Carbon dioxide comes from a surprisingly wide range of sources, both natural and human-made. The single largest source is soil, where microorganisms breaking down dead plant and animal material release roughly ten times more CO2 each year than all fossil fuel burning combined. But that natural flux is largely balanced by photosynthesis, which pulls CO2 back out of the air. The sources that are tipping the balance are primarily human activities: burning fossil fuels, manufacturing cement, and clearing forests.
Living Organisms
Every animal on Earth gives off carbon dioxide simply by being alive. Your cells constantly break down sugars, fats, and proteins for energy in a process called cellular respiration. During this process, carbon atoms from food are gradually stripped of their stored energy and fully bonded to oxygen, producing CO2 as the final waste product. This happens mainly inside your mitochondria, the tiny power generators in nearly every cell.
That CO2 enters your bloodstream, where most of it is converted into bicarbonate for easy transport. When blood reaches your lungs, the reaction reverses: bicarbonate turns back into CO2 gas, which you breathe out. The average adult exhales about 500 liters of CO2 per day, roughly 1 kilogram by weight. Every mammal, bird, fish, reptile, and insect does the same thing at varying scales.
Plants also release CO2. During the day, photosynthesis typically absorbs more CO2 than the plant produces, but at night, when photosynthesis stops, plants continue to respire and release CO2 into the air. Under hot, dry conditions, plants can also lose CO2 through a process called photorespiration, where the key enzyme responsible for capturing CO2 mistakenly grabs oxygen instead, triggering a chain of reactions that releases CO2 without producing useful energy for the plant.
Soil and Decomposition
Soil is one of the planet’s biggest CO2 sources. When leaves fall, trees topple, and animals die, bacteria and fungi get to work breaking down that organic matter. These microorganisms digest carbon-rich material the same way animal cells do, using oxygen and releasing CO2. The collective “breathing” of soil organisms produces a respiratory flux of about 55 billion metric tons of carbon per year, dwarfing the roughly 8 billion metric tons released annually by fossil fuels, cement production, and land-use change combined.
This enormous natural output doesn’t cause a net increase in atmospheric CO2 on its own because it’s part of a balanced cycle. Plants pull roughly 60 billion metric tons of carbon out of the atmosphere through photosynthesis each year, absorbing slightly more than what soil microbes release. The problem arises when human activity adds extra CO2 on top of this natural exchange, or when land-use changes reduce the amount of plant growth available to reabsorb it.
Oceans
The ocean both absorbs and releases CO2, depending on water temperature. Cold water holds dissolved CO2 more easily, so cooler ocean regions (near the poles) tend to pull CO2 out of the atmosphere. Warmer tropical waters hold less dissolved gas, so they release CO2 back into the air. The direction of flow at any given spot depends on whether the concentration of CO2 is higher in the water or in the atmosphere above it.
On balance, the ocean currently acts as a net absorber, soaking up a significant share of the CO2 humans add to the atmosphere. But warming ocean temperatures reduce that absorbing capacity, meaning warmer seas will release more CO2 in the future while taking in less.
Volcanoes and Geological Activity
Deep within the Earth, carbon stored in rocks and magma escapes as CO2 through volcanic eruptions, hot springs, and mid-ocean ridges. According to NOAA, the best estimate for total volcanic CO2 emissions is about 0.6 billion metric tons per year, with a range of roughly 0.15 to 0.45 billion metric tons depending on the study. That sounds like a lot, but it’s less than 2% of what humans emit from fossil fuels alone. Volcanic CO2 has played a major role in shaping Earth’s climate over millions of years, but on the timescale of a human lifetime, volcanoes are a minor contributor compared to industrial activity.
Burning Fossil Fuels
Coal, oil, and natural gas are the remains of ancient organisms whose carbon was locked underground for millions of years. Burning them reverses that storage, releasing CO2 rapidly into the atmosphere. Not all fossil fuels produce the same amount. Per unit of energy, coal releases the most CO2: about 96 kilograms per million BTU. Gasoline comes in at around 71 kilograms, and natural gas is the lowest at roughly 53 kilograms. This is why switching from coal to natural gas for electricity generation cuts CO2 output nearly in half for the same amount of energy, though it still adds carbon to the atmosphere.
Transportation, electricity generation, heating, and industrial processes all burn fossil fuels. Together, they account for the majority of the CO2 humans add to the atmosphere each year.
Cement and Industrial Manufacturing
Cement production is a surprisingly large CO2 source that most people don’t think about. Making cement requires heating limestone to between 2,700 and 3,000 degrees Fahrenheit in a kiln. This triggers a chemical reaction called calcination, where limestone (calcium carbonate) breaks apart into calcium oxide and CO2 gas. This reaction alone accounts for almost two-thirds of the total CO2 emissions from cement production. The remaining third comes from the fossil fuels burned to heat the kiln to those extreme temperatures. Globally, cement manufacturing is responsible for around 8% of human CO2 emissions.
Deforestation and Land Clearing
Tropical deforestation contributes about 20% of annual global greenhouse gas emissions, roughly equal to all fossil fuels burned in the United States each year and more than the world’s entire transportation sector. When forests are cut down and burned, the carbon stored in tree trunks, branches, and roots is released as CO2 almost immediately. Even when trees aren’t burned, the dead wood left behind decomposes over time, with soil microbes converting that stored carbon into CO2.
It’s not just clear-cutting that matters. Selective logging, thinning, and ground fires that leave some canopy standing still reduce a forest’s ability to store carbon. In the Amazon, selective logging increases greenhouse gas emissions by about 25% above what deforestation alone produces, because damaged trees die slowly and the disturbed forest floor releases additional carbon.
Livestock and Agriculture
Livestock contribute to atmospheric carbon in several ways, though their most talked-about emission is actually methane, not CO2 directly. Cattle and other ruminants produce large amounts of methane through digestion (enteric fermentation), which accounts for over half of a typical beef operation’s total greenhouse gas output. The direct CO2 from agriculture comes mainly from energy use: running farm equipment, manufacturing fertilizer, processing and transporting feed. In beef production systems, energy-related CO2 makes up roughly 9 to 10% of total emissions.
The bigger CO2 connection to agriculture is indirect. Clearing forests for pasture and cropland releases the carbon stored in those ecosystems. And when soil that was previously under grassland or forest is converted to annual crop production, it loses organic carbon over time, adding more CO2 to the atmosphere. Including these soil carbon changes shifts the total footprint of livestock production by 11 to 16%.