Overwintering ‘zombie’ fires may become more common as climate changes

The findings underline the vulnerability of boreal peat, which protects underlying permafrost and holds huge stores of sequestered carbon.

ANCHORAGE — In the boreal forests of the planet’s far north, where the climate is warming faster than almost anywhere else in the world, some wildfires are surviving winter snows and sparking back up again in spring.

Now scientists from the Netherlands and Alaska have figured out how to calculate the scope of those “zombie fires” that smolder year-round in the peaty soil.

From 2002 to 2018, an average of about 1 percent of the burning in Alaska and in Canada’s Northwest Territories was caused by overwintering fires that survived from one summer to the next, according to a study published Wednesday in Nature. But in one year, zombie fires accounted for 38 percent of the region’s burning.

“We know that fires can start in the fire season by lightning and humans. Now we can have another cause of burned area,” said co-author Sander Veraverbeke, a landscape ecologist at Vrije Universiteit Amsterdam. “If it happens near a fire scar from the year before, early in the season, and there’s no lightning and it’s not human, then it’s an overwinter fire.”

As climate change dries out landscapes and drives increasingly ferocious summertime blazes, these zombie fires are also likely to become more common, he said.

To calculate the extent of zombie fires in the area, the researchers built a computer algorithm that considers satellite imagery, records of lightning strikes, and human presence and infrastructure. For Alaska and the Northwest Territories, that algorithm produced an estimate of 0.8 percent of burned area over a nearly two-decade period.

Zombie fires have also been recorded in Siberia in recent years, and the new algorithm could be used with local data including satellite imagery to estimate the scope of overwintering fires in northern Russia, Veraverbeke said.

To survive the winter, fires have to burn especially hot and deep, the study suggests. The amount of rain or snow that falls appears to be inconsequential, according to the study.

“The sheer fact that this is happening is already pretty crazy and shows how fast this region is changing because of climate change,” he said.

The findings underline the vulnerability of boreal peat, which protects permafrost below and holds huge stores of sequestered carbon, said Nancy Fresco, a landscape ecologist and climate researcher at the University of Alaska Fairbanks who was not involved in the study.

The potential for increased wildfire in the region threatens to release more greenhouse gases into the atmosphere, she said. Separately, scientists have determined that climate change — and melting sea ice — will lead to an increase in lightning strikes in the region that can also trigger more fires.

“What has been in the past a relatively rare phenomenon might become something more frequent and catastrophic,” Fresco said.