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New research: 2021 Pacific Northwest heatwave had multiple causes, including climate change

Last summer, the jet stream became a "jet wave," which helped form a "heat dome" over the Pacific Northwest. The effect was a catastrophic heat wave that killed as many as 1,400 people.

(CN) — Last summer, a small village named Lytton, in British Columbia, set a record by recording the hottest day on record in Canada — ever. On June 29, temperatures there reached 121 degrees Fahrenheit, eight degrees higher than had ever been recorded anywhere else in the country. The next day, a wildfire broke out just south of the town. Within days, nearly the entire village had burned down. Two civilians died.

The Lytton wildfire was just one small scene in a historic heat wave that hit the Pacific Northwest last summer. It has been estimated that the extreme weather event led to more than 1,400 excess deaths (the number of deaths higher than statistically expected, during the 12-day heat wave). And a New York Times analysis found that on June 28 alone, there were 1,000 more heat related emergency room visits throughout the Pacific Northwest United States, compared to fewer than 10 such visits in 2019.

It was, in short, a catastrophe. In the weeks that followed, many rushed to blame climate change — a sensible conclusion, to be sure. But the unprecedented magnitude of the heat wave suggested there may have been other factors as well.

"It broke these heat records by such huge margins," said Samuel Bartusek, a Ph.D. student at the Columbia Climate School’s Lamont-Doherty Earth Observatory. "It was shocking, even to the broader science community."

Bartusek is the lead author of a paper published Thursday in the journal Nature Climate Change that attempts to perform a thorough autopsy on the 2021 heat wave. To do so, the researchers compiled climate data going back to the 1950s and weather observations from the heat wave and preceding weeks, to form what they call an "intimate portrait" of the disaster.

It's well established that temperatures will continue rise as long as humans keep pumping carbon into the atmosphere. What's less well established is how climate change will affect fluctuations in temperatures or extreme weather events.

The 2021 heat wave in the Pacific Northwest "was so extreme, it’s tempting to apply the label of a ‘black swan’ event, one that can’t be predicted,” said Bartusek. “But there’s a boundary between the totally unpredictable, the plausible, and the totally expected that’s hard to categorize. I would call this more of gray swan.”

The researchers lay out what they believe were the three main causes of the heat wave: the rise in global temperatures, the drying of the soil (another effect of climate change) and, crucially, a phenomenon involving the jet stream, the fast flowing air current high in the air that typically flows west-to-east and creates a sort of barrier between cool and warm temperatures. At times, this jet stream can "buckle," as Bartusek puts it, turning the stream into more of a wave. This can trap temperatures in certain regions, creating what scientists call a "heat dome."

"From time to time, the jet stream does form into wavy patterns," said Bartusek. "It happens in the summer. And it's connected to heat waves." He adds: "It’s not completely known how much climate change interacts with this phenomena. It's the subject of ongoing investigation."

At any rate, Bartusek's paper shows that's exactly what happened last summer: the jet stream became a jet wave, and a heat dome formed over the Pacific Northwest.

he 2021 heat wave over western North America came in part from bending of the Northern Hemisphere’s jet stream into four huge north-south peaks and troughs. Above, redder colors indicate higher temperatures; black arrows show wind directions. (Nature Climate Change, 2022)

Other parts of the world experienced temperature spikes too, including Iceland, western Eurasia and northeast Siberia, which were affected by different waves. But the wave that settled over North American was by fart the most damaging.

The three factors — longterm global warming, dry soil and a bending jet stream — all combined to amplify temperatures. It was, said Bartusek, a rare event, but one we'll likely see more of.

Bartusek and his co-authors write that before global warming, a heat wave like last summer's would be "virtually impossible." Now, after global temperatures have risen approximately 1.8 degrees Fahrenheit since the Industrial Revolution, a heat wave like that is an estimated "200-yearly occurrence."

"Its likelihood is projected to increase rapidly with further global warming, possibly becoming a 10-yearly occurrence in a climate 2 °C warmer than the pre-industrial period, which may be reached by 2050."

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