Deadly wildfires like those raging in northern and southern California have become more common in the United States and other world in recent years. AFP talked with researchers about how climate change could make them worse.
Other factors have also led to an increase in the frequency and intensity of major fires, including human forest landing and doubtful forest management. "The patient was already ill," according to David Bowman, a professor of environmental change biology at the University of Tasmania and a fire brigade expert.
"But climate change is the accelerant."
Good weather for fire
Any firefighter can tell the recipe for "promotional fire": hot, dry and windy.
It is no surprise that many of the tropical and temperate regions destroyed by forest fires are those predicted in climate models to see higher temperatures and more droughts.
"In addition to getting more dry and hot air, climate change – by increasing evaporation and the presence of drought – creates more flammable ecosystems," noted Christopher Williams, Head of Environmental Science at Clark University, Massachusetts.
Over the last 20 years, California and southern Europe have seen several droughts of a magnitude that used to occur once in the century.
Dry weather means more dead trees, bushes and grass – and more fuel for the fire.
"All these extremely dry years create an enormous amount of dried biomass," said Michel Vennetier, engineer at France's National Science and Technology for Environment and Agriculture (IRSTEA) research.
"It's an ideal combustible."
Change of landscape
To make matters worse, new species grow better suited to semi-dried conditions in their place.
"Plants that have disappeared as moisture, replaced by more flammable plants that can withstand dry conditions, such as rosemary, wild lavender and thyme," said Vennetier.
"The change happens quite quickly."
With rising mercury and less rain, water-laden trees and bushes root deeper into the soil and suck each drop of water as they can to nourish the leaves and needles.
This means that moisture in the soil that may have helped to brake a fire sweeping through a forest or garrigue is no longer there.
In the northern hemisphere's temperate zone, the hot season was historically short – July and August, in most places.
"Today, the period of fires has increased from June to October," said IRSTEA researcher Thomas Curt, referring to the Mediterranean.
In California, just recently emerging from a five-year drought, some experts say that there is no season anymore – fires can happen all year long.
"The hotter, the more lightning you have," says Mike Flannigan, professor at the University of Alberta, Canada and head of the Western Partnership for Wildland Fire Science.
"Especially in the northern areas, it means more fires."
At the same time, he noted that 95 percent of wild fires worldwide are started by humans.
Weakened jet current
Normal weather patterns across North America and Eurasia depend strongly on the powerful high-air currents – produced by contrast between polar and equatorial temperatures – known as the jet current.
But global warming has increased the Arctic temperatures twice as fast as the global average, which weakens these currents.
"We see more extreme weather because of what we call blocked ridges, which is a high pressure system where the air drops, gets hotter and drier along the way," says Flannigan.
"Firemen have known for decades that these contribute to fire activity."
Climate change not only increases the likelihood of fires but also their intensity.
"If the fire gets too intense" like in California right now, and in Greece last summer – "there is no direct action you can take to stop it," says Flannigan.
"It's like spying on a campfire."
With rising temperatures, beetles have moved north to Canada's boreal forests and cause chaos – and kill trees – along the way.
"Bark beetle eruptions increase temporary forest fires by increasing the amount of dead material, such as needles," Williams says.
Globally, forests hold about 45 percent of the earth's lockable coal and drink up to a quarter of human greenhouse gas emissions.
But when forests die and burn, some of the carbon is released into the atmosphere and contributes to climate change in a bad loop, as researchers call "positive feedback".