Across much of the Northern Hemisphere this year, winter was a shadow of its former self — and climate change is partly to blame.
According to data released Wednesday by the Copernicus Climate Change Service, Europe’s average temperature for December through February was 6.1 degrees Fahrenheit above the 40-year average, shattering the previous record by more than two degrees. In the United States, temperatures were above average for every state but Alaska.
The season was the second-warmest on record for the globe as a whole — putting 2020 on track to be one of Earth’s top-10 hottest years.
This winter weirdness was in part driven by normal variations in global weather patterns, scientists say. But climate change, which tilts the planetary scales in favor of high-temperature extremes, exacerbated the variation and makes future warm winters more likely.
Karsten Haustein, a meteorologist at the University of Oxford, said climate change was the “only way to explain” these extraordinary highs. As long as humans continue to emit planet-warming gases, he added, winters as we once knew them will be fewer and farther between.
A Washington Post analysis of global temperature records has found that 10 percent of the planet has already warmed by more than 3.6 degrees Fahrenheit (2 degrees Celsius) since the end of the 19th century. The majority of these new hot spots are at far northern latitudes, where winter tends to be the fastest-warming season.
The past three months, a weather pattern known as the Arctic Oscillation caused a powerful polar vortex in the upper atmosphere, keeping cold air bottled up over the North Pole. Ripples in the vortex that cause winter’s typical Arctic outbreaks never came or only briefly visited places known for their frigid cold seasons.
As a consequence, the jet stream that pushes weather systems across the hemisphere flowed fast and straight from west to east — a situation that traditionally produces mild weather. (This February, the roaring jet stream enabled a plane to fly from New York to London in an unheard-of four hours and 56 minutes.)
It is not yet known whether this strong, persistent jet stream will become more commonplace as the world heats up, Haustein said. But winters that tend to be warm will become even warmer and sometimes wetter — taking a toll from ski slopes to farms, from bustling cities to windswept tundra landscapes.
This year’s strong Arctic Oscillation contributed to persistently warm, high-pressure air above the eastern United States. Winter storm tracks angled from the central United States toward Canada, missing much of the Appalachians and Mid-Atlantic, and the nor’easters that normally surge up the coastline were nowhere to be found.
“A third of the U.S. just wasn’t getting that deep, cold air,” said Karin Gleason, a climate scientist for the National Oceanic and Atmospheric Administration.
Yet the same phenomena that stole winter from the East Coast contributed to extreme cold in Alaska, heavy snowfalls in the Rockies and far northern New England and record-dry conditions in California.