Climate Change Juiced the Deadly Pacific Northwest Heatwave

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Last week’s deadly heat in the northwestern U.S. and British Columbia would not have reached the highs it did in a world without greenhouse gas pollution, according to climate scientists who conducted a rapid-response study of the heatwave.

“We’ve never seen a jump in record temperature like the one in this heatwave,” said Geert Jan van Oldenborgh, a weather and climate researcher at the Royal Netherlands Meteorological Institute. 

The event was so rare that the researchers were unable to estimate a full range of probabilities, concluding only that humanity’s warming of the planet made it at least 150 times more likely. They also provided a glimpse of the future. Carbon dioxide and other warming gases have lifted global average temperatures by 1.2° Celsius since industrialization. After another 0.8°C—which could arrive by the 2040s—heatwaves of this magnitude could strike every five to 10 years. 

While the heat was still raging, the collaboration of climate scientists known as World Weather Attribution brought together 27 researchers from seven countries to look for fingerprints of greenhouse gas pollution, using a now-standard approach. They focused their study on a single element, maximum daily temperatures, as experienced by the 9.4 million people who live in and around Vancouver, Seattle, and Portland. An extraordinary high-pressure system, or “heat dome,” served as the event's meteorological driver. That was itself record-breaking, but nonetheless a not entirely foreign feature of the area's circulation. 

Scientists have conducted dozens of studies on heatwaves over the last two decades, all of which point to a single idea: Every heatwave is made more likely and intense by climate change. 

The extraordinary nature of the Pacific Northwest heat made analysis challenging and in some ways impossible for the time being. There is no recorded event that comes close as a comparison, the scientists said. A conventional analysis of the temperature record found that the observed temperatures were 4°C higher than what data suggested was possible—a reminder that when it comes to climate change, the past can be a poor gauge of the future.

Two possibilities framed their work. First, the heatwave could have been a “freak” event juiced a couple degrees Celsius higher by global warming, “the statistical equivalent of really bad luck, albeit aggravated by climate change,” they said. Or, it could represent a dramatic step-change in how quickly the world is warming, a possibility that requires further study.

Consider Lytton, British Columbia, which last week became a global symbol of climate change. The Canadian national heat record of 45°C (113° Fahrenheit) had stood for 84 years. The small town shattered it on June 27 by 2°. And then obliterated that the next day by another 3°. And then again on the 29th by still another 3°. The next day, the town was forced to evacuate as wildfire incinerated most of it. 

“You're not supposed to break records by four or five degrees Celsius. This is such an exceptional event that we can't rule out the possibility that we're experiencing heat extremes today that we only expected to come at higher levels of global warming," said Friederike Otto, associate director of the University of Oxford's Environmental Change Institute, in a statement.

The scientists will continue to refine this estimate that climate change made the heatwave at least 150 times likelier, and publish more definitive findings in a peer-reviewed journal. 

“The big problem is that 90% of people forget about the ‘at least’” before “150 times likelier,” van Oldenborgh said. “I hope this doesn’t happen again here.”

©2021 Bloomberg L.P.

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