When we think about wildfires, we think of summertime: hot, dry weather and flames leaping from tree to tree. In some forests, though, fires can reach deep underground, smoldering all winter long and waiting to re-emerge. These are known as "zombie" fires, and according to new research they are increasingly threatening northern regions, including the Arctic.
A new study in Nature on Tuesday found that, in parts of Alaska and Canada, wildfires can hide in the soil between fire seasons, reigniting as soon as the weather warms. Researchers from Vrije Universiteit Amsterdam in the Netherlands teamed up with the University of Alaska and the Bureau of Land Management, trying to determine what factors are making these underground fires more common. Mixing satellite data with on-the-ground reports, they developed an algorithm to identify re-emerging fires and found that the hotter summers are setting the stage for these zombie fires.
“It seemed strange that these fires could sustain themselves for 8 or 9 months under the snow, where it’s minus 40 degrees and wet,” said Rebecca Scholten, lead author on the study, in an interview. “But then I started looking into it and it became a really coherent picture.”
Though we typically imagine fires as huge, orange flames, fires in the northern boreal forests often burn underground. Northern boreal forests have deep, rich, organic soil—and that organic matter is flammable. It also stores over twice as much carbon as Earth’s atmosphere, and burning it releases huge amounts of greenhouse gases. A large portion of carbon emissions from northern fires comes from underground burning, as smoldering fires emit more methane (a more potent greenhouse gas than carbon dioxide) than flaming fires do.
Just as these fires contribute to climate change, climate change makes the fires worse in turn. As summers get hotter in Alaska and Canada, fires burn bigger and deeper into the soil. And because fires are burning deeper in the soil, they are more likely to survive under all the winter snow.
“When we look at the last 40 years, we see temperatures are rising and, in places like Alaska, there's a significant increase in burned areas,” Scholten said. “The spike of these fires is probably due to climate change.”
The phenomenon of soil smoldering isn’t new, but the fact that these fires can survive until the next season is only now being studied in depth. Such work is critical because the Arctic, already beset by wildfires, is also warming faster than the rest of the world. Zombie fires contribute to a climate change feedback loop, and they typically burn early in the season, before fire managers are ready to battle blazes.
To investigate the phenomenon, researchers developed an algorithm that maps “overwintering” fires. Once they reginite, these fires have certain characteristics that make them identifiable: they tend to be near the original burn scar and they require no additional ignition source (like lighting or human-caused fires). The researchers trained an algorithm on 45 overwintering fires, finding that 89 percent were started within the bounds of the previous year’s fire.
The researchers also looked at how quickly these fires emerged, and found that the average was 27 days after the onset of snowmelt. In most cases, that means these zombie fires return around the end of May.
The team validated their findings against other overwintering fires, and then used it to identify 20 previously unreported overwintering fires in Alaska and northwest Canada from 2002 to 2018. In that time frame, zombie fires were responsible for 0.8 percent of the burned area and 0.5 percent of the total carbon emissions. But overwintering fires aren’t always small: in 2008, one zombie fire burned nearly 40 percent of the total burned area in Alaska that year.
Currently, sneaky soil-burning fires are still quite rare. But climate change is likely to make them more common in some regions. An extended fire season paves the way for larger, more severe fires. Drier summers mean drier vegetation, which adds fuel to these already-outsized flames. The researchers hope that, as we learn more about how these fires work, we can control flare-ups.
“This work is useful because the more that we know about where and when these fires appear, the easier it will be for fire managers to get there early,” Scholten said. “Then, they can safeguard the carbon that is in the soil.”