WASHINGTON — Tree limbs snap, the power goes out, the car needs digging out again. Along with the grumbling about winter snow there's also a common curiosity: So what does all this say about global warming?
How can the average world temperature be inching up and 2010 be tied for the warmest year ever, when places from North Carolina to New England get buried by whopper winter storms?
There are several scientific explanations that help sweep away the snow confusion.
The latest report on climate change by federal scientists, "Global Climate Change Impacts in the U.S.," said precipitation has increased by an average of about 5 percent in the past 50 years. It projects that northern areas generally will become wetter, and southern areas, especially in the West, will become drier.
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Meanwhile, two current ocean and atmosphere patterns help explain winter cold and snow.
The climate phenomenon La Nina is linked to a cooling in sea-surface temperatures in the equatorial Pacific. Its opposite is El Nino, the warming of those waters.
La Nina is mainly responsible for this year's weather patterns, said Ignatius Rigor, an atmospheric scientist at the Polar Science Center at the University of Washington.
La Nina was expected to bring more snow to the Pacific Northwest, much of the Upper Midwest and New England.
"Even in a warming world we will still have natural oscillations like day and night, winter and summer, and in this case El Nino and La Nina," he said.
Another possible reason for colder winters and more snow is linked to the loss of Arctic sea ice and the effect that change has on the winds that circle the North Pole.
In 2010, the extent of sea ice cover in the Arctic at its lowest point in September was 31 percent less than the 1979-2000 average. It was the third-lowest year after 2007 and 2008.
Open water and water covered by thin ice allows heat stored in the ocean to escape to the atmosphere. The autumn release of heat weakens the winds that normally circle the North Pole and act as a fence keeping cold air in. When the circle of winds, the Polar Vortex, breaks down, cold air spills south.
Scientists have observed that the past five years have been the warmest period recorded in the Arctic. Last month, temperatures were unusually warm in areas that were ice-free in summer, including waters north of Alaska, and also in Greenland.
This year, for the second time in row, a warm Arctic is again influencing the Polar Vortex, allowing more cold wind to escape to the south and bringing cold to the U.S. East Coast, East-Central Asia and Europe.