Greg Laden of ScienceBlogs now has a good article, and image macro meme that you can use on your Limbaughtomized Facebook frienemies and in-laws when they repeat that usual derp talking point "It's snowing, therefore global warming is a hoax!"
Below the fold, I'll mention some of the science that explains why the age of global climate change may bring more blizzards.
In short, because the Arctic has warmed (warmed is a relative term - we're talking about going from -40 to -35), the forces that keep the Arctic air in the Arctic aren't nearly as strong. The jet stream, which is a river of air that's the boundary between the warmer air of temperate-to-tropic climes, and the colder air of temperate-to-arctic regions, thus meanders a lot more, or as the analogy goes, staggers around drunkenly. And when the jet stream staggers south, cold Arctic air comes with it. No, the Arctic cold air masses are not expanding. They're moving around more, and visiting us more.
So, if you're wondering why the temperatures have dropped below zero, and there's feet of snow on the ground all of the sudden, yes, you can blame man-made global warming. Not every scientist agrees that global warming may be causing more blizzards, but a lot of them think this theory has merit.
From Laden's article:
...Chris Mooney, writing for Mother Jones, elaborates:
What is happening instead is the cold air mass that usually sits up on the Arctic during the northern Winter has moved, drooped, shifted, gone off center, to engulf part of the temperate region. Here in the Twin Cities, it is about 8 below zero F as I write this. If I go north towards the famous locality of International Falls (famous for its cold temperature readings often mentioned on the national news) it will in fact be colder. If I go even farther north, at some point it will start to get warm again, as we leave the giant blob of cold air that has engulfed us. In fact, it is relatively warm up on the North Pole right now. Alaska and Europe are relatively warm as well.
The graphic above from the European Centre for Medium-Range Weather Forecasts shows what is happening. The Polar Vortex, a huge system of swirling air that normally contains the polar cold air has shifted so it is not sitting right on the pole as it usually does. We are not seeing an expansion of cold, an ice age, or an anti-global warming phenomenon. We are seeing the usual cold polar air taking an excursion.
So, this cold weather we are having does not disprove global warming.
In fact, the cold snap we are experiencing in the middle of the US and adjoining Canada may be because of global warming. The Polar Vortex can go off center any given winter, but we have been having some strange large scale weather activity over the last few years that is thought to be related to global warming and that may have contributed to this particular weather event (explained here). This may be an effect of this strangeness, though the jury is still probably out on this particular weather event.
To understand how it works, it first helps to think of the jet stream as a river of air that flows from west to east in the Northern Hemisphere, bringing with it much of our weather. Its motion—sometimes in a relatively straight path, sometimes in a more loopy one—is driven by a difference in temperatures between the equator and the north pole. Southern temperatures are of course warmer, and because warm air takes up more space than cold air, this leads to taller columns of air in the atmosphere. "If you were sitting on top of a layer of atmosphere and you were in DC, looking northward, it would be like looking down a hill, because it's warmer where you are," explains Francis.
The jet stream then flows "downhill," so to speak, in a northward direction. But it's also bent by the rotation of the Earth, leading to its continual wavy, eastward motion.
As the Arctic rapidly heats up, however, there's less of a temperature difference between the equator and the poles, and the downhill slope in the atmosphere is accordingly less steep. This creates a weaker jet stream, a jet stream that meanders more or, if you prefer the new analogy, staggers around drunkenly. "As the Arctic continues to warm, we expect the jet stream to take these wild swings northward and southward more often," says Francis. "And when it does, that's when we get these particularly wild temperature and precipitation patterns, and they tend to stay in place a long time." (For a more thorough explanation, see here.)
That's not to say the jet stream never staggered around drunkenly in the past. It did. But Francis thinks this is happening more often, and the result is all manner of weather extremes, including both cold snaps and also record heat. (Not every scientist agrees; for the debate over Francis's work, see here.)