In the boreal forests of the far north of the hemisphere, where the climate is warming faster than almost anywhere else, some wildfires survive winter snows and reignite in spring.
Now scientists in the Netherlands and Alaska have figured out how to calculate the extent of those “zombie fires” that smolder year-round in murky soil.
The study, published in the journal Nature, found that winter fires remain relatively rare in boreal forests: between 2002 and 2018 they were responsible for only 0.8% of the total area burned. But this varied dramatically based on the heat of the summers, the authors found, with the number rising to 38% of the area burned in one year.
This potentially suggests more fires hibernating as the weather warms, landscapes dry up and the flames of summer grow increasingly fierce, the researchers said.
“We know that fires can start in the lightning and human fire season. Now we can have another cause of burn zone. If it occurs near a fire scar from the previous year, early in the season, and there are no lightning strikes and it is not human, then it is a winter fire, ”said Sander Veraverbeke, landscape ecologist at Vrije Universiteit Amsterdam and co-author of the study.
“I think a general perception of people when they think of wildfires, they think of burning trees,” he said. “But in these areas of the upper north, in the boreal forest, about 90% of the carbon that is emitted comes from the soil.”
To calculate the extent of zombie fires in the area, the researchers built a computer algorithm that takes into account satellite images and records of lightning and human presence and infrastructure. For Alaska and the Northwest Territories, the algorithm produced an estimate of 0.8% of the area burned over a period of nearly two decades.
Zombie fires have also been recorded in Siberia in recent years, and the new algorithm could be used with local data to estimate the extent of winter fires in northern Russia, Veraverbeke said.
To survive the winter, fires have to be especially hot and deep, the study suggests. The amount of rain or snow that falls appears to be inconsequential, according to the study.
“The simple fact that this is happening is crazy enough and shows how fast this region is changing due to climate change,” Veraverbeke said.
Nancy Fresco, a landscape ecologist and climate researcher at the University of Alaska Fairbanks who was not involved in the study, said the findings underscored the vulnerability of boreal peat, which protects the permafrost below and contains large stores of sequestered carbon. The potential for an increase in wildfires in the region threatens to release more greenhouse gases into the atmosphere, he said.
On the other hand, scientists have determined that climate change and melting sea ice will cause an increase in lightning in the region, which could also trigger more fires.
“What has been a relatively rare phenomenon in the past could become something more frequent and catastrophic,” Fresco said.
George is Digismak’s reported cum editor with 13 years of experience in Journalism