A zombie fire may have broken out in Alaska and Canada

Like every winter snow blankets Alaska and northern Canada, summer fires are extinguished and tranquility prevails — at least on the surface. Beneath all this white tranquility, some of these fires continue to burn underground, chewing on a high-carbon peat, devoting their time. When spring arrives and the cold landscape thaws, these “winter” fires come out from below — which is why scientists call them zombie fires.

Now, new analysis in the magazine Nature quantifies the extent of these for the first time, and shows what conditions may be involved in reviving fires. Using terrestrial satellite data and reports, the researchers developed an algorithm to detect fires that spanned more than a decade (tens in total) in the territories of Alaska and Northwestern Canada, snowed and re-ignited in the spring. Basically, the scars of the burn were related to the surrounding area and then a new fire broke out. (They ruled out cases that could have coincided with a lightning storm, as well as enough that happened to the public as a result of an unexpected outbreak). Between 2002 and 2018, winter fires were estimated to account for 0.8% of the total area burned on these lands. That may seem small, but it stood out for a year: in 2008, only one zombie fire was responsible for the formation of 38 percent of the total burned area.

This type of appearance can be a sign of things happening in the rapidly warming Arctic. Although 2008 was a bad year, it was not an unfortunate one. Instead, it was part of a pattern of conditions where zombie fires are most common. “They appear more often after hot summers and big fires,” says Rebecca Scholt, a systems systems scientist at VU Amsterdam research university, the lead author of the new paper. “And we can actually show that it’s increased in the last 40 years.” For example, in 2009 and 2015 they were particularly active during the fire years in Alaska and in 2014 in the Northwest Territories, with numerous winter fires occurring the following spring.

The northern soils are overgrown with peat, dead vegetation essentially concentrated carbon. When a fire burns in an arctic landscape, it also burns vertically on that ground. Once the surface fire has depleted the plant’s fuel, the peat fire continues to burn beneath the dirt, move deeper downward, and move from side to side. In the study, Scholten and his colleagues found that it is likely to occur after the hottest summers, which makes the vegetation drier, thus triggering more disasters. “The more severe it burns, the deeper it is in that earth,” says Sander Veraverbeke, a VU Amsterdam earth systems scientist who is the author of the new paper. “And the deeper it burns, the greater its chances of hibernating that fire.” Even when it rains in the fall or the surface freezes in the winter, the water can’t completely shut off the soil.

Then spring arrives and the ice recedes. These hot spots can be turned on to look for more vegetation on the edges of the original burn scar. “Basically, after the snow melts, we already have dry fuel available,” Scholten says.

It is believed to have occurred frequently in 2008 and other years with zombie fires. Large fires burned deeper into the ground, which gave them a better chance of surviving the winter. And researchers believe these conditions are becoming more common. “We really show that the big fire years, associated with hot summers, have become more and more frequent since 1975, and we hope that this trend will continue,” says Veraverbek. “That would lead to more frequent fires during the winter.”