Washington D.C.'s getting slammed by record snowfall right now, which means that in addition to unplowed roads and Mad Max-style scenes at Safeway, we also have to suffer through a flurry of Al Gore jokes and Republicans snorting about how this proves global warming is all fake. I guess the prim, boring response is that a single weather event, even an extreme one, simply doesn't tell us much about long-term climate trends.
But blah, blah, everyone's heard that line before. A more thoughtful reply comes from meteorologist Jeff Masters, who explains why massive snowstorms in the Northeast aren't inconsistent with a steadily warming world:
There are two requirements for a record snow storm:
1) A near-record amount of moisture in the air (or a very slow moving storm).
2) Temperatures cold enough for snow.
It's not hard at all to get temperatures cold enough for snow in a world experiencing global warming. According to the 2007 Intergovernmental Panel on Climate Change (IPCC) report, the globe warmed 0.74°C (1.3°F) over the past 100 years. There will still be colder than average winters in a world that is experiencing warming, with plenty of opportunities for snow.
The more difficult ingredient for producing a record snowstorm is the requirement of near-record levels of moisture. Global warming theory predicts that global precipitation will increase, and that heavy precipitation events--the ones most likely to cause flash flooding--will also increase. This occurs because as the climate warms, evaporation of moisture from the oceans increases, resulting in more water vapor in the air.
According to the 2007 Intergovernmental Panel on Climate Change (IPCC) report, water vapor in the global atmosphere has increased by about 5% over the 20th century, and 4% since 1970. This extra moisture in the air will tend to produce heavier snowstorms, assuming it is cold enough to snow. Groisman et al. (2004) found a 14% increase in heavy (top 5%) and 20% increase in very heavy (top 1%) precipitation events in the U.S. over the past 100 years, though mainly in spring and summer. However, the authors did find a significant increase in winter heavy precipitation events have occurred in the Northeast U.S.
This was echoed by Changnon et al. (2006), who found, "The temporal distribution of snowstorms exhibited wide fluctuations during 1901-2000, with downward 100-yr trends in the lower Midwest, South, and West Coast. Upward trends occurred in the upper Midwest, East, and Northeast, and the national trend for 1901-2000 was upward, corresponding to trends in strong cyclonic activity."
Meanwhile, it's worth noting the U.S. Global Change Research Program actually predicted stronger winter storms for the Northeast, in its 2009 report on potential climate-change impacts for the United States:
Storm tracks have shifted northward over the last 50 years as evidenced by a decrease in the frequency of storms in mid-latitude areas of the Northern Hemisphere, while high-latitude activity has increased. There is also evidence of an increase in the intensity of storms in both the mid- and high-latitude areas of the Northern Hemisphere, with greater confidence in the increases occurring in high latitudes (Kunkel et al., 2008). The northward shift is projected to continue, and strong cold season storms are likely to become stronger and more frequent, with greater wind speeds and more extreme wave heights."
Still, we can't definitively say that global warming caused this snow monstrosity—again, it's impossible to attribute any single weather event to long-term climate shifts. (For instance, El Niño may be playing a bigger role right now in feeding these snowstorms.) At most, we can say that a warming climate is expected, over time, to create the conditions that make fierce winter storms in the Northeast and mid-Atlantic more likely. Or at least it will for awhile: If the planet keeps heating up, then in theory freezing conditions in the Northeast will become rarer, at which point snowstorms would, too. But we're not at that point—the Earth hasn't warmed that much yet.
On the other hand, climate models do project that snowstorms in the southernmost parts of the United States should become less frequent in the coming decades: There's plenty of moisture down south, but freezing temperatures are likely to decrease and the jet stream is expected to shift northward. So if those regions start seeing a sustained uptick in snowfall, then something's gone awry in climate predictions. But one blizzard in the Northeast, while miserable and incredibly disruptive, isn't out of whack with long-term forecasts. (That's not exactly cheerful news for those of us who have to live here.)