OSLO/LONDON The icy winters suffered by Europe and North America for the last two years contrast with unusually mild weather in the Arctic, in a pattern first noted by a Danish missionary in Greenland in the 1770s.
Some scientists suggest climate change may be intensifying a natural oscillation. Others say that verdict would be premature and the pattern appears to be the same old natural one.
A deep freeze struck the eastern United States, northern Europe and parts of Russia in the last two winters. But the Arctic had high temperatures and last year was one of the hottest on record globally, according to the U.N.'s World Meteorological Organization.
In a paradoxical side-effect of global warming, a less chill Arctic may be changing wind patterns, diverting cool northern air south, some researchers say. One theory is that melting ice in the Arctic exposes warmer water that disrupts polar winds.
"The overall warming of the Earth's northern half could result in cold winters," said a study led by Vladimir Petoukhov of the Potsdam Institute for Climate Impact Research in Germany.
He told Reuters that a retreat of sea ice in the Barents and Kara Seas in the Arctic had tripled the risks of cold winters in Europe and northern Asia and doubled risks over North America.
But other experts say the deep freeze may simply be part of a natural variation first recorded by Denmark's Hans Egede Saabye, a missionary who lived in Greenland from 1770-78.
"Every winter in Greenland is severe, but they are not equally so," he wrote in his journal. "The Danes have observed that, if the winter in Denmark has been severe, that in Greenland was, in its kind, more mild, and vice versa."
That see-saw pattern is now known as the North Atlantic Oscillation (NAO). In a so-called negative phase, it disrupts westerly winds across the Atlantic and sends warmer air towards the Arctic while parts of Europe and North America freeze.
"The NAO has been negative for longer than at any time since the winter of 1969-70," said John Cappelen, a Senior Climatologist at the Danish Meteorological Institute, who happens to be a distant descendant of Saabye.
"It's a natural variation. That's the explanation for the very strong winter last year and this year," he said.
He said that cold winters were sometimes bunched together, like three years during World War Two. Still, that could mean a quick return to milder winters for Europe and North America.
The NAO was very positive -- meaning milder winters for Europe and North America -- during the 1990s. A link between climate change and recent cold is unproven, said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research.
"There are some important questions about, does decreased Arctic sea ice and a little extra snow in northern Siberia lead to this kind of pattern? I don't think we have good answers at the moment."
The past two winters hit record NAO values in an index dating back more than a century, but that just be chance, said James Hansen at NASA's Goddard Institute for Space Studies.
"Whether these last two years were a fluke or not will not be known until more years have passed. High latitude atmospheric dynamics is very chaotic, so I wouldn't bet on anything," he told Reuters.
The contrasting temperature pattern noticed by Saabye appear to still hold true.
In 2010, Greenland's capital Nuuk had an average temperature of 2.6 degrees Celsius (36.68F), a huge 4.0 degrees C (7.2 F) above normal. Denmark, by contrast, had an average temperature of 7.0 degrees C, 0.7 degree C below a long-term average.
The area of Arctic sea ice in January 2011 was at its smallest since satellite records began, at 13.55 million sq km (5.23 mln sq miles), 1.27 million sq km less than the 1979-2000 average, the U.S. National Snow and Ice Data Center said.
Bjorn Lomborg, a Danish statistician and author of "The Skeptical Environmentalist", said that chill winters were a cautionary tale about not being too certain about dire predictions of more heatwaves, droughts and rising seas.
"We have been told the science of climate change is settled. And then something happens and we have a very different attempt to try to explain it," he said.
(Additional reporting by Timothy Gardner in Washington and Daniel Fineren in London; editing by Andrew Roche)