Carbon is transferred between CO2 and living or dead organic material by the very basic photosynthesis / respiration reaction (shown here in simplified form).
CO2+H2O+energy <=> CH2O + O2
When this reaction proceeds to the right, it is the fixation of carbon to organic matter by plants via photosynthesis; and when it proceeds to the left, it is respiration or combustion of that organic matter. Fossil fuels are the remnants of dead organic matter that lived millions of years ago.
The Global Carbon Cycle
The carbon cycle is a complex system of biological, chemical and physical processes. A schematic from the IPCC AR4 report is shown here. The schematic shows the major reservoirs of carbon in gigatons of carbon, GtC (1 GtC = 1 PgC: Petagram of Carbon) and the major fluxes in GtC/yr. The numbers shown are the best estimates for the 1990’s.
IPCC AR4 (2007) Fig 7.3. The global carbon cycle for the 1990s, showing the main annual fluxes in GtC yr –1 : 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). Net anthropogenic exchanges with the atmosphere are from Column 5 ‘AR4’ in Table 7.1. 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.