ABSTRACT
The Earth's crust is the largest environmental reservoir of carbon. The main form of carbon in the atmosphere is carbon dioxide the most naturally abundant 'greenhouse gas' in dry air. Carbon occurs in freshwaters and seawaters mainly as dissolved inorganic carbon and as dissolved organic carbon and carbonaceous particulates, such as fragments of eroded carbonate rock and undecomposed organic solids. The natural carbon cycle is characterised mainly by fluxes to and from the atmosphere from the lithosphere, hydrosphere and, in particular, the biosphere. This is mainly because carbon compounds are used extensively by living organisms in the fundamental processes of photosynthesis and respiration. The natural greenhouse effect is enhanced by anthropogenic emissions of greenhouse gases, leading to global warming as more of the Earth's outgoing thermal radiation are retained in the atmosphere. Radiative forcing is the change in the Earth's net energy budget resulting from changes in the atmospheric concentrations of greenhouse gases and aerosols.
