<P> The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes . The ocean contains the largest active pool of carbon near the surface of the Earth . The natural flows of carbon between the atmosphere, ocean, terrestrial ecosystems, and sediments is fairly balanced, so that carbon levels would be roughly stable without human influence . </P> <P> Carbon in the Earth's atmosphere exists in two main forms: carbon dioxide and methane . Both of these gases absorb and retain heat in the atmosphere and are partially responsible for the greenhouse effect . Methane produces a larger greenhouse effect per volume as compared to carbon dioxide, but it exists in much lower concentrations and is more short - lived than carbon dioxide, making carbon dioxide the more important greenhouse gas of the two . </P> <P> Carbon dioxide is removed from the atmosphere primarily through photosynthesis and enters the terrestrial and oceanic biospheres . Carbon dioxide also dissolves directly from the atmosphere into bodies of water (oceans, lakes, etc .), as well as dissolving in precipitation as raindrops fall through the atmosphere . When dissolved in water, carbon dioxide reacts with water molecules and forms carbonic acid, which contributes to ocean acidity . It can then be absorbed by rocks through weathering . It also can acidify other surfaces it touches or be washed into the ocean . </P> <P> Human activities over the past two centuries have significantly increased the amount of carbon in the atmosphere, mainly in the form of carbon dioxide, both by modifying ecosystems' ability to extract carbon dioxide from the atmosphere and by emitting it directly, e.g., by burning fossil fuels and manufacturing concrete . </P>

The process by which trees remove carbon from the air and store it in their biomass is known as