Combustion is the process that moves carbon from living or recently living material into the atmosphere, primarily as carbon dioxide and methane. Every time a forest burns, a grassland catches fire, or a furnace consumes coal, carbon that was locked in solid form enters the air as a gas and rejoins the atmospheric pool of the carbon cycle. Globally, landscape fires alone release roughly 3.4 billion metric tons of carbon per year, and fossil fuel burning adds several times more on top of that. But the story is far more layered than “fire puts carbon up, plants pull it back down,” because combustion also creates long-lived charcoal, reshapes soils, alters atmospheric chemistry, and triggers feedback loops that can either stabilize or destabilize the climate.
How Fire Moves Carbon Between Pools
The carbon cycle is essentially a set of reservoirs and the pathways connecting them. Carbon sits in the atmosphere, in ocean water, in rocks and sediments, in soils, and in living things. Combustion is one of the fastest pathways: it takes carbon stored in biomass or fossil deposits and converts it to atmospheric CO₂ in minutes to hours. Photosynthesis works the other direction, pulling CO₂ back into plant tissue over months and years. Decomposition does the same job as fire but far more slowly, as microbes gradually break down dead organic matter.
What makes combustion distinctive is its speed and its byproducts. A wildfire can release in a single day the carbon a forest accumulated over decades. Along with CO₂, fires produce methane, carbon monoxide, black carbon soot, and charcoal. Satellite measurements have captured these emissions in real time: wildfire plumes contain both CO₂ and methane, with methane-to-CO₂ emission ratios varying by ecosystem, from higher ratios in boreal forests to lower ones in savannas.1Geophysical Research Letters. First satellite measurements of carbon dioxide and methane emission ratios in wildfire plumes The type of vegetation, how wet it is, how intensely it burns, and how much oxygen reaches the fire all determine the chemical mix that goes into the atmosphere.
Wildfire Combustion Versus Fossil Fuel Combustion
Not all combustion is equal in the carbon cycle. This is one of the most important distinctions the general public tends to miss. When a forest burns, the carbon released was already part of the active cycle: the trees pulled it from the atmosphere years or decades ago, the fire sends it back, and regrowth eventually recaptures it. When coal or oil burns, the carbon being released was locked underground for millions of years. Fossil fuel combustion is a one-way transfer from geological reservoirs into the active atmosphere-ocean-land system.
Research has shown that these two types of emissions should not be compared side by side as though they have equivalent climate effects. The average lifetime of the atmospheric CO₂ increase is longer when it comes from fossil fuels than from wildfire, because vegetation regrowth following fire pulls the released carbon back relatively quickly. Fossil carbon, by contrast, has no natural return pathway on any human timescale. Even the secondary effects differ: fire changes land surface reflectivity in ways that fossil fuel burning does not.2Biogeosciences. Non-deforestation fire vs. fossil fuel combustion: the source of CO2 emissions affects the global carbon cycle and climate responses When you see headlines comparing wildfire emissions to the annual output of a country’s cars and power plants, take them with a grain of salt. The carbon reservoirs involved operate on fundamentally different timescales.
That said, the numbers are still enormous. Between 1997 and 2010, biomass burning released about 2.2 billion metric tons of carbon per year on average, roughly a third of fossil fuel emissions over the same period.3PubMed Central. Global combustion: the connection between fossil fuel and biomass burning emissions (1997–2010) Updated estimates using newer satellite data put the landscape fire figure at about 3.4 billion metric tons of carbon per year for the 2002–2022 period.4Scientific Data. Landscape fire emissions from the 5th version of the Global Fire Emissions Database (GFED5) Whether that increase reflects genuinely more fire or better measurement tools is an active area of research, but either way, it is a massive annual flux of carbon.
What Happens After a Fire Burns
Combustion does not just release carbon and leave a blank slate. The aftermath is its own chapter in the carbon cycle, and it plays out over years to decades. Immediately after a fire, a burned landscape is a net carbon source: the charred soil keeps releasing CO₂ as remaining organic matter decomposes, and there are not enough living plants yet to offset those losses through photosynthesis. In one study of a chaparral ecosystem, wildfire immediately cut one major fraction of soil carbon in half and another by a third. A year later, soil carbon dropped even further as soil microbes ramped up their activity and consumed organic matter faster than new plant growth could replace it. That deficit persisted for over four years.5PubMed Central. Wildfire-Induced Losses of Soil Particulate and Mineral-Associated Organic Carbon Persist for Over 4 Years in a Chaparral Ecosystem
Recovery speed depends heavily on the ecosystem. In boreal forests, modeling work suggests the carbon compensation point, the moment when cumulative regrowth has absorbed as much carbon as the fire and its aftermath released, can take up to about 21 years. Under warming scenarios, that recovery may actually accelerate, because warmer conditions speed up plant growth.6Global and Planetary Change. Modelling wildfire and post-fire carbon budgets of a boreal forest under a changing climate In fire-adapted ecosystems, recovery can be far faster. A semi-arid Australian woodland dominated by mallee eucalyptus bounced back to pre-fire levels of carbon uptake relatively quickly because the trees resprouted leaves within months of burning, a trait shaped by millennia of regular fire.7Agricultural and Forest Meteorology. Rapid recovery of net ecosystem production in a semi-arid woodland after a wildfire
The Charcoal Residue That Fire Leaves Behind
Here is where combustion gets genuinely counterintuitive as a carbon cycle process. Not all the carbon in a burning landscape goes into the atmosphere. Incomplete combustion produces charcoal and soot, collectively called pyrogenic carbon, and some of it stays put in the soil for centuries or millennia. This leftover material is chemically resistant to decomposition, which means it effectively removes carbon from the fast-cycling part of the system and parks it in long-term storage.
In savanna fires, field measurements found that roughly 0.6 to 1.5 percent of the carbon exposed to fire ended up as black carbon, with more than 90 percent of it staying on the ground rather than becoming airborne. Scaled globally, savanna fires alone may produce 10 to 26 million metric tons of black carbon per year, and because this material resists breakdown, its formation acts as a net sink for atmospheric CO₂.8Journal of Geophysical Research: Atmospheres. Black carbon formation by savanna fires: Measurements and implications for the global carbon cycle In Amazonian forests, charcoal particles from both ancient and modern fires have accumulated through the soil profile, forming a long-term carbon reservoir with a meaningful role in the global carbon balance.9PubMed. Soil charcoal as long-term pyrogenic carbon storage in Amazonian seasonal forests
In Arctic and boreal regions, pyrogenic carbon in mineral soils has been dated to thousands of years old, with some samples from continuous permafrost zones reaching back nearly 13,000 years. Across sites in northern Canada, pyrogenic carbon consistently made up about 7 percent of the total soil organic carbon, and it always lasted longer in the soil than the surrounding non-charred organic matter.10Communications Earth & Environment. Millennial-aged pyrogenic carbon in high-latitude mineral soils So combustion simultaneously releases carbon to the atmosphere and locks some of it into a slow-release form underground. The net climate effect of any given fire depends on the balance between those two outcomes.
Soil Microbes and the Stability of Fire-Derived Carbon
The assumption that charcoal sits inert in the soil forever turns out to be an oversimplification. Post-fire microbial communities include organisms that can actually break down some of those supposedly resistant aromatic carbon compounds. Research on wildfire-affected soils found that microbes in severely burned areas actively degraded fire-derived organic matter, with implications for how much pyrogenic carbon truly counts as long-term storage.11Nature Microbiology. Wildfire-dependent changes in soil microbiome diversity and function
Genomic studies have gone further, showing that early-successional microbes in burned soils are especially adapted to degrade pyrogenic carbon compounds. These organisms carry genes for breaking down aromatic molecules produced by pyrolysis, giving them a competitive edge in the charcoal-rich post-fire environment. Given that an estimated 54 to 109 billion metric tons of pyrogenic carbon is stored in soils worldwide, the ability of microbial communities to chip away at this pool may play a critical role in the long-term stability of fire-derived carbon.12The ISME Journal. Ecological and genomic responses of soil microbiomes to high-severity wildfire: linking community assembly to functional potential The charcoal is not permanent, it is just slow. How slow depends on the local microbial community, soil moisture, temperature, and how chemically altered the charcoal was during the original fire.
Smoldering Peatland Fires
Not all combustion looks like roaring flames. Some of the most carbon-consequential burning on Earth happens underground, in peatlands, where thick layers of partially decomposed plant material smolder for weeks or months. These smoldering fires burn at lower temperatures than flaming fronts but can consume enormous amounts of carbon because peat is essentially concentrated organic matter built up over thousands of years.
In one temperate peatland fire, the smoldering front continued burning for more than a month after the initial surface fire, surviving multiple episodes of heavy rain. The fire burned through an average peat depth of about 17.5 centimeters, in some spots reaching over half a meter deep. The total carbon loss was estimated at roughly 96 metric tons per hectare of burned area.13Forest Ecology and Management. Peat consumption and carbon loss due to smouldering wildfire in a temperate peatland That is a staggering figure compared to surface fires in the same region. Smoldering peat fires also produce the largest wildfires on Earth by area and pose severe air quality and health problems due to prolonged smoke production.14Current Opinion in Environmental Science & Health. Smouldering wildfires in peatlands, forests and the arctic: Challenges and perspectives
In the Arctic, these fires are particularly alarming because they can persist underground through the winter. Experiments have shown that even heavy natural snowfall cannot suppress a shallow smoldering peat fire; extinguishing one required a minimum snow layer thickness of about 9 centimeters packed at sub-zero temperatures.15Environmental Science & Technology. Impact of Snow on Underground Smoldering Wildfire in Arctic-Boreal Peatlands These “zombie fires” can re-emerge in spring, igniting new surface fires and releasing carbon that has been accumulating in the peat for millennia.
The Permafrost Feedback Loop
Arctic and boreal fire regimes sit at the center of a troubling positive feedback cycle. As the climate warms, permafrost thaws, and the thickening of the soil’s active layer, the top portion that undergoes seasonal freeze-thaw cycles, has cascading effects. Thawing reduces surface reflectivity and soil moisture, which intensifies summer warming and atmospheric dryness, which in turn promotes more vegetation growth and dries out soil organic matter. The result is more fire, more burning, and more carbon release, which further destabilizes the climate and drives more thaw.16Nature Geoscience. Amplified Arctic–boreal fire regimes from permafrost thaw feedbacks
At high latitudes, the carbon stored by regrowing forests after fire may be largely or completely canceled out by these warming feedbacks: decreased surface reflectivity from darker forest canopy, increased emissions from thawing permafrost, and the increased fire itself.17Annual Review of Environment and Resources. Feedbacks of Terrestrial Ecosystems to Climate Change There is also a chemical feedback at work. Forest fires release reactive gases that reduce the atmosphere’s capacity to break down methane, a potent greenhouse gas. As fires intensify, more methane persists in the atmosphere, amplifying warming and creating conditions for still more fire.18Nature Geoscience. Climate feedback of forest fires amplified by atmospheric chemistry
Black Carbon in the Atmosphere and Stratosphere
The soot particles that do go airborne during combustion have their own climate story. Black carbon absorbs sunlight, warming the air around it. Modeling work has suggested that the direct warming effect of black carbon may be the second largest contributor to global warming after CO₂ itself, potentially large enough to nearly balance the cooling effect of all other human-made aerosol particles combined.19Nature. Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols Over continental regions where black carbon concentrations are high, the atmospheric heating rates can be at least three times what they would be without the soot present.20Journal of Geophysical Research: Atmospheres. Black carbon aerosols over an urban region: Radiative forcing and climate impact
Perhaps more surprising is how high this material can travel. Intense wildfires can generate their own thunderstorms, called pyrocumulonimbus clouds, which punch smoke directly into the stratosphere. Analysis of 13 years of airborne observations found that these fire-generated storms are responsible for 10 to 25 percent of the black carbon and organic aerosols in the lower stratosphere, with similar impacts in both hemispheres.21PubMed. Pyrocumulonimbus affect average stratospheric aerosol composition Once in the stratosphere, this smoke lingers: emissions from the 2017 British Columbia fires were observed in the lower stratosphere for 8 to 10 months afterward.22Journal of Geophysical Research: Atmospheres. Pyrocumulonimbus Events Over British Columbia in 2017: An Ensemble Model Study of Parameter Sensitivities and Climate Impacts of Wildfire Smoke in the Stratosphere If these events become more frequent as fire seasons intensify, stratospheric aerosol composition could shift in ways that are still poorly understood.
From Fire to Ocean
Combustion’s reach extends well beyond the atmosphere and soils. Some of the black carbon produced by fire dissolves in water and eventually makes its way to the ocean via rivers. This dissolved black carbon is chemically stubborn: studies in coastal waters have found that it travels conservatively through estuaries, meaning it is neither significantly added to nor removed from the water column during transport. In the northwestern Pacific marginal seas, dissolved black carbon tracked closely with salinity, confirming that rivers are the primary delivery route.23Nature Communications. Conservative behavior of dissolved black carbon in the northwestern Pacific marginal seas Similar behavior has been observed along the Brazilian coast, where a large fraction of dissolved black carbon survived its journey from headwaters through the estuary and onto the continental shelf.24Frontiers in Earth Science. Dissolved Black Carbon in the Headwaters-to-Ocean Continuum of Paraíba Do Sul River, Brazil
Once in the deep ocean, dissolved black carbon becomes part of the marine dissolved organic carbon pool, where the oldest measured samples have radiocarbon ages exceeding 20,000 years.25PubMed Central. Variable aging and storage of dissolved black carbon in the ocean In other words, fire on land can ultimately move carbon into one of the longest-term storage pools on the planet. This is a part of the combustion story that rarely makes it into textbook diagrams of the carbon cycle, but it closes a loop that connects terrestrial burning to deep-ocean carbon chemistry.
Fire as a Carbon Cycle Force Across Geological Time
Combustion is not a recent addition to the carbon cycle. Fossil charcoal appears in the geological record from the Late Silurian period, roughly 420 million years ago, and its presence at any point in time tells scientists that atmospheric oxygen was within a specific window: above 13 percent (below which plant material will not sustain combustion) and below 35 percent (above which even wet vegetation burns so readily that forests could not survive).26PubMed Central. The diversification of Paleozoic fire systems and fluctuations in atmospheric oxygen concentration27Journal of the Geological Society. Fossil charcoal as an indicator of palaeoatmospheric oxygen level Because both forests and charcoal have been documented continuously since the Late Devonian, scientists can constrain oxygen levels within that 13–35 percent band for the past 360 million years or so.
The relationship between fire and the carbon cycle deepened dramatically around 300 million years ago, when plants evolved lignin for structural support. This tough molecule was difficult for decomposers of that era to break down, so enormous quantities of organic carbon were buried rather than being respired back to the atmosphere, forming the vast coal deposits of the Carboniferous period. But those same large woody structures also meant more fuel above ground. Combined with rising oxygen levels that eventually approached 30 percent, the first forests coincided with a dramatic global increase in fire activity.28Environmental Research Letters. Biological and geophysical feedbacks with fire in the Earth system Fire and the carbon cycle have been shaping each other ever since: more organic carbon means more fuel, more fuel means more fire, and more fire means more carbon cycling through the atmosphere.
Managing Combustion Through Prescribed Fire
If wildfire combustion releases huge pulses of carbon, can controlled burning reduce the damage? Forest managers have tested this idea for decades, and the evidence is nuanced. Prescribed burns reduce the amount of fuel available for future wildfires, which can lower the total emissions when an unplanned fire eventually arrives. Modeling for the western United States has suggested that wide-scale prescribed fire could reduce wildfire CO₂ emissions by 18 to 25 percent overall, and by as much as 60 percent in certain forest types.29PubMed. Prescribed fire as a means of reducing forest carbon emissions in the western United States
The carbon math of these treatments matters. Thinning out small trees and reducing surface fuels can increase a stand’s resistance to severe fire while retaining most of the existing carbon in the large, fire-resistant trees that dominate the canopy. Research has recommended focusing treatments on removing surface fuels and small trees rather than aggressively thinning larger ones, because the gains in fire resistance come mainly from understory work, while the carbon cost of removing big trees is high.30PubMed. Fire suppression and fuels treatment effects on mixed-conifer carbon stocks and emissions In fire-tolerant forests, though, the picture is less tidy: one study found that recent prescribed fire did not markedly change how a subsequent wildfire affected short-term carbon stability at the stand level.31PubMed. Assessing fire impacts on the carbon stability of fire-tolerant forests Prescribed fire is a useful tool, but it is not a silver bullet for keeping combustion carbon out of the atmosphere.