* Respiration: All living organisms respire, releasing carbon dioxide into the atmosphere.

* Decomposition: When plants and animals die, they decompose, releasing carbon dioxide into the atmosphere.

* Volcanic eruptions: Volcanic eruptions release carbon dioxide and other gases into the atmosphere.

* Forest fires: Forest fires release carbon dioxide into the atmosphere.

Human contributors:

* Burning fossil fuels: The burning of fossil fuels, such as coal, oil, and natural gas, releases carbon dioxide into the atmosphere.

* Deforestation: Deforestation reduces the number of trees that can absorb carbon dioxide from the atmosphere.

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

* Agriculture: Some agricultural practices, such as the use of nitrogen fertilizers, release carbon dioxide into the atmosphere.