carbon enters the atmosphere as carbon dioxide from respiration and combustion. carbon dioxide is absorbed by producers to make carbohydrates in photosynthesis. animals feed on the plant passing the carbon compounds along the food chain. most of the carbon they consume is exhaled as carbon dioxide formed during respiration. the animals and plants eventually die. the dead organisms are eaten by decomposers and the carbon in their bodies is returned to the atmosphere as carbon dioxide. in some conditions decomposition is blocked and the plant and animal material is stored and may then be available as fossil fuel in the future for combustion. in the sea, marine animals may convert some of the carbon in their diet to calcium carbonate which is used to make their shells. over time the shells of dead organisms collect on the seabed and form limestone. due to Earth movements this limestone may eventually become exposed to the air where it’s weathered and the carbon is released back into the atmosphere as carbon dioxide. volcanic action may also release carbon dioxide.