How Is CO2 Made? Natural and Human Production Methods

Carbon dioxide forms whenever carbon-containing material reacts with oxygen or breaks down, and the sources range from invisible soil microbes to massive power plants. Nature produces far more CO2 each year than humans do, primarily through biological respiration in soils, but the natural carbon cycle has been roughly balanced for millennia. What changed the equation is the human addition on top of that balance: burning fossil fuels, manufacturing cement and steel, and running chemical processes that release carbon stored underground for millions of years. Understanding where CO2 actually comes from, and in what proportions, clears up common confusion about why human emissions matter even though they are smaller in absolute terms than natural flows.

Soil Microbes and Decomposition

The single largest flux of CO2 into the atmosphere comes from a source most people never think about: the ground beneath their feet. Trillions of bacteria, fungi, and other microorganisms living in soil consume dead plant material, breaking it down for energy and releasing CO2 as a byproduct. This process, called soil respiration, also includes CO2 released by plant roots and by small soil-dwelling animals. Together, these biological activities send an enormous pulse of carbon dioxide into the air every year.

How enormous? The soil-derived CO2 flux overshadows annual fossil fuel emissions by roughly tenfold.¹ That number sounds alarming until you realize the cycle works both ways. Plants pull CO2 out of the air through photosynthesis, build their tissues from it, die, and return that carbon to the soil, where microbes release it again. The flow in and the flow out have been roughly matched over long stretches of time. Fossil fuel emissions, by contrast, inject carbon that was locked away underground into a cycle that has no corresponding natural removal pathway on human timescales.

Soil respiration rates are not constant. They increase with warmth and moisture, which is why tropical soils release far more CO2 than arctic ones. This creates a feedback loop that climate scientists watch closely: as global temperatures rise, soils in previously cold regions may start decomposing organic matter faster, releasing more CO2, which drives further warming.

Volcanoes, Metamorphism, and the Deep Earth

Deep inside the planet, carbon stored in rocks and the mantle finds its way to the surface through volcanic eruptions, hot springs, and the slow cooking of buried rock. Volcanoes get the most attention, but they are just one part of a broader geological CO2 system. At volcanoes like Mt. Vulture in Italy, researchers have traced the emitted CO2 to a mix of biological carbon from shallow soils and mantle-derived carbon rising from deep within the Earth.² Even volcanoes that haven’t erupted in thousands of years continue to leak CO2 through the ground.

A less obvious geological source is metamorphism, the process by which buried sedimentary rocks are heated and compressed until their minerals break down and release gases, including CO2. Over geological timescales spanning millions of years, metamorphic CO2 release has been a major driver of planetary climate. Research into the Proterozoic era (roughly 2.5 billion to 541 million years ago) suggests that the types of carbonate rocks present and the hotter conditions of Earth’s crust at that time would have enhanced this kind of CO2 release significantly.³

Rock weathering at the surface contributes too. When ancient organic carbon locked inside rocks is exposed to air, it oxidizes, essentially rusting and releasing CO2. This process releases an estimated 40 to 100 million metric tons of carbon per year, which over geological time scales is a major atmospheric CO2 source alongside volcanism.⁴ Mountain-building events that expose fresh rock accelerate it. Glaciers grinding over bedrock are especially effective at cracking open organic-rich stone and letting oxygen get at the carbon inside.

Breathing and Biological Respiration

Every animal on Earth, including you, produces CO2 simply by being alive. Cellular respiration converts the carbon in food into energy and exhales CO2 as the main waste product. This is true not only for mammals but for insects, fish, birds, and virtually every organism that consumes organic material for energy.

Human respiration has actually been quantified in surprising detail. The average American adult takes in about 67 kilograms of carbon per year through food and releases roughly 59 kilograms of that as exhaled CO2, with most of the rest leaving the body as solid and liquid waste.⁵ Scale that up to the entire U.S. population in 2000, and you get about 15 million metric tons of carbon exhaled as CO2 in a single year. That sounds like a lot, but it is dwarfed by fossil fuel emissions and, as with soil respiration, is part of a short-term cycle: the carbon you exhale came from food that recently pulled it from the atmosphere via photosynthesis.

Plants also release CO2, which surprises people who think of them purely as carbon absorbers. All plants respire around the clock, burning sugars for energy and releasing CO2 just like animals do. They also lose CO2 through a process called photorespiration, which happens when the enzyme responsible for capturing CO2 during photosynthesis mistakenly grabs an oxygen molecule instead. This triggers a chain of reactions that consumes energy and releases CO2, making it a wasteful side effect of photosynthesis.⁶ Despite the inefficiency, photorespiration may serve as a kind of safety valve, preventing the plant’s internal energy-processing machinery from overloading under stressful conditions like drought or intense heat.

Wildfires and Biomass Burning

When forests, grasslands, or peatlands burn, their stored carbon combines with oxygen and enters the atmosphere primarily as CO2. Wildland fuels typically contain 35 to 55 percent carbon by dry weight, and fires convert about 95 percent of the carbon released into CO2, carbon monoxide, and methane, with CO2 dominating.⁷ In a natural fire regime, the CO2 released is eventually recaptured as vegetation regrows. The concern arises when fire frequency or severity increases beyond what ecosystems can recover from, or when fires consume peat soils that took thousands of years to accumulate.

Wildfires also release particulate matter, black carbon, and other gases that affect air quality and climate in ways beyond CO2 alone. But from a pure carbon-accounting perspective, fire is one of the fastest natural mechanisms for moving carbon from biomass into the atmosphere. A single large wildfire can release in days what a forest absorbed over decades.

Oceans as Both Sink and Source

The oceans hold about fifty times more carbon than the atmosphere and constantly exchange CO2 with the air above them. Whether a given patch of ocean absorbs or releases CO2 depends on water temperature, biological activity, and the concentration of dissolved CO2 already in the water. Warm tropical waters tend to release CO2 because gases become less soluble as temperature rises. Cold polar waters tend to absorb it. Globally, the ocean currently acts as a net sink, pulling in more CO2 than it releases, but this was not always the case, and regional outgassing is still a significant natural CO2 source.

As the ocean absorbs more CO2 from the increasingly carbon-rich atmosphere, it becomes more acidic, which in turn reduces its ability to keep absorbing. This is not a theoretical future concern; ocean pH has already dropped measurably since the start of the industrial era. If the ocean’s capacity to absorb CO2 weakens, a larger share of human emissions will remain in the atmosphere.

Fossil Fuel Combustion

Burning coal, oil, and natural gas is the best-known human source of CO2 and the largest single driver of the rise in atmospheric carbon dioxide since the mid-1800s. The chemistry is straightforward: carbon in the fuel combines with oxygen during combustion, forming CO2 and releasing heat. The amount of CO2 per unit of energy varies by fuel. Natural gas produces roughly 56 kilograms of CO2 per gigajoule of energy, while coal and petroleum coke release substantially more for the same energy output. This is the main reason why switching from coal to natural gas for electricity generation cuts CO2 emissions per kilowatt-hour, even though natural gas is still a fossil fuel.

What makes fossil fuel CO2 different from, say, the CO2 released by a forest fire is timescale. Fossil fuels are carbon that was removed from the atmosphere hundreds of millions of years ago and buried underground. Burning them moves ancient carbon into a modern atmosphere that had no corresponding removal mechanism in place. The natural carbon cycle cannot reabsorb this extra load on any timeline relevant to human civilization.

Cement, Steel, and Heavy Industry

Fossil fuels get most of the blame for industrial CO2, but some manufacturing processes produce CO2 through the chemistry of the materials themselves, even apart from the energy used to run the factory. Cement is the classic example. Making cement requires heating limestone (calcium carbonate) to extremely high temperatures in a kiln, which causes the calcium carbonate to decompose into calcium oxide and CO2. This “process emission” accounts for a substantial share of cement’s carbon footprint, on top of whatever fuel is burned to heat the kiln. Researchers have explored ways to capture this CO2 stream separately by using different kiln designs that keep combustion gases and calcination gases apart, making the CO2 easier to collect and store.⁸

Steel production through the blast furnace route is another heavy emitter. The blast furnace uses coke (derived from coal) both as a fuel and as a chemical reducing agent to strip oxygen from iron ore. The carbon in the coke bonds with the oxygen from the ore, forming CO2. A global analysis of plant-level data found that emissions ranged from about 1.2 to 2.1 tons of CO2 per ton of hot metal produced, depending on the mix of fuels and reductants each plant used.⁹ The blast furnace process accounts for over 90 percent of global iron ore-based steel production, which makes it one of the hardest industrial sectors to decarbonize. The coal isn’t just providing heat; it’s a chemical ingredient in the reaction itself.¹⁰

Fermentation and Chemical Processing

Fermentation is one of humanity’s oldest technologies, and it is also a CO2 factory. When yeast converts sugar into ethanol, CO2 is the other main product. Every brewery, winery, and distillery vents CO2 during fermentation. With the growth of fuel ethanol production, this stream has become large enough to attract commercial interest. The CO2 coming off an ethanol fermentation tank is nearly pure and close to saturation, making it relatively easy to capture and sell compared to the dilute CO2 in power plant exhaust.¹¹ Much of the food-grade CO2 used to carbonate beverages and freeze food comes from ethanol plants and ammonia production facilities.

Other chemical processes also generate CO2 as an inherent byproduct. Steam reforming of methane, the most common method for producing industrial hydrogen, involves reacting methane with steam at high temperatures. The reaction yields hydrogen and carbon monoxide, and a follow-up reaction converts the carbon monoxide into more hydrogen and CO2.¹² This means that most hydrogen on the market today, sometimes called “grey hydrogen,” carries a CO2 burden. Separating and capturing that CO2 turns it into “blue hydrogen,” which has a smaller climate footprint but still isn’t emission-free.

How Scientists Tell CO2 Sources Apart

With so many sources pumping CO2 into the same atmosphere, how do researchers figure out which ones are responsible for the observed rise? One of the most powerful tools is carbon isotope analysis. Carbon atoms come in slightly different weights: carbon-12 is the most common, carbon-13 is a bit heavier, and carbon-14 is radioactive and rare. Plants preferentially absorb the lighter carbon-12 during photosynthesis, so plant-derived carbon, and by extension fossil fuels made from ancient plants, is depleted in carbon-13 relative to the atmosphere.

As fossil fuel burning has increased, the ratio of carbon-13 to carbon-12 in atmospheric CO2 has been dropping, a trend known as the Suess effect.¹³ This isotopic fingerprint is essentially a smoking gun: the shift toward lighter carbon in the atmosphere matches exactly what you would expect if fossil fuel combustion were the main new addition. Volcanic CO2, by contrast, has a heavier isotopic signature, and its addition to the atmosphere would push the ratio in the opposite direction. The observed trend rules out volcanoes as the explanation for rising CO2 levels.

Researchers have also proposed that measuring both carbon-13 and carbon-14 together can distinguish modern changes from natural variability in the past, because the pattern of isotopic shifts created by fossil fuel burning is distinct from anything that occurred before industrialization.¹⁴ Carbon-14 is especially useful because fossil fuels, being millions of years old, contain no carbon-14 at all. Adding their carbon to the atmosphere dilutes the concentration of carbon-14 in atmospheric CO2, creating a measurable signal.

Direct Air Capture and Engineered CO2 Production

While most of the world is trying to reduce CO2 emissions, some industries need a reliable supply of pure CO2 for applications like carbonated drinks, fire extinguishers, greenhouses, and enhanced oil recovery. Traditionally this CO2 has been sourced as a byproduct of ammonia production, ethanol fermentation, or natural gas processing. But a newer approach flips the problem: pulling CO2 directly from the ambient air.

Direct air capture (DAC) technology uses either solid sorbent materials or liquid solvents to grab CO2 molecules out of the atmosphere. The captured CO2 sticks to the material’s surface through physical or chemical bonds and is then released by heating or lowering the pressure, producing a concentrated CO2 stream.¹⁵ That stream can be stored underground permanently, which makes the process a net removal of CO2 from the atmosphere, or it can be used as a feedstock for making synthetic fuels, building materials, or other carbon-containing products.

The challenge with DAC is energy. Atmospheric CO2 is present at only about 420 parts per million, far more dilute than the exhaust from a power plant or cement kiln. Concentrating something that dilute takes significant energy, and if that energy comes from fossil fuels, the process can end up releasing as much CO2 as it captures. Current DAC facilities are small and expensive compared to the scale of global emissions, but the technology is advancing as new sorbent materials are developed that require less energy to regenerate. Whether DAC becomes a meaningful part of the climate solution depends largely on how cheaply it can be powered by low-carbon energy sources.

Why Natural Emissions Dwarf Human Ones but Don’t Drive the Rise

A persistent point of confusion in public discussions is the fact that natural CO2 sources are far larger than human ones. If soil respiration alone outpaces fossil fuel emissions by a factor of ten, why worry about the smaller human contribution? The answer lies in balance. Natural emissions are matched by natural absorption. Soil microbes release CO2, but plants pull it back. The ocean releases CO2 in some regions and absorbs it in others. Volcanoes add CO2 slowly, and chemical weathering of silicate rocks removes it over geological time. These flows have been roughly in equilibrium, give or take ice ages and volcanic episodes, for hundreds of thousands of years.

Human fossil fuel burning adds carbon on top of that equilibrium, and the natural sinks cannot absorb the extra load fast enough. The result is a net accumulation: atmospheric CO2 has risen from about 280 parts per million before industrialization to over 420 parts per million today. The isotopic evidence confirms that this increase is driven by fossil carbon, not by a sudden change in natural emissions. Thinking of the carbon cycle as a bathtub helps: natural processes fill and drain the tub at roughly equal rates, keeping the water level stable. Human emissions are like a second faucet that has been turned on without opening a matching drain.