In the far North, fire season usually doesn’t start until June, when snow has melted away and summer lightning storms sweep into the region. So scientist Sander Veraverbeke was confused when in May of 2016 he saw little flecks of fire on some satellite images from Alaska and the Northwest Territories.
“I was like, what the hell is going on?” says Veraverbeke, an Earth scientist at the Vrije Universiteit of Amsterdam in the Netherlands.
What he saw on the satellite images were “zombie fires,” remnants of burns from the previous year that somehow stayed alive, smoldering underground, through the long, cold winter.
Zombie fires aren’t an entirely new phenomenon in the Arctic; fire managers have noted occasional flare-ups in past decades. But Veraverbeke’s team found that their occurrences are tightly linked to climate change, happening more often after hot, long summers with lots of fire and suggesting that these still-rare events could become more frequent.
“The sheer fact that this is happening is a testament to how quickly the region is changing,” he says.
Arctic fire, changing
Like all forests, the wooded stretches of the Arctic sometimes catch on fire. But unlike many forests in the mid-latitudes, which thrive on or even require fire to preserve their health, Arctic forests have evolved to burn only infrequently.
Climate change is reshaping that regime. In the first decade of the new millennium, fires burned 50 percent more acreage each year in the Arctic, on average, than any decade in the 1900s. Between 2010 and 2020, burned acreage continued to creep up, particularly in Alaska, which had its second worst fire year ever in 2015 and another bad one in 2019. Scientists have found that fire frequency today is higher than at any time since the formation of boreal forests some 3,000 years ago, and potentially higher than at any point in the last 10,000 years.
Fires in boreal forests can release even more carbon than similar fires in places like California or Europe, because the soils underlying the high-latitude forests are often made of old, carbon-rich peat. In 2020, Arctic fires released almost 250 megatons of carbon dioxide, about half as much as Australia emits in a year from human activities and about 2.5 times as much as the record-breaking 2020 California wildfire season.
Hibernating in peat
Peat is made up of dead vegetation—mosses, flotsam of trees and shrubs, other Arctic plants— that hasn’t broken down all the way. It forms in damp and chilly parts of the world, where organic material degrades slowly. Peat reserves build over centuries or millennia; soils just a few feet deep can be thousands of years old. Today, peatlands cover some 4 million acres of the Arctic and store an estimated 415 billion tons of carbon, many times more than the forests above them and as much as all the trees on Earth.
Elsewhere in the world, soils don’t usually contain much burnable organic material, so fires feed on what they encounter—trees, shrubs, homes—at the surface. In the Arctic, fires usually start at the surface too, sparked by summer lightning or occasionally by humans. But under the kinds of climate-changed conditions becoming more common there—long, hot summers with particularly acute heat waves that suck the moisture out of plants and soils—the damp underlying peat can ignite.
“These are ancient soils,” says Jessica McCarty, an Arctic scientist at Miami University in Ohio. “The peat in Siberia is really old. These fires are starting to burn soils that evolved alongside Homo sapiens,” she says. Most peat in North America is younger, but still thousands of years old.
Once it’s on fire, peat can provide a habitat for fires to smolder on long after their surface flames have abated—for days, weeks, months, or even years.
The “peat has everything a fire needs to sustain itself,” says Rebecca Scholten, from the VU Amsterdam, the lead author of the new research. “It can burn quite intensely, enough to have enough heat to sustain itself over the winter,” essentially “hibernating” until the following spring, when snow melts away and gives the fire an opportunity to emerge at the surface once again.
The long burn
The existence of zombie fires—called “overwintering” or “holdover” fires by most experts—has been known for a while. In 1941, for instance, a human-caused fire along an Alaskan railroad line burned nearly 400,000 acres. The following May it popped up again; by the time it was finally extinguished it had eaten roughly another 300,000 acres. In more recent decades, officials in Alaska and the Northwest Territories have kept track of dozens of overwintering fires their crews have encountered.
But scientists hadn’t previously known whether there were more zombie fires smoldering away unrecorded, or whether they were getting more frequent as the Arctic climate rapidly warmed—though they suspected that was likely.
“Ten years ago, someone asked me, how often do these happen? And I said ehhh, they’re interesting but they don’t happen very often,” says Randi Jandt, a fire ecologist with the University of Alaska, Fairbanks.
But her opinion has changed. “We definitely seem to be seeing them more, in my 30 years of observation and asking people up there about before that,” says Jandt.
The penny really dropped for her in 2019, when enormous fires scorched over 2.5 million acres in Alaska. Fire teams worked around the clock to contain burn after burn, and by the end of the season, they thought they’d gotten all the blazes put out safely.
But following spring, as the snow melted, plumes of smoke began popping up in early May, well before the burn season was supposed to start.
“You compare this image that one day is just completely white, then you look at the same place two days later and it’s a little hotspot with smoke coming out of it,” says Mark Parrington, a researcher at the Copernicus Atmospheric Monitoring Service, a group within the European Center for Medium-Range Weather Forecasting.
He keeps careful track of fires across the Arctic by seeking out hotspots in satellite imagery. The last few years, in which Alaska, Siberia, and northern Canada have burned aggressively, the community of researchers keeping track of both holdover and regular-season fires has expanded.
“There hadn’t been so much focus on Arctic fire activity because there hadn’t been so many fires,” he says, but that’s changing quickly.
To make a more complete record of North American zombie fires and see if they were linked to climate change, Veraverbeke, Scholten, and their team mined 20 years of satellite images of the boreal forest regions of Alaska and the Northwest Territories. They built an algorithm that could pick out teeny flecks of fire that popped up near fire scars from the previous summer.
Because underground peat fires generally move slowly, a little over an inch per hour at most—about 100 times as fast as hair grows—they could rule out fires that were far away from the previous year’s burn. They limited their search to flames that popped up early in the year, before lightning started igniting new fires.
From 2002 to 2018, they found evidence of 20 large overwintering fires that hadn’t previously been detected by fire managers. In all, the zombie fires made up less than one percent of all the area burned in the regions—a tiny fraction. As Jandt says, “these aren’t keeping fire managers up at night.”
But the holdover fires showed a troubling if unsurprising relationship with climate change. Hot summers spawned big fires that often burned until late in the season, and remnants of those burns were much more likely to survive through the winter. In the Northwest Territories, zombie fires emerged after each of the six hottest summers on record, while none made it through winters after the coolest summers.
“There was such a clear relation of the overwintering fires with hot temperatures and burned area,” says Scholten. Those patterns are only likely to strengthen in the future as climate change intensifies the fire potential around the Arctic.