What exactly are 'zombie' fires, and how are they affecting wildfires this spring?

The wildfire season is off to an early start in some parts of Canada partially due to "zombie" fires that have carried over from last year.

But what are they, exactly? And are they unusual?

Here's a breakdown.

So, what are they?

Fires that smolder below the surface in winter are sometimes called "zombie" fires.

But experts tend to prefer the more specific and less ominous term, "overwintering" or "holdover" fires.

"You think they're dead and then they're back. They come back in the spring when the ground dries out," said Lori Daniels, a forest ecology professor at University of British Columbia in Vancouver.

Without much oxygen, theyspread slowly, but they can flare up in spring, if the conditions are right, creating a potential hazard early in the wildfire season. High winds and dry, warm weather, in particular, can lead these fires to come alive again, said Daniels.

"When you get the wind fanning it, it becomes an open flame and then spreads fire again," she said.

WATCH| Holdover fires part of wildfire threat in B.C.'s northeast:

Holdover fires part of wildfire threat in B.C.s northeast

4 months ago

Duration 1:55

As wildfire threatens Fort Nelson, B.C., from the west, there are also much larger fires burning to the east of the town. They're known as holdover fires ones that never completely went out last year. Chad Pawson has more on those fires.

Is this season any different?

Last year's record-setting wildfire season and the drought-like conditions in parts of Western Canada created conditions for overwintering fires to reignite. At least 100 holdover wildfires were still smouldering in February.

Daniels said some of last year's fires had perimeters that were "very large and convoluted," making tracking them over the winter difficult.

"We expect that there's going to be places along that perimeter that we just couldn't get to and ensure the fire was out at the end of fire season," she said.

When there are fires this early in the season, they are "either overwintering fires flaring up" or they are human-caused, said Jennifer Baltzer, a biology professor at Wilfrid Laurier University.

She pointed out there are already fire bans in place in some regions "to try to reduce the latter, given how extraordinarily dry it is throughout Western Canada."But she said there were roughly 200 overwintering fires in Western Canada this past winter, making them a potentially major factor this season.

One of the clearest examples so far is a fire threatening the community of Fort Nelson, in northeastern British Columbia, which had an unusually warm winter and low snowpack.

The Patry Creek fire went dormant over the winter and then reignited in early May. (The Parker Lake fire, also threatening Fort Nelson, started when high winds blew a tree onto a power line).

B.C. Wildfire Service director of operations Cliff Chapman told The Canadian Press that holdover fires aren't a new problem. But he said last year's record-setting season, coupled with the abnormally dry conditions, have "amplified" the risk stemming from such wildfires.

The cause of a fire that led to evacuations in Fort McMurray, Alta., remains under investigation.

Are they becoming more common?

Research suggests such fires are becoming more common in the boreal forest as the climate warms.

The same hot, dry conditions that contribute to powerful, large-scale wildfires during the summer can also lead to deep burning in carbon-rich soils like peat that allow them to endure through the winter.

In such cases, a fire can simmer underground for months after the flames subside deep in the dried-out peatlands.

Scientists have found this phenomenon could be an increasing threat in the Arctic. Researchers also note that these fires are little understood and difficult to detect.

An added concern is that fires smouldering in the peatlands of Canada's far north release carbon that took hundreds of years to accumulate, Daniels said. (Already this year, an unusually early start in B.C. resulted in record-setting carbon emissions from wildfires in the province for May.)

"The recovery of those ecosystems and the ability to grow back vegetation and store carbon into the future is altered, and maybe permanently," she said.

"And so, we get this feedback cycle. We release all this carbon into the atmosphere which adds to global warming, which increases the drought, which contributes to the fires. It's a positive feedback with a negative consequence and one that has us very concerned."