November 12 – Deadly fires such as those that are raging in northern and southern California have become more common in recent years across the country and elsewhere in the world. AFP spoke to scientists about ways climate change could worsen.
Other factors have also contributed to increasing the frequency and intensity of major fires, including human intervention in wooded areas and questionable forest management. "The patient was already ill," says David Bowman, a professor of environmental biology at the Tasmanian University and a crash expert.
"But climate change is an accelerant."
Good weather for fire
Every fireman can tell you the recipe for "favorable fire weather": Hot, dry and windy.
It is not surprising that many of the tropical and temperate areas devastated by the sharp increase in forest fires are those predicted in climatic models to see higher temperatures and more droughts.
"In addition to the increased dry and hot air, climate change – by increasing the rate of evaporation and the spread of drought – also creates more flammable ecosystems," said Christopher Williams, director of environmental sciences at Clark University in Massachusetts.
In the last 20 years, California and Southern Europe have experienced droughts of magnitude that occurred only once in a century.
Dry weather means more dead trees, shrubs and grasses – and more fuel for fire.
"All these extremely dry years create a huge amount of dried biomass," said Michel Vennetier, an engineer at the French National Science and Technology Research Center for Environment and Agriculture (IRSTEA).
"That's the ideal fuel."
Changing the scenery
To make it even worse, new species more suited to semi-dry conditions are growing in place.
"Plants that disappeared as moisture have been replaced by more flammable plants that resist dry conditions like rosemary, wild lavender and thyme," said Vennetier.
"The change happens quite quickly."
With rising mercury and less rain, trees and shrubs of water stress send roots deeper into the soil and ingests each drop of water that can feed leaves and needles.
This means that the moisture in the ground, which could help slow down the fire by sweeping the forest or gargle, is no longer.
In the temperate zone of the northern hemisphere, the fire season was historically short – July and August, in most places.
"At the present time, the period that is prone to fires has expanded from June to October," said Thomas Curt, a scientist at IRSTEA, on the Mediterranean.
In California, which has recently emerged from a five-year drought, some experts say there is no longer a period when fires can occur all year round.
More lightning strikes
"The warmer the more light you have," said Mike Flannigan, a professor at the University of Alberta, Canada, and director of the Western Partnership for Wildland Fire Science.
"Especially in the northern areas it becomes more fire."
At the same time, he said that 95% of fires around the world are initiated by humans.
Low current current
Normal weather patterns in North America and Eurasia depend strongly on strong airflows – high altitudes – produced by the contrast between polar and equatorial temperatures – known as currents.
However, global warming has increased temperatures twice as fast as the global average in the Arctic, and has weakened these currents.
"We see extreme weather due to what we call blocked ridges, a high-pressure system where the air is falling, making it warmer and drier on the way," Flannigan said.
"Firefighters know for decades that it has led to a fire."
Climate change not only increases the likelihood of fires but also their intensity.
"If the fire is too intense," as in California right now, and in summer summer in Greece – "there is no direct action you can take to stop him," Flannigan said.
"It's like spitting on a campfire."
Infestation of beetles
As temperatures grew, the beetles moved north into the Canadian boreal forests, causing a trigger – and killing trees – along the way.
"Bee bark beetles temporarily increases forest flammability by increasing the amount of dead material such as needles," Williams said.
Worldwide forests hold about 45% of Earth's earthed carbon and draw a quarter of greenhouse gas emissions.
But when forests are dying and burning, part of the carbon is released back into the atmosphere and contributes to climate change in a vicious loop called "positive feedback" by scientists.