Carbon from the Earth
The largest carbon reservoir is in the rocks of the Earth. This reservoir is largely 'locked up' and unavailable to the atmosphere or living organisms. However, there are a few ways that carbon does get into the atmosphere.
Carbon emerges from volcanoes in the form of carbon dioxide and thus enters the atmosphere.
Carbon is subducted into the Earth's mantle during plate tectonics and erupted from volcanoes. Carbon is removed from ocean water and used to make the shells of marine organisms which later die and fall to the ocean bottom as sediment.
Carbon is moved by man into the atmosphere. In the lithosphere, carbon is stored for millions of years as fossil fuels, including oil, natural gas, and coal. By burning fossil fuels, people move carbon from long-term geologic storage into the atmosphere.