Zombie fires: how Arctic wildfires that come back to life are ravaging forests | Wildfires

IIn May 2023, lightning struck the Donnie Creek forest in British Columbia and the trees began to burn. There was a forest fire early in the year, but a dry fall and warm spring had turned the forest into a tinderbox and the flames were spreading quickly. By mid-June, the fire had become one of the largest in the province’s history, ravaging an area of ​​boreal forest almost twice the size of central London. That year, more of Canada burned than ever before.

The return of cold and snow at the end of the year usually marks the end of wildfire season. But this time the fire didn’t stop. Instead, it smoldered in the underground soil, insulated from freezing by the snowpack. The following spring, it reemerged as a “zombie fire” that continued to burn until August 2024. By that time, more than 600,000 hectares (1.5 million acres) had been destroyed.

Zombie fires, sometimes betrayed by a plume of steam emerging from the boiling frozen forest floor, were once a rare phenomenon in the boreal regions stretching from the Far North across Siberia, Canada and Alaska. But in a rapidly warming world, they are becoming more and more common. Winter burns are minor – and often difficult to detect – but they turn fires into multi-year events and fundamentally change the ecology of the land where they burn, making it harder for forests to regrow.

“It’s a huge problem,” says Lori Daniels, a professor of forestry and conservation sciences at the University of British Columbia. “Zombie fires, also called lingering fires, are fires that spread through soil organic matter and smolder. They burn very slowly, but hot, over a prolonged period of time, and then they resurface. In December 2023, we had over 100 fires still burning, and in spring 2024, they continued to burn,” she says.

Current estimates show that only about 15 percent of the Northern Hemisphere is underlain by permafrost, and yet these frozen soils contain about twice as much carbon as is currently in the atmosphere. By burning slowly and at a lower temperature, they release far more polluting particles and greenhouse gas emissions than flaming fires.

“These swampy soils take hundreds or even thousands of years to accumulate the carbon stored in the organic layers of the soil. In some cases, it burns down to the rock,” says Daniels. “In a very short period of time, we’re burning all this ancient carbon in the atmosphere. It becomes a feedback loop with extremely negative consequences for the ecosystem. It changes the hydrology – you lose the substrate and the seed banks that are in them, you turn the soils into a sort of mineral base instead of an organic base,” she says.

Intense and prolonged burning has serious consequences on soil seed reserves. Low and medium intensity fires – the type of “ordinary” burns that periodically sweep through forests – provide space for the forest to quickly regenerate. After these fires, peaty soils are full of seeds, and burned areas usually come back to life once the fire is over. But repeated fires and warming caused by zombie fires can destroy many seed species, making it much more difficult for ecosystems to quickly regenerate. After intense fires, only mineral soils remain – mainly sand, clay and silt.

Lacking easy detection, scientists are working to quickly develop methods to identify persistent fires in the Arctic using satellite imagery. Researchers say that while the vast majority of fires are caused by humans and lightning, zombie fires pose a significant challenge in fire management, with many crews working throughout the year to contain the fires. Scientists say similar phenomena are likely occurring in Siberia, which has also seen huge fires in recent years, but are not as well studied due to a lack of international collaboration with Russian researchers.

The problem may also get worse. Patrick Louchouarn, a professor in the School of Earth Sciences at Ohio State University, says: “This could become really problematic. The more the permafrost dries out and some of these organic-rich soils dry out in the Arctic, and the more you increase the temperature, there’s a greater chance that this fuel will be ready to burn.”

In 2025, the fire season has started early again in Canada. As of mid-September, 8.8 million hectares (21 million acres) had burned in hundreds of fires across the country.. Most of the early fires were likely zombie fires that spread from the previous year, say the researchers, who point out that Earth’s northern regions are the areas warming the fastest, rising several degrees warmer than average.

Communities on the ground are already feeling this change. Jennifer Baltzer, professor at Wilfrid Laurier University, says: “In 2023, we had this crazy fire season in the Northwest Territories. Seventy percent of the region was evacuated and it was very traumatic. People were driving through crazy fires to get away from their homes. There were many fires that overwintered around communities that had to be evacuated.

“This adds to the stress that people in these ecosystems face. Usually, winter is the season when you can feel safe from fire.”

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