Climate change caused last summer’s heat dome—and extreme weather will only get more likely, says study

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      Last year’s deadly heat dome in the Pacific Northwest and BC couldn’t have happened without the effects of man-made climate change, according to a new study from Columbia Climate School.

      In June 2021, Canada recorded an all-time high of 49.6 C in Lytton, BC, shortly before a wildfire tore through the region and destroyed countless homes in the village and surrounding First Nations. 

      Across the rest of BC and the wider Pacific Northwest (PNW), temperature records tumbled as a high-pressure weather system camped out for a week in mid-summer. At least 619 people in BC died, making it the deadliest weather event in modern Canadian history. 

      According to the study—authored by Samuel Bartusek, Kai Hornhuber, and Mingfang Ting and funded by grants from the National Oceanic and Atmospheric Administration and the National Science Foundation—a combination of factors caused such horrifically high temperatures. 

      “It was so extreme, it’s tempting to apply the label of a ‘black swan’ event, one that can’t be predicted,” said Bartusek in a statement. “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 a gray swan.”

      Climate change was a core factor of the heat wave. “Over the past 70 years, such an event has multiplied in probability from virtually impossible to a multi-hundred-year event,” reads the study. 

      The 2021 heat wave was 10.4 C over normal temperatures. Modelling suggests that the probability of this event increased 13-fold with 2021 temperatures compared to those from 1986-2021, and 500,000-fold when compared to 1950-85. The current odds of another heat wave like 2021 are at about 1 in 200 years. But, if global temperatures rise to between 2 to 4.5 C above pre-industrial times, the study warns that the chance of an event “exceeding the magnitude of 2021” increases rapidly, to 1 in 10 years by 2050. 

      In contrast, a study from researchers at the University of California Los Angeles, published in October, found that the heat wave was a 1 in 10,000-year event, and subsequent events of the same scale were at about 1 in 1,000 years between 2021 and 2100. 

      In other words, the Columbia study found that climate change that has already happened was necessary for the chance of a 10 C temperature anomaly to even occur—and future climate change makes those anomalies even more likely to happen. 

      Another important factor was soil moisture. The PNW is a wet region that keeps a lot of moisture in the soil. However, long-term soil drying is starting to shift the region towards a “transitional climate,” where the soil shifts between wet and dry. Damp soil and vegetation can help cool the air, as water absorbs heat until it evaporates. In 2021, soil was significantly drier than normal, meaning the soil itself heated the area above it. That effect was more pronounced in places where the soil had traditionally been damp.

      “In the PNW, drying may increase temperature variability more than in already arid regions like the southwestern United States,” the study reads. “The 2021 heatwave may represent an alarming manifestation of a shifting regime across much of the PNW from wet to transitional climate, making such events more likely.”

      Bartusek says soil dryness may have an impact on other kinds of extreme weather, too. In 2021, the heat dome was followed by catastrophic storms that caused mass floods. 

      “It's likely that dry soils can exacerbate flooding from rainfall events that follow them,” he writes in an email to the Straight. “It's definitely possible that the heatwave and the dry and wildfire-active summer of 2021 could have contributed to flooding later that year. This is a really important area for future research.”

      A report from the Canadian Centre for Policy Alternatives released Wednesday estimated that the combined cost of the 2021 heat dome, wildfires, flooding, and landslides in BC is between $10.6 billion and $17.1 billion, making it potentially the most expensive climate disaster in Canadian history (so far).

      The 2021 Lytton fire destroyed hundreds of homes in the region.

      While drier soil and long-term global warming effects set the stage for the heat wave, there were also short-term causes. The paper points towards an unusual jet stream as a major driving force. The North American jet stream normally carries air west to east across the Northern hemisphere, but before the 2021 heat wave it changed its pattern into four huge peaks and troughs that went north-south, “a pattern historically associated with North American wildfires.”

      The peaks create high-pressure weather systems that compress the air underneath it, heating it up. The higher the peak, the more pressure being exerted, and the hotter the air becomes. 

      One of these four peaks camped out over the PNW, and rather than moving across the continent, it stalled out. Although high temperatures were recorded under all four of the high-pressure systems, “the PNW experienced markedly stronger temperature … anomalies,” the report adds.

      BC is likely to see more of these extreme heat events going forward. While cooling centres can be helpful and air conditioners can lower the risk of fatalities, Bartusek says that BC and the broader PNW wasn’t prepared for such extreme heat. 

      “Air conditioning access is very low in the PNW, and additionally, in many PNW counties there are very high outdoor agricultural worker populations and high social vulnerability, putting residents at very high risk,” he says. Local governments had also failed to do enough: “Much of the critical work of preparing people for heat impacts and helping those at risk was carried out by community organization networks.”

      The solution, Bartusek says, isn’t just to change policy to protect against the worst of the heat dangers. It’s to stop the emissions that drive up temperatures in the first place.

      “To prevent extreme heat waves from getting even more intense, numerous, and long-lasting, the fossil fuel industry and those who profit from emitting further CO2 have to be challenged by collective and government action, and governments need to take decisive steps to transition our energy systems to renewable power sources.”