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Burn, sleep, repeat – Arctic wildfires overwinter

19 May 2021
Earth system scientists at Vrije Universiteit Amsterdam present the first scientific evidence of overwintering fires in boreal forests in Alaska and the Northwest Territories. Due to climate change, this bizarre phenomenon seems to be increasingly common.

The study was published today in the scientific journal Nature. The researchers found that extreme summer temperatures and intense burning enable some wildfires to remain in a smouldering phase in peat under the snow throughout winter, even when the temperature drops to -40 degrees Celsius. When warm and dry conditions arrive in spring, these fires flare up anew in spring. Because of this ‘rising from the dead’ these fires are also called ‘zombie fires’.

Finding patterns
The study combined field data from fire managers with satellite imagery to identify overwintering fires. Even though these fires are invisible to satellites while smouldering underground, the location and timing of their re-emergence gives them away. “During my work on lightning wildfires in the region, I discovered a particular pattern in satellite imageries, where new ignitions would spark up in spring within or at the edge of old fire scars”, explains Sander Veraverbeke, Associate Professor in Climate and Ecosystems Change at VU Amsterdam and senior author of the article. 

By analysing how far these fires usually spread underground, and how fast they re-emerge after the snowmelt, the researchers developed an algorithm that can reliably distinguish between overwintering fires and new ignitions by lightning or human activities.

“We studied if the re-emergence of the overwintering fires we identified with our algorithm was connected to landscape characteristics such as a forest and peat cover, and topography”, says VU PhD student Rebecca Scholten, first author of the study. The researchers found that overwintering predominantly happens in peaty soils that experienced deep burning. “We also saw a clear relationship between warm summers, that create large and intense wildfires, and the number of fires that manage to overwinter”, Scholten explains.

Support for fire management
Fire managers can directly use the results of this study. Although new to the scientific community, overwintering fires are not entirely new to fire managers. “I've worked in fire in Alaska for 30 years and at one time I would have said the overwinter holdover was a very rare phenomenon”, says co-author Randi Jandt, a fire ecologist at the Alaska Fire Science Consortium who works closely with fire managers in the region. “After the large fire seasons in 2004, 2005, 2009 and 2015, however, we noticed a surprising number of these overwintering fires.”

Intensifying wildfire regimes and population expansion into former wildland in Canada and Alaska put the limited fire management budgets under pressure. “Overwintering fires flare up at a time when fire management units are not yet fully staffed”, explains Jandt. “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.”

Global warming
With global warming, temperatures in the Arctic are expected to increase much faster than in other parts of the world. The effects of this are already seen in an earlier snowmelt and in a measurable increase in fire activity and severity in many parts of the high North. So can we expect all northern wildfires to overwinter in the near or far future?

“There are a lot of unknowns”, says Veraverbeke. “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.”

The study reveals that overwintering fires still are a relatively rare event, which causes a little less than one percent of the total burned area and carbon emissions from fires in Alaska and the Northwest Territories. Yet, they may exert considerable damage to the soils, thereby impacting the regeneration of vegetation. Scholten: “A lot is still unknown about these fires, for example what happens in the soils during winter, or why some of them grow into large, flaming forest fires, whereas others extinguish early on. We found an indication that early season drought is an important factor for growth, but since these fires re-emerge close to the original fire scar, fuels and wind direction play an important role as well.”

The study was principally funded by a Vidi grant from the Netherlands Organisation for Scientific Research (NWO), with additional support from NASA’s Arctic-Boreal Vulnerability Experiment.