The monstrous fires, now charring large areas of western North America, are not only colossal and fast-moving, but have also produced their own thunderstorms - an example of exotic fire behavior, which scientists believe is becoming more common with climate change.

Both the Park Fire, which has burned more than 160,000 acres in northern California, and the Jasper Fire, which has destroyed about a third of the resort town of Jasper, Canada, have produced "pyrocumulonimbus" clouds, towering formations that can throw lightning bolts, potentially igniting more fires.

Reports of such clouds have been relatively rare in recent years. Before 2023, the global record was 102 specimens seen in one year, 50 of them in Canada, says Mike Flannigan, who studies wildfires at Thompson Rivers University in Kamloops, Canada. During last year's extremely active fire season, 140 were reported in Canada alone. This year too, an above-average number have already been reported. “We can expect more of the same in the future if we don’t change our behavior,” Flannigan says.

Both the Park and Jasper fires surprised fire officials with the speed of their spread - the Jasper fire reached the city in half the time that models had predicted.

“The sobering reality is that in some ways these are not extreme outliers,” says Daniel Swain, a climate scientist at the University of California, Los Angeles. “We have seen many fires over the last few years that have behaved like this, which is not reassuring at all.”

Wildfire

When the firefighters on July 24th Park Fire near Chico, California, they had a chance of defeating him. The fire, which according to some reports was deliberately started, was still relatively small, only 1-2 hectares in size. But it quickly grew out of control, becoming too big and moving too quickly to be contained. Within three days it had burned more than 140,000 hectares. It is now the fourth largest fire in the state's history.

A few days earlier and further north there was a crash another fire through Jasper National Park in Canada before destroying hundreds of buildings in the city of Jasper. Witnesses reported firefighters battling a 100-meter-high wall of fire that was moving toward the city. So far, one person has died in the Jasper Fire and no deaths have been reported in the Park Fire.

According to James Gomez, a graduate student at the University of California, Riverside, there are three essential ingredients for a major wildfire: dry fuel; hot, dry, windy weather; and an ignition source. Climate change is making two of these more common. “The conditions will be ideal for fires like this,” Gomez said.

Before the Park and Jasper fires began, both regions were suffering extreme heat waves, which left the forests bone dry. Compared to cool air, warm air holds more moisture – about 7% more per degree Celsius – drawing more water from vegetation on the ground. A warmer climate therefore leads to drier fuel.

“Dry plant material burns more easily and more intensely, so you have faster fires that are more intense in terms of their heat production, and the likelihood that they will exhibit exotic and extreme fire behaviors,” says Swain.

Fire begets fire

These exotic behaviors include the formation of pyrocumulonimbus (pyroCb) clouds 3, which Swain calls “dynamoes of combustion.” PyroCb clouds produce lightning that can start dozens of new fires many kilometers away from the main fire, Flannigan says. They can also trigger winds that cause fires to spread more quickly and less predictably.

This means fires that produce pyroCb clouds are intense and fast, says Flannigan. The Jasper Fire is an example: When it started, models suggested it would take four days to reach the town of Jasper. He made it in two, also due to the pyrocumulonimbus effect. The model system “is designed for wind-driven fires, not PyroCbs,” Flannigan says.

The Smoke and soot produced by large fires, also traps sunlight, making days warmer and drier than normal, Gomez says. That alone can fuel the fire. “It creates a feedback loop where fires can become more self-sustaining,” he says. It can also create conditions conducive to new fires starting 4.

Undesirable consequences

Land use practices over the past 150 years have also contributed to what is known as a fire deficit across much of the western U.S., Swain says. Strict fire management practices and the displacement of indigenous peoples from their lands mean there has been less wildfire than in past millennia. As a result, the forests have become denser.

“Because of these policies, there is more fuel for fires than there would have been,” Swain said. “This is not what the forest would look like in its natural state.”

In Canada, where forests are naturally denser and population densities are lower, this is less of a problem, says Anthony Taylor, a forest management expert at the University of New Brunswick in Fredericton.

The most important measure to prevent these extreme fires is to slow the effects of climate change, scientists say. “We’re on a certain course,” Taylor says. “But there is still time not to go down this disastrous path.”