Last year brought one of the harshest winters in recent memory across much of the eastern and central U.S., and this year winter seems to be getting off to an unusally potent – and early – start. So is this the start of a trend, and should we should expect weather like this every year?
Don't count on it, according to a number of climate scientists who've been researching the prevalence of weather extremes in recent years.
In fact, over the long term we're likely to experience far fewer extreme cold events like the one now being felt across much of the northern U.S., and by the end of the century extreme cold events like this week's could be something most of us who live in the Lower 48 will know about only from history books.
"We find no evidence that central and eastern U.S. cold extremes will increase in the future," said Dr. James Screen, a climate researcher with the U.K.-based University of Exeter who is currently in the middle of a multi-year study on the impacts of climate change in the Arctic, in an email interview with weather.com.
"On the contrary, based on simulations from numerous models we fully expect that cold extremes will become less frequent in the future due to global warming," he added. "This has been shown numerous times and follows very simple logic: as the planet warms, you get fewer cold extremes and more hot extremes."
Extreme cold events like this week's really aren't unusual when you look at climate data going back decades, Screen says. "These bouts of cold are 'normal' and in fact occurred quite frequently in the not-so-distant past."
He pointed out that last winter, if you take the temperature average over the central and eastern U.S., the coldest day dropped to -16.8°C (about 1.8°F). That was indeed bitterly cold, but Screen adds that between 1980 and 1999, there were 20 days that got this cold or even colder.
"We should not be so surprised when a cold snap occurs," he said. "It is normal weather variability."
A NASA visualization of the jet stream over North America.
(NASA)
What hasn't been normal in recent years is the pattern of the jet stream, the fast-moving river of air that swirls high above Earth's surface and marks the boundary between cold, polar air to the north and warm, moist tropical air to the south.
Here's how it affects our weather: When the jet stream sweeps southward, its looping waves drag cold Arctic air down from the north. That's what has happened this week. And when the jet stream moves north, it clears the way for warm tropical air to sweep in from the Gulf of Mexico.
Over the past decade, a number of scientists have noted that the jet stream has been getting stuck in unusual patterns for days or even months at a time, permitting weather patterns to stick around much longer than they normally would have.
Scientists like Rutgers University's Dr. Jennifer Francis suggest that global warming is playing a big role in the changes we're seeing in the jet stream, and that those changes could be the cause of some historic extreme weather events in recent years, including last year's exceptionally warm winter across much of Europe and the 2010 Russian heat wave that killed more than 55,000 people.
The theory goes like this: Because the Arctic is warming faster than anyplace else on Earth, sea ice is being lost at a rapid pace (summertime Artic sea ice is down by about 50 percent since 1900, according to Weather Underground's Dr. Jeff Masters). That means that vast swaths of the Arctic that were once covered in ice and snow -- which reflect sunlight -- are now instead made up of dark ocean waters that absorb it.
And as the Artic heats up, the temperature difference between the North Pole and the lower latitudes shrinks. So in this scenario, there's a direct connection between the loss of Arctic sea ice and the weakening of the jet stream, which causes it to get "stuck" in patterns that bring about extreme cold events like the one we're experiencing this week, Dr. Francis says.
The weakened flow of the jet stream could also make it more vulnerable to outside influences like Typhoon Nuri, which slammed into the Pacific jet stream a couple of weeks ago and is believed to have played a role in pushing the jet stream (and this week's blast of Arctic air) southward ever since.
Francis – whose research has been the subject of vigorous debate over the past year, as chronicled in this February 2014 letter in Science Magazine – acknowledged in an email interview that this a "very active" area of research, and added that it's possible a weaker jet stream might be more easily knocked about by a wide range of atmospheric influences.
"An outstanding question in my mind is whether the now weaker westerly flow in the jet stream – owing to Arctic amplification – is resulting in a jet stream that is more easily deflected by impulses from a variety of factors, including tropical storms, intense tropical convection, and mountain ranges," she said. "If so, these highly amplified patterns will become more frequent (and our research suggests they already are)."
Screen disagrees with Francis's conclusions about the ways in which the loss of Arctic sea ice may or may not be influencing weather at lower latitudes. In his research, Dr. Screen has found that Arctic ice loss reduces, rather than increases, the chances that events like this week's will occur.
In the email interview, Screen added this:
"In my view there are two far more important factors that will determine the frequency of extreme cold in the future. The first factor is that the U.S. (like the rest of the globe) is warming and is expected to continue to do so in the long-term (with short-term fluctuations due to natural variability).
"The second factor is that disproportionately large warming in the Arctic compared to the mid-latitudes (e.g. US) reduces the average temperature gradient between the two regions.
"This means that if an Arctic air mass is displaced southward into the US, the resulting temperature anomaly is smaller than in the case for a larger north-south temperature gradient (essentially the cold Arctic air is no longer as cold as it used to be, so the cold air outbreaks are less severe).
"Both of these factors reduce the risk of extreme cold.
In a paper he recently submitted to the Bureau of the American Meteorological Society, Dr. Screen and his fellow researchers found that if greenhouse gas emissions keep rising at their current pace, by the end of the century extreme cold will be an extremely rare event.
"Days as cold or colder than the 7 January 2014 are still projected to occur in the mid twenty-first century (2030-49), albeit less frequently than in the late twentieth century (1980-99)," the paper says. "However, such events will cease to occur by the late twenty-first century (2080-99), assuming greenhouse gas emissions continue unabated.
"Projected Arctic sea ice loss alone reduces the odds of such an event by one quarter to one third by the mid twenty-first century, and to zero (or near-zero) by the late twenty-first century," the paper adds.
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A side-by-side comparison of Grinnell Glacier in Montana's Glacier National Park. The black-and-white photo on the left dates from 1938, while the color photo on the right was taken in 2009. (T.J. Hileman and Lindsey Bengtson, USGS)