Judging by the weather, the world seems to have flipped upside down. For two winters running, an Arctic chill has descended on Europe, burying that continent in snow and ice.

Last year in the United States, historic blizzards afflicted the mid-Atlantic region. This winter the deep South has endured unusual snowstorms and severe cold, and a frigid Northeast is bracing for a snowstorm. Yet while people in Atlanta learn to shovel snow, the weather 2,000 miles to the north has been freakishly warm the past two winters.

Throughout northeastern Canada and Greenland, temperatures in December ran as much as 15 to 20 degrees Fahrenheit above normal.

The immediate cause of the topsy-turvy weather is clear enough. A pattern of atmospheric circulation that tends to keep frigid air penned in the Arctic has weakened during the last two winters, allowing big tongues of cold air to descend far to the south, while masses of warmer air have moved north.

The deeper issue is whether this pattern is linked to the rapid changes that global warming is causing in the Arctic, particularly the drastic loss of sea ice. At least two prominent climate scientists have offered theories suggesting that it is. But many others are doubtful, saying the recent events are unexceptional, or that more evidence over a longer period would be needed to establish a link.

Surface ice decline

Since satellites began tracking it in 1979, the ice on the Arctic Ocean’s surface in September has declined by more than 30 per cent. It is the most striking change in the terrain of the planet in recent decades, and a major question is whether it is starting to have an effect on broad weather patterns. Ice reflects sunlight, and scientists say the loss of ice is causing the Arctic Ocean to absorb more heat in the summer. A handful of scientists point to that extra heat as a possible culprit in the recent harsh winters in Europe and the United States.

Their theories involve a fast-moving river of air called the jet stream that circles the Northern Hemisphere. Many winters, a strong pressure difference between the polar region and the middle latitudes channels the jet stream into a tight circle, or vortex, around the North Pole, effectively containing the frigid air at the top of the world. When that pressure difference diminishes, however, the jet stream weakens and meanders southward, bringing warm air into the Arctic and cold air into the midlatitudes – exactly what has happened the last couple of winters. The effect is sometimes compared to leaving a refrigerator door open, with cold air flooding the kitchen even as warm air enters the refrigerator. This has happened intermittently for many decades. Still, it is unusual for the polar vortex to weaken as much as it has lately.

James E Overland, a climate scientist with NOAA in Seattle, has proposed that the extra warmth in the Arctic Ocean could be heating the atmosphere enough to make it less dense, causing the air pressure over the Arctic to be closer to that of the middle latitudes. Judah Cohen, director of seasonal forecasting at a company called Atmospheric and Environmental Research in Lexington, Mass., has spotted what he believes is a link between increasing snow in Siberia and the weakening of the polar vortex. In his theory, the extra snow is creating a dense, cold air mass over northern Asia in the late autumn, setting off a complex chain of cause and effect that ultimately perturbs the vortex. Cohen said in an interview that the rising Siberian snow might, in turn, be linked to the decline of Arctic sea ice, with the open water providing extra moisture to the atmosphere – much as the Great Lakes produce heavy snows in cities like Buffalo and Syracuse.  

What about the strange winters?

But Cohen acknowledges, as does Overland, that some of his ideas are tentative and need further research. The uncertainty about what is causing the strange winters highlights a core difficulty of climate science. While mainstream researchers are sure that greenhouse gases released by humans are warming the Earth, they acknowledge being on shakier ground in trying to predict the regional effects of that change. It is entirely possible, they say, that some regions will cool temporarily, because of disruption of the atmospheric and oceanic circulation, even as the Earth warms overall.

Bloggers who specialise in raising doubts about climate science have gleefully pointed to the recent winters in the United States and Europe as evidence that climatologists must be mistaken about a warming trend. These commentators have not been as eager to write about the strange warmth in parts of the Arctic, a region that scientists have long predicted will warm more rapidly than the planet as a whole.

Without doubt, the winter weather that began and ended 2010 was remarkable. Two of the 10 largest snowstorms in New York City history occurred last year. The two snowstorms that fell on Washington and surrounding areas within a week in February had no known precedent in their overall impact on the region, with total accumulations of 40 inches in some places.

That was a strange year...

Even without them, 2010 would have gone down as one of the strangest years in the annals of climatology, thanks in part to a weather condition known as El Nino, which dumped heat from the Pacific Ocean into the atmosphere early in the year. Later, the ocean surface cooled, a condition known as La Nina, contributing to heavy rainfall in many places. Despite cooling from La Nina, newly compiled figures show that 2010 was among the two warmest years in the historical record. It featured a blistering heat wave in Russia, all-time high temperatures in at least 17 countries, and devastating floods in Pakistan, China, Australia, the United States and other countries.

“It was a wild year,” said Christopher C Burt, a weather historian for Weather Underground, an Internet site.