Climate Change

Deadly North American heat wave tests the limits of climate change models

In Portland, Oregon, temperatures exceeded 46°C during the last week of June.
In Portland, Oregon, temperatures exceeded 46°C during the last week of June. © Nathan Howard, Getty Images via AFP

In a preliminary study released Wednesday, scientists said the heat wave that swept across Western North America in late June, killing hundreds of people, would have been “virtually impossible” without climate change. But researchers are still struggling to explain such a spike in temperatures.


There was never any question that the temperatures that hit cities including Seattle, Portland and Vancouver in late June were extreme. But preliminary research published by a team of scientists Wednesday showed just how remarkable they were, with the scientists describing the heat wave as “a 1 in 1000 year event in today’s climate.”

The temperatures recorded — as high as 49.7°C in the Canadian province of British Columbia and 46.1°C in Portland — exceeded seasonal norms to such an extent that the researchers had a hard time finding a historical comparison point.

“We estimated the probability of such an extreme event on the scale of a millennium, but it could very well be more,” Robert Vautard, director of the Paul-Simon Laplace Institute of climate science and a co-author of the study, told FRANCE 24.

‘Virtually impossible’ without climate change

The international team of researchers ran the late June temperatures through some twenty climate models in an effort to understand the unprecedented spike. They found that the heat wave would have been “virtually impossible without human-caused climate change”.

“That means we can’t see any other explanation, but there remains a minute chance that such an extreme event could have happened randomly without climate change,” said Vautard.

The scientists’ models, however, “couldn’t explain how the temperature could climb so high,” Dim Coumou, a climatologist at VU Amsterdam and co-author of the study, told FRANCE 24.

The temperatures recorded exceeded previous records by five to six degrees Celsius — “much more than the two to three degrees that we would have expected under current climate conditions,” said Vautard.

Indeed, the “heat dome” that caused the soaring temperatures — a convergence of pressure conditions in the atmosphere — wasn’t vastly different from the one that drove the European heat wave of 2019. And while that heat wave set records, they were not nearly as stark as those set last week, which fueled major forest fires in British Columbia and left hundreds dead.

“It’s clear that the dome alone isn’t enough to explain what just happened, and now we must seek to understand which other factors played a role,” said Coumou, who specialises in the impact of climate change on heat waves.

‘Really bad luck’, or a step into uncharted territory?

For now, the scientists have two main theories. The first is what the study calls “really bad luck” — in other words, that all the possible factors that cause temperatures to rise converged at the same time and place.

For one, the heat dome formed in a region which has lately seen little rain.

“The ground was very dry, meaning that there was very little water evaporating, which normally allows the air to cool a bit when the temperatures start to rise,” explained Coumou. Add to this the hot winds that came down from the surrounding mountains, which made the air even more suffocating.

But the scientists are also weighing another, potentially more worrying, scenario. The heat wave that struck the Pacific Northwest could signal “that we might have crossed a threshold in the effect that climate change has on extreme weather events”, said Vautard.

In that case, the deadly heat would mark a sort of step into uncharted territory, where “climate change produces heat waves more intense than we thought”, said Coumou. He stressed the urgent need for further research, in order to determine whether aspects of current climate models need revising.

For Vautard, the task is all the more pressing now that the North American case has proven that “it’s absolutely possible for temperatures to rise as high as 50°C in temperate regions”.

“This could happen in France next,” Coumou agreed. And the likelihood of such extreme events will only continue to increase if climate change continues on its current path. If the targets of the 2015 Paris Agreement are not met and global temperatures reach 2°C above pre-industrial levels, the study predicts that similar heat waves “would occur roughly every 5 to 10 years”.

Barring drastic changes over the next decade, even that scenario could be optimistic, as current levels of greenhouse gas emissions put the planet on track for at least three degrees of warming.

Vautard says global North countries have no time to waste in preparing for extreme heat.

“We’re not at all sure that we could stand up to temperatures of 45°C or more in the Paris area,” he said.

This article was adapted from the original in French.

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