That kind of outbreak may be a sign of things to come in a rapidly warming Arctic. While 2008 was a notably bad year, it was no fluke. Instead, it was part of a pattern of conditions in which zombie fires are most likely to arise. “They appear more often after hot summers and large fires,” says earth systems scientist Rebecca Scholten of the research university VU Amsterdam, lead author on the new paper. “And indeed, that is something that we could show has increased over the last 40 years.” For example, the particularly active fire years of 2009 and 2015 in Alaska, and 2014 in the Northwest Territories, generated multiple overwintering fires the following spring.
Northern soils are loaded with peat, dead vegetation that’s essentially concentrated carbon. When a wildfire burns across an Arctic landscape, it also burns vertically through this soil. Long after the surface fire has exhausted the plant fuel, the peat fire continues to smolder under the dirt, moving deeper down and also marching laterally. In their analysis, Scholten and her colleagues found this is most likely to happen following hotter summers, because that makes vegetation drier, thus igniting more catastrophically. “The more severe it burns, the deeper it can burn into that soil,” says VU Amsterdam earth systems scientist Sander Veraverbeke, coauthor on the new paper. “And the deeper it burns, the higher the chances that that fire will hibernate.” Even when autumn rain falls or the surface freezes in the winter, water isn’t able to penetrate the soil enough to entirely extinguish it.
Then spring arrives and the ice retreats. These hot spots can flare up, seeking more vegetation to burn at the edges of the original burn scar. “Basically, right after the snow melts, we already have dry fuel available,” says Scholten.