A new study has concluded unusual weather systems, supercharged by climate change, were behind the record 2021 western North America heatwave

The heatwave that hammered western North America in late June and early July 2021 was not just any midsummer event. Over nine days, from British Columbia through Washington and Oregon and beyond, it exceeded average regional temperatures for the period by 10°C (50°F). On single days in some locales, that figure was an astounding 30°C, or 86°F.

Among many new daily records, it set a new national benchmark for all of Canada, at 49.6°C or 121.3°F in Lytton, British Columbia. The next day, the entire town burned down amid 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 blamed the event’s extremity largely on climate change. Now, a new study affirms that conclusion, and for the first time comprehensively elucidates the multiple mechanisms – some strictly climate-related, others more in the realm of disastrous coincidences – that they say led to the mind-bending temperatures.

Lead author Samuel Bartusek, a PhD student at the Columbia Climate School’s Lamont-Doherty Earth Observatory, said: “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 categorise. I would call this more of grey swan.”

See also: Locations that have suffered cyclone-strength winds more likely to suffer forest fires

human-induced warming

The study pulled climate data starting in the 1950s together with daily weather observations from the weeks preceding and during the heatwave to form an intimate portrait. A core conclusion: such an event would have been virtually impossible without human-induced warming. It was impossible in the 1950s, but atmospheric warming since then has moved the needle to a prospective 1-in-200-year event – still rare, but now feasible.

The researchers predict that if warming continues at even a moderate pace, such heatwaves could hit the region about every ten years by 2050.

One major driver, they say, was a disruption of the jet stream, which normally carries air west to east across the northern hemisphere mid-latitudes along a more or less circular path. Preceding the heatwave, though, the jet stream stalled and bent into huge waves, with four north-south peaks and troughs. These concentrated high-pressure systems underneath each peak; high pressure compresses air more and more as it approaches the surface, and this generates heat. One of those systems settled on western North America, then stayed there day after day, creating what meteorologists call a ‘heat dome’.

Some scientists believe big jet-stream waves are becoming more frequent and extreme due to human-induced warming. The jet stream normally forms a boundary between frigid polar air and warmer southern air, but recent outsize warming in the Arctic is breaking down the temperature difference, destabilising the system, they say. This idea is still being debated.

A longer-term key factor, the researchers say, is climate-driven drying that has overtaken much of the US and Canadian west in recent decades, reducing soil-moisture levels in many areas. During the heatwave, that meant reduced evaporation of water from vegetation that previously would have helped counteract heating of the air near the surface. With less evaporation, in some places the surface more effectively heated the air above it. Indeed, the researchers found that the heatwave was most extreme in areas with the driest soils.

“Global warming is gradually making the Pacific Northwest drier,” said study co-author Mingfang Ting, a Lamont-Doherty professor, pushing it into a long-term state where such extreme events are becoming ever more likely.

worst air quality

Extraordinary heat and drought continue to affect the region. In mid-October of this year, many daily temperature records were shattered with spikes more characteristic of high summer than mid-autumn. These included 31°C (88°F) in Seattle on 16th October, a full 16°C (61°F) above the previous daily record. The same day, there were records in Vancouver 30°C (86°F); Olympia, Wash. 29°C (85°F); and Portland, Ore. 30°C (86°F), its fifth consecutive day above 27°C (80°F).

The hot, dry weather has sparked forest fires so fierce and widespread that on 20th October, smoke caused Seattle to see the worst air quality of any big city in the world, ahead of usual favourites like Beijing and Delhi.

Bartusek concluded: “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.”

The study is published in Nature Climate Change. 

Image: Hot, dry weather during the heatwave sparked numerous wildfires, destroying large areas and worsening air quality. © Kari Greer/ USDA.