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With snow blanketing much of the country, the topic of global warming has become the butt of jokes. Climate skeptics built an igloo in Washington, D.C. during last weeks storm and dedicated it to former Vice President Al Gore, who's become the public face of climate change. There was also a YouTube video called "12 Inches of Global Warming" that showed snowplows driving through a blizzard.

For scientists who study the climate, it's all a bit much. As NPRs Christopher Joyce reports, they're trying to dig out.

CHRISTOPHER JOYCE: Snowed-in Washington is where much of the political debate over climate change happens. So it did not go unnoticed when a Washington think-tank that advocates climate action had to postpone a climate meeting last week because of inclement weather.

That kind of irony isnt lost on climate scientists. Most don't see a contradiction between a warming world and lots of snow. Heres Kevin Trenberth, a prominent climate scientist at the National Center for Atmospheric Research in Colorado.

Mr. KEVIN TRENBERTH (Scientist, National Center for Atmospheric Research): The fact that the oceans are warmer now than they were, say, 30 years ago, means there's about, on average, 4 percent more water vapor lurking around over the oceans than there was, say, in the 1970s.

JOYCE: Warmer water means more water vapor rises up into the air. And what goes up, must come down.

Mr. TRENBERTH: So one of the consequences of a warming ocean near a coastline like the East Coast and Washington, D.C., for instance, is that you can get dumped on with more snow, partly as a consequence of global warming.

JOYCE: And Trenberth notes that you don't need very cold temperatures to get big snow. In fact, when the mercury drops too low, it may be too cold to snow.

There's something else fiddling with the weather this year: a strong El Nino. That's the weather pattern that, every few years, raises itself up out of the western Pacific Ocean and blows east to the Americas. It brings heavy rains and storms to California and the South and Southeast. It also pushes high-altitude jet streams farther south, which brings colder air with them.

Trenberth also says El Nino can lock in weather patterns like a meteorological highway, so that storms keep coming down the same track. True, those storms have been big ones - record breakers. But meteorologist Jeff Masters, with the Web site Weather Underground, says it's average temperatures - not snowfall -that really measure climate change.

Mr. JEFF MASTERS (Meteorologist): Because if it's cold enough to snow, you will get snow. We still have winter, even though the temperatures have warmed on average, oh, about 1 degree Fahrenheit over the past 100 years.

JOYCE: Masters says that 1 degree average warming is not enough to eliminate winter or storms. A storm is part of what scientists classify as weather. Weather is largely influenced by local conditions and changes week to week. It's fickle, fraught with wild ups and downs.

Climate is the long-term trend of atmospheric conditions across large regions, even the whole planet. Changes in climate are slow and measured in decades, not weeks.

Masters and most climate scientists say a warming climate would be expected to affect the weather, sometimes drastically, but exactly where and when is hard to predict.

Mr. MASTERS: In that kind of a climate, youll have more frequent extreme events, heat waves and so on. But again, none of those individual events is proof in itself that the climate is changing.

JOYCE: Climate scientists say they can't prove any single weather event is due to climate change. Thus, they say, Katrina or the heat wave in Vancouver that's dogging the Olympics isn't proof that climate change is happening. Nor can eastern and southern snowstorms prove that it's not.

Christopher Joyce, NPR News.

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