Heat waves, wild fires and zombie fires in the Arctic | Polarjournal
Satellite image (Terra/MODIS) of Alaska on July 9, 2019. The red dots indicate wildfires with smoke drifting across the Bering Strait. Image: NASA

In recent years, there have been numerous large-scale wildfires in the Arctic regions of Russia, Canada and the USA, which firefighters have had little to counter. Some of the fires broke out again the following year after long cold winters. What firefighters already suspected, scientists at the Free University of Amsterdam have now been able to prove: the fires continue to smolder in the peat-rich soil, even under a thick snow cover, until the ground is snow-free and dry in the spring and they can break out again.

The natural triggers of fires in the boreal forests are usually lightning strikes. More and more often, however, it is humans who are causing them, through arson or carelessness. Normally, wildfires do not last through the Arctic’s long winter season. But the soils of boreal forests consist of a thick layer of peat and decomposing spruce needles, which contain much more carbon than the trees growing on them. This layer contains just enough oxygen for the fires to continue smoldering and “resurrect” even in deep subzero temperatures and under a snow cover, which is why they are called “zombie fires”.

“We also saw a clear relationship between warm summers, that create large and intense wildfires, and the number of fires that manage to overwinter.”

Rebecca Scholten, PhD student at the Free University of Amsterdam

Rebecca Scholten, a doctoral student at the Free University of Amsterdam and lead author of the study, and her colleagues wanted to investigate the causes of overwintering fires and studied wildfire regions in Alaska and Canada’s Northwest Territories. In their study, published in the journal Nature, they found that extreme summer temperatures promote fire overwintering. High temperatures in the boreal zone lead to increased drought, which in turn causes a longer fire season with fast-spreading, large-scale fires. In addition, the ground dries out to greater depths, allowing fires to penetrate deeper.

Satellite imagery (Landsat) of a forest fire started in 2015. The fire (a, white outline) appeared to be extinguished but continued to smolder under the snow cover (b) until conditions were favorable for it to flare up again (c) and more area burned (d, blue outline). Images: Scholten et al. 2021

Support for the fire fighters
Using data from local fire managers and satellite imagery, scientists were able to identify the overwintering fires. To satellites, the underground fires are invisible, but they show the location and timing of the resurgence. Sander Veraverbeke, professor of climate and ecosystem change at the Free University of Amsterdam and co-author of the study, discovered a pattern in the satellite images. Apparently, the fires reappeared in the spring within or at the edges of old fire scars. The researchers were even able to develop an algorithm that could reliably distinguish between overwintering fires and new ignitions caused by lightning or human activity.

With this new information, scientists can make a significant contribution to future fire management. “Overwintering fires flare up at a time when fire management units are not yet fully staffed,” explains Randi Jandt, a fire ecologist at the Alaska Fire Science Consortium who works closely with fire managers in the region. “By focusing early-season monitoring on lowland forested peatlands, especially after a year with large and intensive wildfires, fire management units can make better use of their scarce resources.”

In the video, the authors explain the origins of “zombie fires” (German subtitles available). Video: Free University of Amsterdam

Global warming as a cause
To date, the proportion of zombie fires has mostly been very small. Between 2002 and 2018, “zombie fires” were responsible for just under one percent of the total area burned. One exception was in 2008, when 38 percent of the area burned in Alaska during the year was due to a overwintered fire.

Global warming and more frequent temperature extremes are already showing their effects clearly in the Arctic, with earlier onset of snowmelt and a measurable increase in fire activity and intensity.

“There are a lot of unknowns”, Veraverbeke says. “We know that Alaskan forests are expected to transition to more deciduous tree species in a warmer climate, which may lower fire activity, but also the storage of carbon in the soil. These ecosystem changes may reduce the occurrence of overwintering fires in the future.”

In another study published in the journal Environmental Research Letters, an international team of scientists from Japan, South Korea and the U.S. examined the connections between forest fires, air pollution and climate patterns in the Arctic. They found that the heat waves were due to simultaneously persistent or developed high-pressure systems over Europe, Siberia and subpolar North America. The researchers called this atmospheric circulation pattern the “circum-Arctic wave pattern” (CAW) and see it as a driver for the simultaneous occurrence of heat waves in Europe and wildfires in Siberia, Alaska and Canada. A CAW-like pattern could also be observed in early summer 2019.
The CAW pattern only became apparent after 2002, according to the researchers, indicating its increasing climatic significance.

The relationship between circumarctic wave patterns, heat waves, wildfires and air pollution. Anomalous anticyclones characterize the atmospheric circulation that develops simultaneously over the three remote regions surrounding the summer Arctic (July and August). These anticyclones cause warm and dry conditions from the surface to the mid-troposphere. Graph: Yasunari et al. 2021

Air pollution from wildfires
In the same study, scientists also evaluated air pollution resulting from forest fires and found significant levels of particulate matter, especially in summer. According to Teppei J. Yasunari, assistant professor at Hokkaido University and lead author of the study, the levels correlated with higher concentrations of organic carbon aerosol, indicating active forest fires.

“Wildfires lead to extensive air pollution, primarily in the form of inhalable particulate matter with diameters of 2.5 micrometers or smaller (PM2.5). Arctic hazes during winter and spring are typical phenomena due to aerosols existing in the Arctic. In our scientific field, it is also known that deposition of light-absorbing aerosols onto snow surfaces can induce the so-called snow darkening effect, contributing to accelerated snow melting. For these reasons, long-term assessments of PM2.5 and aerosols in the Arctic and surrounding regions are required,” Yasunari says.

Julia Hager, PolarJournal

Link to study on zombie fires: Scholten, R.C., Jandt, R., Miller, E.A. et al. Overwintering fires in boreal forests. Nature 593, 399-404 (2021). https://doi.org/10.1038/s41586-021-03437-y

Link to the study on Circum-Arctic Atmospheric Circulation: Teppei J. Yasunari, et al. Relationship between circum-Arctic atmospheric wave patterns and large-scale wildfires in boreal summer. Environmental Research Letters. 2021.

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