Holdover fire (zombie fire)
Holdover fires, often referred to as "zombie fires," are wildfire remnants that can reignite after a period of apparent dormancy, primarily found in boreal forests of arctic and subarctic regions. These fires develop when surface flames extinguish, but smoldering remains persist in peat-rich underground soils. They can be reactivated during fire-favorable weather conditions, particularly in spring when the soils dry out, even after lying dormant through winter. The phenomenon is notable for its potential to cause significant damage, as these smoldering fires can burst back to life, leading to new wildfires.
The term "zombie fire" specifically describes holdover fires that reignite across multiple seasons, particularly following intense wildfire activity. Concerns have grown among scientists regarding the increasing frequency of these fires, which is believed to be influenced by climate change, as warmer temperatures accelerate the drying of peat soils. These areas are significant carbon reservoirs, containing substantial amounts of carbon that can be released into the atmosphere upon ignition, further exacerbating climate change. The implications of holdover and zombie fires extend beyond environmental impact, marking a critical area of study in understanding wildfire dynamics and climate interactions.
On this Page
Subject Terms
Holdover fire (zombie fire)
Holdover fires are wildfire remnants capable of reigniting after an extended period of apparent dormancy. Mainly associated with wildfires affecting boreal forests in arctic and subarctic regions, holdover fires develop after the surface-level flames of a wildfire are extinguished. The wildfire’s remains may then smolder in peat-rich underground soils, and the wildfire can reactivate in the presence of fire-favorable weather and soil conditions.
Smoldering wildfire remains can also become insulated while packed under snow during the winter, rekindling when dry and milder weather conditions return in spring. Observers informally describe these fires as “returning from the dead,” a characteristic that inspired the commonly used informal term “zombie fire.”
Overview
The broadest definitions of a holdover fire describe a fire that lapses into dormancy or goes undetected for an extended period after igniting. These fires can be caused by both natural and artificial catalysts but are commonly associated with lightning strikes. As used in relation to the phenomenon of wildfires reigniting after going dormant, the term “holdover fire” technically describes a fire that rekindles during the same season in which it appeared to be extinguished. Meanwhile, “zombie fires,” which are known to scientists as “overwintering fires,” specifically refer to holdover fires that rekindle across multiple seasons.
“Zombie” holdover fires appear in wildfire-prone, vegetation-rich areas during dry weather conditions. After burning during wildfire season, zombie fires lapse into apparent inactivity during the winter but actually continue smoldering or burning in the deep underground layers of thick, peat-rich soil deposits. When spring returns and these soils dry out, zombie fires can ascend back to the surface level and reignite, potentially causing widespread damage and returning to wildfire status.
Experts note that overwintering fires are more common after particularly active or intense wildfire seasons. Scientists believe that climate change is contributing to an apparent increase in the worldwide frequency of overwintering fires, as elevated temperatures cause peat-rich soils to dry out from within at accelerated rates. According to the National Audubon Society, in 2023 Canada experienced the worst fire season on record, and this included its boreal forests.
The overwintering fires affecting the world’s boreal forests are also of particular concern to scientists because of the high carbon concentrations found in the peat-rich soils where they occur. Researchers believe that the uppermost 10 feet (3 meters) of soil found in boreal forests contains 200 percent more carbon than Earth’s entire atmosphere. As a result, zombie fires are capable of releasing very high concentrations of carbon into the atmosphere when they reignite into large wildfires. Elevated levels of atmospheric carbon, which functions to trap solar heat that would otherwise escape into space, is believed to be the primary driver of climate change.
Bibliography
Aziz, Saba. “What Are ‘Zombie Fires’ and Are They Becoming More Common?” Global News, 9 Aug. 2022, globalnews.ca/news/9047512/zombie-fires-canada-explainer/. Accessed 13 Mar. 2023.
Borunda, Alejandra. “‘Zombie’ Fires in the Arctic Are Linked to Climate Change.” National Geographic, 19 May 2021, www.nationalgeographic.com/environment/article/zombie-fires-in-the-arctic-are-linked-to-climate-change. Accessed 13 Mar. 2023.
Kelly, Ryan, et al. “Recent Burning of Boreal Forest Exceeds Fire Regime Limits of Past 10,000 Years.” PNAS, vol. 110, no. 32, Aug. 2013, pp. 13055–13060.
Leman, Jennifer. “Hot New Environmental Threat: Zombie Fires that Come Back to Life.” Popular Mechanics, 19 May 2021, www.nationalgeographic.com/environment/article/zombie-fires-in-the-arctic-are-linked-to-climate-change. Accessed 13 Mar. 2023.
Neustaeter, Brooklyn. “What Is a ‘Zombie Fire?’ Experts Describe the Cause and Concerns.” CTV News, 9 Aug. 2022, www.ctvnews.ca/climate-and-environment/what-is-a-zombie-fire-experts-describe-the-cause-and-concerns-1.6018530. Accessed 13 Mar. 2023.
Scholten, Rebecca C., et al. “Overwintering Fires in Boreal Forests.” Nature, vol. 593, 2021, pp. 399–404.
Sentner, Rebecca and Jeff Wells. "Boreal Forest Experiences Record-Breaking Wildfire Season." National Audubon Society, 25 July 2023, www.audubon.org/news/boreal-forest-record-breaking-wildfire-season. Accessed 13 Nov. 2024.
“Wildfire Glossary.” British Columbia Wildfire Service, 2023, www2.gov.bc.ca/gov/content/safety/wildfire-status/about-bcws/glossary#H. Accessed 13 Mar. 2023.
“‘Zombie Fires’ Partially Explain the Resurgence of the Wildfire in Gironde, but What Are They?” Le Monde, 13 Aug. 2022, www.lemonde.fr/en/environment/article/2022/08/13/what-are-the-zombie-fires-which-partially-explain-the-revived-fire-in-gironde‗5993464‗114.html. Accessed 13 Mar. 2023.