Zombie fires are wildfires that burn underground in carbon-rich organic soils, persisting through months of winter snow and cold before resurfacing as new blazes the following spring. They smolder slowly beneath the surface in peat and similar fuel layers, consuming organic material without visible flames, sometimes for an entire season before anyone realizes they never actually went out. The phenomenon has drawn increasing scientific attention as warming temperatures dry out northern soils and extend the conditions that allow these fires to survive from one year to the next.
How a Surface Fire Moves Underground
Most wildfires involve both flaming and smoldering combustion at different stages. Flaming combustion is the visible, fast-moving process people picture when they think of a wildfire. Smoldering combustion is slower, flameless, and sustained by the gradual thermal decomposition of organic material. The two processes transition back and forth during any large fire, and when a surface fire passes over ground rich in organic matter, the heat can ignite a smoldering front that creeps downward into the soil.1Europe PMC. Smouldering wildfires in peatlands, forests and the arctic: Challenges and perspectives
Once that happens, the fire no longer needs wind, open air, or continuous fuel at the surface. It feeds on layers of partially decomposed plant material buried anywhere from a few centimeters to over a meter deep. This organic layer acts as both fuel and insulation, trapping enough heat to keep the smoldering front alive even as the surface cools, rain falls, or snow covers the ground. Temperatures in the smoldering zone can range from roughly 400 to 700 degrees Celsius, hot enough to sustain combustion but cool enough that no flames break through to the surface.
Surviving Winter and Re-emerging in Spring
The defining trick of a zombie fire is overwintering. At the end of a boreal fire season, a surface fire may appear to be fully extinguished. Firefighters leave, satellite monitoring flags the area as inactive, and snow buries the burn scar. But if smoldering combustion has taken hold in deep organic soils, the fire continues underground throughout the winter months. It creeps laterally and downward, consuming soil carbon at a pace measured in centimeters per day or even per week. Then, when spring arrives and snowmelt exposes dry surface fuels again, the smoldering fire re-emerges as a new flaming wildfire.2IOP Publishing. Overwintering fires rising in eastern Siberia
This re-emergence typically happens before lightning season begins, which is one of the key ways researchers identify zombie fires from satellite data. If a new fire appears in early spring in the same area as a previous year’s burn, at a time when no lightning or human ignition source can explain it, overwintering smoldering combustion is the most likely explanation. Researchers studying eastern Siberia have documented a rise in these overwintering events, linking them to longer and hotter fire seasons that give smoldering fronts more time to establish themselves deep underground before winter arrives.2IOP Publishing. Overwintering fires rising in eastern Siberia
Why Peat and Organic Soils Are the Key Ingredient
Zombie fires are overwhelmingly a peatland and organic-soil phenomenon. Peat is made up of partially decomposed plant matter that has accumulated over thousands of years in waterlogged conditions. When that peat dries out, whether from drought, drainage, or warming temperatures, it becomes an extraordinarily effective fuel for smoldering combustion. A dried peat layer can burn for months on a tiny energy budget because its dense organic content releases heat slowly and steadily.
Moisture is the single most important factor determining whether peat will ignite and sustain combustion. Research on peaty substrates has shown that the probability of combustion drops as moisture levels increase, for both smoldering and flaming ignition types.3PubMed. Conserving mire ecosystems: linking flammability and moisture content of peaty substrates In a healthy, water-saturated peatland, the water table sits near the surface and keeps the organic layers too wet to burn. But when the water table drops, whether because of a dry summer, deliberate drainage for agriculture, or long-term climate drying, the upper peat layers lose their moisture protection. Once those layers dry below a critical threshold, a passing surface fire or even sustained radiant heat can initiate smoldering that works its way deeper.
This is why zombie fires are not just an Arctic problem. Any landscape with deep organic soils can be vulnerable if conditions dry out enough. Tropical peatlands in Southeast Asia, temperate bogs in Europe, and boreal forests across Canada and Russia all contain the raw fuel. The boreal zone gets the most attention because it holds the largest peat reserves on Earth and is warming faster than anywhere else, but the underlying mechanism is the same wherever thick organic soil meets dry conditions.
Where Zombie Fires Have Been Documented
Siberia has become ground zero for zombie fire research. The vast boreal forests of eastern Siberia sit atop deep organic soils and permafrost, and warming temperatures are creating conditions that favor overwintering fires more frequently than in the past. In 2020, underground smoldering fires resurfaced across the Arctic early in the season, contributing to an unprecedented wave of spring and summer wildfires.4Nature Geoscience. Arctic fires re-emerging That year was striking because fires appeared in areas far north of the usual wildfire belt, in tundra regions where fires had historically been rare.
Alaska and northern Canada have also reported overwintering fire events. In Canada, provincial fire agencies have started monitoring previous-year burn scars in early spring specifically to catch zombie fire re-emergence before it can spread. Local and Indigenous communities living near boreal forests have long observed these phenomena. Research combining community observations with remote sensing data in eastern Siberia has highlighted the value of ground-level knowledge from people living along roads and rivers who can spot the early signs of re-emergence, like wisps of smoke rising from snow-free patches, well before satellites pick up a heat signature.5Arctic Science. Combining community observations and remote sensing to examine the effects of roads on wildfires in the East Siberian boreal forest
The Carbon Problem Nobody Was Counting
One of the most concerning aspects of zombie fires and underground smoldering in general is how much carbon they release and how badly standard tracking systems underestimate it. Conventional wildfire emission models are calibrated primarily for flaming combustion of surface vegetation. They estimate how much biomass burns on the surface and calculate emissions from that. But when fire moves underground and consumes soil carbon that took millennia to accumulate, those models miss a huge share of the total output.
A study of the 2022 wildfires in France illustrates this gap vividly. Researchers measuring atmospheric carbon found that the fires emitted roughly 6 million tonnes of COâ‚‚, and belowground smoldering accounted for about half of that. For carbon monoxide, the underground share was even more dominant, around 85% of total CO emissions. When everything was tallied, the total carbon footprint of those fires was about twice what the standard global fire tracking system had estimated for the country.6Biogeosciences. Soil smoldering in temperate forests: a neglected contributor to fire carbon emissions revealed by atmospheric mixing ratios Those are temperate forests in western Europe, not Arctic tundra. The belowground carbon problem is not limited to the far north.
The climate feedback loop is straightforward but alarming. Warming dries out organic soils. Drier soils burn more easily and burn deeper. Deeper burning releases ancient carbon. That carbon accelerates warming, which dries out more soil. In the Arctic, this feedback is compounded by permafrost thaw, which exposes organic material that has been frozen and protected from decomposition for thousands of years. Once thawed and dried, it becomes fuel for the same smoldering process.
What Smoldering Smoke Does to Air Quality and Health
The smoke from smoldering fires is chemically distinct from the smoke produced by flaming combustion, and in some ways it is worse. Smoldering combustion produces substantially more particulate matter per unit of fuel consumed. Depending on fuel type, smoldering can emit anywhere from four to nearly fifty times more particulate mass than flaming combustion of the same material.7PubMed Central. Mutagenicity and Lung Toxicity of Smoldering vs. Flaming Emissions from Various Biomass Fuels: Implications for Health Effects from Wildland Fires The particles are also larger on average, with smoldering-phase particles centering around 2 micrometers compared to about 0.6 micrometers for flaming-phase particles.
The chemical composition differs too. Smoldering smoke contains higher concentrations of methoxyphenols, a class of compounds released when lignin in plant material breaks down slowly at lower temperatures. Smoldering peat in particular produces smoke enriched in long-chain hydrocarbons. All of this matters for public health because smoldering fires can burn for weeks or months in a single location, producing a continuous low-level plume that settles over nearby communities. Unlike a fast-moving surface fire that creates intense but brief smoke exposure, a zombie fire or deep peat fire creates chronic air quality problems that can persist through an entire season.
For people living in boreal communities near peatlands, this is not a theoretical concern. Siberian villages have reported smoky conditions lasting weeks during overwintering fire events. In Indonesia, smoldering peat fires have produced haze events that blanketed entire regions for months, sending hospital admission rates for respiratory illness sharply upward. The health burden from smoldering fires is probably underestimated for the same reason the carbon emissions are: the standard monitoring tools were designed with flaming surface fires in mind.
Why These Fires Are So Hard to Extinguish
Putting out an underground fire is fundamentally different from fighting a surface wildfire. Water dumped on the surface may not penetrate deep enough to reach the smoldering front, especially in dense peat where the fire can be burning a meter or more below ground level. Even when water does reach the fire, peat’s sponge-like structure can make it surprisingly resistant to thorough saturation. Research testing different suppression approaches on smoldering peat has found that extinguishing air-dried peat requires roughly 90 kilograms of water per square meter, which is an enormous volume to deliver across a large burn area.8ScienceDirect. Suppressing underground peat fire and smoldering spread via water, ice, dry ice, and liquid nitrogen
Alternative approaches show some promise in laboratory settings. Dry ice, for instance, achieved the highest probability of extinguishment in one study by sublimating slowly and displacing oxygen at the combustion front. It required about 41 kilograms per square meter for air-dried peat, less than half the water requirement, and its cooling effect lasted up to about three hours. Liquid nitrogen cooled the soil almost instantly to around minus 175 degrees Celsius, but the effect wore off so quickly that the fire often rekindled once temperatures rebounded.8ScienceDirect. Suppressing underground peat fire and smoldering spread via water, ice, dry ice, and liquid nitrogen
Scaling any of these methods to real-world conditions is a massive logistical challenge. Zombie fires occur in some of the most remote landscapes on the planet, far from roads, water supplies, and infrastructure. In Siberia, some overwintering fires burn in areas accessible only by helicopter. Firefighters sometimes have no practical option but to monitor the fire and try to prevent its spring re-emergence from spreading, rather than attacking the underground smoldering directly. Prevention, in the form of maintaining high water tables in peatlands and reducing the severity of the initial surface fires that seed underground combustion, is generally considered more effective than attempting suppression after the fact.
Detection Challenges and the Limits of Satellites
Finding a zombie fire before it re-emerges is genuinely difficult. Satellites that detect active wildfires rely on thermal anomalies and visible flame signatures, but a smoldering fire buried under soil or snow may not produce a surface temperature anomaly large enough to trigger detection. The fire might warm the ground a few degrees above ambient, which is lost in the noise of normal temperature variation across a landscape. Smoke plumes from underground fires can also be too thin or too dispersed to show up on satellite imagery, especially during winter when atmospheric conditions scatter and dilute the signal.
Researchers have developed methods to identify probable zombie fires retrospectively by overlaying spring fire detections with previous-year burn scar maps and filtering out locations where lightning or human activity could explain the new ignition. But this approach only confirms a zombie fire after it has already re-emerged. Catching it during the overwintering phase, when intervention might still be possible, requires either ground-based monitoring or specialized thermal imaging from aircraft or drones. Some experimental approaches use soil temperature sensors installed in burn scars at the end of the fire season to flag subsurface hotspots, but deploying sensors across the vast areas of boreal forest where zombie fires might occur is not yet practical.
What Happens to the Soil and Ecosystem Afterward
When fire burns through the organic soil layer rather than just passing over the surface, the ecological damage is qualitatively different from a conventional wildfire. A surface fire may kill trees and shrubs but leave the soil structure and seed bank largely intact, allowing regrowth within a few years. A deep smoldering fire can consume the organic layer entirely, exposing mineral soil that may take centuries to rebuild. In peatlands, this means the destruction of carbon stocks that accumulated over thousands of years, with no realistic prospect of recovery on a human timescale.
The soil microbial community also shifts dramatically. Research in boreal forests has found that wildfire causes microbial communities to transition from fungal networks that support living trees to communities dominated by decomposer organisms. Bacterial growth declines, and the specialized fungi that help trees absorb nutrients from the soil are reduced substantially, with losses increasing as fire severity goes up.9Biogeochemistry. Wildfire and post-fire management reshape soil microbial guilds and carbon dynamics at a boreal forest site in Sweden This matters for recovery because young trees depend heavily on those fungal partnerships to establish themselves. When the fungi are gone and the organic soil that supported them has burned away, the site may recover as grassland or shrubland rather than returning to forest.
In permafrost regions, the damage extends below the fire itself. When smoldering combustion removes the insulating organic layer, the underlying permafrost loses its thermal protection and begins to thaw. This can lead to ground subsidence, altered drainage patterns, and the release of additional greenhouse gases from decomposing permafrost carbon. A single zombie fire event can permanently alter the hydrology and vegetation of a site, creating conditions that make future fires more likely rather than less.
Evidence in the Deep Past
Zombie fires are not a new phenomenon, even if the name is recent. Researchers studying peat cores have found charred aggregates of decomposed peat material at various depths in bogs dating back thousands of years, physical evidence that underground peat burning has occurred repeatedly throughout the Holocene period.10Mires and Peat. Holocene fire history: can evidence of peat burning be found in the palaeo-archive? These charred fragments look different from the charcoal left by surface vegetation fires. They show signs of the organic soil itself having burned, not just the plants growing on top of it.
What has changed is not the phenomenon but its frequency and scale. The boreal and Arctic regions that hold most of the world’s peatland are warming two to four times faster than the global average. Longer fire seasons give surface fires more opportunity to ignite deep smoldering. Drier conditions lower the moisture barriers that historically kept peat too wet to burn. And thawing permafrost exposes new organic material to the combustion cycle for the first time in millennia. Scientists tracking overwintering fire events in Siberia have documented an upward trend that aligns with warming temperatures and drought patterns, suggesting that what was once an occasional ecological event is becoming a regular feature of the boreal fire regime.