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The first week of January was the coldest such week on record in most locations in the Eastern United States. It was so frigid that week, and the week preceding it, that sea ice formed around Cape Cod and Chesapeake Bay, sharks froze to death on Massachusetts beaches, and alligators went into a resting state while entombed in ice.

One might think that a cold snap like this one all but disproves global warming, or at least refutes the more dire scenarios about winter all but disappearing as the globe responds to sharp increases in greenhouse gases, such as carbon dioxide and methane.

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However, the reality is far more complex, scientists say. In fact, it's getting harder to pull off a cold outbreak of the severity and longevity of the late December and early January Arctic blast, according to a new analysis published on Thursday.

Data visualization showing very cold temperatures gripping large portions of North America on Jan. 1, 2018Credit: noaa/nnvl

The study, by the World Weather Attribution project, an international consortium of researchers that analyze the role global warming may have played in extreme events, concludes that a cold outbreak like the one that just occurred is 15-times less likely to take place today due to global warming.

Scientists from Princeton University, the Royal Netherlands Meteorological Institute, University of Oxford, and Climate Central examined the two-week cold wave, between Dec. 26, 2017 and Jan. 8, 2018, over the northeastern U.S. and southeastern Canada. They found that this was a "relatively rare event" now that global warming has made such cold snaps less frequent and severe.

In fact, the attribution analysis, which has not yet been peer-reviewed, found that the effects of global warming on cold outbreaks like this is to make them warmer than they otherwise would be, by about 4 degrees Fahrenheit.

Trend in the coldest two weeks of the winter as a multiple of the global mean temperature rise during the 1880 to 2017 time period.Credit: Berkeley Earth/ ERA-interim

For the study, scientists compared the temperatures during the cold wave to readings during the past 30 years, as well as the time series of the temperature of the coldest two weeks of the year, dating back to 1880. They found that there were many equally cold or colder two-week periods in this region in the past, but none have occurred since the winter of 1993-94.

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“Cold waves like this occurred more frequently in the climate of a century ago and the temperature of two-week cold waves has increased throughout North America, which is consistent in a climate of global warming,” said Geert Jan van Oldenborgh, senior researcher at the Royal Netherlands Meteorological Institute (KNMI), in a press release.

Many records were broken during this cold snap, with the most frigid conditions found on the back side of the "bomb cyclone" that slammed the East Coast with snow and high winds on Jan. 3 through 5. New York City’s temperature remained at or below 32 degrees Fahrenheit for two weeks, which ranks among the top five records for the most consecutive number of days at or below freezing. Chicago’s 12 consecutive days below 20 degrees Fahrenheit tied a record seen only twice before in 1895 and 1936

The study's findings are likely cold comfort to the millions who just experienced bone-chilling conditions, though. But they are important, since they show how winter cold spells are changing as the climate warms.

Temperature of the coldest two-week period averaged over land points in 40º–50ºN, 65º–95ºW. The green line is a 10-year running mean.Credit: WWA

The scientists found that the temperature of the coldest two-week period has increased about two times faster than the global mean temperature has increased. In other words, the coldest periods are warming faster than the overall warming rate, making cold extremes less severe and more rare.

In fact, the researchers calculated that a cold wave like this occurred about once every 17 years at the beginning of the 20th century, but now can be expected to occur just once out of every 250 years. In other words, there used to be a 5.8 percent chance of such a cold wave occurring in a given year, but now the odds are down to 0.4 percent.

While some scientists contend that melting Arctic sea ice is causing colder air to leak southward into the midlatitudes during the winter, thereby intensifying winter weather in the U.S. and Europe, this study argues against that.

The researchers found that the weather pattern that caused the two-week cold period has not been occurring more frequently lately.

In any case, winter has only just started. While the odds of another cold snap of similar severity are long, they're not zero. So keep that heavy coat and long underwear handy for a little longer.

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