Next time your weird uncle or least favorite Twitter user decides to make a joke about cold weather and global warming, offer them some historical perspective. A new attribution analysis finds that extreme cold outbreaks comparable to the one that swept the Eastern U.S. last week are fifteen times less likely today than they were in the early 20th century. You’ll never guess why.

The analysis, released Thursday by Climate Central, looked at the long-term context for the bone-rattling cold spell that swept the northeastern U.S. and southeastern Canada toward the start of the year. It’s part of an emerging field of weather attribution, which seeks to understand linkages between individual extreme weather events and climate change, sometimes by placing them in the context of their historical frequency.

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While many recent attribution studies have looked at events that we think of as increasing in a hotter world—extreme downpours or heatwaves, for instance—the same techniques can be used to look at weather that’s expected to become less frequent, including severe cold snaps.

“In general, we feel like attribution should tackle any event that appears to be important,” study co-author Claudia Tebaldi, a climate statistician at the National Center for Atmospheric Research, told Earther. “This was a case where the event was so unusual, we felt that it would be good to say something about it one way or the other.”

Indeed, the cold snap that gripped the eastern U.S. during the last week of 2017 and first of 2018 saw temperatures of nearly 20 degrees Fahrenheit colder than average for this time of year. To find out how weird this truly was, Tebaldi and her co-authors first compared the cold outbreak to similar cold outbreaks from 1981 to 2010. They used this comparison to define a geographic region where severe cold spells tend to occur, and then looked at temperature records of the coldest two weeks within that region, going all the way back to 1881.

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“From there everything falls out,” Tebaldi said.

The analysis showed that by the standards of 2018, the last two weeks were very unusual—a 1 in 250 year event, statistically speaking. But at the beginning of the 20th century, a cold wave like this occurred on average every 17 years, or roughly 15 times more frequently. Overall, the two coldest weeks of winter have gotten warmer over time, as expected in a warming world.

Temperature of the coldest two week period for the northeastern U.S. in years going back to the 1880s. Image: van Oldenborgh et al. 2018

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It’s important to note that this analysis has not yet gone through peer-review, and there’s still quite a bit of debate over the best approaches to attribution. Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research, told Earther he thinks the study’s approach is “wrongly framed,” because by looking just at statistics, “it makes underlying assumptions that the climate has not changed, but it has.” In this case, however, he thought the study’s conclusions “are about right.”

Others may not be on board with all of study’s conclusions. One particular sticking point is that the study seems to contradict a long-debated theory about how rapid Arctic warming could lead to more cold blasts at low latitudes.

But Rutgers climate scientist Jennifer Francis, a leading proponent of that theory, doesn’t think the new findings are at odds with it.

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“I wholeheartedly agree with one conclusion of this study, which is that cold spells should become less intense,” Francis told Earther in an email. “However, our hypothesized linkage with Arctic amplification [rapid Arctic warming] is not that record-breaking cold should happen more often, but instead that cold spells should become more persistent.”

Francis thinks cold snaps should last longer when they do come, thanks to a warmer Arctic yielding a wavier jet stream. That’s something the new study, which limited itself to two week cold periods, didn’t look at. Tebaldi added that the new study’s geographic restriction makes it hard to test ideas about jet stream wobbles, which could affect weather across all of North America.

“We cannot see the Arctic influence that supposedly makes these [cold outbreaks] worse,” she said. “But we are looking at this geographic box, these two weeks. We’re not saying the theory’s wrong, just that we can’t say we see it in this study.”

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As for the theory that climate change is affecting cold weather, period? The evidence seems pretty clear that it is.