The term “zombie fire” is a colloquial name for a phenomenon scientifically known as an overwintering fire or holdover fire. These are wildfires that begin during a typical fire season but survive through the cold, wet winter months. Lurking beneath the surface, they remain hidden and dormant, only to re-emerge and reignite the landscape in the following spring, often weeks before the usual start of the fire season. The ability of these fires to survive sub-zero temperatures and snowpack makes them a growing concern, particularly as high-latitude regions experience accelerated climate warming. This survival mechanism is rooted in a specific type of combustion and the distinct nature of the fuel found in the Arctic and Boreal regions.
The Mechanism of Overwintering Fires
The core scientific concept that allows a fire to achieve its “zombie” quality is called smoldering combustion. Unlike the visible, high-temperature flaming combustion seen in a typical wildfire, smoldering is a slow, flameless process. It occurs when oxygen reacts directly with the surface of a solid fuel. This type of burning operates at a much lower temperature and requires significantly less oxygen, allowing it to persist in subterranean environments.
A surface fire, often started by lightning or human activity during the warmer months, can burn deep into the organic soil layer. Once the surface flames are extinguished by rain, cold weather, or firefighting efforts, the fire transitions into its smoldering state underground. This low-temperature burn survives through the winter, insulated by the surrounding soil and the layer of snow or ice above it.
The surrounding earth and snow act as a blanket, helping to retain the residual heat needed to sustain the reaction. Even heavy snowfall may not be enough to suppress the fire entirely, as the smoldering front can continue to creep slowly underground. When spring arrives and the snow melts, exposing the smoldering front to more oxygen and drier surface fuels, the fire can spontaneously transition back into a flaming, full-scale wildfire.
The Unique Fuel Sources Sustaining Them
The persistence of overwintering fires is directly linked to the specific, carbon-rich materials found in the Northern Boreal forests and Arctic regions. The primary fuel source is peat, a dense accumulation of partially decomposed organic matter that has built up over thousands of years in water-saturated environments. Peatlands are the most carbon-dense ecosystems on Earth, and this material is exceptionally effective at sustaining a slow, smoldering burn.
These deep organic layers provide a massive, continuous fuel bed. The process of climate warming facilitates the burning of this ancient fuel by causing permafrost to thaw and drying out the overlying peat soil. Once this historically wet material dries out, it becomes readily ignitable and capable of sustaining the subterranean, low-oxygen combustion that characterizes a zombie fire.
The regions most affected, including Alaska, Canada’s Northwest Territories, and Siberia, contain soils that are thousands of years old. When a fire burns deep into these layers, it consumes carbon that has been locked away for millennia. This deep-burning behavior is a characteristic of high-severity fires and is more likely to lead to a smoldering holdover through the winter months.
Environmental Impact and Climate Feedback
The major concern surrounding zombie fires is their profound environmental impact, particularly their role in accelerating global warming. These fires release vast quantities of carbon that have been sequestered in the ground for thousands of years, effectively turning a natural carbon sink into a carbon source. When the ancient peat burns, it releases greenhouse gases, primarily carbon dioxide and methane, directly into the atmosphere.
The volume of emissions can be staggering; for instance, Arctic fires in 2020 released an estimated 250 megatons of carbon dioxide. This release creates a positive climate feedback loop, which is a self-reinforcing cycle that compounds the problem. Warmer temperatures lead to drier soils and more intense fires, which in turn release more carbon, driving further atmospheric warming.
The smoldering nature of these fires is also less efficient than flaming combustion, meaning they produce more particulate matter and incompletely burned gases. This contributes to air quality issues and the climate crisis. The degradation of these carbon-rich peatlands means the land cannot easily recapture the emitted carbon, locking the region into a trajectory of increased fire frequency and intensity.
Monitoring and Future Trends
Scientists rely heavily on advanced technology to study and predict the occurrence of these elusive fires, which are often too small to be easily detected. Remote sensing and satellite imagery are the primary tools used to track these deep, smoldering events. Researchers look for temperature anomalies, or “hotspots,” that appear in the early spring, often near the burn scars from the previous year’s fire season.
By analyzing satellite data, scientists can identify fires that flare up unusually early, ruling out common ignition sources like lightning or human activity. This approach helps fire managers target specific locations where overwintering fires are most likely to re-emerge. Studies have shown that overwintering fires are more likely to occur after warmer-than-normal summers and in areas that burned deeply into the organic soil.
The frequency of zombie fires has increased significantly over the past two decades, coinciding with the accelerated warming of the Arctic. This trend suggests that shorter, warmer winters and earlier snowmelt are creating more favorable conditions for these fires to survive and reignite. Understanding and anticipating these early-season blazes is becoming increasingly important for fire management in high-latitude regions.