In late June and early July 2021, the heat wave that struck western North America was not just any midsummer event. For more than nine days, from British Columbia through Washington and Oregon and beyond, the temperature surpassed average regional temperatures for the period by 10°C (18°F), and on single days in few locales, by an incredible 30°C or 54°F.
The 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. Under the peaks, western Eurasia and northeast Siberia experienced temperature spikes, but North America (inside box) saw the worst. Within a fourth peak, Iceland also saw elevated temperatures. Image Credit: Bartusek et al., Nature Climate Change 2022.)
This occurrence created a new national record for Canada, at 121.3 F in Lytton, British Columbia. The following day, almost the full town burned to ashes amid an unmanageable wildfire—one of several sparked by the hot and dry weather. Throughout the region, over 1,400 people died from heat-related reasons.
Within weeks, researchers blamed the extremity of the event hugely on climate change. A new study performed in the journal Nature Climate Change asserts that conclusion, and for the first time, extensively clarifies several mechanisms. These include a few rigorously climate-related mechanisms and others in the domain of catastrophic coincidences that researchers say resulted in the mind-bending temperatures.
It was so extreme, it’s tempting to apply the label of a ‘black swan’ event, one that can’t be predicted. 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.
Samuel Bartusek, PhD, Study Lead Author and Student, Columbia Climate School, Lamont-Doherty Earth Observatory
The study took climate data starting in the 1950s collectively with daily weather observations from the weeks prior and during the heat wave to develop an intimate portrait. They found that such an event would have been impossible without human-induced warming.
It would not have been possible in the 1950s, but atmospheric warming has shifted the needle to a potential 1-in-200-year event that is still occasional but currently possible. Scientists anticipate that if warming remains at a moderate pace, such heat waves could occur in the region nearly every decade by 2050.
In the past century, average global temperatures have increased below 2°F. However, small upward increments might change interactions between the air and land, forcing chances of extreme temperature spikes much more than the average temperature increase.
Boiled down to the easiest terms, the study says that much of the 2021 heat wave arises from the multiplying effects of greater overall temperatures. This includes drying soils in a few areas.
The researchers say one significant driver of the heat wave was an interruption of the jet stream, which generally carries air west to east throughout the Northern Hemisphere midlatitudes along a more or less circular path. Before the heat wave, though, the jet stream stopped and bent into huge waves, along with four north-south peaks and troughs.
Such concentrated high-pressure systems are present below every peak; high pressure squeezes air more and more as it reaches the surface, producing heat. One of those systems settled in western North America, then stayed there day after day, making what meteorologists call a so-called “heat dome.”
Few researchers think big jet-stream waves are turning out to be highly frequent and extreme as a result of human-induced warming. Generally, the jet stream shapes a boundary between the rigid polar air and warmer southern air. However, recent outsize warming in the Arctic is the separation of the temperature difference and instability of the system, say the researchers. This concept is still a matter of discussion.
That said, part of the foundation for the new study was placed by coauthor Kai Kornhuber, who published a 2019 study determining such meanders as hazards to world food security should they hit multiple significant agricultural regions concurrently.
In 2021, coinciding major heat waves hit not just North America but covered much of Eastern Europe, western Russia, Scandinavia, the Caucasus, and another over northwestern Siberia.
So far, Western North America’s heat wave has been the worst. The authors say that one factor was a range of smaller-scale atmospheric waves produced in the western Pacific Ocean. These moved east and, subjecting to hitting land, locked onto the bigger jet-stream wave and amplified it. Meteorologists observed such patterns coming around ten days prior and had warned of the heat wave much earlier.
The scientists state that a longer-lasting key factor is climate-driven drying that has overcome much of the US and Canadian west in the past few years. This decreases soil-moisture levels in several regions.
When the heat wave was caused, it decreased water evaporation from vegetation that earlier would have helped neutralize the heating of the air next to the surface. Having less evaporation in a few places, the surface was efficiently heated by the air above it. The scientists discovered that the heat wave was extreme in regions with the driest soils.
Global warming is gradually making the Pacific Northwest drier.
Mingfang Ting, Study Co-Author, Lamont-Doherty professor, Columbia Climate School Climate, Earth, and Society
Such extreme events are becoming ever more likely.
Remarkable heat and drought remain to impact the region. In mid-October of this year, several daily temperature records were broken with spikes more characteristic of high summer than mid-autumn. These included 88° in Seattle on October 16th, 2022—a full 16° above the earlier daily record.
On the same day, there were records in Wash (85); Vancouver (86); Olympia, and Portland, Oregon. (86), its fifth successive day in the 80s. The hot, dry weather sparked fierce forest fires. This smoke caused Seattle to note the worst air quality of any big city in the world.
We can certainly expect more hot periods in this area and other areas, just due to the increase in global temperatures, and the way it shifts the probability of extreme events by huge amounts.
Samuel Bartusek, PhD, Study Lead Author and Student, Columbia Climate School, Lamont-Doherty Earth Observatory
Journal Reference:
Bartusek, S., et al. (2022) 2021 North American heatwave amplified by climate change-driven nonlinear interactions. Nature Climate Change. doi.org/10.1038/s41558-022-01520-4.