We knew that the "heat dome" that fell in late June over the northwestern United States and part of the Canadian coast was extraordinary… but perhaps not to this extent.
The heat wave in cities like Seattle, Portland or Vancouver is an event that "only occurs once in a millennium", said twenty scientists in an analysis on the link between this heat wave and global warming, published Wednesday July 7 by the World Weather Attribution network.
Recorded temperatures - up to 49.7 ° C in British Columbia (Canada) or 46.1 ° C in Portland - were so above seasonal norms that the researchers struggled to find points of historical comparison.
So "we have estimated the probability of such an extreme event occurring on the scale of a millennium, but it could quite be even more", recognizes Robert Vautard, director of the Paul-Simon Laplace Institute, specialized in climate science, who participated in the study, contacted by France 24.
How did the thermometer get so high?
This international team of scientists subjected the temperature rises to some twenty climate models to try to understand the dynamics of this exceptional heat wave. Result: their analysis confirms with "almost certain" that this episode of intense heat is linked to global warming favored by human activity. "This means that we do not see any other explanation but that there is a tiny probability that such an extreme event happened by chance without global warming", explains Robert Vautard.
On the other hand, their machines to model the climate "did not make it possible to explain how the thermometer could rise so high", recognizes Dim Coumou, specialist in the impact of climate change on heat waves at the free university from Amsterdam and co-author of the study of the World Weather Attribution network, contacted by France 24.
That's the mystery of this heat wave: temperatures recorded in western North America have broken previous temperature records by five to six degrees.
"This intensity of the heat wave is much higher than the increase of two to three degrees which one would have expected in this climatic context", explains Robert Vautard.
The "heat dome" of the American Northwest - that is to say the anticyclonic phenomenon at the origin of the explosion of temperatures - is indeed not much more impressive than that which fell on Europe during the heat waves of 2019. And at the time, the thermometer had not recorded such dramatic increases in temperature.
"It is clear that the dome in itself is not enough to explain what has just happened, and we must now seek to understand what are the other factors that weighed," says Dim Coumou.
Bad luck or the entry into unknown territory
So far, scientists have two main assumptions.
The first is what the scientific study calls "bad luck".
In other words, all the possible factors that can cause the thermometer to rise have come together at the same time, in the same place.
On the one hand, there has been the formation of the heat dome in an area that has experienced very little humidity lately.
"The soils were very dry, which means that there was very little water evaporation which generally allows the air to be cooled a little when the temperatures start to rise", specifies Dim Coumou.
On the other hand, hot winds came down from the surrounding mountains, helping to make the atmosphere even more stifling.
But there is another scenario mentioned by researchers that is potentially more worrying.
The heat wave that has just experienced the northwest of America could indicate "that we have perhaps crossed a threshold in the effect that global warming has on extreme events", underlines Robert Vautard.
What has just happened would then be a kind of entry into unknown territory where "global warming produces more intense heat waves than we thought", notes Dim Coumou.
For him, it is urgent to explore this hypothesis in order to know whether it is necessary to review certain aspects of climate models.
This is all the more urgent as the North American heat wave proves that it "has become quite possible for the thermometer to rise to 50 ° C in temperate regions," says Robert Vautard.
In other words, "it is something which can happen soon in France", assures Dim Coumou.
And the likelihood of such an extreme event will only increase if global warming continues on its way.
If the goal of the 2015 Paris climate agreement - to keep temperature increases below the threshold of 2 ° C more by 2100 - is not met, heat waves of an intensity similar to that of June 2021 "may occur every five to ten years," write the authors of the study.
It therefore becomes absolutely necessary, for Robert Vautard, that the countries of the northern hemisphere prepare for this prospect because "we are not at all sure that we would withstand temperatures of 45 ° C or more in the Paris region" .
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