Considerable amount of carbon returns to the atmosphere as carbondioxide through respiratory activities of the produces and consumers. Decomposers also contribute substantially to carbondioxide pool by their processing of waste materials and dead organic matter of land or oceans.
Burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity are additional sources for releasing carbondioxide in the atmosphere.
Rapid deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere.