Carbon dioxide enters the atmosphere from both natural processes and human activities. Natural sources, including ocean outgassing, volcanic eruptions, and the breathing of every living organism, have cycled CO2 for billions of years. Human activities now add roughly 40 billion metric tons of CO2 on top of that natural cycle every year, overwhelming the planet’s ability to reabsorb it.
Natural Sources of CO2
Every animal on Earth exhales CO2 as a byproduct of converting food into energy. An average person breathes out about 0.66 kilograms of CO2 per day. Multiply that across eight billion people and it sounds enormous, but this carbon was already in the active cycle: it came from plants that absorbed CO2 while growing, which we then ate. The same logic applies to all animals, insects, and microorganisms. When dead plants and animals decompose, bacteria and fungi break down organic matter and release stored carbon back into the air as CO2.
Oceans are both the largest natural absorber and a significant emitter of CO2. In most regions, seawater pulls carbon dioxide out of the atmosphere. But in the Southern Ocean, deep water that has been storing carbon for centuries rises to the surface through upwelling and releases it. This deep water carries CO2 concentrations well above current atmospheric levels. The release is strongest in winter, when deep mixing brings carbon-rich water to the surface. Sea ice, cold temperatures, and biological activity in the water partially limit how much escapes, but the Southern Ocean remains a major natural exhaust vent for ancient carbon.
Volcanoes also release CO2, both during eruptions and through slower seepage from underground magma. Estimates put volcanic emissions somewhere between 0.3 and 0.6 billion metric tons of CO2 per year. That sounds like a lot until you compare it to human output: we emit at least 60 times more CO2 annually than all the world’s volcanoes combined. Erosion of carbon-rich rock adds a smaller, slower trickle.
Burning Fossil Fuels
Fossil fuel combustion is the single largest human source of CO2. Coal, oil, and natural gas all release carbon that was locked underground for millions of years, but they differ significantly in how much CO2 they produce per unit of energy. According to EIA data from 2022, coal is the dirtiest: it emits about 96 kilograms of CO2 per million BTU of energy produced. Diesel and gasoline fall in the middle at roughly 70 to 76 kilograms. Natural gas is the cleanest of the three at about 53 kilograms, nearly half the carbon intensity of coal.
These fuels power electricity generation, heating, manufacturing, and transportation. Globally, burning fossil fuels and producing cement together account for more than 35 billion metric tons of CO2 per year, a figure that has climbed steadily since the Industrial Revolution.
Transportation
Transportation is the largest source of greenhouse gas emissions in the United States, responsible for 28% of the national total in 2022. Within that sector, passenger cars and light trucks dominate, producing 57% of transportation emissions. Medium and heavy-duty trucks account for another 23%, reflecting the carbon cost of shipping goods by road. Aircraft contribute 9%, and ships and boats about 3%.
The outsized role of personal vehicles explains why fuel efficiency standards and electric vehicle adoption get so much policy attention. Cars and trucks together account for 80% of all transportation CO2 in the U.S.
Cement and Industrial Processes
Cement production is one of the less obvious but significant CO2 sources. Making cement requires heating limestone (calcium carbonate) to around 1,500°C. That heating triggers a chemical reaction called calcination, which splits the limestone into calcium oxide and CO2. This reaction alone accounts for 40 to 50% of cement’s total emissions. Another 40% comes from burning the fossil fuels needed to reach those extreme temperatures. The remaining 10% comes from cooling, grinding, and transporting the finished product.
What makes cement particularly hard to decarbonize is that about 85% of its CO2 comes from the chemical reaction itself and the heat required to drive it. Switching the electricity grid to renewables won’t solve this problem, since the CO2 is baked into the chemistry of the raw material.
Deforestation and Land Use
Forests act as carbon sinks, pulling CO2 out of the air and storing it in wood, roots, and soil. When forests are cut down or burned, that stored carbon goes back into the atmosphere. Between 1961 and 2020, land use changes released a net average of about 4.4 billion metric tons of CO2 per year globally (1.21 billion metric tons of carbon, which converts to roughly 4.4 billion metric tons of CO2). Deforestation, particularly the conversion of forests to farmland or grazing land, was the dominant driver.
Some of that loss was offset by reforestation and regrowth in other areas, but the net balance remained firmly on the emissions side for decades.
Agriculture and Soil
Farming releases CO2 in ways most people don’t think about. Tilling soil, the standard practice of turning and breaking up earth before planting, exposes buried organic carbon to air and accelerates microbial activity. Research from the USDA found that tilled soil released 58% more CO2 than untilled soil on average. After heavy rain or irrigation, tilled fields released 1.5 to 2.5 times more CO2 than no-till fields, as water displaced dissolved CO2 from the soil in sudden bursts.
The effect of each tilling event is short-lived, typically fading within 24 hours. But repeated tilling across an entire growing season adds up. Reduced tillage and no-till farming are considered among the most effective agricultural practices for cutting CO2 release and keeping carbon locked in the ground. Crop type and nitrogen fertilization, by comparison, had little measurable effect on CO2 emissions in the first year of study.
How Human and Natural Emissions Compare
Natural CO2 sources are massive in absolute terms, but they’ve been roughly balanced by natural absorption for millennia. Oceans absorb CO2, plants pull it from the air, and chemical weathering of rocks slowly locks it away. Human emissions disrupt that balance by adding carbon that was stored underground and out of the active cycle. The 40 billion metric tons humans emit annually is a net addition, not a replacement for natural flows. That’s why atmospheric CO2 concentrations have risen from about 280 parts per million before industrialization to over 420 today, even though natural sources dwarf human ones in raw volume.