Carbon cycles between the earth and atmosphere in a repeating process. Plants absorb carbon dioxide from the air through photosynthesis, and animals release carbon back into the air when they exhale or when dead plants and animals decompose. Some carbon becomes trapped underground in deposits like oil and coal, and is slowly released back into the atmosphere over many years through natural processes or faster when humans burn fossil fuels. Maintaining the balance of the carbon cycle is important for regulating the earth's climate.