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Fatal fires such as those in northern and southern California have become more common in the United States and elsewhere in the world in recent years. AFP spoke to scientists about how climate change can make matters worse.
Other factors have also resulted in an increase in the frequency and intensity of large fires, including human encroachment on forested areas, and dubious forest management. "The patient was already sick," said David Bowman, professor of environmental change biology at the University of Tasmania and expert on forest fires.
"But climate change is the accelerator."
Good weather for a fire
Any firefighter can tell you the recipe for "fire weather": hot, dry and windy.
It is therefore not surprising that many of the tropical and temperate regions devastated by an upsurge in forest fires are those predicted by climate models to cope with higher temperatures and more droughts.
Heavy smoke covers the forest where the campfire burns near Paradise, California on November 11, 2018.
AAP
"In addition to bringing dry, warm air, climate change – by increasing evaporation rates and the prevalence of drought – also creates more flammable ecosystems," said Christopher Williams. , director of environmental science at Clark University of Massachusetts.
Over the past 20 years, California and southern Europe have experienced several droughts of a magnitude that only occurred once a century.
More fuel
Dry weather means more dead trees, shrubs and grass – and more fuel for the fire.
"All these extremely dry years are generating a huge amount of desiccated biomass," said Michel Vennetier, engineer for scientific and technological research for the environment and agriculture (IRSTEA).
"It's an ideal fuel."
Change of scenery
To make matters worse, new species better adapted to semi-arid conditions grow in their place.
"Plants that like moisture have disappeared, replaced by plants that are more flammable and able to withstand dry conditions, such as rosemary, wild lavender and thyme," said Vennetier.
"The change is happening fairly quickly."
Thirsty plants
With increasing mercury and decreased rainfall, trees and shrubs under water stress send the roots deeper into the soil, sucking up every drop of water possible to feed the leaves and needles.
This means that the moisture in the ground that could have helped to slow down a fire sweeping through a forest or scrubland is no longer there.
Longer season
In the temperate zone of the northern hemisphere, the fire season was historically short – July and August, in most places.
"Today, the forest fire-prone period has stretched from June to October," said Thomas Curt, IRSTEA's scientist, about the Mediterranean Basin.
In California, which has just emerged from a five year drought, some experts say that there is no more season, fires can occur all year long.
More lightning
"The hotter it gets, the more lightning you get," said Mike Flannigan, a professor at the University of Alberta, Canada, and director of the Western Partnership for Wildland Fire Science.
Fires spreading across California have already killed nine people and forced tens of thousands of people to flee.
AAP
"Especially in the northern regions, this results in more fires."
At the same time, he noted that 95% of the world's forest fires are caused by humans.
Jet stream weakened
Normal weather conditions in North America and Eurasia are highly dependent on the high-altitude atmospheric currents produced by the contrast between polar and equatorial temperatures, known as jet flows.
But global warming has raised temperatures in the Arctic twice as fast as the global average, weakening these currents.
"We are seeing more extreme weather conditions because of what we call blocked ridges, which are a high-pressure system in which the air goes down, heats up and dries up along the way," Flannigan said. .
"Firefighters have known for decades that they are conducive to fire activity."
Unmanageable intensity
Climate change is increasing not only the risk of fire, but its intensity.
"If the fire gets too intense," as in California right now and in Greece last summer, "there's no direct step you can take to stop it," Flannigan said.
"It's like spitting on a campfire."
Coleoptera infestations
With rising temperatures, beetles moved north into Canada's boreal forests, causing havoc – and killing trees – along the way.
"Bark beetle outbreaks temporarily increase the flammability of forests by increasing the amount of dead materials, such as needles," Williams said.
Positive feedback
Globally, forests contain about 45% of terrestrial land-locked carbon and account for a quarter of man's greenhouse gas emissions.
However, as forests die and burn, some of the carbon is released into the atmosphere, contributing to climate change in a vicious circle called "positive feedback" by scientists.
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