Carbon is stored in the biosphere within living or recently dead plants, animals and microorganisms in the ocean and on land. Forests contain 86 percent of the planet's carbon on top of the ground. This biomass is released into the carbon cycle through natural decomposition and respiration.
Carbon stored in the biosphere gets there through photosynthesis in green plants. The process of photosynthesis includes removing carbon dioxide from the atmosphere. After producing their food, plants release oxygen back into the air.
Animals receive carbon when they eat plants and then release carbon dioxide into the atmosphere upon exhalation. When animals and plants die, their carbon turns to fossil fuels after millions of years in the form of coal, oil and natural gas. The burning of these hydrocarbons in power plants and automobiles releases more carbon into the air and contributes to global warming.
The carbon cycle also includes carbon dioxide dissolved in water, carbonate in sedimentary rocks like limestone and methane in the atmosphere. The element recycles through the atmosphere, down to plants, then into rocks and water before going back into the air. Carbon is not one of the most abundant elements on Earth, but it is necessary for all life to exist.