The 2021 Pacific Northwest heatwave was sixth most extreme in the modern record, according to a new study published today.
The research focused specifically on heatwaves which were the most divergent from an area’s regular temperature - a factor which can have dangerous impacts for the health of humans and plant and animal species.
“If you’re not used to the temperature, it will have a bigger impact, even if it’s not as hot as in other parts of the world,” Vikki Thompson, a climate scientist at the University of Bristol and author on the paper, told The Independent.
The June 2021 Pacific Northwest heatwave shattered temperature records across the region, sparking a series of destructive wildfires and killed hundreds of people in the US and Canada. Portland, Oregon hit 46 degrees Celsius (115 Fahrenheit), for example, while Lytton, Canada got up to 49.6C (121F).
These temperatures were a drastic departure from the region’s typical climate. Average temperatures in Portland at that time of year usually reach a highs in the mid-70s (F).
That level of divergence made the heatwave one of the most “extreme” in the past century, according to the study.
The study, published in the journal Science Advances, determined that the normal high temperature in the Pacific Northwest is around 23C (7F) throughout the summer, ranging from 19C (66F) and 28C (82F).
Researchers found the 2021 heatwave was a significant departure from that norm, - a level of difference outdone by only five other heatwaves in the past sixty years.
Interestingly, two of those heatwaves reached temperatures with significantly lower temperatures than the 2021 Pacific Northwest heatwave. The 2019 Alaska heatwave and the 2016 Peruvian heatwave only reached temperatures of around 23C (74F) — but compared to the norm in those areas, these heatwaves were vastly out of character.
Other heatwaves considered “more extreme” than last year’s Pacific Northwest heatwave include the 1980 heatwave in the US South, the Brazilian heatwave of 1985 and the 1998 Southeast Asia heatwave.
It’s important to look at heatwaves both in terms of how hot they get, and how different they are from the local norms, Noah Diffenbaugh, a climate scientist at Stanford University in California who was not involved in the study, told The Independent.
In any location, there are infrastructure and systems in place to deal with extreme conditions based on the location’s normal range of weather, he says.
For example, areas in the far north likely have more snowploughs and salt storage in anticipation of snowy weather than areas closer to the tropics. On the other hand, hotter areas might have more widespread air conditioning than places with more temperate climates.
“The prevalence of air conditioning is much higher in Phoenix than it is in Portland or Seattle or Vancouver,” Diffenbaugh says.
But if the range of possible weather dramatically changes, the local infrastructure may not be able to cope.
Divergence from the norm can also impact local wildlife which may be adapted to a specific range of temperatures, Dr Thompson says.
“If it suddenly jumps higher, even if it’s a temperature that doesn’t matter for humans, it can have a big impact,” she adds.
Dr Thompson threats posed to Australian bats in recent years. In 2018, ABC News reported that a single heatwave in Queensland killed around one third of the country’s spectacled flying fox population after temperatures pushed over 42 C (108 F).
But temperature measurements of heatwave extremes matter too — as hotter temperatures reach closer to thresholds of heat tolerability for the human body, Dr Diffenbaugh says.
“There’s been a lot of evidence that there are absolute thresholds for heat with, you know, human physiology,” he says.
As the world delves deeper into the climate crisis, heatwaves — and extreme heatwaves —are becoming more common. The most recent report from the Intergovernmental Panel on Climate Change, considered the definitive view of climate science, noted that heatwaves that once occurred every 10 years now happen almost every three years on average.
With two degrees of warming, those heatwaves will occur once every 5-6 years — and almost every year, on average, if the world reaches four degrees of warming. According to the Climate Action Tracker, the world is currently on pace for around 2.7C of warming.
Last year, an analysis found that the 2021 Pacific Northwest heatwave would have been “virtually impossible” without the climate crisis. That analysis noted that the heatwave would occur once every 1,000 years in today’s climate.