ABSTRACT

In its simplest formulation, the global carbon cycle consists of four terms (atmosphere, land, ocean, and fossil fuels) (Table 23.1). The natural fluxes of carbon between land and atmosphere are 120–150 PgC/year (1 petagram carbon equals 1015 gC, or 1 billion metric ton C, or 3.67 billion metric ton CO2), as a result of global photosynthesis and respiration (including fire). Similar fluxes of 90–120 PgC/year occur between ocean and atmosphere as a result of physical, chemical, and biological processes. These are not the fluxes of the global carbon budget, however. Instead, the global carbon budget usually refers to the anthropogenic perturbation to the global carbon budget. The fluxes resulting from anthropogenic perturbation are 1–2 orders of magnitude smaller than the natural flows (Table 23.1).