The record-crushing cold that rang in 2018 was like a blast from the past that will become increasingly rare.

For much of the Eastern United States, the polar vortex unleashed the coldest start to a calendar year in recorded history. The punishing cold was exceptional for both its strength and duration, shattering scores of records and persisting two weeks after its invasion on Christmas Eve.

The frigid spell was particularly noteworthy for occurring when the planet is steadily warming because of the accumulation of greenhouse gases in the atmosphere. Climate warming from the increase in these heat-trapping gases does not eliminate cold waves, but it tends to weaken them.

An international group of scientists has concluded the recent piercing chill was 15 times less likely than it would’ve been 100 years ago, when temperatures in such cold waves averaged about 4 degrees lower.

The cold was “highly unusual in the current climate,” wrote the scientists who analyzed the temperatures.

Temperatures were close to 25 degrees colder than normal in a zone that included Minneapolis, Detroit, Buffalo and Boston over the period from Dec. 26 to Jan. 7. Boston’s high temperature did not escape the teens for seven days, tying its longest streak in recorded history.

Given how much the climate has warmed, the scientists called the recent cold “exceptional.” Yet, as frigid as it was, the scientists found that there were “many similar or colder two-week periods in that region in earlier periods.” They made clear that, on balance, cold outbreaks like these are getting less frequent “due to global warming, but cold waves still occur somewhere in North America almost every winter.”

Their finding that the intensity of Arctic cold is easing in a warming world is supported by many other studies. For example, Jonathan Martin, a meteorology researcher at the University of Wisconsin, has documented considerable shrinkage of the pool of frigid air surrounding the Arctic in recent decades.

The scientists involved in the analysis rejected the hypothesis that climate change is making these cold extremes more likely by warming the Arctic more than the mid-latitudes and altering the atmosphere’s circulation.

“We do not find any evidence for an intensification of these types of cold waves due to the Arctic warming faster than the midlatitudes,” they concluded. In fact, they said, less Arctic air is coming south than it used to.

The rapid-turnaround analysis was published as part of the “World Weather Attribution” project, in which — in a matter of days — scientists assess whether a recent weather event was made more or less likely by climate change. The project is a partnership of Climate Central, a nonprofit science communication organization in Princeton, N.J., and several international research institutions.

Even as the planet warms overall, some scientists who were not involved in this analysis have published studies that show climate change may be altering the atmosphere’s steering currents in ways to concentrate punishing cold in certain locations at certain times.

Daniel Swain, a researcher at UCLA, published a study late in 2017 that showed a warm-West, cold-East weather pattern over the United States has become more frequent in recent decades. A subsequent study linked the prevalence of this pattern to Arctic sea-ice loss driven by climate change.

Generally, a growing body of literature has suggested that as the Earth warms and Arctic ice melts, the jet stream may slow down and become more erratic, which could lead to pockets of extreme winter conditions, even as most places thaw.

Michael Mann, a climate scientist at Pennsylvania State University, described the recent winter weirdness as “in no way evidence against climate change.” In fact, he wrote, “it is an example of precisely the sort of extreme weather we expect because of climate change.”

The government’s National Climate Assessment cited human influence as the "dominant cause of the observed warming since the mid-20th century." (Patrick Martin/The Washington Post)