The Carbon Cycle
The global carbon cycle involves the carbon in and exchanging between the earth's atmosphere, fossil fuels, the oceans, and the vegetation and soils of the earth's terrestrial ecosystems.
Each year, the world's terrestrial ecosystems withdraw carbon from the atmosphere through photosynthesis and add it again through respiration and decay.
A more detailed look at the global carbon cycle for the 1990s is shown below. The main annual fluxes in GtC yr–1 are: pre-industrial "natural" fluxes in black and "anthropogenic" fluxes in red (modified from Sarmiento and Gruber, 2006, with changes in pool sizes from Sabine et al., 2004a). The net terrestrial loss of –39 GtC is inferred from cumulative fossil fuel emissions minus atmospheric increase minus ocean storage. The loss of –140 GtC from the "vegetation, soil and detritus" compartment represents the cumulative emissions from land use change (Houghton, 2003), and requires a terrestrial biosphere sink of 101 GtC (in Sabine et al., given only as ranges of –140 to –80 GtC and 61 to 141 GtC, respectively; other uncertainties given in their Table 1).
Gross fluxes generally have uncertainties of more than ±20% but fractional amounts have been retained to achieve overall balance when including estimates in fractions of GtC yr–1 for riverine transport, weathering, deep ocean burial, etc. "GPP" is annual gross (terrestrial) primary production. Atmospheric carbon content and all cumulative fluxes since 1750 are as of end 1994.