Why Berkshire County’s coldest December in 15 years doesn’t contradict climate change | Central Berkshires

Berkshire County shivered through its coldest December in 15 years, raising questions about whether global warming had stalled.

But scientists say the deep freeze, which pushed average temperatures well below historical norms, is consistent with climate change, as a warming Arctic destabilizes the polar vortex and increases the odds of prolonged cold spells alongside record warmth.

December 2025 averaged 23 degrees at the National Weather Service observation station at Pittsfield Municipal Airport, well below the historical norm of 28.

Records at the airport dating to 1939, along with data from the Williams College environmental observatory at Hopkins Memorial Forest in Williamstown, which extends back to 1892, show that December 1989, 1963 and 2000 were the coldest on record, averaging 12, 18 and 20 degrees, respectively.

At the Northeast Regional Climate Center hosted by Cornell University, climatologist Jessica Spaccio pointed out that last month’s polar outbreak from Canada covered a massive geographical area, extending from Maine to West Virginia — 10 states and around 90 million people in all.

The center located on the Ithaca, N.Y., campus partners with the National Oceanic and Atmospheric Administration, parent of the National Weather Service, to collect and distribute regional weather and climate data.

How climate change figured into that extreme weather event is “an active area of research,” Spaccio told The Eagle. There’s no clear consensus yet on how the kink in the polar vortex that caused the brutal Arctic intrusion will affect future outbreaks in frequency and intensity, she added.

“Even though we’re seeing an overall warming trend in our recent winters, we can still get these cold temperatures and year-to-year variability,” Spaccio said.

However, the polar outbreaks are trending fewer and farther between, she said. “We’re seeing more record warmth than record cold, so that would fit in with people’s perceptions.”

Of the 10 warmest Decembers on record, six have been in this century, according to National Weather Service records at the Pittsfield airport. The warmest was in 2015, with an average temperature of nearly 40.

“Climate reflects the long-term averages of weather,” said Alice Bradley, associate professor of geosciences at Williams College. “Even in a warming climate, you can have the occasional cold year or cold month in this case.”

However, as she pointed out, “it’s just that the cold months are getting less and less likely. We’re having more and more of the warmest ever and fewer and fewer of the coldest ever.”

Since Berkshire County is at the intersection of cold air masses from Canada and warmer air coming up the Atlantic coast, there can be sharp contrasts such as the early January thaw this weekend.

The circular polar vortex, which traps and spins frigid winds west to east 10 to 30 miles atop the North Pole in the winter, is becoming disrupted more often, splitting into chunks that can partner with the jet stream and invade the Northeast.

Weatherwise, “we sit in a wobbly place,” said Bradley, resulting in sharp swings between the cold and the warm, “so it’s mostly just luck which direction we go.”

Those disruptions make the jet stream — a continuous river of air from west to east six to nine miles high in the atmosphere — more likely to wobble, creating extreme cold and hot weather contrasts.

“We think that’s connected to climate change in several ways,” said Jennifer Francis, a senior scientist at Woodwell Climate Research Center in Falmouth, in a YouTube presentation.

Explaining the impact of the polar vortex, Bradley cited sea ice encasing the Arctic Ocean as creating strong winds circling the North Pole, resulting in a sharp contrast with milder air to the south.

But with sea ice thinning out, “the polar vortex in the Arctic has gotten wobblier and wobblier over time,” Bradley noted. The result: “Arctic air masses formed in the polar area dislodge” and make their way into the Northeast and points south.

Is climate change accelerating?

While scientists caution against drawing conclusions from short-term events, some data suggest the pace of warming itself is increasing.

Longer-term studies are needed, but Bradley said what really matters is the extent of fossil fuel discharges into global carbon dioxide.

The Williams College geosciences department is now working on digitizing Hopkins Forest weather data back to 1816 in order to study long-term trends, according to Bradley. 

Climate change driven by global warming is accelerating, based on a worldwide increase of nearly 3 degrees F from 2023 to 2025, with similar trends dating back several decades, said Michael Rawlins, associate professor of earth, geographic and climate sciences at the University of Massachusetts Amherst.

“That’s sobering,” he said. “These are really, really worrisome impacts of warming climate change. We don’t actually know what normal is anymore. Research shows that a rapidly warming Arctic is believed to be impacting the polar vortex and the jet stream.”

Rawlins is on board with an explanation for last month’s big freeze — a sudden warmup high above the Arctic in early December set the stage to dislodge chunks of polar air, sending that frigid air into the Northeast.

That view, while not unanimous among climate scientists, is shared by many, though others are cautious.

“Even if climate change isn’t accelerating, if it just continues apace, it’s still a problem,” Bradley said. “It’s not slowing down, for sure.”