The fires ravaging the Los Angeles, California area are on a scale « unprecedented »according to the governor of the American state. On January 13, the death toll was 24, more than 105,000 people still subject to mandatory evacuations and 15,000 hectares gone up in smoke. The power of these fires is linked to the presence of very rare weather conditions, the intensity of which is reinforced by climate change. This is the conclusion of a rapid attribution study, published on January 10 by the European collaboration ClimaMeter.
We already knew that California was suffering from more and more summer fires, when conditions are hot and dry in summer, and that this development was due to climate change. But what makes the current catastrophe « unprecedented »it is mainly the fact that it occurs in winter.
Between 1is and January 9, 2025, more than 60 fire alerts were recorded in Los Angeles County alone, or… approximately 40 times more than the average of the last 12 years for this same period, according to the World Resources Institute.
A known phenomenon, but more intense than before
The central phenomenon currently fueling the Californian fires is called the « Santa Ana winds » : powerful, hot and dry, they favor the spread of fires. Normally, the winds that blow over California come from the Pacific Ocean. But when an isolated area of low pressure attaches itself above the region, and forms what is called a « cold drop »this favors the Santa Ana winds, which come from inland and blow in the other direction, towards the ocean.
The weather conditions that trigger these winds have been around for a long time. By comparing similar events of the past (over the period 1950 to 1986) to those of the present (1987-2023), ClimaMeter scientists have highlighted an evolution: the same phenomenon produces temperatures 5° higher today. C, drier conditions of 15 % and winds 5 km/h more violent than in the past.
A burning building in Los Angeles, California on January 12, 2025.
© Lokman Vural Elibol / Anadolu / Anadolu via AFP
Intensifying temperature and drought are directly linked to climate change. « The winds that blow from the Rocky Mountains towards California are less capable than before of carrying freshness, the drought means that we do not have the usual snow on these mountains. Without it, the Santa Ana winds descend towards California hotter and drier than before »explains Davide Faranda, climatologist at the Pierre-Simon Laplace Institute and co-author of the attribution study.
The third factor identified on the rise, wind speed, is more difficult to analyze. « We see that this is increasing, but we cannot conclude on the link with climate change ; the phenomenon depends a lot on each valley and the topography. We have no clear results on the influence of climate change on this. »specifies the researcher.
When rain leads to fire
The drier, hotter and more violent winds in any case contribute to drying out vegetation, which then forms fuel for fires, and to dispersing embers and flames, spreading these destructive fires even more quickly.
Over a few years, the multiplication and intensification of droughts, also favored by climate change, also fuel these fires. But, less obviously, the increase in rain also accentuates the problem.
This is what shows another study carried out by American researchers, published on January 9 in the journal Nature Reviews Earth & Environment. For every 1°C the atmosphere gains, it can contain 7 % additional humidity, they recall. Consequence: the atmosphere becomes like a big sponge. It can, on the one hand, absorb more water, and on the other, release more when it is saturated. In other words: warming of the atmosphere accentuates both droughts and extreme rains.
« The sponge grows exponentially »
The phenomena of rapid transitions between extreme drought and humidity have already seen their frequency increase under the effect of climate change (+31 to 66 % since the middle of XXe century) and will further intensify in the future. It will even go faster and faster, the authors warn: the rate of occurrence of the phenomenon could double if we reach 3°C of global warming, as we are heading towards.
However, this has a perverse effect on the risk of fire: periods of heavy rain nourish plants, which grow more. Then, when drought occurs, this vegetation dries up and provides abundant fuel for fires.
It is also this phenomenon that is at work today in the Los Angeles region: the winters of 2022-2023 then 2023-2024 broke precipitation records in southern California, before the summer of 2024 breaks heat records, followed by an extremely dry winter of 2024-2025, explains the University of California.
« The problem is that the sponge grows exponentiallyunderlines Daniel Swain, first author of the American study. And the rate of increase increases for every fraction of a degree of warming. »
If the spotlight is now on California, the problem is obviously far from being confined there: « In the Mediterranean, we currently have somewhat the same conditions as in California in 2023: a lot of precipitation in Sicily, Tunisia or on the Spanish or Algerian coasts. As a result, bushy vegetation grows there very quickly. »notes Davide Faranda. Severe drought conditions will eventually return here too, providing ideal and abundant fuel for new devastating fires.
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