A study provides new insights into record-setting 2021 western North American heatwave

November 24, 2022

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A study provides new insights into record-setting 2021 western North American heatwave

Combined with unusual weather systems, affected by climate change

The 2021 heat wave hitting western North America in late June and early July is not just any midsummer event. For nine days, from British Columbia to Washington and Oregon and beyond, it exceeded the average regional temperature for the period by 10 degrees Celsius (18 degrees Fahrenheit), and on days in some areas, exceeded An astonishing 30 degrees Celsius or 54 degrees Fahrenheit. Many new daily records, it set a new national benchmark for the whole of Canada at 121.3 F in Lytton, British Columbia. The next day, the entire town burned in an uncontrollable wildfire — one of many sparked by the hot, dry weather. Across the region, at least 1,400 people died from heat-related causes.

Within weeks, scientists blame the extremity of events Mostly about climate change. Now, a new study in the magazine natural climate change Affirming this conclusion, and for the first time fully elucidating the multiple mechanisms—some closely related to climate, others more of a catastrophic coincidence—mechanisms they say are responsible for the mind-boggling temperatures.

“It’s so extreme, it’s easy to label a ‘black swan’ event, an event that can’t be predicted,” said the lead author Samuel BatusekPhD Student at Columbia Climate Institute Lamont-Doherty Earth Observatory“But there’s an indefinable line between the totally unpredictable, the plausible, and the fully expected. I prefer to call it a gray swan.”

Map with arrows showing global wind direction

The 2021 heat wave in western North America comes in part from the jet stream in the northern hemisphere bending into four huge north and south peaks and valleys. In the picture above, the redder the color, the higher the temperature; the black arrow indicates the wind direction. Below the peak, temperature spikes were experienced in western Eurasia and northeastern Siberia, but most severe in North America (boxed). Temperatures also rose in Iceland during the fourth peak period. (Adapted from Bartusek et al., Nature Climate Change 2022.)

The study combined climate data starting in the 1950s with daily weather observations in the weeks leading up to and during the heatwave to create an intimate portrait. A central conclusion: Such an event would be nearly impossible without human-caused warming. That was impossible in the 1950s, but atmospheric warming since then has shifted the pointer to an expected once-in-200-year event—still rare, but now doable. The researchers predict that such heat waves will hit the region roughly every 10 years through 2050 if warming continues at a moderate rate.

Average global temperatures have risen by less than 2 degrees Fahrenheit over the past century. But small increases could alter the interaction between the atmosphere and land, making extreme temperature spikes far more likely than average temperature increases. Boiled down to the simplest of terms, the study says most of the 2021 heat wave will be caused by the multiplier effect of the overall temperature increase, including drying out of the soil in some areas. What’s more, about a third of heatwaves come from what the researchers call “nonlinear” forces — short-term weather patterns that help lock in the heat, which can also be amplified by climate change.

A major driver, they say, is the disruption of the jet stream, which typically follows a more or less circular path that carries air from west to east across the mid-latitudes of the northern hemisphere. Before the heat wave hits, however, the jet stream stops and bends into huge waves with four north-south crests and troughs. These are concentrated high-pressure systems below each mountain peak; as the air approaches the surface, the high pressure compresses the air more and more, which creates heat. One of these systems settled in western North America, then stayed there day in and day out, forming what meteorologists call a “thermal dome.”

The hot and dry weather has sparked multiple wildfires, destroyed large areas and deteriorating air quality. (Photo by Kari Greer, USDA)

Some scientists believe that large jet waves are becoming more frequent and extreme due to human-caused warming. The jet stream normally forms a boundary between cold polar air and warm southern air, but recent unusual warming in the Arctic is breaking up the temperature gap and destabilizing the system, they said. The idea is still being debated.That said, part of the groundwork for the new study was laid by co-authors Kay CohenhuberWho published a 2019 study These twists and turns were identified as a threat to world food security if they hit multiple major agricultural regions simultaneously. In 2021, a simultaneous major heatwave associated with the meander hit not only North America, but also Scandinavia, Eastern Europe, western Russia, and much of the Caucasus; another was in northwestern Siberia.

North America West is by far the worst. One factor, the authors say, is a series of smaller-scale atmospheric waves generated by the western Pacific Ocean. They moved eastward and caught and amplified larger jet waves as they hit land. Meteorologists can see these patterns about 10 days later, so heat waves can be accurately warned in advance.

A key long-term factor, the researchers say, is the climate-driven drought that has swept much of the western United States and Canada in recent decades, reducing soil moisture in many areas. During heat waves, this means less evaporation of water from vegetation, which previously helped to counteract the heating of the air near the surface. With less evaporation, in some places the earth’s surface can heat the air above it more efficiently. In fact, the researchers found that heat waves were worst in areas with the driest soil.

“Global warming is gradually making the Pacific Northwest drier,” said the study’s co-authors Mingfang Tinga Lamont-Doherty professor, pushes it into a secular state in which such extreme events become increasingly likely.

Unusually high temperatures and drought continue to affect the region.In mid-October this year, many Daily temperature records broken Compared with Mid-Autumn Festival, Midsummer has more peak characteristics. These included 88 degrees in Seattle on Oct. 16 — a full 16 degrees above the previous daily record. On the same day, there were records in Vancouver (86); Olympia, Washington (85); and Portland, Oregon (86), for the fifth straight day in the 80s. The hot, dry weather fueled wildfires so intense and widespread that on Oct. 20, smog caused Seattle to have the worst air quality of any major city in the world, surpassing the usual favourites, Beijing and Delhi. .

“We can certainly expect more hot periods in this region and elsewhere, simply because of rising global temperatures and the way it dramatically changes the probability of extreme events,” Bartusek said.