence of La Niña conditions in the Pacific to force warm air north and hold it there for an unusually long time.

The building consensus that links the Oscillation to ice levels may seem to have come about suddenly, but Greene said signs of what he termed an "Arctic Warm Period" have been apparent since he started working in the region in 1989. Back then, he had to drill through 14 feet of permanent ice to reach the water, and that permanent ice covered about 80 percent of the Arctic. "Now, that's just completely flip-flopped," Greene told Ars, with permanent ice being the exception. The dramatic loss of ice may have started in the middle of the last decade, but Greene said that "changes have been going on longer than that."

"It takes a while for the scientific community to recognize what we call a 'regime shift,'" he told Ars, implying that one was likely to have taken place in the Arctic.

As for the public, the awareness of Arctic climate change is there, but Greene suggested that there's little awareness of the consequences. "People have heard about Arctic climate change and all the sea ice melting, but we hear about it representing a threat to the polar bears. Most people, if it doesn't affect their everyday lives, don't worry about it too much. The changes that are going on in the Arctic climate system are changing our everyday lives now, and we have to think about that."

Oceanography, 2012. DOI: 10.5670/oceanog.2012.58 (About DOIs).