Photograph by Yuri Gripas, AFP/Getty
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People enjoy unseasonably warm weather near the White House on December 11, 2015.

Photograph by Yuri Gripas, AFP/Getty

What’s Behind the Freakishly Warm Winter in the Eastern U.S.?

Global warming, El Niño, and the jetstream are giving many Americans a balmy December. 

People from Massachusetts to Mississippi cut down their Christmas trees while wearing shorts and t-shirts this year. Now, the chances for a white Christmas are slim to none for much of the East Coast, leaving kids disappointed and adults scratching their heads.

Two weekends ago, temperatures were 20 to 30 degrees higher than normal in much of the East, shattering records in some places. Warm temperatures have returned in many places Wednesday, and forecasts for the near future suggest more of the same.

What’s going on? One big factor is the strong El Niño taking place this year. The periodic warming of the Pacific Ocean every few years tends to bring more moisture and warmer temperatures to the western coasts of North and South America. Globally, El Niño boosts air temperatures by at least 0.1°C, writes meteorologist Jeff Masters with the Weather Underground.

El Niño has spurred warmer temperatures around the planet for several months. October was the warmest on record, at 0.98°C (1.76°F) above the 20th century average for the month.

Not only does El Niño bring warmer temperatures, but it’s also been forcing the jet stream farther to the north. Normally, that band of strong, high winds brings colder air and more moisture down to lower latitudes. This year, El Niño and wind patterns combined to push heavy snow to places such as Denver while still leaving areas further east relatively balmy. (It’s basically the reverse of what happened with the jet stream this time last year, when a blast of cold air from the polar vortex brought unusually low temperatures to the eastern U.S.)

Watch: Learn more about El Niño.

But the severe lack of snow cover in much of the East is also contributing to a feedback loop that’s keeping things warmer, writes meteorologist Eric Holthaus. (Learn more about El Niño’s effects on the weather.)

Finally, it’s likely global warming is contributing at least partly to the warmth, although it can’t be considered fully responsible, notes Holthaus. This year is shaping up to be the warmest year on record, according to the United Nations’ World Meteorological Organization, with global surface temperatures set to breach a 1 degree Celsius rise from pre-industrial levels. 2014 was also the hottest year to date, and the past five years are shaping up to be the hottest such period.

This winter has been unusually warm in much of Europe, too, hurting sales for winter clothing.

Unseasonable weather is one of the many potential effects of global warming that world leaders discussed at the Paris Climate Talks earlier this month, when countries agreed to a new roadmap to reduce greenhouse gas emissions and cope with a shifting climate.

Watch: Learn more about the polar vortex.

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