Why Canada is riddled with wildfires that burn year-round

Even at -40C (-40F), smoke kept billowing from under the snow. All through the winter, Marty Wells, a wolf trapper and fire crew leader, would see plumes of white as he drove north out of Fort Nelson, British Columbia in Canada. He found melt holes venting like hot springs, and flames licking at the foot of white-encrusted trees in the muskeg bogs. In places the ground was burning a foot (30cm) deep or more.

"It gets in the muskeg and burns the muskeg and then it crawls, it creeps underground is what happens, and then pops up somewhere else," Wells says.

When the snow melted in early May, these smouldering fires, often called "zombie" fires, came to life again and began to feed on dry trees and brush. The plumes of smoke north of Fort Nelson became a conflagration of 700 sq km (270 sq miles). The town is now caught in a horseshoe of fire: to the east, another zombie fire has burned an even larger area, while to the west, a new wildfire has encroached to within 2.5km (1.6 miles) of the community, damaging properties. Residents have been evacuated. Wells says he didn't expect the fire to get this bad. "It just happened that day that we had really high winds and right conditions and away it went."

Canada's boreal zone – a mixture of forest and wetlands – makes up more than half of its land area. Wildfires burned a record 185,000 sq km (71,429 sq miles) of the country in 2023, an area the size of Syria. In western Canada, many of these fires went underground and smouldered until this spring, which fire services refer to as "overwintering" or "holdover" fires.

Much of Canada is in its third year of drought, with the western provinces of Alberta, British Columbia and the Northwest Territories the worst hit. Snowpack, the main source of water, was 37% lower than normal in British Columbia this winter, and powerful rivers shrivelled into thin veins of blue. As the snow melted and windy weather arrived to fan the flames in April and May, these zombie fires tore into a smorgasbord of dry fuel. With hotspots flaring up around old burn areas in north-west Alberta, the province had to declare an early start to the fire season on 20 February, rather than the usual 1 March.

Mike Flannigan, a professor of wildland fire at Thompson Rivers University in British Columbia who has been monitoring fires there since the 1970s, says he's never seen an outbreak of zombie fires like this one. "This year, it's extremely significant," he says. "British Columbia has hundreds of thousands of hectares burned, and most of those are due to overwintering fires."

Scientists believe that zombie fires are becoming more common, and that they are a symptom of the growing frequency and intensity of wildfires due to human-caused climate warming. But these fires also kick off the fire season earlier and could potentially result in more carbon emissions per area burned.

"They're leading to significant emissions, significant fire growth the following year," Flannigan says. "As long as conditions are right, dry, it will continue to burn and burn."

The power of peat

Behind two sets of thick metal doors in Imperial College London, there's a race on, and the dried peat is winning. Peat is the dark, damp soil that forms when dead plant matter piles up in waterlogged, oxygen-poor environments like the bogs of Canada, Russia or Scotland. Because this matter can't fully decompose, it's rich in carbon, making it potentially potent fuel for zombie fires. It just depends on how wet it is.

In their "Hazelab," Guillermo Rein, a professor of fire science, and his graduate students have filled two square, foil-wrapped ceramic containers with Canadian peat and ignited the back ends with glowing hot wires. Because of the limited oxygen content within the soil, they're smouldering, or burning without a flame. White smoke wafts out as the fire creeps forward through the brown peat. The lab takes on a smell like Scotch whisky. The peat in the container on the left is unaltered, with a moisture content of about 100%. The peat on the right was dried out in an oven to less than 1% moisture. After about two hours of burning, only a quarter of the moist peat has burned, versus more than half of the dry peat. "This is the natural, pristine condition," Rein says, pointing to the moist peat. "It can still burn, but it's relatively easy to stop because it's propagating so slowly."

Next he gestures to the dry peat. "Now imagine that someone has drained the peatland, or that climate change has arrived and it's a really, really dry year. And this is the situation."

It's the same situation in drought-stricken western Canada. As last year's gigantic fires – some started by humans, some by lightning – burned through the forests, they ignited dried-out peat below. And once peat starts burning, it can go for months, burning up to five metres (16.4ft) below the surface and creeping through the soil. Normally, firefighters in the remote expanses of Alaska, Canada and Siberia rely on the icy grip of winter to put out wildfires. But in peat, winter doesn't always stop the burn. Behind the two ceramic containers in Hazelab is a glass-doored refrigerator where Rein has performed similar peat-burning experiments. "It doesn't care about the cold weather," he says. Even at -20C (-4F), the fire keeps smouldering. "It's very scary, actually."

Overwintering fires have probably occurred for millennia. One of the first to be recorded was in 1941, when hot ashes dumped by an Alaska Railroad train lit a fire in the boggy woods south-west of Fairbanks. That fire came back to life the next spring, burning 2,900 sq km (1,100 sq miles).

But now zombie fires are happening more often, experts believe. After a record fire season in Alaska in 2004, scientists found that fires like one in the Yukon Flats wildlife refuge had kept burning underground through the winter, chewing up the roots and soil until trees toppled. 

Following the ruinous "summer of smoke" in the Northwest Territories in 2014, a dozen holdover fires reignited into "explosive" blazes that stretched fire crews thin in May 2015, far sooner than normal. And in 2020, the year after an unprecedented 100 wildfires occurred above the Arctic Circle in 2019, hotspots began showing up on satellite early in the spring thaw in Siberia, setting the stage for record wildfire carbon emissions that summer.

'Apocalypse of a forest'

The increase in zombie fires is hard to quantify since infrared satellites can't usually spot them under the snow. So fire expert Sander Veraverbeke and his graduate students at the Vrije Universiteit Amsterdam have been teasing them out from on-the-ground reports and spring and summertime satellite imagery.

The first major study of overwintering fires, which they published in 2021, found that these fires accounted for 0.8% of the total area burned by wildfires in Alaska and the Northwest Territories in 2002-18. But the percentage was higher the year after big fire seasons. Hot summers resulted in especially severe wildfires that spread further and burned deeper into the soil, seeding more zombie fires, they discovered.

In Siberia, overwintering fires ate up even more of the total burned area: 3.2% in 2012-20, a study the next year found. That may actually be an underestimate, since it doesn't count the re-burning of previously scorched areas, says Vrije Universiteit graduate student and study co-author Thomas Hessilt.

Veraverbeke says his group's as-yet unpublished data suggests that since 1986, zombie fires have been burning increasingly large swathes of North America. The percentage of wildfires they make up, however, has stayed roughly the same, since wildfires are growing in general as the climate heats up. But if zombie fires continue at high rates, it's possible they could encourage even more widespread and severe fires, according to Veraverbeke. "If you also have more severe fires, then that would also increase the likelihood [of overwintering fires]," he says. "So that could become a feedback loop."

The boreal forest is a "phoenix among ecosystems", author John Vaillant has written, that is "literally reborn in fire". In North America, a major wildfire every 100 years is vital to crack open black spruce cones and make room for the seeds to grow. But now fires are sweeping through more frequently, overwhelming black spruce and paving the way for deciduous aspen and birch.

Zombie fires, which can scorch an area down to the mineral soil, may be striking an even more devastating blow. In 2022, Veraverbeke and scientists Jennifer Baltzer, of Wilfrid Laurier University, and Merritt Turetsky, of the University of Colorado Boulder, went "zombie fire hunting" in the Northwest Territories. They helicoptered into 15 sites that had burned in 2014 and again in 2015. Rather than a cacophony of new saplings, they found an "apocalypse of a forest,", Turetsky says, full of blowing ash and trunks collapsed like a Jenga game. More zombie fires could convert stretches of boreal not into deciduous forest but rather shrubland or meadow, the scientists say. That may further threaten declining woodland caribou populations.

"These sort of repeat fires, bam, bam, they burned back-to-back, but maybe a decade ago. And there was very little evidence of regeneration," Turetsky says. "They sort of looked like moonscapes."

Surprisingly, many of these fires occurred in forests that had a peat layer of only a few centimetres, she adds. That suggests that zombie fires can overwinter not only in peat but also in woody debris or tree trunks and roots.

How often that happens could answer another big question: are zombie fires boosting wildfire emissions?

Boreal peatlands store as much as 30% of the land-based carbon on Earth. Trees, on the one hand, can regrow within a century, drawing down the carbon that was released when they burned. But peat takes thousands of years of cold, wet conditions to regenerate. That means that when zombie fires burn peat, they emit ancient carbon that has very little prospect of being sequestered back into peat. "If there are parts of the world where these are happening predominantly in peatlands and there's a much deeper burn as a consequence, then yes, those parts of the landscape will see higher per area emissions," Baltzer says. "And we have some evidence of that."

Furthermore, peatlands often contain permafrost. If zombie fires set off the thawing and decomposition of that frozen soil, as other wildfires have in Alaska, it could add even more carbon to the atmosphere – though the biggest emissions threat, Baltzer notes, is from more frequent and severe wildfires overall.

The zombies of the fire season are almost as hard to stop as the ones in horror films. But they're more predictable than lightning fires because they spread from the edges of previous burn areas. This winter, aircraft in the Northwest Territories and British Columbia identified potentially dangerous zombie fires with infrared scans, and workers with a tanker truck in Alberta dug up and hosed down holdover fires near populated areas.

It takes a lot of water to put out smouldering peat: firefighters pumped 7.5 billion litres of water from nearby lakes to extinguish a peat fire in North Carolina in 2008. Rein and his colleagues have devised a suppressant that, like soap, reduces the surface tension of water, allowing it to penetrate tiny soil pores and extinguish peat fires 39% faster. That could be potent against peat fires in Indonesia, which often survive the rainy season like zombie fires do the winter. They are often near villages that have drained peatlands to grow palm oil for globally consumed products like candy bars and shampoo.

But many boreal zombie fires are tens of kilometres from the nearest town or road. Provinces typically leave them to burn unless they threaten a community, especially with the fire season growing longer and part-time firefighters stretched thin. The best way to prevent zombie fires, scientists say, is to drastically cut our climate-warming emissions.

Tyanna Steinwand, who works for the Tłı̨chǫ first nation government in the Northwest Territories, says zombie fires like the ones that were smouldering along Highway 3 this winter increase the stress of living with wildfire. She and her three cats had to evacuate their home in Behcheko last summer in the face of advancing flames. 

"Trying to get to exercise, trying to get time on the land… it's all hampered due to the forest fires," she says. "And then seeing the zombie fires smoke all spring, all winter, doesn't let you ever really relax from that."

-bbc