“Zombie fires” haunt Canada as underground blazes reignite across country

“Zombie fires” that have been smouldering beneath Canada’s snowpack since last winter have reignited with the return of warmer weather, contributing to an intense and early start to the wildfire season across the country.

These fires, also known as overwinter or holdover fires, occur when soil ignites during the summer months and continues to burn slowly underground through the winter. Scientists say the organic-rich peat and moss of Canada’s vast boreal forests act as fuel, allowing the flameless blazes to survive even beneath deep snow cover, the Financial Times revealed. 

Rather than extinguishing the flames, the snow actually insulates them. 

“As conditions return to warm and dry conditions of the summer, that burning will continue to propagate and then reach the surface again, potentially re-emerging as a flaming wildfire,” said Thomas Theurer, research fellow and fire expert at the University of Aberdeen.

Because they burn underground, zombie fires are difficult to detect and even harder to suppress. Their re-emergence has already had tangible effects in British Columbia. The province’s wildfire service confirmed that by midweek, 49 active wildfires had overwintered from the 2024 season, all located in the northeastern region near Fort Nelson.

“Holdover fires in northern British Columbia are one of many factors contributing to the current fire situation in the province — if weather conditions were cooler and wetter, they would not be able to become active again,” said Jen Baron, a fire ecologist at the University of British Columbia.

As flames intensified this week, evacuation orders were issued in parts of British Columbia, while Saskatchewan, Manitoba, and Ontario also battled fires amid unusually warm and dry conditions. The smoke has travelled thousands of kilometers, reaching the United States—from New York to the Gulf Coast—and spreading across the Atlantic to Europe. The EU’s earth monitoring agency Copernicus warned that more smoke plumes are expected in the coming days.

The crisis formed the backdrop of a first ministers’ meeting in Saskatoon, where Canadian prime minister Mark Carney acknowledged the efforts of firefighters and the plight of evacuees. The wildfires have also disrupted Canada’s energy sector, threatening around 5 per cent of national crude output as oil sands facilities in Alberta were forced to shut down.

Once rare, zombie fires are becoming increasingly common. “If we go 60–100 years ago, they [zombie fires] would appear very infrequently. But now they’re all over the place,” said Sebastian Wieczorek, a professor at University College Cork.

While some research suggests that snowmelt can quench fires burning close to the surface, deep-burning fires may evade this natural suppression. Moisture can be absorbed by the topsoil or evaporate due to heat from smouldering material below, allowing the subterranean fires to persist.

Though fires are a natural part of the boreal ecosystem, global warming is exacerbating the conditions behind these blazes. “Climate change plays a role because the summer is warmer and allows for more fire to move into the soil, which then finds protection from the snow, but [the fires] come back even sooner when spring is warmer,” said Guillermo Rein, professor of fire science at Imperial College London.

The growing frequency of zombie fires poses a broader climate threat. Peatlands, which store more carbon than all the world’s forests combined, release vast quantities of CO₂ when burned, further fuelling global warming.

Modelling by Wieczorek and his colleagues suggests that zombie fires may even ignite spontaneously under the right conditions, as rapid surface temperature increases activate microbes in the peat, generating enough heat to trigger smouldering underground without a spark.

“People are afraid of [zombie fires] and they are really scratching their heads about how you put them out. It really is underground,” said Wieczorek. “This is what is scary about them.”

By Sabina Mammadli