The Global Terrestrial Carbon Balance

Résumé du livre

1. In the global carbon cycle, carbon is bidirectionally transported between the atmosphere and terrestrial ecosystems (vegetation and soils) and the oceans. The amounts of carbon estimated to be stored in the main compartments (atmosphere, terrestrial biota, soils and oceans) are large and the annual fluxes between the compartments are aIso large although net fluxes are much smaller (Figure I). 2. This cycle has been substantially perturbed by man over the last 130 years through large changes in land use and through the burning of fossil fuels. These changes continue today at an accelerating rate. The resultant anthropogenically-induced fluxes of carbon dioxide are very small in relation to the gross fluxes that occur naturally, but are large enough to modify the net fluxes, leading to an increase in the content of the atmosphere at a current rate of 3.2 gigatonnes (Gt) of carbon pr annum (= 3 -2 x 1015 da). This increase in CO2 content of the atmosphere has been the largest single contributor to the enhanced "greenhouse effect" : cumulatively abut 55% of the total effect up to the present. It is, therefore, vital to know whether the atmospheric content is likely to continue to increase pro rata with increme in the consumption of fossil fuels. 3. The current annual increase in CO2 content of the atmosphere accounts for a little more than half of the known current release of CO2 through the burning of fossil fuels (5.7 Gt of carbon per annum), The fate of the remainder of this CO2 released, plus any released in land use changes, is poorly known. The airborne fraction of future CO2 releases must depend upon the continuing capacity of the sinks for CO2 to take up a substantial part of that released. To be able to evaluate this, and indeed also possibly to manage the sinks to diminish the "greenhouse effect", we must first know the size and whereabouts of the major sinks for at the presnt time, and second, interpret the nature of these sinks and predict their likely ro.