WASHINGTON — Winter's been so mild in much of the United States this year that you can slip out some sunny afternoons to the golf course or bike path. Many snow shovels have stayed in storage. Heating bills have fallen.
Meanwhile, Europe's suffering a brutal winter.
But in the USA, in the East, Midwest, Texas, the Plains and parts of the West, the average mean temperature was 5 degrees or more above normal over the past 30 days. Only Washington state and parts of Oregon showed cooler-than-average temperatures then.
It remains to be seen, however, whether U.S. winter records will be broken.
"We'll have to wait until February is over," said Kathryn Vreeland, a climatologist at the Northeast Regional Climate Center.
Mike Halpert, the deputy director of the government's Climate Prediction Center, said the U.S. had had a number of warm winters recently. But the last two were cold across much of the country, and that's fresh in everyone's mind, he said.
Part of the explanation for this year's mildness stems from the Arctic Oscillation — atmospheric pressure patterns in the Arctic and northern-middle latitudes.
The Arctic Oscillation has been in a warm or positive phase recently, meaning the polar jet stream is stronger than average and has shifted poleward, Halpert said. "Basically, that keeps cold air bottled up over the pole."
The last two winters were the opposite: A negative phase allowed cold air to drop down. That brought heavy snow to New York and New England last year, and the big snows in the mid-Atlantic in 2010.
Record keeping on the Arctic Oscillation began around 1950, Halpert said. In the 1950s, '60s and '70s, there often was a negative phase, bringing colder winters. In the 1980s, '90s and 2000s, many winters had a positive AO.
The Arctic Oscillation is part of the planet's natural cycle, Halpert said; it's too early to tell whether climate change is having an impact on it.
In the last couple of weeks, the Arctic Oscillation turned negative again, bringing severe cold to Europe and Asia, but not to the United States. That's partly due to another pattern of pressure, the North Atlantic Oscillation, Halpert said. That's remained positive.
It's also a La Nina winter, when cooler temperatures in the Pacific typically result in cooler temperatures in the Northwest. La Nina also directs storm tracks, shifting them away from the Southeast toward the west and north, Halpert said.
While weather varies with each season and year, climate scientists worldwide have observed a longer-term global warming trend. They've reached a consensus that it can be explained only by the increase in greenhouse gases in the atmosphere, mainly as a result of burning fossil fuels.
Some climate research aims to see whether there are links between global warming and changes in the Arctic Oscillation.
One study released two weeks ago, for example, showed that the probability of cold winters and a lot of snow in central Europe increases when less sea ice covers the Arctic in summer. Scientists at the Alfred Wegener Institute for Polar and Marine Research in Germany said the shrinking summer ice changed air pressure zones in the Arctic, affecting the weather.
The Arctic sea ice cover was low last summer, the second lowest since satellite records began in 1979. But when the Wegener study was published Jan. 26, the cold spell hadn't hit Germany yet.
"Many other factors naturally play a role in the complex climate system of our Earth, which overlap in part," Ralf Jaiser, the report's lead author, said in a news release.
Jeff Masters, the director of meteorology for Weather Underground Inc., keeps track of weather events and research on climate change and blogs about it at wunderground.com.
"In December, we had the most extreme atmospheric pressure pattern ever observed over the North Atlantic, with records going back to 1865," Masters said in an email. The patterns have been extreme in four of the last six Decembers, he added.
This year, the strong jet stream has kept the cold air bottled up in Canada. The National Weather Service keeps snow data for 166 cities, and 157 of them have had below-average amounts, Masters said.
In addition, a rare tropical disturbance brought heavy rain Monday to the Florida Keys. It's the kind of disturbance usually seen in the summer hurricane season, "and something I've never seen in February during my 30 years as a meteorologist," Masters said.
All this and other extreme weather over the past two years is just too much to be an unusually long run of natural extremes, Masters said.
"Something is definitely up with the weather, and it is clear to me that over the past two years, the climate has shifted to a new state capable of delivering rare and unprecedented weather events," he said. "Human emissions of heat-trapping gases like carbon dioxide are the most likely cause of such a shift in the climate."
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