Geohealth News

The Rise of Zombie Fires

Wildfires can smolder underground through Arctic winters, reigniting at the surface when conditions are right.

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In early May, just as the spring thaw was beginning in the northern reaches of Siberia, Mark Parrington spotted something strange on images captured by instruments aboard NASA’s Terra satellite. Red dots, indicating some kind of thermal anomaly, stood out on a vast white expanse.

Parrington, a senior scientist at the Copernicus Atmosphere Monitoring Service, posted the image on Twitter.
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Thomas Smith, an assistant professor in environmental geography at the London School of Economics, quickly noticed that the hot spots were located in areas that had burned in last year’s epic Arctic fires.

“Whatever they are (land clearance? natural?) they were occurring at the same time last year,” Smith wrote, posting a picture of the same location from 2019.

“Zombie fires?” Parrington replied.

Burning Between Fire Seasons

“Zombie fires” is a new and catchier name for an old and relatively rare phenomenon. Known among Arctic fire managers as holdover or overwintering fires, zombie fires transcend the typical fire season.

After flaming wildfires are extinguished on the surface, they can continue to smolder belowground, burning through peat and other organic matter. Fueled by methane deposits and insulated by a layer of snow, zombie fires can burn all the way through the cold and wet Arctic winters. In the spring, as temperatures begin to climb and the soil dries out, the fires can reignite aboveground.

Although records of zombie fire go back decades, the phenomenon wasn’t extensively studied until recently.

Researchers noticed that these early spring fires seemed to pop up more often after large fire seasons and often near the burn scars left by the previous fire. This can’t be a coincidence, thought Sander Veraverbeke, an assistant professor at Vrije Universiteit Amsterdam. Veraverbeke and Rebecca Scholten, a Ph.D. student at the university, have carried out some of the first scientific studies on zombie fires.

The pair analyzed records from Alaska going back to 2005, showing empirically that zombie fires were more likely after large fire seasons.

Fire managers in Alaska had noticed the trend. “We noticed clusters of these overwintered fires the spring after some of our big fire seasons in Alaska: 2004, 2005, and 2015,” said Randi Jandt, a fire ecologist with the Alaska Fire Science Consortium.

But Veraverbeke and Scholten’s study was the first to attempt to detect holdover fires using satellites, according to Jandt.
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A bird’s-eye view of the brown burn scar crisscrossed by fallen trees a year after a wildfire, surrounded by green vegetation
Toppled trees where the Lower Mouth Fire of 2004 burned underground through soil and roots. Credit: Randi Jandt/Alaska Fire Science Consortium

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Aerial photos of the Yukon Flats region in 2005, a year after the Lower Mouth Fire burned through the region, show trees “felled like toothpicks,” said Jandt, “due to their roots and soil underneath burning out.” That’s where smoke first emerged in the spring of 2005.

A Record-Breaking Season

Last year’s Arctic fire season was one of the biggest on record. In June and July 2019, more than 100 blazes burned in the Arctic circle. Millions of hectares of boreal forests across Siberia, Alaska, Greenland, and Canada went up in smoke. Clouds of soot the size of the European Union stretched across the sky.

Jandt and her colleagues at the consortium have already identified a few overwintered fires that are active now.

Without on-the-ground confirmation, it’s hard to say for sure whether or not the fires Parrington identified in the Siberian Arctic were actually started by the remnants of last year’s blazes. The satellite instruments can detect the fires only once they’ve reignited on the surface. But there are a few reasons why Parrington and Smith suspected they may have been burning there all winter long: Arctic fires are typically started by people or by lightning strikes, but these fires emerged in remote areas far away from human settlements, near the areas that burned last year, and before lightning strikes typically pick up in June.

Like all wildfires, zombie fires are sources of carbon emissions. In 2019, Veraverbeke and his team traveled to Siberia and camped out in the burn scars of past wildfires to study carbon combustion. They took soil core samples from burned and control plots to figure out just how much carbon Arctic fires are releasing.

Most people think that most of the carbon released during wildfires comes from burning trees, but that’s a misconception, according to Veraverbeke. “Seventy to ninety percent comes from the organic soil,” he said. “Trees contribute just a small fraction.”

Veraverbeke thinks that the smoldering phase of these fires accounts for only a small portion of wildfire emissions. Though overwintering fires can burn through organic matter and methane stored in the soil, they also tend to stay put. The team’s previous research shows that on average, zombie fires account for less than 1% of Alaska’s burned land area each year.

They can still be a headache for fire managers. “These fires can pop up early, while we’re still trying to complete fire readiness and training activities, and before lightning season (June–July) when we normally expect to be actively managing fires,” said Jandt. The upside is that the tundra is usually still cool and wet in the spring, and the fires may be easier to control.

But the Arctic region is changing rapidly, and the patterns of the past may not hold going forward.

“We know that these large fire years in the boreal forests are already happening more often,” Veraverbeke said. Temperatures in the Arctic are rising faster than almost anywhere else on Earth, melting the permafrost, drying soils, and providing new fuel for blazes. Global warming is also leading to more thunderstorms and thus more lightning strikes—a common ignition source for fires in remote regions.

Veraverbeke and Scholten are currently working on a study to find out how climate change might impact Arctic blazes and zombie fires.

—Kate Wheeling (@katewheeling), Science Writer

Citation: Wheeling, K. (2020), The rise of zombie fires, Eos, 101, https://doi.org/10.1029/2020EO146119. Published on 30 June 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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