What is a 'zombie fire?' Experts describe the cause and concerns

Two major wildfires in N.L. stabilizing NTV's Marykate O'Neill says air quality is improving but residents are still in limbo with a state of emergency that remains in effect.

As wildfires rage throughout parts of Canada, one fire in particular is highlighting concerns about so-called "zombie fires."

Authorities in the Northwest Territories are monitoring a large fire that has flared up after remaining dormant underground during the winter months.

Fires that persist through the winter in Canada were once considered a rare phenomenon, but experts warn these events are becoming more common as temperatures get warmer and less snow falls.

According to the BC Wildfire Service, a "zombie fire" – more commonly known as an overwintering fire or a holdover fire – occurs when a wildfire that burned deep underground in the previous year has continued to smoulder all winter long.

The agency warns these "residual hot spots" can re-emerge with the onset of warmer, dry weather in the spring.

Jennifer Baltzer, an associate professor of biology at Wilfrid Laurier University and Canada Research Chair in forests and global change, told CTVNews.ca in a telephone interview on Monday "zombie fires" smouldering in the biomass, such as the roots and bowls of trees or in peat soils, of the affected landscape.

Such phenomena can cause wildfire season to begin sooner than expected and last longer, she said. Baltzer added that overwintering fires also have the potential to cause much larger changes and carbon losses in the ecosystem than an ordinary wildfire.

"A single-season fire burns through the season and then ends, whereas a smoldering fire continues to combust wood and peat soils throughout the winter -- very, very slowly -- but we continue to see combustion throughout the winter," Baltzer said.

According to a 2021 study, overwintering fires generally accounted for a small amount (about one per cent) of the total burned area in the Northwest Territories and Alaska between 2022 and 2018.

However, researchers out of the University Amsterdam and the University of Alaska Fairbanks found there was a "surprising" increase in the number of overwintering fires reported in individual years.

In Alaska, for example, overwintering fires accounted for 38 per cent of the burned landscape during the 2008 wildfire season, according to the study.

Experts suggest "zombie fires" could become more common due to climate change, as hot, dry conditions associated with heavy fire years can lead this deep burning of carbon-rich biomass.

A study published in 2019 found increasing summer temperatures associated with climate warming may promote the survival of overwintering fires in the future, threatening boreal regions including the sub-Arctic, Arctic, Northwest Territories and the northern areas in Ontario, Alberta and Saskatchewan.

Baltzer, who is leading a team of researchers in collecting field data on "zombie fires" in the Northwest Territories, hopes to further analyze how these fires affect carbon loss and forest regeneration in Canada.

Baltzer explained shrubs that recover quickly from wildfires can do this because of underground plant systems. However, "zombie fires" damage these systems as well as seed beds for trees, which "inhibit reproduction and recolonization of these sites" post-fire, she said.

"This is a concern is because of the changing fire activity in Boreal forests," Baltzer said. "High latitude systems are warming at about three to four times the rate of the planet… And this really rapid warming is causing these systems to be more flammable… [resulting in] larger areas burned, more severe fires and more frequent fires."

Baltzer said overwintering fires are "inherently linked with climate change," as they become more prevalent following years that see increased wildfire activity, which is steadily rising amid global warming.

With this in mind, Baltzer said preventing "zombie fires" will require a reduction of fossil fuel emissions to slow the overall production of greenhouse gases driving global warming.

While Baltzer acknowledges this requires a global effort and is beyond the scope of her research team, she says they are working to provide fire managers with information to support an enhanced understanding of where "zombie fires" are in Canada to help inform the understanding of the behaviour of theses fires and how to respond.

See the map showing wildfires burning across North America, provided by ESRI Canada, full screen

Information about the map

With files from The Canadian Press