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Climate change made North American heatwave 150 times more likely

Temperatures recently rose to record highs in Lytton, Canada

JENNIFER GAUTHIER/REUTERS/Alamy

The recent deadly and record-breaking heatwave in North America would have been “virtually impossible” without climate change, according to scientists who say they are very worried about the prospect of similar events occurring around the world.

An international team has found that the heatwave, which may have killed hundreds and saw Canada’s temperature record being broken by nearly 5°C in the village of Lytton, was made at least 150 times more likely by global warming.

The temperature highs were 2°C hotter than they would have been without the human activity that has warmed Earth, say the researchers at the World Weather Attribution project. By the 2040s, they warn, such a heatwave could be another 1°C warmer.

“It’s an extraordinary event,” says Geert Jan van Oldenborgh at the Royal Netherlands Meteorological Institute, who contributed to the research. “A lot of people are very worried about this event. Could this also happen here in the Netherlands, France, in other places, suddenly having a 5°C jump? This is something that really needs to be researched, whether we should be prepared for this kind of jump in other parts of the world.”

Van Oldenborgh and his colleagues arrived at their findings using an approach known as extreme event attribution, whittling down 35 computer models to 21 that were best able to reproduce past weather observations in an area incorporating parts of British Columbia, Oregon and Washington. The models were then used to estimate average maximum daily temperatures in the area studied, with and without climate change.

The near-50°C temperatures recorded in Canada don’t appear in statistical models. That forced the team to artificially include the event in their models, making assumptions on the rarity of such a heatwave, which they estimated as roughly a 1 in 1000 event. The models then showed the event was 150 times more probable in a world with climate change.

Up to last year such heat in the region was impossible, says van Oldenborgh. “It’s rather surprising and shaking that our theoretical picture of how heatwaves behave was broken so [dramatically],” he says. “We are much less certain about how the climate affects heatwaves than we were two weeks ago.”

The heatwave could have just been bad luck aggravated by climate change, says the team. An alternative, more worrying, explanation is that it could be due to non-linear interactions in the climate, such as the severe drought in the south of the area studied. More research will be needed to show if such non-linearities – sometimes referred to as tipping points in Earth’s systems as the world warms – were to blame. If they were, that would show today’s climate models are too conservative, says van Oldenborgh.

The attribution report is published on the World Weather Attribution site, but hasn’t yet been peer-reviewed due to the rapid nature of the work. Separate analysis, published today by the Copernicus Climate Change Service in Europe, shows that last month was the warmest June on record in North America.

Article amended on
9 July 2021

We clarified the likelihood of the heatwave in Lytton

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