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The global carbon cycle exchanges carbon (as CO2) between carbon reservoirs. These include (4): • The atmosphere (720 Gt = billion tons) • The terrestrial biosphere (2,000 Gt) • the oceans (38,400 Gt) • marine sediments and rocks (Lithosphere > 60,000,000 Gt) and  fossil fuels (4,130 Gt) = coal (3,510 Gt),  oil (230 Gt),  gas (140 Gt) and others (250 Gt)
Terrestrial ecosystems and the oceans exchange CO2 rapidly with the atmosphere. The carbon exchange from the lithosphere is very slow, although some CO2 is released by volcanoes. CO2 is removed from the atmosphere through photosynthesis and stored in organic matter. When plants grow they utilize sunlight, CO2 and water (H2O) to synthesize organic matter (photosynthesis) and release oxygen (O2, see equation 1).

Equation 1                                                     Light
                                                  CO2 + 6H2O  →  C6H12O6 + 6O2

This organic matter is returned to the atmosphere by decomposition of dead plant tissue or distur-bances, such as fire, in which large amounts of organic matter are oxidized and rapidly transferred into CO2. Terrestrial carbon is primarily stored in forests (5). In undisturbed full-grown forest ecosystems, the turnover time of carbon is on the order of decades and uptake by photosynthesis and release by decay is balanced.

 Figure 2 Sources and sinks of CO2 - The global carbon cycle

Read more about the Human influence on the carbon cycle and greenhouse gases

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