The monstrous fires that are now charring vast areas of western North America aren’t just colossal and fast-moving, they have also created their own thunderstorms — an example of exotic fire behaviour that scientists say is becoming more common as the climate changes.

Both the Park Fire, which has burnt more than 160,000 hectares in northern California, and the Jasper Fire, which has destroyed around one-third of the resort town of Jasper in Canada, have spawned ‘pyrocumulonimbus’ clouds, towering formations that can spit lightning, potentially starting more fires.

Reports of such clouds were relatively uncommon in years gone by. Before 2023, the global record was 102 spotted in a single 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. A higher-than-normal number have also been reported so far this year. “We can expect more of these in the future unless we change our ways,” Flannigan says.

Both the Park Fire and the Jasper Fire also surprised fire officials with the rapidity of their spread — the Jasper Fire, for example, reached the town in half the time that models predicted.

“The sobering reality is that these are not extreme outliers in some ways,” says Daniel Swain, a climate scientist at the University of California, Los Angeles. “We’ve seen a lot of fires behave like these ones in recent years, which I don’t think is reassuring at all.”

Runaway fire

When firefighters first reached the Park Fire near Chico, California, on 24 July, they stood a chance of beating it. The fire, which by some reports had been started deliberately, was still relatively small at just 1–2 hectares. But it soon grew out of control, becoming too large and moving too fast to contain. Within three days, it had burnt more than 140,000 hectares. It is now the fourth-largest fire in state history.

A couple of days earlier and further north, another fire raced through Jasper National Park in Canada before destroying hundreds of buildings in the town of Jasper. Witnesses described seeing firefighters battling a wall of fire 100 metres tall closing in on the town. So far, one person has died in the Jasper Fire; no deaths have been reported in the Park Fire.

There are three essential ingredients for a large wildfire, says James Gomez, a PhD student studying wildfires at the University of California, Riverside: dry fuel; hot, dry, windy weather; and a source of ignition. Climate change is making two of those more common1. “Conditions will be ideal for fires like these more often,” Gomez says.

Before the Park and Jasper fires started, both regions had been sweltering in extended heatwaves, leaving the forests tinder-dry. Compared with cool air, warm air holds more moisture — about 7% more per degree Celsius — and so draws more water from vegetation on the ground. A warmer climate therefore results in drier fuel2.

“Drier vegetation burns more readily and more intensely, so you have faster-moving fires that are more intense in terms of their thermal output, and the likelihood [is] that they create exotic and extreme fire behaviours,” Swain says.

Fire begets fire

Among those exotic behaviours is the formation of pyrocumulonimbus (pyroCb) clouds3, which Swain calls “dynamos of combustion”. PyroCb clouds generate lightning that can spark dozens of new fires many kilometres from the main blaze, Flannigan says. They can also trigger winds that lead to faster and less predictable fire spread.