Carbon dioxide comes from both natural processes and human activities. Nature produces it through breathing, decomposition, volcanic eruptions, and ocean release. Human activities, primarily burning fossil fuels, add roughly 40 billion metric tons on top of that every year. Understanding the full picture means looking at both sides: the natural carbon cycle that has operated for millions of years and the industrial emissions that have pushed atmospheric CO2 to a record 422.8 parts per million in 2024.
Your Body Makes It Right Now
Every living cell that uses oxygen produces carbon dioxide as a waste product. Inside your cells, structures called mitochondria break down sugars, fats, and proteins through a series of chemical reactions. At each step, energy stored in the carbon bonds of food is released, and the leftover carbon atoms bond with oxygen to form CO2. This is the final, fully oxidized form of carbon, essentially the “exhaust” of metabolism.
Your bloodstream carries that CO2 from your tissues to your lungs, where you breathe it out. At complete rest, an average person exhales about 785 grams of CO2 per day. With normal daytime activity, that number climbs to roughly 2.2 kilograms, or close to 5 pounds. Every animal on Earth, from insects to elephants, does the same thing. Plants do it too, especially at night when photosynthesis stops but their cells keep burning fuel.
Decomposition and Soil Respiration
When plants and animals die, bacteria and fungi break down their organic matter. This decomposition is chemically similar to what happens inside your cells: carbon compounds are oxidized, and CO2 is released. The global scale of this process is enormous. Soils alone release an estimated 68 billion metric tons of carbon per year as CO2, driven by microbial activity in forests, grasslands, wetlands, and agricultural land. Warmer, wetter environments tend to produce more because microbes are more active in those conditions.
This soil respiration is part of a balanced cycle. Plants pull CO2 out of the air through photosynthesis at roughly the same rate that decomposition puts it back. The cycle has kept atmospheric CO2 relatively stable for thousands of years, at least until industrialization changed the equation.
Volcanoes and Ocean Outgassing
Volcanoes release CO2 stored deep in the Earth’s crust. When magma rises and pressure drops, dissolved gases escape, including carbon dioxide. This happens during eruptions but also continuously through vents, hot springs, and submarine volcanic ridges. The best current estimate puts total volcanic CO2 emissions at about 0.6 billion metric tons per year, though estimates range from 0.15 to 0.75 billion metric tons depending on how much subsurface magma degassing is included.
Oceans are another natural source. Seawater absorbs and releases CO2 depending on temperature: cold water holds more dissolved gas, and warm water releases it. Tropical ocean surfaces tend to emit CO2, while polar waters absorb it. Over time, the oceans act as both a source and a sink, cycling vast quantities of carbon between water and air.
Burning Fossil Fuels
The largest human source of CO2 is combustion of coal, oil, and natural gas. These fuels are hydrocarbons, molecules made of carbon and hydrogen atoms. When they burn, the carbon bonds with oxygen in the air to form CO2, and the hydrogen bonds with oxygen to form water. A simple example: burning propane (three carbon atoms, eight hydrogen atoms) with oxygen produces three molecules of CO2 and four molecules of water. Every carbon atom in the fuel ends up as part of a CO2 molecule. There is no way around it.
Human activities released roughly 40 billion metric tons of CO2 in 2015, and emissions have stayed in that range. That is at least 60 times more than all the world’s volcanoes combined. The energy sector alone accounts for 75.7% of all greenhouse gas emissions worldwide. Within that sector, electricity and heat generation contribute 29.7%, transportation adds 13.7%, and manufacturing and construction account for 12.7%. Road transportation specifically is responsible for 12.2% of global emissions, and residential buildings contribute 12.5%.
To put the volcanic comparison in sharper terms: a U.S. Geological Survey analysis found that human CO2 emissions were more than 90 times greater than volcanic emissions when using earlier, more conservative volcanic estimates. Even with updated data that roughly doubled the volcanic figure, human output still exceeds volcanic output by a factor of 60 or more.
Cement, Steel, and Industrial Chemistry
Not all industrial CO2 comes from burning fuel. Some comes from chemical reactions in manufacturing itself. Cement production is the clearest example. Making cement requires heating limestone (calcium carbonate) to extremely high temperatures in a process called calcination. The heat breaks the limestone apart, releasing the CO2 that was locked inside it and leaving behind calcium oxide, which becomes the active ingredient in cement. About two-thirds of cement’s CO2 footprint comes from this chemical decomposition, not from the fuel burned to heat the kiln. The remaining third comes from the fuel. In total, producing one metric ton of ordinary cement releases about 850 kilograms of CO2.
Industrial processes outside the energy sector, including cement, steel, and chemical manufacturing, account for about 6.5% of global emissions. That may sound small as a percentage, but it translates to billions of metric tons per year.
Deforestation and Land Clearing
Trees store carbon in their wood, roots, and leaves. When forests are cut down and burned or left to decay, that stored carbon returns to the atmosphere as CO2. Land use change, primarily deforestation, contributes 12 to 20% of global greenhouse gas emissions. This makes it one of the largest single categories of human-caused emissions, comparable in scale to the entire global transportation sector.
The impact is double-edged. Clearing a forest not only releases stored carbon but also removes a system that was actively pulling CO2 out of the air. A standing tropical forest absorbs CO2 year-round through photosynthesis. Once it is gone, that absorption stops and the land may become a net source of emissions, especially if the soil is disturbed or converted to agriculture.
How It All Adds Up in the Atmosphere
Natural sources of CO2 are massive, far larger than human emissions in raw numbers. Soil respiration alone dwarfs fossil fuel combustion. But natural sources are balanced by natural sinks: photosynthesis, ocean absorption, and mineral weathering pull roughly the same amount of CO2 back out. The system was in approximate equilibrium for millennia, keeping atmospheric CO2 between about 180 and 280 parts per million through ice ages and warm periods alike.
Human emissions are the addition that breaks that balance. The 40 billion metric tons we add each year has no corresponding new sink to absorb it. Oceans and forests absorb roughly half of what we emit, but the other half accumulates in the atmosphere. That is why CO2 concentrations have climbed from about 280 ppm before the Industrial Revolution to 422.8 ppm in 2024, with a peak monthly reading of nearly 427 ppm recorded in May of that year. Every source contributes, but human fossil fuel combustion and deforestation are what is driving the number upward.