The recent snowstorms in the northeastern U.S. and a similar blizzard have made some people ask whether global warming is real. The short answer is that it is - and some studies predicted exactly this result.
First to explain how a warmer world could feel colder: just because the planet gets warmer overall doesn't mean that it is going to be the same everywhere, all the time. The global temperature increase of anywhere from 2 degrees Fahrenheit to 12 degrees or more that scientists have predicted is only an average. Climate is very different from weather - Florida had a record cold year in the 1980s, but tourists still flock there in winter.
Second, even a uniform average increase won't make much difference in how a winter temperature feels. The difference between -20 degrees and -18 degrees isn't much said James Overland, of the National Oceanographic and Atmospheric Administration. I mean, it's still darned cold.
That said, there is some research that predicts colder winters with more snow, even though the winter season itself might be shorter.
How does this happen? One reason is moisture. Warmer air holds more moisture, so more precipitation occurs when that warm air hits colder air. That becomes snow when the temperature is below freezing.
The process is illustrated by a 2006 study led by Stanley Chagnon, a professor of geography and atmospheric science at the University of Illinois, tracked where and what time of year snowstorms happened in the U.S. between 1901 and 2001. What he and his colleagues found was that 71-80 percent of heavy snowstorms occurred in warmer-than-normal years. A further 61 to 85 percent of storms happened in when the weather was exceptionally wet.
Another trend the study spotted was that overall, the number of snowstorms went up. The lower Midwest and South had fewer snowstorms over time, but the Northeast and upper Midwest had an increase. In the West, the trend was up, then down for the last decade of the century.
A more recent study, published in December, from the Potsdam Institute for Climate Research in Germany, notes that changes in the temperature of the Arctic might actually increase the odds that a winter in northern Europe will be colder than normal.
Researchers led by Vladimir Petoukhov ran a model that assumed sea ice cover kept on decreasing. What the team found was that without the ice, the sea (specifically north of Russia) transfers more heat to the air, disrupting the usual circulation patterns. This would create blocking events which is essentially when of colder air is trapped over a certain area.
This doesn't mean that the last two years indicate anything like a major shift, at least not yet. Most climatologists would say that it would require many more years of data to point to a real change in climate. But to some in the field the signs are worrying, and not only because of the snow.
Overland notes that the heavy snows last winter, as well as this one, were driven in part by changes in the circulates of air in the Arctic. Ordinarily, there is a pattern of counterclockwise-circulating air called the polar vortex that makes a circle around the region. The circulation is driven by the temperature difference between the low and high latitudes.
As the average global temperature has risen, the difference between the Arctic and the rest of the hemisphere is reduced. That makes the circular pattern less neat, and allows the colder air from the north to move south. When it hits moist, warm air, it becomes rain or snow.
Much of Overland's studies focus on the thinning of arctic ice. That's important to air circulation because as more ice free areas in the Arctic form, more heat gets stored in the sea surface, and that heat is eventually given back to the atmosphere.
That heating disturbs the polar vortex. Overland says that may have sparked another trend he noticed: air flowing north to south as opposed to east to west.
We have had more of this north south meridian flow instead of east west flow - that brings cold air down, he said. What's really intriguing is that last winter that meridinal pattern was the strongest we have seen it since we started taking measurements 145 years ago.
What got his notice was that this is the second winter in a row that unusually warm temperatures have disturbed the flow of Arctic air. It's difficult to prove from just a couple of events, he said.
Overland's impressions are only anecdotal so far. A hard connection between global warming and the increased snowfall we have seen these last two winters would need many more years of data (most climatologists ask for at least a decade to show trends). In addition, Computer climate models don't have the resolution yet to see the atmosphere in blocks smaller than 100-200 kilometers, or about 60-120 miles. But many weather events happen on a smaller scale than that.
Rasmus Benestad, senior researcher at the Norwegian Meteorological Institute, says climate models won't capture most of the snow-making processes in detail. He adds that there have been changes in the atmospheric circulation patterns observed, especially in the tropics. I would expect to see increased snowfall as the temperature increases until it reaches 32 degrees F (0C), and then the proportion of snow to rain will diminish, he wrote in an email. But that expectation may not turn out to be true.
Benestad has blogged that while he isn't sure about Petoukhov's findings, it is more because of the constraints on the computer models used - the 100-200 kilometer resolution might affect the results, and the models assumed a relatively fixed sea surface temperature. But he notes that even with the caveats, the idea that cooler temperatures can exist in some areas doesn't contradict the idea that the planet is getting hotter.