“Zombie fire” describes an Arctic phenomenon where wildfires survive the long, frigid winter months beneath the snow and ice. These blazes, also known as overwintering fires, are not new ignitions but a continuation of the previous summer’s burn. This persistence challenges traditional understanding of fire control and has profound implications for the global climate.
Defining the Overwintering Fire
An overwintering fire is fundamentally different from the visible, fast-moving flaming combustion associated with typical wildfires. These persistent blazes are characterized by smoldering, a slow, low-temperature burn within dense organic material. Smoldering lacks high-intensity flames, often presenting only as a faint plume of smoke. This subterranean activity allows the fire to escape surface extinction efforts, such as heavy rains or snowfalls. The term “zombie fire” reflects their ability to reignite the surface once spring conditions become favorable.
The Arctic Fuel Source
The material fueling these long-lasting fires is the vast deposit of organic soil found across the Arctic and boreal regions, primarily in peatlands. Peat is a dense accumulation of partially decayed vegetation that forms over thousands of years in cold, waterlogged environments. This organic layer is highly carbon-rich, making it a concentrated fuel source once it dries out and ignites.
In the far North, this peat often sits atop permafrost, the permanently frozen ground that locks away ancient carbon. The fire consumes the deep organic matter, sometimes burning several feet into the soil. As the fire burns downward, it can cause the surface layer of permafrost to thaw, making the underlying carbon available as additional fuel. This process shifts the ground from a stable carbon sink to an active carbon source.
Survival Mechanism Through Winter
The persistence of these fires through months of sub-freezing temperatures is possible because of an insulating barrier. After the initial surface fire is extinguished by snow and cold, the smoldering combustion retreats deep into the peat layer. The overlying snowpack acts as a thermal blanket, trapping the heat generated by the slow burn below. This insulation keeps the subsurface temperature high enough, sometimes above freezing, to sustain the combustion reaction.
The peat maintains a porous structure, even when frozen on the surface, allowing a minimal supply of oxygen to diffuse down to the smoldering zone. This restricted oxygen flow forces the fire into a slow, energy-saver mode where it consumes fuel without flaming. Smoldering can survive even when air temperatures drop far below zero, particularly in dry peat with low moisture content. When the snow melts in the spring, the insulating layer vanishes and the ground dries out, allowing the exposed fire to flare up violently on the surface.
The Scale of Carbon Emissions
The burning of this deep organic fuel releases carbon that has been sequestered for thousands of years. Smoldering combustion is an incomplete burning process, resulting in the release of both carbon dioxide and methane. Methane is a potent greenhouse gas, possessing a greater warming potential than carbon dioxide over a shorter time scale. This combined release of greenhouse gases from ancient carbon is the primary environmental concern.
Arctic fires in 2020 alone released an estimated 250 megatons of carbon dioxide into the atmosphere. This release contributes to a positive feedback loop in the climate system. As these fires release ancient carbon, they accelerate global warming, leading to hotter, drier summers in the Arctic. These warmer conditions increase the likelihood of intense summer wildfires and dry the peat, creating more opportunities for overwintering fires to ignite and persist.