What are ‘zombie fires’ and are they becoming more common?

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As the severity and frequency of wildfires increases across Canada, so-called “zombie fires” could become more common with climate change, experts warn.

“Zombie fires” ignite and burn during the wildfire season, go dormant over the winter period before reigniting again the following season in the spring, according to experts. They burn deep underground and remain undetected for a considerable amount of time before re-emerging.

Canada’s wildfire season got off to a subdued start this year but fire activity has picked up pace on both the east and west coasts with firefighters busy battling blazes.
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A team of researchers is collecting the first field data on “zombie fires” – formally referred to as overwintering fires – in the Northwest Territories, where a total of 76 active fires were burning on Tuesday.

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“We tend to see these zombie or overwintering fires in conjunction with really large, intense fire years,” said Jennifer Baltzer, Canada research chair in forests and global change at the Wilfrid Laurier University, who is leading the research in N.W.T.

The view from above a fire where controlled burning was completed to stop progression towards Wekweeti – a community in the Northwest Territories’ North Slave region earlier this season. Photo credit: Government of the Northwest Territories

Climate change is increasing the frequency of such events, said Anthony Farnell, Global News chief meteorologist.

“Warm winters have an effect on the warm summers, so you end up with these bogs and marshes that dry out from the inside,” he explained.

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It’s not a new phenomenon and zombie fires are not limited to N.W.T.

The 2016 Fort McMurray fire in Alberta was considered a “zombie fire” and these forest fires have also been reported in British Columbia in past years.

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“Sometimes fires can continue smouldering underground and then all they need are certain wind conditions or a dry stretch to resurface,” said Farnell.

How common are zombie fires?

Meanwhile, holdover fires can occur in the same season after briefly going dormant. These are different than “zombie” or overwintering fires that cross over different wildfire seasons.

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Authorities in the N.W.T are keeping a close eye on one on the west side of Marian Lake that first ignited on July 25, went dormant for about four or five days before flaring up again.

This naturally-caused holdover fire has grown to nearly 6,500 hectares, according to the Department of Environment and Natural Resources.

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Heavy winds from the south are expected to cause extreme fire behaviour and significant growth, it said in an update on Tuesday.

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Mike Westwick, N.W.T wildfire information officer, said the territory generally sees a few overwintering fires per year and it is a natural feature of the boreal forest.

“It is a reality that deep down in the soil, fires can quietly burn even over winter and then flare back up when conditions change a bit,” he said.

Favourable fire conditions such as high temperatures, dryness and wind play a role, said Erika Berg, B.C. wildfire information officer, adding that such fires tend to be caused by lightning.

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“Zombie fires” generally account for a minute fraction of fire damage, but they can occasionally become a major problem, according to a scientific report published in May 2021 by a group of scientists from the U.S. and the Netherlands.

“Between 2002 and 2018, overwintering fires were responsible for 0.8 per cent of the total burned area in Alaska, USA and the Northwest Territories of Canada,” the report found. “However, in one year this amounted to 38 per cent.”

Behaviour and impact

Over the last week and a half, Baltzer and her team visited a number of sites of overwintering fires that started in 2014 in the Northwest Territories, continued over winter and reignited in 2015.

She said one of the challenges with “zombie” or overwintering fires is that they tend to reignite very early in the spring so it extends the active fire season. But whether or not they can become more widespread will depend on the conditions for that particular year, she said.

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“Zombie fires” can occur in Arctic, subarctic and northern boreal forests.

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Because Canada like most boreal nations is warming at “three to four times the global average” there is greater potential for very large fires, which go hand-in-hand with an increased prevalence of overwintering fires, said Baltzer.

Her research team is trying to evaluate what types of forests these fires tend to flare up in to better understand their behaviours. They are also looking into the impact of so-called zombie fires on the ecosystem.

“That has implications for what the landscape will look like moving forward,’ she said.

— With files from Global News’ Kristi Gordon and The Canadian Press

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