(Los Angeles) Climate change has become the primary factor responsible for the forest fires that regularly ravage the western United States, and human activities are overwhelmingly the cause, says a study released Monday when the COP26 conference on the climate opened in Scotland.
In the American West, the area devastated by fires between 2001 and 2018 averaged 13,500 km2 per year, twice as much as over the 1984-2000 period.
“It happened so much faster than we expected,” told the Los Angeles Times Rong Fu, climatologist at the University of California UCLA, who led this study published by the journal of the National Academy of Sciences of the United States (Pnas).
In an attempt to understand what contributed to this spectacular worsening in such a short time, the team of American researchers led by Mr.me Fu analyzed the various factors at work in the “vapor pressure deficit” (VPD), which reflects the dryness of the air.
The VPD represents the difference between the quantity of water actually present in the atmosphere and the maximum that the latter could contain. The higher this deficit, the more the ambient air sucks in water from the soil and plants, creating conditions that are increasingly conducive to fires.
Scientists have established that the increase in wildfires in the American West is closely related to this deficit during the warm season. Between May and September, the number of days having had a high VPD increased by 94% over the period 2001-2018 compared to the previous period, notes the study.
According to the calculations of Mme Fu and his colleagues, “natural” atmospheric variations only played a role in this worsening of VPD by 32% on average. The remainder (68%) of this surge in water deficit in the atmosphere over the past 20 years is attributable to global warming, caused in large part by human activities.
“Before 2000, we can explain the weather conducive to fires just using traditional weather models”, but this is no longer the case, Rong Fu said in the LA Times.
According to some models, adds the study, anthropogenic warming, that is to say of human origin, could explain up to 88% of the anomalies observed in VPD.
In August 2020, when California suffered the largest fire on record in the region – the August Complex Fire – which alone burned nearly 4,200 km², anthropogenic warming was responsible for nearly half of the deficit. humidity “exceptionally high”, concludes the study.
According to climate experts, due to greenhouse gases generated by humans, mainly through the consumption of fossil fuels, the planet has already gained around 1.1 ° C since the pre-industrial era. Most of this warming has occurred in the past 50 years.