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Carbon Cycle
If we now consider the effects of anthropogenic changes to this natural
cycle, we must focus on the two largest contributions, namely burning of fossil
fuels and deforestation. Fossil fuels represent terrestrial carbon that has
been taken out of the rapidly changing part of the carbon cycle and stored more or
less permanently below the earth's surface where natural processes cannot
release it back to the atmosphere. Fossil fuels account for about 5 to 6 units
of emission into the atmosphere, and the burning of standing carbon in the form
of old growth tropical forests contributes about 1 to 2 units.
The carbon cycle includes other carbon-containing compounds in addition to carbon dioxide. For instance, methane molecules, which have one carbon atom and 4 hydrogen atoms, may be produced rather than carbon dioxide in the decay process, particularly in moist soils, marshes, and boreal tundra. Methane also is produced by ruminant animals, such as cattle and sheep. These animals eat large amount of grain or forage, which goes into the first of 4 stomachs known as the rumen. The digestion process releases significant amounts of methane that are released to the atmosphere. Carbon monoxide, a much more reactive carbon-oxygen molecule, also is released to the atmosphere from the burning of fossil fuels and from decay of plant material. Volcanoes are natural sources of both carbon dioxide and carbon monoxide.
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