Correction to This Article
Earlier versions of this story misquoted James Overland of the National Oceanic and Atmospheric Administration. He said, "The breakdown of the vortex is very unusual." This version has been corrected.
Is severe winter weather related to global warming?

By Ben Harder
Monday, February 7, 2011; 2:56 PM

You may not remember exactly what you were doing one year ago, but odds are good you spent part of the day shoveling, buying an extra pair of gloves or replenishing your emergency stash of batteries. Odds are even better that you remember what happened just before and just after the anniversary we celebrate today. The first of a pair of major blizzards struck Washington on Feb. 5 and 6. The reprise landed on Feb. 9 and 10.

Our region experienced record snowfall last winter, topping the charts dating at least as far back as the late 1800s. In all, more than six feet of snow fell at sites such as Baltimore-Washington International Marshall Airport. Extreme weather nailed other U.S. cities last winter, too, and swaths of Europe saw unprecedented snowfalls and record cold temperatures. This year, the nation's capital has suffered one unusually severe storm. Parts of the East Coast from Atlanta to Boston have been experiencing blizzard conditions. Last week, a vast swath of the country's midsection and East Coast got deluged with sleet and snow, paralyzing travel. What gives?

Some weather scientists suspect that climate change - the menace often called global warming - is partly to blame. Although the link is far from definitive - two years of lousy weather, after all, doesn't make much of a trend - the meteorological dots are beginning to line up.

But wait a second: global warming is about the world getting warmer, right? All those greenhouse gases from burning fossil fuels trap solar radiation in the atmosphere, et cetera. If anything, winter weather seems like it should be on its way to extinction.

Weather isn't that simple, as it turns out. On average, the world is indeed getting warmer. (Even those skeptical about the human effect on Earth's climate don't dispute that fact, which is well established from year after year of upward-trending thermometer readings around the planet.) But global warming doesn't necessarily translate into warming everywhere, all the time.

To understand how warming and snowstorms may be connected, it helps to start with the epicenter of winter weather. Around the North Pole, some of the world's coldest air currents blow in what's typically a tight loop known as the polar vortex. Air masses inside the vortex tend to have not only low temperatures but also low barometric pressures compared with air outside the vortex. The surrounding high-pressure zones push in on the vortex from all sides, helping the cold air stay where it belongs, at the top of the world.

That's what happens, most of the time, at least. Occasionally, pressure inside the vortex strengthens, causing the vortex itself to become unstable, like a top that's losing its spin. When that happens, frigid polar air is more likely to escape the meteorological fence that normally confines it. The result, sometimes felt far to the south, can take the form of severe winter weather.

Over the past two years, the polar vortex has been strikingly unstable, according to meteorological data. James Overland of the National Oceanic and Atmospheric Administration cites a couple of measures in particular: One, called the Arctic oscillation, tracks air pressure and related atmospheric variables over the North Pole. The other, the North Atlantic oscillation, takes into account similar variables in the neighborhood of Iceland. Both indexes are reliable indicators of the strength of the polar vortex.

Last winter, both indexes reflected higher air pressures and therefore less vortex stability than scientists have ever recorded. This year, both were again seriously off-kilter.

Any number of meteorological factors contributed to those anomalies. Some were undoubtedly random, Overland says. But he and other experts suspect climate change is contributing to the unusual pattern, and if they're right, things could get a whole lot worse in the years ahead.

The root of the problem, Overland says, is melting sea ice. Sea ice forms in the Arctic Ocean during the cold, dark days of fall and winter and hangs around, melting slowly but not completely vanishing, throughout the summer. In recent years, more sea ice has melted during the warm months than can be replenished during the chillier ones.

As a result, scientists have found, the total amount of arctic sea ice has shrunk; at the end of the past few summers, it has been consistently down some 30 percent when compared with historical averages of the 1980s and 1990s at that time of year. In the decades ahead, climate scientists have predicted, the amount of sea ice surviving into the late summer may fall as much as 80 percent below historical levels.

Sea ice reflects sunlight, redirecting some of the the sun's energy away from the planet. When sunlight strikes open ocean instead of ice, however, the water absorbs much of the solar radiation. So low levels of sea ice allow water temperatures to increase more than usual during the summer, when the sun is shining on the Arctic. Even as the days grow short and cold, the water's tepidity can release excess heat, which tends to increase pressure in the air above. And remember, excess pressure in the far north poses a threat to the polar vortex.

"The breakdown of the vortex is very unusual," says Overland. "We've had it two years in a row. That makes you wonder.

"My speculation," he says, "is that the extra heat stored in the ocean and given to the arctic atmosphere has a tendency to support the breakdown of the vortex."

In other words, some melting ice and a moat of warmish water in the Arctic may be spawning the blasts of frigid air that keep pummeling us. How's that for a cold irony?

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