NEW YORK - Climate change has made severe cold spells like the one that recently gripped the US Northeast far less common than they used to be, a team of researchers has found.
The reason is straightforward: The Arctic has warmed so much - twice as fast in recent decades than other parts of the world - that when polar air descends to lower latitudes, the cold snaps are warmer on average. So a spell of extremely cold weather like the recent one is rare, about 15 times rarer than a century ago, the scientists said.
"Although this cold spell would not have been unusual before global warming, it is now a relatively rare event in any one region," the study's authors, from a loose-knit international group called World Weather Attribution, wrote.
Put another way, the most intense cold waves are on average about 2 degrees Celsius warmer now than previously, they said.
"The trend in general for these cold waves is that they're warming," said Claudia Tebaldi, a research fellow with Climate Central, a news organisation that focuses on climate science and coordinated the study.
While Arctic air is milder because of climate change, the question whether global warming is actually leading to more cold spells has been much debated.
Some recent studies have suggested that the loss of Arctic sea ice because of warming could be a factor, by influencing air circulation patterns, weakening the jet stream and allowing more polar air to shift southward, at least in some regions. But Tebaldi said her group's study found no connection to sea-ice loss.
World Weather Attribution is one of a number of scientific teams that in recent years has begun to analyse weather events for signs of the influence of climate change.
The group, with members from the Royal Netherlands Meteorological Institute, Princeton University and other institutions as well as Climate Central, has found that global warming increased the likelihood of Hurricane Harvey's torrential rains, a heat wave in Australia and other events.
But some analyses have found little or no effect of climate change, including a study of a Somali drought in 2016-17. The idea behind such rapid studies is to get scientific analysis to the public as soon as possible after an event.
In this case, Tebaldi said, the analysis was even more rapid than usual - published just three days after the cold spell ended - because the researchers used methodologies that had already been developed, and because the study relied only on observations, not on computer models that can take time to run.