What are zombie fires and how dangerous are they?

Like animals, fires can hibernate in the cold, smoldering underground for months on end. Known as holdover or zombie fires, they are not characterized by roaring flames but by plumes of smoke seeping from the ground.

How and where do zombie fires originate?

Zombie fires mainly originate in the cool boreal conifer forests found in Canada, Alaska, Northern Europe, or Siberia, where the ground is covered with a thick layer of needles and other vegetation that burns easily.

The soils in these regions are also often very peaty. As the precursor to coal, peat is highly flammable, so when boreal forests go up in flames, it doesn't take much for the fires to spread underground, where they stay alight for a long time. Particularly when the soil is dry.

Even when it snows, the fires tend not to go out, because not enough moisture gets into the soil to extinguish them.  

In fact, if there is a lot of peat and other plant residue in the ground, these fires can continue to burn throughout the winter.

How are climate change and zombie fires related? 

Wildfires are part of the natural ecological cycle in conifer forests, enabling them to continually regenerate. The thick layer of discarded vegetation makes it hard for seeds to access nutrients in the soil, so fires ensure the release of vital minerals, while also creating a bed of ash that seeds can better penetrate.

But planetary warming as a result of humans burning fossil fuels has led to increasingly frequent and prolonged periods of drought and heat. The Arctic, for example, has warmed about four times faster than the rest of the world over the past 43 years.

Heat and drought don't only lead to more wildfires in the spring and summer, but also cause more zombie fires to continue smoldering in the ground in their wake.

In a 2021 study, researchers found a connection between higher numbers of zombie fires in the boreal forests of Alaska, the United States, and Canada when the preceding summers had been particularly hot.

In early 2024, for instance, there were 10 to 12 times more zombie fires in western Canada than usual. The previous summer, Canada had experienced its worst fire season in history, with over 18 million hectares of forest and grassland burning, displacing around 200,000 people. 

Zombie fires also contribute to climate change as they release greenhouse gases such as carbon dioxide (CO2) into the atmosphere.

What do winter fires mean for spring and summer?

Although a lack of oxygen means they spread slowly in the dense underground, if they are not extinguished by thawing snow, zombie fires significantly increase the risk of wildfires in the following warmer seasons. In spring, a gust of wind is often enough to reignite a surface-level blaze.

Hibernating zombie fires can cause the wildfire season to start in early spring rather than the summer. The risks are heightened in a dry spring or if there is little winter snow to moisten the soil as it melts.   

Stopping such smoldering fires in the ground is difficult. Because they spread in densely compacted material, water alone is not sufficient to put them out. It might not penetrate the complete fire area, but leave behind smoldering nests waiting to reignite. Extinguishing zombie fires requires the removal of entire layers of ground where they are burning.

Edited by: Tamsin Walker