carbon cycle

What is the carbon cycle?

The carbon cycle is the process through which carbon is cycled through the air, ground, plants, animals, and fossil fuels. People and animals inhale oxygen from the air and exhale carbon dioxide (CO2). At the same time, plants absorb CO2 for photosynthesis and emit oxygen back into the atmosphere. Carbon dioxide is also exchanged between the atmosphere and the oceans and is emitted or absorbed in other natural processes.  This natural system of processes is called the carbon cycle, which keeps CO2 levels in the atmosphere stable over time. The carbon cycle figure below shows how carbon moves among land, the atmosphere, and the ocean through various natural- and human-initiated processes. The figure also provides estimated storage values and the various fluxes for specific processes. On land, carbon can be embedded in limestone, a plant/animal, or within soil on a farmland. When a plant/animal decomposes, or soil is plowed on farmland, oxygen (O2) from the atmosphere reacts with the carbon to produce CO2, which subsequently gets emitted into the atmosphere. Once in the atmosphere, the carbon can be absorbed by the ocean or by a land/ocean-based plant or an animal. The ocean has several internal pathways for carbon transformation. It is important to note that only a small amount of the Earth’s carbon moves through the carbon cycle each year.

Carbon Fluxes in Gigatons (a Gigaton is equal to 1 billion tons)
(click image to enlarge)

Nature's carbon cycle normally keeps CO2 levels in balance, but human activity, mostly the burning of fossil fuels, produces more CO2 than nature can absorb. The human contribution is relatively small, but enough to throw the cycle off balance. The Industrial Revolution resulted in human activities that produce anthropogenic CO2, which have accelerated ever since. Since the beginning of the production of anthropogenic CO2, about 150 years ago, humans have increased the carbon content in the everyday carbon cycle by about 0.7 percent.