Carbon dioxide comes from both natural processes and human activities. The planet naturally cycles about 100 billion metric tons of carbon through the atmosphere every year, released by living organisms, oceans, and volcanoes. On top of that natural cycle, humans add roughly 35 billion metric tons annually by burning fossil fuels, clearing forests, and manufacturing materials like cement. That extra load is what’s driving atmospheric CO2 to its current level of about 426 parts per million.
Living Things Breathe It Out
Every animal, plant, fungus, and most bacteria produce carbon dioxide through cellular respiration. Cells break down sugars (primarily glucose) to extract energy, and CO2 is a byproduct of that process. Your body does this constantly: you inhale oxygen, your cells use it to convert food into usable energy, and you exhale carbon dioxide. This is the single largest natural source of atmospheric CO2.
When organisms die, decomposition releases even more. Bacteria and fungi break down dead plant and animal material, consuming the carbon stored in tissues and releasing it as CO2. A fallen tree on a forest floor can take years or decades to fully decompose, slowly returning its carbon to the atmosphere the entire time. Soil itself is a massive carbon reservoir, and disturbances like tilling or erosion speed up the release.
Oceans and Volcanoes
The ocean constantly exchanges CO2 with the atmosphere. Warm surface water releases dissolved carbon dioxide into the air (outgassing), while cold water absorbs it. The balance shifts with temperature and ocean currents, making the sea both a source and a sink depending on location and season. Overall, oceans absorb more CO2 than they release, which is why they act as a net carbon sink, but the outgassing portion still moves enormous quantities of carbon.
Volcanoes, including undersea vents and hydrothermal systems, also release CO2 trapped deep in the Earth’s crust. This is often cited as a major natural source, but the numbers tell a different story. All the world’s volcanoes combined release between 0.13 and 0.44 billion metric tons of CO2 per year. According to the U.S. Geological Survey, human activities produce roughly 80 to 270 times more CO2 than volcanoes do. Put another way, it takes humans only about 2.7 days to emit as much CO2 as all global volcanism produces in an entire year.
Burning Fossil Fuels
Coal, oil, and natural gas are the remains of plants and marine organisms that lived hundreds of millions of years ago. Over geologic time, heat and pressure converted that organic material into carbon-dense fuels buried underground. When you burn gasoline in a car engine or coal in a power plant, the carbon that was locked away for millions of years combines with oxygen and enters the atmosphere as CO2 in seconds.
This is the dominant human source. Electricity and heat generation alone account for about 29.7% of global greenhouse gas emissions. Transportation adds another 13.7%, and manufacturing and construction contribute 12.7%. Since 1990, emissions from industrial energy use have grown by 225%, electricity and heating by 88%, and transportation by 66%. The sheer speed of this release is what distinguishes it from natural processes: the Earth’s carbon cycle took millions of years to store that carbon underground, and we’re releasing it over decades.
Cement, Steel, and Industrial Processes
Not all industrial CO2 comes from burning fuel. Some comes from chemical reactions in the manufacturing process itself. Cement production is the clearest example. To make cement, manufacturers heat limestone to temperatures between 600 and 900°C in large rotating kilns. The heat triggers a chemical reaction that converts calcium carbonate (the main component of limestone) into calcium oxide, releasing CO2 as a byproduct. Even if the kiln were powered by a zero-emission energy source, this step would still produce carbon dioxide because the CO2 is baked into the raw material itself.
Industrial processes like these account for about 6.5% of global emissions, separate from the energy used to power them. Cement is responsible for a significant share, but other chemical and metal production processes release CO2 through similar unavoidable reactions.
Deforestation and Land Use
Forests act as carbon sinks, absorbing CO2 as trees grow and storing it in wood, roots, and soil. When forests are cleared, that stored carbon gets released. Trees that are burned release their carbon immediately. Trees that are left to rot release it gradually through decomposition. Either way, the carbon ends up back in the atmosphere.
Tropical deforestation for agriculture releases about 2.6 billion metric tons of CO2 per year, roughly 6.5% of global emissions. The main drivers are expansion of croplands, cattle pasture, and tree plantations for logging. Beyond the trees themselves, disturbing forest soil exposes organic carbon that microbes then break down into CO2. This makes land clearing a double hit: you lose a carbon sink and gain a carbon source at the same time.
Agriculture and Livestock
Farming contributes greenhouse gases through several pathways. Livestock, particularly cattle, produce methane during digestion, which is a different greenhouse gas but roughly 80 times more potent than CO2 over a 20-year period. Rice paddies also generate methane from waterlogged soils where bacteria break down organic matter without oxygen. Agricultural soils release nitrous oxide, another powerful greenhouse gas, especially when synthetic fertilizers are applied.
CO2 specifically comes from farm machinery burning diesel, electricity powering irrigation and grain drying equipment, and the production of fertilizers and pesticides. The indirect emissions from electricity use in agriculture run about 5% of the sector’s direct emissions. While agriculture’s CO2 contribution is smaller than energy or transportation, its combined greenhouse gas footprint, including methane and nitrous oxide, makes it a significant part of the overall picture.
How Natural and Human Sources Compare
The Earth’s natural carbon cycle is enormous, moving around 100 billion metric tons of carbon per year between the atmosphere, oceans, soil, and living things. But that cycle is roughly balanced: natural sources release CO2 and natural sinks (forests, oceans, soil) absorb it back. For thousands of years before industrialization, atmospheric CO2 levels stayed relatively stable because these flows canceled each other out.
Human emissions, at about 35 billion metric tons per year, are smaller in absolute terms than the natural cycle. But they sit on top of a balanced system. Natural sinks absorb some of that extra CO2, but not all of it, so the excess accumulates. That’s why atmospheric concentrations have climbed steadily, reaching 426 parts per million as of late 2025, up from about 280 ppm before the Industrial Revolution. The issue isn’t that human sources are larger than natural ones. It’s that they’re additional, and the planet’s absorption capacity can’t keep up.