* Fossil fuel combustion: This is the process of burning fossil fuels, such as coal, oil, and natural gas, for energy. When fossil fuels are burned, they release carbon dioxide into the atmosphere. Fossil fuel combustion is the largest human-caused source of carbon dioxide emissions.

* Deforestation: Trees absorb carbon dioxide from the atmosphere and store it in their trunks and leaves. When trees are cut down, the carbon that is stored in them is released into the atmosphere. Deforestation is the second largest human-caused source of carbon dioxide emissions.

* Land use changes: When land is cleared for agriculture, urbanization, or other uses, the vegetation that is removed releases carbon dioxide into the atmosphere. Land use changes are the third largest human-caused source of carbon dioxide emissions.

* Industrial processes: Some industrial processes, such as cement production and iron and steel production, release carbon dioxide into the atmosphere.

* Agricultural practices: Some agricultural practices, such as the use of nitrogen fertilizers and the burning of crop residues, release carbon dioxide into the atmosphere.

Other processes that can add carbon dioxide to the atmosphere include volcanic eruptions and the release of methane from thawing permafrost.