WASHINGTON, USA — They're often described in media reports as "zombie wildfires" - though they're more commonly referred to as "holdover" or "overwintering" fires, especially by scientists.
By any name, they are fires that can burn underground for months and return above ground when conditions are right - and they've happened in the Pacific Northwest.
Susan Prichard, Ph.D., is a research scientist at the University of Washington studying fire ecology. She said she's witnessed one of these fires in her own neighborhood.
"We actually, when we moved to Methow Valley in eastern Washington, bought a house near a wetland, and our neighbor unwittingly had a pile burn that got covered in snow, we're talking about 3 feet of snow," Prichard said. "And in spring, there was a hot spring day with some wind, and we were on the opposite side of the valley coming back to our house, and saw a smoke plume and had to call 911 about it because you could see it was the old pile burn that had reignited. That was my first experience, and since then, we've seen quite a bit of that activity."
Prichard said the phenomena occurs when fire burns into deep organic layers, such as the buried root systems of a tree, that can allow a fire to smolder in a low-oxygen environment but not extinguish. It combusts slowly, burning inefficiently for a long time.
"Where I live, sometimes the old stumps or snags of dead trees catch fire, and then the fire will actually go subterranean in the old root systems," Prichard said. "That's the perfect setup for overwintering fires because, as you said- all fires need oxygen. It's part of the combustion equation. Even low amounts of oxygen are good enough for these smoldering fires."
Prichard's best advice to prevent holdover fires is to be sure to dig properly, hitting mineral soil around a campfire or pile burn. She also advises people to check their old pile burn sites the next season.
Remote sensing technologies such as satellites or airborne surveys from airplanes can detect smoke from these underground fires.