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. Historic blizzards last year afflicted the United States' mid-Atlantic region. The Deep South has endured unusual snowstorms and severe cold this winter, and a frigid Northeast is bracing for what could shape into another major snowstorm at midweek.
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. Temperatures in northeastern Canada and Greenland ran as much as 15 or 20 degrees above normal in December. Bays and lakes have been slow to freeze.
Iqaluit, capital of the remote Canadian territory of Nunavut, had to cancel its New Year's snowmobile parade. Deputy Mayor David Ell said people in the region had been looking with envy at snowbound American and European cities. "People are saying, 'That's where all our snow is going!' " he said.
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 past 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 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.
Since satellites began tracking it in 1979, ice on the Arctic Ocean's surface in the bellwether month of September has declined by more than 30 percent. 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 affect 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 note that extra heat is 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. During 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 frigid air at the top of the world.
"It's like a fence," said Michelle L'Heureux, a researcher in Camp Springs, Md., with the National Oceanic and Atmospheric Administration (NOAA).
When that pressure difference diminishes, however, the jet stream weakens and meanders southward, bringing warm air into the Arctic and cold air into the mid-latitudes — exactly what has happened the past two winters. The effect sometimes is compared with 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, the polar vortex usually doesn't weaken as much as it has lately. One index related to the vortex hit its lowest wintertime value last winter since record-keeping began in 1865, and it was quite low again in December.
James 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 air pressure over the Arctic to be closer to that of the middle latitudes. "The added heat works against having a strong polar vortex," he said.
But Overland acknowledges his idea needs further research.
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. His theory: The extra snow is creating a dense, cold air mass over northern Asia in late fall, setting off a complex chain of cause and effect that ultimately perturbs the vortex.
He is publishing seasonal forecasts based on his work, supported by the National Science Foundation. Those forecasts correctly predicted the recent harsh winters in the mid-latitudes. But Cohen acknowledges, as does Overland, that some of his ideas need further research.
While mainstream researchers are sure that greenhouse gases released by humans are warming Earth, they acknowledge being on shakier ground in trying to predict 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 Earth warms overall.
Without doubt, the winter weather that began and ended 2010 was remarkable. Two of the 10 largest snowstorms in New York history occurred last year, including the one that disrupted travel right after Christmas. The two snowstorms that hit Washington, D.C., 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.
But the winters were not the whole story. 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 Niño, which dumped heat from the Pacific Ocean into the atmosphere early in the year. Later, the ocean surface cooled, a condition known as La Niña, contributing to heavy rainfall in many places.
Despite cooling from La Niña, newly compiled figures show 2010 was among the two warmest years in the historical record. The year featured a heat wave in Russia, all-time high temperatures in at least 17 countries, the hottest summer in New York history, and devastating floods in Pakistan, China, Australia and the United States.
However erratic weather may have become, it is not obvious to most people how global warming could lead to frigid winters. Many scientists are hesitant to back such assertions, until they understand what is going on in the Arctic.
Several scientists recalled that in the decade ending in the mid-1990s, the polar vortex seemed to be strengthening, not weakening
At the time, some climate scientists wrote papers attributing that change to global warming. Newspapers printed laments for winter lost. But the apparent trend soon went away, an experience that has made many researchers more cautious.
John Wallace, an atmospheric scientist at the University of Washington, wrote some of the earlier papers. This time, he said, it will take a lot of evidence to convince him that a few harsh winters in London or Washington, D.C., have anything to do with global warming.
"Just when you publish something and it looks like you're seeing a connection," Wallace said, "nature has a way of humbling us."