Natural processes move vastly more carbon dioxide into the atmosphere each year than human fossil-fuel burning does. Soil microbes alone exhale roughly ten times the CO₂ that comes from burning coal, oil, and gas. The reason this enormous natural flux does not cause runaway warming on its own is that it is largely balanced by equally enormous natural sinks: photosynthesis, ocean absorption, and mineral weathering pull CO₂ back out at roughly the same rate. Understanding where all that natural CO₂ comes from helps clarify why even a comparatively small human addition can tip the balance.
Soil Microbes and Decomposition
The single largest natural source of atmospheric CO₂ is the ground beneath your feet. Bacteria, fungi, and other microorganisms in soil break down dead plant material, root exudates, and other organic compounds, releasing carbon dioxide as a metabolic byproduct. This soil-derived respiratory flux overshadows annual fossil-fuel CO₂ emissions by about tenfold.1PubMed Central. The role of soil microbes in the global carbon cycle: tracking the below-ground microbial processing of plant-derived carbon for manipulating carbon dynamics in agricultural systems That number shocks many people when they first encounter it, but it makes sense once you consider the sheer volume of dead leaves, roots, and woody debris that accumulates in soils worldwide every growing season. Microbes process that material continuously, and the CO₂ they produce seeps upward into the air.
Decomposition rates depend on temperature, moisture, oxygen availability, and the chemical makeup of the organic matter involved. Warm, moist tropical soils decompose material quickly; cold or waterlogged soils slow the process down, which is why peatlands and permafrost can lock carbon away for thousands of years. When conditions change, though, those stores can become sources, a point we will return to later in the article.
Respiration by Plants and Animals
Photosynthesis gets most of the attention in popular accounts of the carbon cycle, but every plant also breathes. In a process called dark respiration (it happens around the clock, not just at night), plant cells in leaves, stems, and roots convert sugars and oxygen into CO₂ and water, generating the energy the plant needs to grow and maintain itself.2PubMed Central. Dark Respiration Measurement from Arabidopsis Shoots During daylight hours photosynthesis more than compensates for this respiratory loss, so the plant is a net CO₂ absorber overall. But the respiration half of the equation is substantial. Studies measuring the rate of CO₂ released from leaves in the dark have shown it to be a meaningful fraction of gross photosynthetic uptake.3PubMed. Effects of elevated atmospheric CO2 concentration on leaf dark respiration of Xanthium strumarium in light and in darkness
Animals contribute too, of course. Every organism that eats and metabolizes food exhales CO₂. Insects are often overlooked in this accounting, but they process enormous quantities of carbon. Subterranean termites, for instance, emit about 42% of the carbon in the wood they consume as gas, mostly carbon dioxide.4Ecosphere. The fate of carbon utilized by the subterranean termite Reticulitermes flavipes Termite colonies number in the trillions globally, and their collective contribution to the atmospheric CO₂ flux is far from trivial. When you add in all other insects, soil invertebrates, and larger animals, the combined respiration of the animal kingdom forms a steady, diffuse source of atmospheric carbon dioxide.
Volcanic and Tectonic Emissions
Volcanoes are probably the first natural CO₂ source most people think of, and for good reason. Magma rising from the mantle carries dissolved gases, including carbon dioxide, and releases them both during dramatic eruptions and through quieter, continuous degassing at vents and fumaroles. Volcanoes emit more gas than can be accounted for by the magma that actually reaches the surface, a phenomenon known as excess degassing.5Reviews of Geophysics. Excess degassing from volcanoes and its role on eruptive and intrusive activity This means that even volcanoes that are not actively erupting can release significant amounts of CO₂ over long periods.
During large eruptions, the numbers become more tangible. Satellite observations of the 2018 Kilauea eruption in Hawaii estimated CO₂ emissions at roughly 77 kilotons per day during one measurement period, with enhancements of one to two parts per million detected far downwind of the eruption plume.6Geophysical Research Letters. Carbon Dioxide Emissions During the 2018 Kilauea Volcano Eruption Estimated Using OCO‐2 Satellite Retrievals Impressive as that sounds, individual eruptions are brief on geological timescales, and the total volcanic CO₂ output worldwide in a given year is a small fraction of human emissions. The significance of volcanic CO₂ lies more in its role over millions of years in regulating the long-term carbon cycle than in any single eruption’s contribution.
Beyond volcanic craters, CO₂ also escapes through fault zones in the Earth’s crust. Research mapping gas discharges across the globe has found a strong spatial link between CO₂ seeping out of the ground and the presence of active extensional fault systems, where the crust is being pulled apart. These faults create pathways that connect the deep crust to the surface, allowing CO₂ generated by deep geological processes to rise and enter the atmosphere.7PubMed Central. Global-scale control of extensional tectonics on CO2 earth degassing In tectonically active regions, this diffuse degassing can rival the output of nearby volcanoes.
Ocean Outgassing
The ocean is both a massive sink and a massive source of CO₂, depending on location, season, and ocean conditions. Cold water absorbs more CO₂ than warm water, so the high-latitude oceans tend to pull carbon dioxide in, while warmer tropical and subtropical waters tend to release it. The net effect globally is that the ocean absorbs more than it emits, making it a net sink. But the outgassing half of this exchange is enormous in its own right.
Several mechanisms drive CO₂ release from the sea surface. Warming of surface waters reduces the solubility of dissolved CO₂, allowing it to escape. Upwelling brings carbon-rich deep water to the surface, where its dissolved CO₂ can equilibrate with the atmosphere. Observations off the coast of southern California illustrate how these processes interact: at nearshore upwelling sites, the strength of upwelling controls whether the ocean is a local source or sink, while at offshore sites, sea surface temperature anomalies dominate. Warm events like El Niño and marine heatwaves cause increased outgassing, while cooler La Niña conditions pull CO₂ back in.8Journal of Geophysical Research: Oceans. Open Ocean Versus Upwelling Regimes: Air‐Sea CO2 Fluxes and pCO2 Inter‐Annual Variability in the Southern California Current System
Deep-sea hydrothermal vents add another dimension to ocean carbon cycling. Along mid-ocean ridges, hot fluids enriched in CO₂ (and highly acidic compared to surrounding seawater) pour out of the seafloor into the deep ocean.9Waste Management. CO2 supply from deep-sea hydrothermal systems Most of this carbon stays dissolved in the deep ocean rather than reaching the atmosphere directly, but it feeds into the larger ocean carbon reservoir. Over geological timescales, this hydrothermal input matters for the total carbon budget of the ocean.
What Happens to Sinking Carbon in the Deep Ocean
The ocean’s biological pump constantly shuttles organic carbon downward as dead plankton, fecal pellets, and other particles sink from the sunlit surface toward the deep. But much of that material never reaches the seafloor. In the twilight zone, the region roughly between 50 and 1,000 meters deep, heterotrophic microbes intercept sinking particles and convert the organic carbon back into dissolved CO₂. Prokaryotes (bacteria and archaea) are responsible for an estimated 70 to 92 percent of this remineralization.10PubMed. Reconciliation of the carbon budget in the ocean’s twilight zone These organisms use hydrolytic enzymes to break down proteins and polysaccharides in the sinking particles, solubilizing them and then metabolizing them.11PubMed Central. Illuminating Key Microbial Players and Metabolic Processes Involved in the Remineralization of Particulate Organic Carbon in the Ocean’s Twilight Zone by Metaproteomics
The CO₂ produced in the twilight zone does not immediately return to the atmosphere. It can stay dissolved in deep water for centuries before ocean circulation eventually brings it back to the surface. But this microbial recycling is a critical natural process that prevents most of the ocean’s exported organic carbon from being permanently buried, and it sets the baseline for how much carbon the ocean stores at depth versus how much eventually vents back to the atmosphere.
Permafrost Thaw
Permafrost soils in the Arctic and sub-Arctic contain an immense stockpile of organic carbon, frozen plant and animal material that accumulated over thousands to tens of thousands of years. When these soils thaw, microbes get to work on material that has been essentially preserved in a deep freeze. The result is CO₂ and methane production from carbon that has been out of the active carbon cycle for millennia.
Measurements from thaw slumps in Arctic Siberia have recorded CO₂ fluxes ranging from about 0.24 to 2.6 grams of carbon per square meter per day during summer, with models estimating annual fluxes of roughly 160 to 184 grams of carbon per square meter, including significant wintertime emissions that account for a quarter to a third of the total.12Journal of Geophysical Research: Biogeosciences. Carbon Dioxide and Methane Release Following Abrupt Thaw of Pleistocene Permafrost Deposits in Arctic Siberia The CO₂ fluxes from Pleistocene-age permafrost (material tens of thousands of years old) were higher than from younger Holocene permafrost, which surprised researchers because it showed the ancient organic matter had been well-preserved and had not been slowly consumed by microbes in place. Laboratory experiments have confirmed that dissolved organic carbon leached from permafrost roughly 36,000 years old is rapidly converted to CO₂ once it thaws. In bioreactor experiments, dissolved organic carbon concentrations dropped by about half in just 200 hours, fueling a more than sevenfold increase in dissolved inorganic carbon. Low-molecular-weight organic acids like acetate and butyrate accounted for 87% of the carbon consumed.13PubMed Central. Ancient low-molecular-weight organic acids in permafrost fuel rapid carbon dioxide production upon thaw
What makes permafrost thaw different from the other natural sources discussed here is its trajectory. Soil respiration, plant respiration, and volcanic degassing have been operating at roughly steady rates for a long time. Permafrost thaw is accelerating as the Arctic warms, converting what was a long-term carbon reservoir into an active and growing source. This creates a positive feedback loop: warming thaws permafrost, the thawed carbon becomes CO₂, and the additional CO₂ drives further warming.
Wildfires and Smouldering Peat
Lightning-ignited wildfires have been releasing carbon from vegetation and soils for as long as terrestrial ecosystems have existed. Among thousands of lightning-ignited fires studied in the United States, the vast majority were preceded by specific types of lightning flashes capable of sustained heating, confirming lightning as the dominant natural ignition source.14Nature Communications. Variation of lightning-ignited wildfire patterns under climate change When a forest or grassland burns, the carbon stored in trees, shrubs, leaf litter, and surface soils is rapidly converted to CO₂ and released into the atmosphere. In many ecosystems, this is part of a natural cycle: vegetation regrows and recaptures the lost carbon over decades.
Peat fires are a different beast entirely. Peat is a dense accumulation of partially decomposed plant matter, often meters thick, that has built up over thousands of years. When peat catches fire, it smoulders below the surface rather than flaming above it, and these smouldering fires can burn for months. They produce some of the largest fires on Earth in terms of total area and fuel consumed.15PubMed Central. Smouldering wildfires in peatlands, forests and the arctic: Challenges and perspectives In the Arctic, where peatlands are widespread, these fires are increasing in frequency, and they are remarkably difficult to extinguish. Research has shown that even heavy natural snowfalls cannot suppress a shallow smouldering peat fire; the minimum snow thickness needed to extinguish one was about 9 centimeters at sub-zero temperatures.16PubMed Central. Impact of Snow on Underground Smoldering Wildfire in Arctic-Boreal Peatlands Because peat fires burn ancient carbon rather than recently fixed biomass, they release carbon that has been sequestered for centuries or millennia, making them a net addition to the active carbon cycle in the same way permafrost thaw is.
Sunlight Breaking Down Dead Plant Matter
Not all CO₂ release from dead organic material requires microbes. In arid and semi-arid environments, ultraviolet and short-wavelength visible light can break down plant litter directly, a process called photodegradation. Laboratory experiments exposing grass and oak litter to UV radiation found that litter receiving UV produced ten times the CO₂ of litter shielded from UV, and sterilizing the litter to remove microbes made no difference, confirming the process is purely photochemical. When litter was placed outdoors in natural sunlight near the summer solstice, UV radiation accounted for about 55% of the photochemical CO₂ release, with short-wavelength visible light contributing the remaining 45%.17Journal of Geophysical Research: Biogeosciences. Photochemically induced carbon dioxide production as a mechanism for carbon loss from plant litter in arid ecosystems
Field measurements across different ecosystems show this is not just a desert curiosity. In one grassland study, photodegradation contributed almost 60% of the dry-season CO₂ flux, and up to 92% of the CO₂ released during summer midday hours.18Global Change Biology. Photodegradation leads to increased carbon dioxide losses from terrestrial organic matter Even peatlands with exposed organic surfaces showed a meaningful photodegradation signal. This process complicates efforts to model carbon cycling in drylands, because traditional decomposition models assume microbes do all the work. In regions where litter sits on the surface exposed to strong sunlight, photodegradation can be the dominant pathway for returning carbon to the atmosphere.
Chemical Weathering as a CO₂ Source
Weathering of rocks is usually discussed as a CO₂ sink: rainwater absorbs CO₂ from the atmosphere, becomes slightly acidic, and dissolves silicate minerals, locking the carbon into dissolved bicarbonate that eventually ends up in ocean sediments. Over millions of years, this process acts as a thermostat for the planet. But not all weathering draws CO₂ down. When tectonic uplift exposes sulfide minerals alongside carbonate rocks, the sulfides oxidize to produce sulfuric acid, which then dissolves the carbonates. That reaction releases CO₂ rather than consuming it.
A study of the upper Changjiang (Yangtze) River basin found that sulfide oxidation coupled with carbonate dissolution was widespread and that the CO₂ released by this process exceeded the CO₂ drawn down by silicate weathering across the basin as a whole.19Journal of Geophysical Research: Biogeosciences. CO2 Release Driven by the Combination of Sulfide Oxidation and Carbonate Dissolution in the Upper Changjiang River This finding is important because many global carbon models assume that mountain uplift and increased weathering always act as a CO₂ sink. In regions with abundant sulfide-bearing rocks, the opposite can be true. The effect plays out over timescales of hundreds of thousands to millions of years, so it does not factor into annual carbon budgets, but it reshapes our understanding of how plate tectonics and atmospheric CO₂ interact over Earth history.
Methane Oxidation
Methane is itself a potent greenhouse gas, but it does not stay methane forever. In the atmosphere, methane reacts with hydroxyl radicals, which break it down through a series of chemical steps that ultimately produce CO₂ and water. This atmospheric oxidation of methane converts a more powerful short-lived greenhouse gas into a less powerful but longer-lived one.20PubMed Central. Atmospheric methane and nitrous oxide: challenges along the path to Net Zero Methane comes from wetlands, termites, ocean sediments, and geological seeps, all natural sources. When that methane is oxidized to CO₂ in the atmosphere, it represents one more natural pathway adding carbon dioxide to the air. The quantities involved are modest compared to soil respiration or ocean outgassing, but the process matters because it links the methane and carbon dioxide cycles together: anything that increases natural methane emissions eventually adds to atmospheric CO₂ as well.
Lessons From the Last Ice Age
The paleoclimate record provides a dramatic illustration of natural CO₂ release. During the last glacial maximum, roughly 19,000 to 23,000 years ago, the deep ocean stored a large reservoir of CO₂ that had been pulled from the atmosphere by the biological pump and kept isolated by sluggish ocean circulation and extensive sea ice. As the ice age ended, shifts in wind patterns, particularly a southward migration of the westerly winds over the Southern Ocean, increased upwelling and ventilated these deep waters, sending their stored carbon back toward the surface and ultimately into the atmosphere.21Paleoceanography and Paleoclimatology. Rapid Loss of CO2 From the South Pacific Ocean During the Last Glacial Termination Climate modeling work suggests that the breakdown of deep-water stratification around Antarctica, triggered by changes in sea ice formation, played a central role in releasing this abyssal carbon.22Climate of the Past. Impact of oceanic processes on the carbon cycle during the last termination
Hydrothermal activity along the eastern equatorial Pacific also contributed. Increased fluxes of carbon from hydrothermal sources passed through the upwelling system and exchanged with the atmosphere during the deglaciation, affecting both the carbon-14 content and the CO₂ concentration of the air.23Environmental Research Letters. Hydrothermal carbon release to the ocean and atmosphere from the eastern equatorial Pacific during the last glacial termination Together, these ocean processes drove atmospheric CO₂ from about 180 parts per million during the ice age to around 280 ppm in the preindustrial era, entirely without human involvement. That 100-ppm swing took thousands of years and was enough to help push the planet from deep glaciation to the warm interglacial conditions of the last ten millennia.
Why These Natural Fluxes Do Not Explain Current Warming
A common misconception is that because natural CO₂ sources are so enormous, human emissions are insignificant by comparison. The raw numbers seem to support that view: soil respiration alone dwarfs fossil-fuel combustion. But natural sources are matched by natural sinks operating at comparable scales. Photosynthesis recaptures most of the CO₂ released by respiration and decomposition. The ocean absorbs CO₂ at roughly the same rate it releases it, with a small net uptake. Terrestrial ecosystems globally absorb an estimated two to three billion tonnes of carbon per year beyond what they emit, though tropical deforestation offsets much of that gain.24Journal of Ecology. Understanding and managing the global carbon cycle
Human fossil-fuel burning adds carbon that has been locked underground for millions of years, injecting it into a system that was otherwise approximately in balance. The natural sinks have absorbed roughly half of what humans have emitted, which is why atmospheric CO₂ has risen by about 50% since preindustrial times rather than doubling. But the sinks cannot keep pace with the rate of addition indefinitely. Modeling projections consistently show that natural systems will not be able to balance fossil-fuel emissions on their own.25Biogeosciences. Biological control of the terrestrial carbon sink The natural carbon cycle is massive, intricate, and self-regulating over long timescales, but it was not designed to absorb a sudden pulse of geologically sequestered carbon released over just a few centuries.