Scientists track 'zombie fires' to predict where they'll rise from the earth

'Zombie fires' smoulder underground through the winter and rise from the earth in spring to set the boreal forest ablaze again. A new study tracked zombie fires in the Northwest Territories and Alaska over 16 years to see how just much damage such fires are doing in North America.

The North has right conditions for overwintering fires, which may be increasing with climate change

A specialist sprays water while extinguishing a forest fire in the Krasnoyarsk region of Siberia, Russia in August 2020. Zombie fires — fires that smoulder underground through the winter and rise from the earth in the spring — were blamed for some of the extreme fires in Russia last year. (Russia's Aerial Forest Protection Service/Reuters)

"Zombie fires" that smoulder underground through the winter and rise from the earth in the spring to set the boreal forest ablaze are expected to become more common in the North as the climate warms. 

In a new study, Dutch and U.S. researchers have, for the first time, found a way to detect and track these remote fires. The research, published Wednesday in the science journal Nature catalogued such fires in Alaska and the Northwest Territories between 2002 and 2018 to estimate how much damage these fires do. They say what they found could help fire managers detect and deal with them sooner.

What are zombie fires?

Most wildfires in the North are started by lightning or humans in summer and extinguished by rain and snow by winter.

But some fires "somehow manage to survive winter by smoldering and thick organic layers under the snow and then re-emerge again and start a new forest fire in the next season," said Rebecca Scholten, lead author of the new study.

Such "zombie fires" in Siberia made headlines last spring, and were blamed for helping ignite some of the worst wildfires the region has ever known amid a record heat wave.

These composite satellite images from Landsat show three stages of an overwintering fire in Alaska: a seemingly extinguished fire at the end of the fire season in 2015, left, a snow-covered fire scar during winter, middle, and a re-emerging zombie fire during the spring of 2016, right. ( Carl Churchill/Woodwell Climate Research Center)

While this type of fire is popularly called a "zombie fire," scientists generally refer to it as an "overwintering fire." 

"It's not really dead," said Scholten, a PhD student at Vrije Universiteit Amsterdam in the Netherlands. "It's just in a different state."

Richard Olsen, manager of fire operations with the government of the Northwest Territories said fire managers prefer the term "holdover fire," which has been used for decades.

Why do they seem to be prevalent in the North?

Scholten said this type of fire is tied to the boreal forest for two reasons:

  • The ground has a thick peat layer that provides fuel to allow the fire to smoulder underground over the winter.

  • The snow cover in the northern winter may help protect the fire by keeping moisture on top of the soil instead of soaking and extinguishing it.

The researchers found overwintering fires tend to smoulder in lowland areas with thick soils full of organic material and higher tree cover that allows for fires to become really intense.

How did researchers identify and count the fires?

That's a challenging problem because the fires are so remote, so they've remained "quite mysterious to us," said Merritt Turetsky, director of the Institute of Arctic and Alpine Research at the University of Colorado Boulder, who researches wildfires in Canada's North, but wasn't involved in the new study.

To spot the zombie fires, Scholten and her colleagues examined daily satellite images between 2002 and 2018. They:

  • Compared the locations of fires to where fires were seen the year before and to observations on the ground.

  • Checked how early in the season those fires were occurring — overwintering fires tend to happen earlier in the spring, before most storms and lightning begin.

  • Ruled out other causes of ignition by checking if lightning strikes had been detected in the region, and the locations of roads and other infrastructure that might suggest the fire was started by humans.

How big a deal are these fires?

That was what the study was mainly trying to find out.

Sander Veraverbeke, associate professor in climate and ecosystem change at Vrije Universiteit Amsterdam and senior author of the new study, said many news reports made it sound like almost all the burned area in Siberia came from zombie fires, which he said "obviously isn't true." Most fires are still started by lightning or humans. But it wasn't clear how widespread zombie fires were.

The researchers found that in the Northwest Territories and Alaska, overwintering fires tend to be small and normally don't contribute much to total fire activity, accounting for less than one per cent of burned area on average.

However, they do have the potential to do a lot of damage. In Alaska in 2008, a single overwintering fire ended up burning 13,700 hectares or about 38 per cent of the burned area that year, the study reported.

Could they become more of a problem with climate change? 

While the study didn't look at a long enough time period to say for sure, the data suggests that climate change could increase their frequency, Veraverbeke said. 

That's because they tend to occur after hot summers and large, deep-burning fires. "And we know that these are occurring more frequently."

The setting sun is partially obscured by smoke from an out of control wildfire on the Parks Highway near Willow, Alaska, on June 14, 2015. Zombie fires, also known as overwintering fires or holdover fires, are more common after hot, dry fire seasons with large fires. (Stefan Hinman/Mat-Su Borough/REUTERS)

What can fire managers do with this information?

Scholten hopes the information about where and when the fires tend to occur will make it easier to predict where they might be expected and help fire managers spot them early.

In general, wildfires are considered part of natural cycles and are left to burn unless they threaten human lives and property.

But Veraverbeke said given the increasing amount of carbon released into the atmosphere by wildfires, fire managers may consider putting some of the fires out to prevent that carbon from being emitted — and may want to target certain types of fires.

"These overwintering fires may be low-hanging fruit because they are predictable," he said.

Turetsky said the Dutch researchers' new techniques could be used to create a monitoring system to track whether and how often zombie fires lead to big wildfires that generate lots of carbon emissions. "That would be the worst-case scenario," she said. "Obviously, if that does happen, we want to know about it."

Olsen said fire managers already watch for holdover fires, especially in areas where fires posed a threat the year before. But he said the new research provides useful information to help with tracking the fires and predicting them based on weather observations.

"Maybe it's something that we need to track a little bit more to get a better understanding [of] if and how the number and intensity or severity of the holdover fires is actually changing," he said, noting that new information in the study about greenhouse gases generated by such fires is also useful for the territory's calculations. 

Mike Flannigan, professor of wildland fire at the University of Alberta, suggested the new tracking technique could also be used to see how big a factor zombie fires are in other places such as Siberia, where Flannigan suspects they could be causing more damage.

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