Whether you call them holdover, overwintering or “zombie” fires, new research says climate change could be worsening fires that last through the winter.
The study, published in the journal Nature on Wednesday, found extreme summer temperatures that extend and intensify wildfire season could be a key driver of the so-called zombie fires.
“Zombie fires are nothing new. People in the forest fire community refer to them as holdover fires,” said David Martell, a professor at the Institute of Forestry and Conservation at the University of Toronto.
Martell was not involved in the study.
He said most fires in Canada are caused by lightning strikes. When lightning hits a tree, it passes through to the ground and can ignite the layer of organic debris on the forest floor.
But most lightning brings rain, and prospective wildfires can smolder at the base of a tree for days or even weeks. The delayed blaze is called a holdover fire. Extend that delay through the winter, and it becomes an overwintering or zombie fire.
“There’s nothing going on in the woods now that hasn’t been going on for a long time,” said Martell.
“Things have changed because of climate change. Some things may be going on differently than they have in the past.”
Normally, overwintering fires aren’t a major concern.
The Nature study included 45 fires reported by the Alaska and NWT fire service between 2005 and 2007. Those fires accounted for less than one percent of total burned area and carbon emissions during that period.
Their impact was worse in some years than others. Twice in the NWT, overwintering fires accounted for more than five percent of the yearly totals.
Researchers found three drivers linked to climate change that help explain why overwintering fires are worse in some years than in others.
Extreme summer temperatures, large burn areas, and deep burning all increased the likelihood that smoldering peat survived the winter and reignited when the snow thawed.
But it isn’t clear that these findings mean overwintering fires are bound to get worse in the future.
“We know that because of climate change, we’re going to have more fires. Because of climate change, they’re going to burn larger areas. And because of climate change, more large fires will sit in the landscape in the boreal forest,” said Martell.
“Then the question is – is that going to cause more holdover fire problems?”
Martell said there are many factors, like the amount of rainfall, that make predicting the future impact of overwintering fires difficult.
“It’s an important question to ask, but I don’t have an answer,” he said.
The new study doesn’t exactly have an answer, either.
“The fate of overwintering fires in the changing boreal biome will depend on counteracting processes that facilitate or constrain their occurrence,” the researchers wrote.
Climate change means hotter, longer summers. But it could also mean the arrival of new tree species that are less vulnerable to fires.
The researchers identified overwintering fires as an understudied area of fire management.
“In the NWT, you have deep concentrations of peat. To the extent that holdover fires are going to be a problem, it’s going to happen in peat,” said Martell.
“If I were interested in studying holdover fires, then the NWT would be a good place to start.”