Every winter, as Snow caps in Alaska and northern Canada, summer fires are extinguished, and calm – at least on the surface. Underneath all that white serenity, some of those fires keep burning underground, chewing up carbon-rich peat, and waiting for its time. When spring arrives and the cold landscapes melts, these “winter” fires pop up from below – which is why scientists call them zombie fires.
Now, a file analyzing In the magazine nature It determines how widespread it is for the first time and indicates the conditions under which fires are likely to re-start. Using satellite data and reports from the ground, researchers have developed an algorithm that can detect more than a decade of fires – dozens in total – that broke out in Alaska and the northwestern provinces of Canada, snowed, and flared up again in spring. Basically, they attached their burn scars to nearby areas where a new fire would later catch on. (They excluded cases that might have coincided with a thunderstorm, as well as cases close enough to people to be caused by accidental ignition.) They calculated that between 2002 and 2018, winter fires accounted for 0.8 percent of the total. A burnt area in these lands. That sounds small, but one year stood out: 2008, when a single zombie fire was actually responsible for charring 38 percent of the total burned area.
This kind of outbreak could be a sign of things to come in the rapidly warming Arctic. Although 2008 was a remarkably bad year, it was not a fluke. Instead, it was part of the pattern of circumstances in which zombie fires are most likely to arise. “They appear more often after hot summers and big fires,” says Rebecca Schulten, Earth systems scientist from VU Amsterdam Research University and lead author on the new paper. “Indeed, this is something that we can show that has increased over the past 40 years.” For example, the particularly active fire years of 2009 and 2015 in Alaska, and 2014 in the Northwest Territories, caused multiple fires in the winter the following spring.
Northern soil is laden with peat and dead plants It is mainly concentrated carbon. When wildfires burn across arctic landscapes, they also burn vertically across this soil. Long after a surface fire has exhausted plant fuel, a peat fire continues to burn under the dirt, moving to a greater depth and also running sideways. In their analysis, Schulten and her colleagues found that this most likely occurs after the hottest summers, as this makes the vegetation drier and thus ignites catastrophically. “The more burns it burns, the deeper the combustion will be in that soil,” says Sander Vierverbeek, a geologist at the University of Amsterdam in Amsterdam, co-author of the new paper. “The deeper the fire burns, the greater the chances of that fire dormant.” Even when autumn rains fall or the surface freezes in winter, the water is unable to penetrate the soil enough to douse it completely.
Then spring comes and the ice recedes. These hot spots can ignite in search of more plants to burn at the edges of the original burn scar. “Basically, right after the snow melts, we already have dry fuel,” says Schulten.
This is what they think happened in 2008 and in other years with frequent zombie fires. The large bonfires blazed deeper into the ground, giving them better prospects for winter survival. Researchers believe these conditions are becoming increasingly common. “We’ve already shown that years of big fires, associated with hot summers, have become more frequent since 1975, and we expect this trend to continue,” says Ververbeek. “This would also lead to more frequent fires in the winter.”