A peat fire is a slow, flameless burn that creeps through thick layers of partially decayed plant matter buried in the ground, sometimes smoldering for weeks, months, or even through an entire winter under snow. Unlike the dramatic wall of flames people picture when they think of wildfire, a peat fire does most of its damage out of sight, consuming soil itself and releasing enormous quantities of carbon dioxide, fine particulate matter, and toxic metals into the air. Peat fires are considered among the largest fires on Earth by the sheer mass of fuel they consume, and their combination of invisibility, persistence, and pollution makes them uniquely dangerous to both human health and the global climate.
How Peat Burns Differently From Other Fuels
Most wildfires are dominated by flaming combustion, where visible fire races through surface vegetation driven by wind and heat. Peat fires work on a completely different principle. They are driven by smoldering combustion, a slow, low-temperature process that does not need much oxygen and produces no visible flame. Smoldering occurs when organic material undergoes a sustained chemical reaction with just the limited air available in the soil. Both flaming and smoldering processes can exist in the same wildfire and transition between each other, but in peatlands the smoldering phase dominates because the fuel is dense, wet, and buried underground where airflow is restricted.1PubMed Central. Smouldering wildfires in peatlands, forests and the arctic: Challenges and perspectives
With natural air supply and limited oxygen, peat smolders. Only when excess air reaches the fuel, such as through cracks in the soil surface or high winds, does the reaction tip over into open flame.2IOP Conference Series: Earth and Environmental Science. Smoldering behavior of peat fire This is why peat fires look so strange to anyone used to conventional wildfires. You can stand on ground that is actively burning a meter below your feet and see nothing but a faint haze or a whiff of acrid smoke. The surface might feel warm, and in some cases the ground collapses without warning where the peat beneath has been consumed.
Smoldering fires burn at lower temperatures than flaming fires, roughly in the range of a few hundred degrees Celsius rather than over a thousand. But they compensate with duration. A flaming grass fire might pass through a landscape in minutes. A smoldering peat fire in the same area can persist for months, steadily consuming the organic soil layer from within. That long burn time is part of what makes them so destructive: they remove the soil itself, not just the vegetation on top of it.
What Starts a Peat Fire
Moisture is the single most important factor determining whether peat will ignite and whether a fire can sustain itself underground. Laboratory experiments have shown that peat with a moisture content up to about 160% (on a dry-weight basis) can still support self-sustained smoldering, but once moisture rises above roughly 180 to 200%, ignition becomes essentially impossible.3Proceedings of the Combustion Institute. Experimental study of the ignition conditions for self-sustained smouldering in peat Those thresholds also depend on the type of peat. Different peat compositions, with varying bulk density and carbon content, have different critical moisture levels for ignition and fire spread.4International Journal of Wildland Fire. For peat’s sake! Peat type influences critical moisture thresholds that prevent combustion of organic soils in Western Australia
In practice, ignition sources range from lightning strikes to human activity. A glowing firebrand, the kind thrown ahead of a surface fire by wind, can ignite dry peat if its moisture content is below about 35%. Between 35 and 85% moisture, ignition still happens but requires wind to fan the ember hot enough to heat the surrounding peat. Above 85% moisture, a single firebrand cannot ignite peat at all.5International Journal of Wildland Fire. The initiation of smouldering peat fire by a glowing firebrand This is why drought is so central to the peat fire problem. When water tables drop and peat dries out, what was essentially a fireproof sponge becomes an enormous underground fuel load waiting for a spark.
Human land management dramatically increases the risk. Converting peatlands for agriculture or plantations usually involves digging drainage canals to lower the water table, which dries out the peat and makes ignition far more likely.6iScience. Effective restoration can avoid peatland fires: Large scale counterfactual assessment in Kalimantan, Indonesia In tropical regions, fire is also used deliberately to clear land, and during dry years those fires can escape into the peat layer below.
The Scale of What Burns
Peat is not just soil in the ordinary sense. It is compressed organic matter, sometimes thousands of years old, that stores carbon at concentrations far higher than mineral soil. The carbon stocks involved are staggering. Finnish peatlands alone hold an estimated 5,618 teragrams of carbon, most of it locked in the peat itself.7Mires and Peat. The influence of anthropogenic land use on Finnish peatland area and carbon stores 1950–2015 Even a relatively small peatland complex, like the one in Australia’s Snowy Mountains covering about 8,000 hectares, stores an estimated 3.55 teragrams of carbon built up over thousands of years at rates of roughly one tonne or less per hectare per year.8Mires and Peat. Peatland Carbon Stores and Fluxes in the Snowy Mountains, New South Wales, Australia
When peat burns, that ancient carbon goes straight into the atmosphere. A single fire episode on Indonesian peatland has been estimated to release around 842 tonnes of COâ‚‚ equivalent per hectare, with huge variability depending mainly on how deep the fire burns.9Global Biogeochemical Cycles. Estimating Greenhouse Gas Emissions From Peat Combustion in Wildfires on Indonesian Peatlands, and Their Uncertainty That depth of burn matters more than anything else; it accounts for over 94% of the variability in emissions estimates. A shallow peat fire and a deep one burning through the same area can differ in their carbon output by an order of magnitude.
What Peat Fire Smoke Does to People
The smoke from smoldering peat is different from the smoke of a typical forest fire, and in some ways worse. Fine particles smaller than 2.5 micrometers in diameter, the size that penetrates deep into the lungs, make up about 91% of the total particle mass emitted by smoldering peat.10PubMed Central. Particles emitted from smouldering peat: size-resolved composition and emission factors Those particles carry redox-active and carcinogenic metals that have been linked to serious health effects when inhaled. Because smoldering fires burn at lower temperatures and for longer periods, they tend to produce proportionally more of these fine particles and associated toxins than flaming fires do.
Peat smoke is also a vector for toxic metals that were historically deposited in peatlands by industrial pollution and mining. As peat burns, it releases metals like lead, cadmium, and zinc, which tend to concentrate on fine particles that travel long distances. Mercury behaves differently: it is released mostly as a gas, though smoldering conditions shift more of the mercury onto particulate matter as well.11Earth-Science Reviews. Peat fires and legacy toxic metal release: An integrative biogeochemical and ecohydrological conceptual framework This means peat fires can remobilize centuries of accumulated pollutants in a single burn event, sending them downwind as breathable particles.
The human toll is clearest in Southeast Asia, where peatland fires produce haze events covering millions of square kilometers. Research on Indonesian peatland fires estimated that the resulting air pollution causes, on average, around 33,100 premature adult deaths and 2,900 premature infant deaths per year, along with roughly 635,000 severe childhood asthma cases, about 4,400 additional hospitalizations for respiratory disease, and nearly 9 million lost workdays annually.12PubMed Central. The health impacts of Indonesian peatland fires These are not one-off disaster figures. They represent the average annual burden because peat fires recur with every drought.
Zombie Fires That Survive the Winter
One of the most unsettling properties of peat fires is their ability to overwinter. A fire that appears to go out in autumn can persist underground through months of snow and freezing temperatures, then resurface the following spring to ignite new vegetation. Researchers have started calling these “zombie fires,” and they are not rare curiosities. In boreal forests, overwintering fires are associated with hot summers that produce large fire years and deep burning into organic soils, conditions that have become more frequent in recent decades.13PubMed. Overwintering fires in boreal forests
The fires survive primarily in spots where the groundwater table stays more than 60 centimeters below the surface, particularly under tree roots, compacted soil, or elevated terrain that blocks moisture from reaching the smoldering front.14Fire. Overwintering Peat Fires in Russia’s Boreal Forests: Persistence, Detection, and Suppression The insulation of the overlying peat, snow, and frozen ground keeps enough heat trapped to maintain the slow chemical reaction through winter. When the snow melts, the fire breaks out again in what appears to be a spontaneous new ignition but is actually a continuation of last year’s burn. This makes fire management planning far more complex, because managers cannot assume that a fire season truly ends with the arrival of cold weather.
Why Peat Fires Are So Hard to Fight
Conventional firefighting tactics fail against peat fires for several reasons. Water dropped from aircraft runs off the surface or is absorbed only shallowly; it rarely reaches the smoldering front, which can be a meter or more underground. You cannot see the fire’s edges, so establishing a firebreak is difficult when you do not know exactly where the burning zone extends. Heated peat also becomes hydrophobic, meaning the very soil you are trying to wet actually repels water, making suppression even less efficient.15PubMed Central. An experimental method to investigate the water-based suppression of smoldering peat fire
Detection is equally challenging. Smoldering fires produce little visible smoke in the early stages and generate temperatures that may not trigger satellite-based hotspot detectors designed for flaming fires. Thermal infrared cameras mounted on drones offer a promising alternative, since they can survey large areas and detect the heat transferred to the surface above a subsurface fire. Pilot studies have confirmed that underground peat fires can be identified this way, and more recent work has developed deep-learning pipelines to automatically detect and map fire boundaries in thermal images.16Drones. Thermal-Drones as a Safe and Reliable Method for Detecting Subterranean Peat Fires17Drones. Detection and Geolocation of Peat Fires Using Thermal Infrared Cameras on Drones The technology is still being scaled up, though. For now, most peat fire detection relies on local reports, visible haze, or satellite data that picks up fires only after they have grown large.
Indonesia and the El Niño Connection
No country illustrates the peat fire problem more vividly than Indonesia. Its tropical peatlands are among the deepest in the world, and decades of drainage for palm oil and pulpwood plantations have left vast areas of dried, fire-prone peat. When El Niño events suppress monsoon rainfall, the result is catastrophic. During drought conditions, particularly when precipitation drops below about 4 millimeters per day for prolonged periods, fire activity in Indonesia increases in a nonlinear way: a modest further drop in rainfall produces a disproportionately large spike in burning.18PubMed Central. Indonesian fire activity and smoke pollution in 2015 show persistent nonlinear sensitivity to El Niño-induced drought
The type of El Niño matters too. Eastern Pacific El Niño events tend to produce more intense and prolonged droughts in Indonesia, with carbon emissions from fires nearly doubling compared to Central Pacific events.19Journal of Geophysical Research: Atmospheres. Connecting Indonesian Fires and Drought With the Type of El Niño and Phase of the Indian Ocean Dipole During 1979–2016 The 1997–1998 El Niño triggered fires across 8 million hectares of Indonesia, with estimated losses between 4.5 and nearly 20 billion US dollars. The 2015 fires cost an estimated 16.1 billion dollars, and even the smaller 2019 fires ran to about 5.2 billion in damages spanning agriculture, forestry, trade, tourism, transport, health care, and school closures.20PubMed Central. Assessing costs of Indonesian fires and the benefits of restoring peatland
Paleoecological records compiled from dozens of charcoal profiles across tropical peatlands show that 20th-century burning in the Indomalayan and Australasian regions exceeded anything in the previous two millennia. Meanwhile, Neotropical and Afrotropical peatlands saw declining fire trends during the same period, underscoring that the modern crisis in Southeast Asia is driven by land use as much as by climate.21PubMed Central. Unprecedented Burning in Tropical Peatlands During the 20th Century Compared to the Previous Two Millennia
Raising the Water Table Back Up
Because moisture is the key to preventing ignition, the most effective long-term strategy against peat fires is rewetting: blocking drainage canals to raise groundwater levels back toward the surface. In Indonesian peatlands where canal dams and deep wells have been installed, restoration work significantly reduced fire hotspot density. Canal dams and deep wells together accounted for about 38% of the total fire hotspot reduction observed between 2015 and 2019 across roughly 3,920 square kilometers of restored peatland.22Geophysical Research Letters. Tropical Peatland Restoration Reduces Fire Occurrence
A large-scale study in Kalimantan found that canal blocks with multiple overlapping rewetted zones were the most effective configuration. The combined impact of the most effective blocks reduced greenhouse gas emissions by about 90,550 tonnes of COâ‚‚ equivalent over a six-year period, and scaling up the best practices could deliver economic returns of up to 225%.23iScience. Effective restoration can avoid peatland fires: Large scale counterfactual assessment in Kalimantan, Indonesia However, that same research cautioned that the projected effectiveness of scaling canal blocks was lower than earlier cost-benefit studies had assumed, a useful reality check for policymakers who might overestimate how quickly rewetting can solve the problem.
At the ground level, the mechanics are straightforward. Monitoring near canal dams showed an average groundwater level of about 27 centimeters below the surface, compared to roughly 59 centimeters in unblocked areas. That difference translated directly into lower carbon emissions, because wetter peat does not burn and also decomposes more slowly under waterlogged conditions.24IOP Conference Series: Earth and Environmental Science. The Impact of canal blocking to reduce fire risks and carbon emissions on tropical peatland, Siak District, Riau Province
Transboundary Haze and the Limits of Law
Peat fire smoke does not respect national borders. The haze episodes from Indonesian fires regularly blanket Singapore, Malaysia, and southern Thailand, creating a diplomatic problem that has resisted decades of negotiation. ASEAN member states concluded the Transboundary Haze Pollution Agreement, which entered into force in 2003, but its enforcement architecture is weak. Disputes are limited to consultation or negotiation, and the principle of non-interference between member states has blunted the agreement’s practical impact.25Chinese Journal of Transnational Law. Extra-territorial Liability and Enforcement: Finding Ways to Tackle Haze Pollution in Southeast Asia26Global Journal of Law, AI & Ethics. State Responsibility and Corporate Accountability for Transboundary Haze Pollution: a Comparative Analysis Of International and Indonesian Law
Singapore took the unusual step of passing its own Transboundary Haze Pollution Act in 2014, which is extraterritorial in scope and imposes criminal and civil liability on entities responsible for haze pollution that damages Singapore. The law was symbolically important, but practical enforcement against companies operating in another country’s jurisdiction remains difficult. The haze problem illustrates a broader tension in environmental governance: the source of harm and the site of damage sit in different countries, and existing international frameworks lack teeth to compel action across that divide.
What Happens to Peatlands After They Burn
Even when a peat fire is finally out, the damage persists in ways that ordinary wildfires do not cause. A forest fire removes the canopy and understory, but the soil remains, and seeds and roots can regenerate. A peat fire removes the soil itself, sometimes a meter or more of it, permanently lowering the ground surface and eliminating the seed bank and root systems stored within. The carbon released took millennia to accumulate and cannot be replaced on any human timescale.
Vegetation recovery in burned peatlands follows a slow and specific sequence. In boreal peat fens, pioneer bryophytes adapted to colonize burned environments appear first, species like liverworts and certain mosses that can tolerate the post-fire chemical conditions. Vascular plants that survive from unburned root structures and rhizomes regrow relatively quickly, but species that reproduce mainly from seed recover only partially even five years after a fire.27Canadian Science Publishing. Post-fire peatland vegetation recovery: a case study in open rich fens of the Canadian boreal forest True peat accumulation, the rebuilding of the carbon-storing soil layer, is measured in millimeters per year at best. A severe burn can set the ecological clock back centuries or millennia.
In tropical peatlands the picture is even grimmer, because burned and drained areas are often converted to other land uses rather than allowed to regenerate. Without active restoration, the drained peat continues to decompose and subside even without fire, releasing carbon steadily through microbial breakdown. The choice is often between rewetting now, which at least stabilizes the peat, or watching the remaining deposit slowly oxidize away over decades.