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” is a broad category that can include fires that rekindle during the same season or persist across multiple seasons. 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 soil found in boreal forests contains roughly as much or up to twice as much carbon as 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

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