(CNN) – In line with rising global temperatures, the Arctic has faced a streak of massive wildfires in recent years.
And while winter cold and heavy snow are enough to put out most fires, scientists say the right conditions can create fires that won’t die.
How do “zombie fires” work?
Like their namesakes, these so-called “zombie fires” are difficult to kill.
Zombie fires – fueled by fuel-rich soils in the Northern Hemisphere and subsisting on the little oxygen available under the snow – can burn for months, long after the flames above the surface have died out.
And sometimes fires that have burned all winter can ignite new flames the following year after the snow melts.
Sander Veraverbeke, associate professor at the Free University of Amsterdam, first suspected several years ago that this phenomenon was causing forest fires. Veraverbeke said that, when scanning satellite images for a previous study examining the role of lightning in the arctic fires, noticed new fires were starting near land that had burned the previous year.
“I saw that on the edges of the scars from the fires of the previous year, the flames would reappear in spring and new forest fires would start on the edges. And for me that was really intriguing, ”he said.
Veraverbeke said local fire managers confirmed that they too had observed fires that appeared to survive the winter.
However, it was still unclear how widespread these zombie fires were or if they were becoming more frequent.
In a new study published this week in the journal Nature, lead author of the Free University of Amsterdam Rebecca Scholten, together with Veraverbeke and other co-authors, attempt to answer these questions.
They warn of a possible increase in “zombie fires”
Generally speaking, they found that zombie fires are responsible for only a fraction of the land burned by fire in most years.
However, they warn that they could become a force firefighters need to be reckoned with, as human-caused climate change favors hot, dry conditions that can trigger large wildfires.
“We will have more of these extreme fire seasons, which also means that we will probably have more zombie fires,” Veraverbeke said. “So even though it’s a relatively small percentage now, it’s likely to be higher in the future.”
Through field observations and satellite monitoring, the study examined forests in Alaska and the Northwest Territories of Canada between 2002 and 2018.
During the years examined, the study found that zombie fires were responsible for only 0.8% of the total land burned and 0.5% of carbon emissions released by fires in Alaska and the Northwest Territories.
However, there were some years when they made up a much more significant part.
The importance of boreal forests
In 2007, 2008 and 2010, zombie fires were responsible for more than 5% of the land that was burned and the carbon emissions that were generated by the fires in Alaska. In 2008, a single zombie fire in Alaska burned more than 130 square km while simmering during the winter, an area equivalent to 38% of the total land that was burned by wildfires that year in Alaska, according to the study.
Both regions host large boreal forests with huge reserves of pines, firs and some species of deciduous trees, as well as soils rich in the flammable mixture of decaying vegetation known as peat.
In addition to being a powerful fuel for fires, peatlands (acidic wetlands) around the world store enormous amounts of carbon that trap heat. By some estimates, peatlands contain twice the carbon of all the world’s forests combined.
When peat is burned, that carbon is released into the atmosphere, where it remains and contributes to global warming.
The researchers found that years with large fires and above-average temperatures strongly correlated with a higher prevalence of zombie fires.
Zombie fires were observed during the six winters after the six hottest summers in the Northwest Territories. In contrast, none were observed after the seven coldest summers included within the study period.
Rising temperatures in the Arctic
Since 2000, the Arctic has warmed twice as fast as the rest of the planet.
And while these fires that can survive the winter remain rare, the researchers argue that they should continue to be monitored, along with the rest of the changes that are transforming these forests.
“I think the mere fact that this is happening shows that this region is changing very, very fast,” Veraverbeke said. “It really is a testament to the rapid warming of the Arctic and boreal (forests).”