Natural CO2 sources account for the majority of CO2 released into the atmosphere.

Natural CO₂ sources account for the majority of CO₂ released into the atmosphere. Oceans sometimes release dissolved CO₂ into the atmosphere to maintain equilibrium between the two media. Oceans provide the greatest annual amount of CO₂ of any natural or anthropogenic source. During animal and plant respiration, oxygen and nutrients are converted into CO₂ and energy. Decomposition of organic matter (i.e., plant or animal matter) is a process in which bacteria breaks down the organic compounds and releases CO₂ into the atmosphere. Forest fires are caused naturally when hot and dry weather affects an area for an extended length of time. When conditions become both hot and dry enough, vegetation is easily ignited. As it burns, the carbon within the vegetation combines with the oxygen in the air, forming CO₂. Magma in volcanic eruptions experiences a drop in pressure as it spews into the atmosphere. The high concentrations of dissolved gases within the magma, including CO₂, begin bubbling out with this pressure relief.

Automobiles are one example of anthropogenic CO2 sources.

Anthropogenic CO₂ sources such as fossil fuel burning and the use of limestone to make cement both produce CO₂. Fossil fuel burning is the foundation of many everyday activities, including driving a car, taking a warm shower, drying clothes, and heating a home. Another significant anthropogenic CO₂ source is farmland plowing, where soil is exposed to air, and bacteria within the soil breaks down the organic matter in the presence of oxygen, releasing CO₂ in the process. 

Although anthropogenic CO₂ sources release less CO₂ into the atmosphere than do natural sources, it is important to realize that certain anthropogenic sources, like fossil fuel burning, add carbon into the natural carbon cycle, causing atmospheric CO₂ to steadily increase. Some scientists are concerned that this gradual increase in atmospheric CO₂ above natural levels could lead to potential climate change.