A study offers new insights into the record 2021 Western North America heat wave

The heat wave that hit western North America in late June and early July 2021 was not just a midsummer event. Over nine days, from British Columbia to Washington and Oregon and beyond, the region’s average temperature for the period was 10 degrees C (18 F), and on single days in some places, it exceeded 30 C, or 54 F. new daily record, it set a new national record for all of Canada, at 121.3 F in Lytton, British Columbia. The next day, the entire town burned in an uncontrollable fire – one of those caused by the hot and dry weather. Across the region, at least 1,400 people died from heat-related causes.

Within weeks, scientists blamed the severity of the event largely on climate change. Now, a new study in the journal Natural Climate Change confirms that conclusion, and for the first time comprehensively illuminates the multiple mechanisms—some strictly climate-related, others more in the realm of catastrophic coincidences—that they say have caused the global warming.

“It was so extreme, it’s tempting to label it a ‘black swan’ event, an unpredictable one,” said lead author Samuel Bartusek, Ph.D. student at Columbia’s Lamont-Doherty Earth Observatory School of Meteorology. “But there’s a line between the totally unpredictable, the reasonable and the totally expected, which is hard to categorize. I’d call it more of a gray swan.”

The study combined climate data dating back to the 1950s and daily weather observations from the previous weeks and during the heat wave to create an intimate portrait. A basic conclusion: Such an event would have been almost impossible without human-induced warming. That was unlikely in the 1950s, but since then atmospheric warming has shifted the needle to a 1-200 year event – still rare, but now possible. Researchers predict that if warming continues at a moderate pace, heat waves will occur in the region every 10 years by 2050.

Global average temperatures have risen by less than 2 degrees F over the past century. But small upward increases may change the relationship between the atmosphere and the land in ways that increase the chances of more severe than average temperature increases. In the simplest terms, the study says that most of the 2021 heat wave resulted from the increased effects of higher overall temperatures, including drying of the soil in some areas. In addition, about a third of the heat wave came from what researchers call “nonlinear” forces — short-term weather patterns that helped trap heat that may also be amplified by climate change.

A major driver, they say, was a disruption of the jet stream, which normally moves air from west to east across the midpoints of the northern hemisphere in a more or less circular path. Before the heat wave, however, the jet stream stopped and moved in large waves, with four peaks and north-south ridges. These high pressure systems under each peak; The high pressure makes the air more and more hot. One of those systems settled over western North America, then stayed there for days, creating what meteorologists call a “heat dome.”

Some scientists believe that large solar jet waves are becoming more frequent and intense due to human warming. The jet stream normally creates a boundary between cold polar air and warmer southern air, but recent large warming in the Arctic is breaking the temperature difference, destabilizing the system, they say. This idea is still debated. That said, part of the groundwork for the new research was laid by colleague Kai Kornhuber, who published a 2019 study that identified meanders as threats to global food security if they hit multiple large agricultural regions simultaneously. . In 2021, large synchronous heat waves associated with meanders occur not only in North America, but in a dome covering most of Scandinavia, Eastern Europe, western Russia, and the Caucasus; and another in northwestern Siberia.

The North American West was by far the worst. One factor, the authors say, was a series of smaller atmospheric waves that formed in the western Pacific Ocean. These moved eastward, and after hitting land, caught the largest wave of the jet-stream and amplified it. Meteorologists were able to see these patterns emerging for 10 days, and thus accurately warn of the heat wave early.

A key longer-term factor, the researchers say, is the drier climate that has swept much of the western United States and Canada in recent decades, reducing land-burn levels in many areas. During a heat wave, this means that the evaporation of water from the plant is reduced which would previously help to warm the air near the earth’s surface. With less evaporation, the surface warms the upper air more effectively in some areas. In fact, the researchers found that the heat wave was the highest in the areas with the driest soils.

“Global warming is gradually making the Pacific Northwest drier,” said study co-author Mingfang Ting, a Lamont-Doherty professor, adding that it is leading to a long-term situation where such extreme events are becoming more frequent.

Extreme heat and drought are affecting the region. In mid-October this year, many daily temperature records were broken with waves that are higher than mid-autumn typical of summer. They included a high of 88 degrees in Seattle on October 16 – 16 degrees above the previous daily record high. On the same day there were records in Vancouver (86); Olympia, Wash. (85); and Portland, Ore. (86), his fifth consecutive day in the 80s. The hot, dry weather made the wildfires so fierce and widespread that on Oct. 20, smog caused Seattle to have the worst air quality of any major city in the world, ahead of popular favorites like Beijing and Delhi.

“We certainly can expect warmer periods in this area and elsewhere, just because of global warming, and the way that it’s increasing the likelihood of extreme events,” Bartusek said.