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.