Fires in Los Angeles: the ingredients of increasingly devastating wildfires

Southern California is facing what is already, two days after it started in a hill near Los Angeles, the most devastating winter fire in four decades, according to calculations by the American agency Associated Press.

An abnormal episode for the season, linked to a cocktail of factors: a high temperature, a succession of particularly dry months – it has only rained 5 millimeters since July 1 – and an episode of wind specific to California: the Santa Ana winds coming down from the mountains starting in the fall. Harmless when the trees are wet, these winds which blew on dry vegetation were devastating. Especially since this episode is particularly violent: some gusts exceeded 160 km/h… All it took was a spark, the causes of which remain to be determined.

As with every extreme episode, an attribution study will be required to quantify the link between the increase in global temperatures and this episode. What is certain, however, is that California has experienced a spectacular increase in burned areas for several years, and that several academic studies have established a very clear link with global warming.

Increase in episodes of magnitude

Over the period 1972-2018, the number of fires in California increased fivefold, the areas burned eightfold, highlighted a study published in 2019 in the journal Earth Future. “We note an increase in extreme phenomenaexplains Jean-Baptiste Filippi, researcher at the CNRS and the University of Corsica. In California, seven of the ten largest fires since 1932 occurred after 2020.”

How can we explain that the Golden State is particularly prone to forest fires? As in other regions of the world, warming increases the frequency, intensity and duration of droughts… and therefore increases the probability of conditions favorable to fires, in an already hot and arid geographical area.

“Wildfires require the alignment of a number of factors, including temperature, air humidity and lack of moisture in fuels (trees, grasses, forest debris)summarizes NOOA, the American Oceanic and Atmospheric Administration. All of these factors are closely linked, directly or indirectly, to climate variability and climate change. »

Overlapping wind and drought conditions

Summer remains the season with the highest risk. Winter fires are rarer, but can be devastating, due to these Santa Ana winds. “Climate change has not been shown to increase or decrease the likelihood of these wind events”explained on the YouTube platform Daniel Swain, climate scientist at the University of California at Los Angeles. But with droughts becoming longer and longer, he continues, “climate change increases the overlap between wind and drought conditions”. This same configuration led to equally devastating fires in December 2017.

But if populations are also at risk, it is also because of past urban planning choices. “Strangely, California’s vulnerability is linked to the fact that it managed fires very well in the last centurynotes Jean-Baptiste Filippi. Having succeeded in reducing the number of fires, vegetation has greatly increased and the population has settled massively in or near forests. » So much so that it is now estimated that one in ten homes in this American state is in an “extreme” fire risk zone, according to the American Insurance Institute for Business and Home Safety. And that, between 1990 and 2019, 12 million houses were built in high-risk areas.