Carbon dioxide comes from both natural processes and human activities. Natural sources, including ocean outgassing, decomposing plants, volcanoes, and the breathing of every living organism, have cycled CO2 through the atmosphere for billions of years. Human activities, primarily burning fossil fuels, now add roughly 36 billion metric tons of CO2 on top of that natural cycle each year. The atmosphere currently holds about 429 parts per million of CO2, up from around 280 ppm before the industrial era.
Every Living Thing Produces CO2
The most fundamental source of carbon dioxide is life itself. Inside your cells, a process called the citric acid cycle breaks down the sugars, fats, and proteins from food by gradually stripping energy from the carbon bonds that hold those molecules together. The end product, once a carbon atom has been fully stripped of its stored energy and bonded to two oxygen atoms, is CO2. This happens in the mitochondria of virtually every cell in your body.
At rest, you exhale about 785 grams of CO2 per day. With normal activity (16 waking hours and 8 hours of sleep), that figure rises to roughly 2.2 kilograms. Every mammal, bird, insect, and fish on the planet does the same thing at its own scale. So does every bacterium and fungus breaking down dead leaves on a forest floor. This decomposition of organic matter is one of the largest natural CO2 sources on Earth, releasing the carbon that plants absorbed during their lifetimes back into the atmosphere.
Oceans, Volcanoes, and Wildfires
The ocean is both the largest natural absorber and a significant emitter of CO2. Warm surface water releases dissolved carbon dioxide into the air (a process called outgassing), while cold water absorbs it. Over geologic time, this exchange has kept atmospheric CO2 relatively stable, though the balance shifts with ocean temperatures.
Volcanoes contribute a comparatively small amount. The U.S. Geological Survey estimates that all the world’s volcanoes, both on land and underwater, release between 0.13 and 0.44 billion metric tons of CO2 per year, with a best estimate around 0.26 billion metric tons. That’s less than 1% of what human activities produce annually.
Wildfires can release enormous pulses of CO2 in a short time. Canada’s extreme 2023 fire season alone released about 640 million metric tons of carbon. There’s an important distinction between fire emissions and fossil fuel emissions, though: as a burned forest regrows, it reabsorbs most of the carbon that was released. Fossil fuel CO2 has no equivalent natural offset. That said, scientists are watching whether increasingly frequent and severe fires could outpace regrowth, weakening forests’ ability to reabsorb what they lose.
Fossil Fuels: The Dominant Human Source
CO2 makes up about 74% of all greenhouse gas emissions, and 92% of that CO2 comes from burning fossil fuels: coal, oil, and natural gas. When these fuels burn, carbon that was locked underground for millions of years combines with oxygen and enters the atmosphere as CO2. The three biggest sectors driving this are electricity and heat production, transportation, and industry.
The reason fossil fuels matter so much more than natural sources is timing. Natural CO2 sources are largely balanced by natural CO2 sinks: plants absorb it, oceans dissolve it, and the cycle stays roughly in equilibrium. Fossil fuel combustion adds carbon that was outside of that active cycle entirely, tipping the balance so that more CO2 enters the atmosphere each year than natural processes can remove.
Cement, Deforestation, and Other Sources
Not all human CO2 emissions come from burning fuel. Cement manufacturing is a significant and often overlooked source. Making cement requires heating limestone (calcium carbonate) to 750 to 900 degrees Celsius, which chemically splits it into calcium oxide and CO2. This chemical reaction alone accounts for 60 to 70% of the CO2 released during cement production, separate from the fuel burned to generate that heat. Globally, cement is responsible for roughly 8% of human CO2 emissions.
Deforestation works differently. When forests are cleared and burned or left to decay, the carbon stored in the trees and soil escapes as CO2. In Brazil, deforestation and agriculture account for about half of the country’s total greenhouse gas emissions. Research on tropical forests worldwide has found that between 2003 and 2014, tropical forests as a whole emitted more carbon than they absorbed, largely because of clearing in Latin America and Southeast Asia. Unlike a wildfire in an intact forest, deforested land typically doesn’t regrow into dense forest, so that carbon stays in the atmosphere.
Why the Balance Has Shifted
For most of Earth’s history, CO2 levels fluctuated within a range driven by volcanic activity, ocean chemistry, and the slow weathering of rocks. The carbon cycle was a closed loop: what went into the atmosphere eventually came back out. Human activity has broken that loop. By extracting and burning fossil fuels, manufacturing cement, and clearing forests, we’ve added carbon to the atmosphere faster than any natural process can remove it. The result is a steady climb from about 280 ppm before industrialization to 429 ppm today, a level not seen in at least 800,000 years of ice core records.