The heat wave that hit the western United States and Canada at the end of June would have been "almost impossible" without the global warming caused by humans.
This is the conclusion made Wednesday by a group of researchers.
Scientists at the World Weather Attribution, an initiative of experts from various research institutes around the world, estimated that climate change has made this event at least 150 times more likely to occur.
“There is no doubt that climate change has played a major role,” said Friederike Otto of the University of Oxford, one of the study's authors.
2 ° C cooler without human intervention
The temperatures experienced were so abnormal compared to the usual averages in this region that it was difficult for the researchers to calculate how quickly such an event could recur. But according to them, with the current climate, such a heat wave could statistically occur once every 1000 years. They also determined that the recorded temperatures were about 2 ° C higher than they would have been if this episode of extreme heat had occurred at the beginning of the industrial revolution, in the 19th century.
Last month, Canada broke its all-time high record several times, which finally stood at 49.6 ° C in the village of Lytton on June 30.
The US states of Washington and Oregon also found themselves under this "heat dome," caused by high pressures trapping hot air.
This climatic phenomenon was not unprecedented in itself, but much more powerful than observed so far.
The exact human toll is not yet known but amounts to at least several hundred deaths.
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The World Weather Attribution has made a specialty of analyzing the possible link between a specific extreme weather event and global warming, by calculating in a very short time the probability that it has occurred even without the climate change linked to emissions. greenhouse gases.
Here, 27 researchers from seven different countries participated in the analysis.
June 2021 was the hottest month for North America since measurements began, the European Copernicus Climate Change Service (C3S) announced earlier on Wednesday.
It was 1.2 ° C above the average for the period 1991-2020, i.e. 0.15 ° C above the previous warmest month of June for this region, in 2012.
For their work on the last heatwave, researchers from the World Weather Attribution studied, over several decades, the temperature readings of the most densely populated affected area, around the American cities of Seattle and Portland, and the Canadian Vancouver.
They then used modeling to compare the current climate with that of the past, using peer-validated methods.
A series of factors made this event possible, but scientists particularly emphasize the role of the drought that hit this region in the spring.
Dry soils lead to less evaporation, which would have made it possible to limit the heat.
Either the event remains rare, or a threshold has been crossed and the warming is exponential
For the future, two scenarios are possible, according to the researchers.
The first is that even if the climate crisis has amplified and made such an event much more likely, it is still very rare.
The second hypothesis is more worrying: the general rise in temperatures could have crossed a threshold, leading to an exponential warming more important than what was predicted so far and that current climate models fail to predict.
VIDEO.
Record heat wave in Canada: "France will experience heat waves of this type"
This dome of heat “is something that no one saw coming, nor thought possible. We feel that we don't understand heat waves as well as we thought, ”said Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute. While the world has already gained at least + 1.1 ° C compared to the pre-industrial era, the Paris Agreement aims to contain this warming below + 2 ° C, if possible 1.5 ° C. . With a warming of 2 ° C, the World Weather Attribution calculated that temperatures in late June in the American West would have been even higher, by 1 ° C. Above all, such an event would then have a probability of recurring every 5 to 10 years.
Scientists are calling for action. Measures must be taken to adapt to these new conditions, in particular high heat alert systems for populations and the construction of suitable buildings. But above all, in the long term, they ask to reduce greenhouse gas emissions at all costs.