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The wildfires in Canada are abnormally early and widespread this year. What’s at play?

The smoky haze was thick Wednesday at the George Washington Bridge in Fort Lee, N.J.Seth Wenig/Associated Press

In recent years, the word “wildfire” has conjured heartbreaking images that became grimly predictable: California ablaze, from its mighty forests to gracious vineyards and traffic-clogged highways of fleeing people.

It’s different this year, as the East Coast chokes on smoke blown south from abnormally early and widespread wildfires in Canada. Pennsylvania and New Jersey are now facing critical risks of fire danger, and more broadly the Northeast and Midwest also face elevated risks of wildfires, while California is at lower risk thanks to a record-high snow pack from this past winter.

What’s at play, climate scientists said, is an atmosphere increasingly roiled by conditions that can unpredictably shift areas of drought to deluge, as has happened in California, only to sow drought and excessive heat in another.

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According to some emerging science, the culprit could be a jet stream destabilized by factors including the loss of arctic sea ice and warmer arctic air. This phenomenon could potentially be responsible for seemingly unrelated weather in disparate parts of the continent: Cool wet weather in California, a heat dome in Western Canada, and a storm system off Nova Scotia that is pushing wildfire smoke south to cities along the Atlantic seaboard.

“It’s all connected, all part of the same pattern,” said Jennifer Francis, a senior scientist at the Woodwell Climate Research Center.

A wildfire needs two main ingredients: prolonged heat and prolonged drought. Throw in a few others, such as windy conditions, and the odds of a wildfire go up.

In western Canada in May, a heat dome — made five times more likely by climate change — settled in, pushing temperatures to highs not typically seen until mid to late summer. Fires in Alberta, British Columbia, and Saskatchewan took off, burning some 1,800 square miles by mid-month, 10 times the typical amount that burns by that time of year, according to NASA.

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By late May a heat wave settled in over Quebec as well, sending temperatures into the mid-90s and sparking fires. Fires started across the province, and in Nova Scotia as well. Those fires are burning out of control, and spewing the smoke now seen in Boston and spreading as far south as Atlanta. On Wednesday, the air quality in New York City was the worst in the world, prompting Mayor Eric Adams to urge residents to wear masks and stay inside. On Thursday, the smoke is expected to swirl back across Massachusetts.

“Each afternoon the fires flare up, and then the following evening and into the night you can clearly see the smoke moving southward into the Northeast,” said Eric James, a meteorologist at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder.

As the fires rage and the smoke swirls around the Eastern United States, the health risks associated with this aspect of climate change mount. In addition to the dangers faced by residents and firefighters in Canada, the smoke carries minuscule air particles that irritate eyes, lungs, and throats, can trigger asthma attacks, and, with extended exposure, can lead to lung cancer, according to the Centers for Disease Control and Prevention.

The winds that carry the smoke south are governed by the jet stream — a river of wind in the atmosphere that determines the weather below. And that is where things look particularly strange right now, said Francis, who studies the relationship between the warming climate and the jet stream.

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In the summer, there is typically just one jet stream in the atmosphere, but right now, there are two, Francis said. And the one wreaking havoc across the country, she said, “is in an incredibly wavy pattern.” While scientists are still working to understand what’s at play, Francis said the main factor appears to be an ocean heat wave that formed in the North Pacific in late January and is still going on — far longer than is typically expected, according to NOAA.

That excess heat appears to be causing a northward bulge in the jet stream as it snakes west to east, Francis said, pushing cooler air, even rain south into California while bringing hotter temperatures to Canada. “It’s been sitting there for quite a while,” she said. “And that’s contributed to the fires there, because it’s been warm and dry.”

Then that same jet stream, as it bends southward again, brings the smoke from Canada to the Eastern United States.

The phenomenon of wavy jet streams predates climate change and the precipitous rise of carbon dioxide from the burning of fossil fuels. What has changed, said Francis, is the frequency. Scientists have been working for years to better understand how extreme warming in the Arctic appears to be fueling this phenomenon by weakening the jet stream and creating more opportunities for extreme weather to appear in unexpected places for extended periods of time.

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“Big waves in the jet stream tend to hang around a long time, and so the weather that they create is going to be very persistent,” she said. “If you happen to be in the part of the wave in the jet stream that creates heat and drought, then you can expect it to last a long time and raise the risk of wildfire.”

As far as the smoke goes, Kristie Smith, a meteorologist with the National Weather Service in Massachusetts, said the state can expect it to stick around until at least Saturday. “We’ll be in and out of smoke,” she said.


Sabrina Shankman can be reached at sabrina.shankman@globe.com. Follow her @shankman.