What are the different ways that atmospheric carbon (in the form of CO2) moves between plants, animals, the oceans, soils, and rocks? These movements are called the 'global carbon cycle', but complete answers to these questions are not known at this time.
Carbon dioxide quantities in the environment are usually measured by scientists in units of 'tons of carbon' rather than tons of CO2, since researchers are most interested in tracking the movements of the carbon atom in each CO2 molecule. The conversion from tons of CO2 to 'tons of carbon' is a simple one. The carbon atom C, has an atomic weight of 12, while the molecular weight of CO2 is 44. (Each of the two oxygen atoms has a weight of 16.) The ratio of 12 to 44, or 12/44 = .273 is the conversion factor. So 10 tons of CO2 is the same as 2.73 tons of carbon.
One obvious way CO2 is removed from the air is through the mechanism of photosynthesis in green plants. Plants use sunlight, water, and CO2 to manufacture cellulose (plant tissue) and oxygen. In the case of plants that live a long time, such as trees, carbon is stored in the cellulose for many years as the tree grows. Eventually the plant dies, or is eaten by an animal. The tissues are broken down by bacteria in the soil, air, or in the animal's digestive tract. Depending on the availability of oxygen and the type of bacteria, the plant's carbon gets converted to either CO2 or methane (CH4). Aerobic bacteria operate in oxygen rich environments and generate CO2. Where oxygen is scarce, such as in wet swamps and inside the guts of animals, anaerobic bacteria turn plant carbon matter into methane.
Large quantities of CO2 are absorbed into the oceans, both by sea plants and small animals called plankton. When these living things die, their tissues and bodies fall to the ocean floor, are compressed and eventually may contribute to the formation of new petroleum deposits. This movement of carbon takes place over a very long time scale, and is part of a biogeochemical cycle, since it involves biological, chemical, and geological mechanisms.
Another way CO2 moves to the oceans involves the atmospheric combination of water and CO2 to form carbonic acid. The acid dissolves limestone and other rocks to produce bicarbonate ions that are eventually washed into the oceans. Once in the oceans, certain marine animals may use the bicarbonate to make their shells. When the animals die, new limestone is formed from their shells on the ocean floor.
There are other mechanisms that are not well understood. The carbon cycle is a very complex one, since the element carbon is an essential building block for all life forms here on Earth. Much scientific research into global warming now centers on improving our understanding of the carbon cycle. ***
