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Climate change made North America’s deadly heatwave 150 times more likely

Wildfire burns above the Fraser River Valley near Lytton, British Columbia, Canada.

Wildfire burns near the Canadian village of Lytton.Credit: James MacDonald/Bloomberg via Getty

The devastating heatwave that struck parts of Canada and the United States late last month would have been extremely unlikely without global warming, researchers have concluded.

The chance of temperatures in the Pacific Northwest region coming close to 50 °C has increased at least 150-fold since the end of the nineteenth century, found a rapid analysis conducted in response to the heatwave.

“This heatwave would have been virtually impossible without the influence of human-caused climate change,” says Sjoukje Philip, a climate scientist at the Royal Netherlands Meteorological Institute (KNMI) in De Bilt and a co-author of the analysis. “It was probably still a rare event, but if global warming might exceed two degrees, it might occur every five to ten years in the future.”

The record-breaking heatwave lasted from 25 June to 1 July, and affected large cities that rarely experience extreme heat, including Portland, Oregon; Seattle, Washington; and Vancouver, Canada. More than 500 excess deaths and 180 wildfires were recorded in the western Canadian province of British Columbia. The region’s peak temperature of 49.6 °C, recorded on 29 June in the village of Lytton, was the highest ever reported in Canada. Lytton’s inhabitants were evacuated before a devastating blaze almost completely destroyed the village.

A group of 27 scientists with the World Weather Attribution (WWA) project rushed to analyse whether global warming had influenced the likelihood of such an intense heatwave occurring in the region.

Their analysis reveals an unambiguous footprint of human-caused climate change. The team compared the observed heat with maximum daily temperatures predicted by climate models, including simulations of temperatures in an atmosphere unaltered by the effect of rising greenhouse-gas concentrations. They concluded that the global average temperature increase of 1.2 °C since pre-industrial times made the extreme heatwave at least 150 times more likely to happen.

Unusual conditions

The analysis was more challenging than similar studies, including those on heatwaves in the past few years in western Europe and Russia, says co-author Geert Jan van Oldenborgh, a climate modeller at the KNMI. The peak temperatures observed were up to 5 °C higher than previous records in the region. These extremes made it hard to pin down precisely how rare a heatwave of such strength might have been in cooler periods of the past, and how often it might be expected to occur in the current climate, he says.

The severity of the heatwave could have been partly due to the effects of earlier drought and unusual atmospheric circulation conditions, says van Oldenborgh. It is also possible — although unproven — that climate change is causing local heat extremes to become more frequent and intense than they would be in a cooler climate. If that is the case, record-breaking heatwaves might already be more likely to happen than existing climate models predict, says WWA co-lead Friederike Otto, a climate researcher at the University of Oxford, UK.

“Here’s a strong warning that we have to study heatwaves more,” she adds. “Climate change is an absolute game changer for extreme events — and models might not be a good indicator of what might still come in a warming world.”

Global warming means that heat anomalies do naturally increase by a large factor, says James Hansen, a climate scientist at Columbia University in New York. “Compared to what was ‘normal’ in the 20th century, the bell curve for climate anomalies is not only shifted to higher temperatures, it becomes asymmetric with a long tail on the hot side,” he says.

City planners and emergency workers worldwide need to prepare more effectively for the impacts of more frequent heatwaves on human health, agriculture and infrastructure, says co-author Maarten van Aalst, director of the Red Cross Red Crescent Climate Centre in The Hague, the Netherlands.

“Heatwaves are topping global disaster death charts, although we are probably missing many cases,” he says. “Clearly, local heatwave plans need to be ready for more extreme conditions than in the past.”

doi: https://doi.org/10.1038/d41586-021-01869-0

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