Forest fires are mostly measured in the context of a summer fire season. But there are always fires that have gone out on the surface, but continue to smolder in the ground beyond the fire season. In a few cases, these so-called “zombie” forest fires overwinter underground and ignite again on the surface in spring, at the beginning of the next fire season. Boreal coniferous forests in northern regions are particularly prone to this, as 90 percent of the combustible carbon is released from the forest floor there. So far this phenomenon has hardly been researched.
Scientists from the Netherlands and the United States have now shown in a study published in the magazine "Nature" that the conditions for hibernating forest fires are favored by global warming. To do this, they developed an algorithm that uses field and remote sensing data to identify the proportion of burned forest areas in Alaska and the Northwest Territories of Canada that can be traced back to zombie forest fires between 2002 and 2018.
Sander Veraverbeke, landscape ecologist at Vrije University in Amsterdam and co-author of the study, says: “We know that forest fires can be caused by lightning strikes and people during the fire season. If a fire breaks out with burn scars from the previous year, early in the fire season, there was no lightning strike and no people were there, then it is a wintered fire. "
The researchers found that within the 16 years of their study period, only 0.8 percent of the forest areas burned can be attributed to the phenomenon. In addition, however, it was shown that the value can increase enormously in individual years - in one year even to 38 percent. The scientists also write in their study that, according to their data, these outliers follow previous heat and dry waves. From this, the researchers derive the thesis that hot, dry summers favor “zombie” forest fires and that global warming could lead to an accumulation of these phenomena in the future.