Global warming could make blizzards worse

A winter storm approaching the eastern United States is seen in a NOAA GOES satellite image released Monday. The National Weather Service (NWS) issued a blizzard warning for New York City and surrounding areas between coastal New Jersey and Connecticut, beginning 1 p.m. EST (18:00 GMT) on Monday and worsening overnight into Tuesday morning. (Reuters/NOAA/Handout)

We’re on the verge of another historic blizzard, with as much as two or three feet of snow expected in parts of the Northeast. Already, the press is throwing around terms like “Snowmageddon” to describe what’s coming. Next comes politics: Whenever the East Coast sees an extreme snow event, the weather is perfect for snow trolling – e.g., trying to use one cold event to refute a warming climate trend.

In this case that’s particularly inappropriate, though, because if anything, extreme snowfall may actually be enhanced by global warming. I know it sounds counterintuitive, but remember that even in a warming world, our hemisphere will still spend part of the year tilted away from the sun, with shorter days and colder temperatures — and winter storms.

The question scientifically, then, is what happens to those storms in a warmer world. So let’s examine what science can say about that question.

Technically, the storm about to slam the Northeast is called — that’s right — a nor’easter. What’s special about these storms is that they draw their energy from a temperature clash between freezing Arctic air on the one hand, and warmer Gulf Stream waters on the other.

Which, in turn, means a warming ocean could potentially intensify winter nor’easters.

“The U.N. Intergovernmental Panel on Climate Change has found that nor’easters like this one may grow stronger [with] human-caused climate change, as they are driven by the contrast between cold Arctic air masses and ever-warming ocean surface temperatures,” says Penn State climatologist Michael Mann.

“We also know that ocean surface temperatures off the U.S. east coast right now are unusually warm, and there is no doubt that a component of that anomalous warmth is due to human-caused climate change,” Mann adds. “Those warm ocean temperatures also mean that there is more moisture in the air for this storm to feed on and to produce huge snowfalls inland.”

That does not, of course, mean the current storm is caused by climate change. Rather, says Mann, it means that climate change may make certain aspects of the event worse, such as its snowfall.

Kevin Trenberth, a climate researcher at the National Center for Atmospheric Research, is willing to get very specific about just how much climate trends may influence this storm. “At present sea surface temperatures are more [than] 2 F above normal over huge expanses (1000 miles) off the east coast and water vapor in the atmosphere is about 10 percent higher as a result,” he says. “About half of this can be attributed to climate change.”

According to Trenberth, the atmospheric configuration that’s now preparing to dump massive snow on New England is quite reminiscent of 2010′s “Snowmageddon” — only, it’s occurring farther north. On that occasion, too, Trenberth notes, a winter storm came through at a time when the Atlantic Ocean was particularly warm — 3 degrees F warmer than normal in that case.

“That led to exceptional amounts of moisture being fed into the circulation of the storm and resulted in exceptional snow amounts in the Washington, D.C., area,” says Trenberth.

This point about atmospheric moisture is crucial. A physical equation called the Clausius-Clapeyron equation states that with warmer atmospheric temperatures, the air can hold more water vapor, setting up the chance for increased precipitation. “The atmosphere can hold four percent more moisture for every 1 F increase in temperature,” says Trenberth.

Sure enough, we’ve seen increasing trends toward extreme heavy precipitation in all regions of the United States (except Hawaii), and most of all the Northeast:

Source: U.S. National Climate Assessment.

In general, a warmer world is therefore expected to be a wetter world, and a downpour, when it occurs, can indeed be more intense. But in order for precipitation to fall as snow, temperatures must also be adequately cold. However, that’s not a very high hurdle: “It’s not hard at all to get temperatures cold enough for snow in a world experiencing global warming,” notes the Weather Underground’s Jeff Masters.

So, where a global warming world could see really enhanced snowstorms is when it remains cold enough for snow, even as there’s more precipitable water in the atmosphere.

There’s also some suggestive evidence of an actual trend in extreme winter storms. Overall, notes the U.S. National Climate Assessment, the northern hemisphere has seen a trend toward more frequent and intense winter storms since 1950. So has the United States. According to a 2013 consensus report published in the Bulletin of the American Meteorological Society, “The number of severe regional snowstorms that occurred since 1960 was more than twice the number that occurred during the preceding 60 years” in the United States.

None of this, by the way, addresses another major way that global warming could potentially intensify winter weather events – by interfering with the flow of the jet stream. (For more on that, read here.)

Granted, there are also ways that global warming may weaken some aspects of winter. With warmer average temperatures, snow may melt away more quickly. There’s also an idea that we might get “less snow, more blizzards” in a warmer world — in other words, less snow on average but more extreme snow events when they do occur.

Trenberth takes a similar view. As long as it’s cold enough — in the height of winter — he expects that more water vapor in the atmosphere will enhance snowfall. “So as long as it does not warm above freezing, the result is a greater dump of snow,” he comments. But at the opening and closing of the winter season, as things are on the warmer side, you might get less snow and more rain.

So in sum: While I wouldn’t call this a very settled scientific area, there are certainly reasons to think that in a warming world, we might get more snowfall, on average, in certain extreme winter storms.

That doesn’t mean global warming caused the event now unfolding. It just means that global warming is now affecting the background conditions of all of our lives — emphatically including winter weather.