The release of CO2 from the terrestrial biota, including soil emissions and clearing and burning of forests, have contributed significantly to the present atmospheric CO2 concentration. At present, however, the main anthropogenic source of CO2 is the combustion of fossil fuels. Future emissions of CO2 resulting from fossil fuel combustion are highly uncertain. The estimated CO2 emission for 1987 is 5.7 Gt C y hoch -1. (Gr = gigaton; 1 Gt = 10 hoch 15 g). In addition, biotic sources of CO2 (mainly deforestation) contribute about 1 to 3 Gt C y hoch -1, including 0.2 to 0.9 Gt C y hoch -1 from soil carbon loss. Major problems in assessing the role of the terrestrial biota in the CO2 cycle are the lack of knowledge of the present (and future) magnitude of the CO2 fertilizing effect, uncertainty in estimates of the extent of forest clearing and carbon stocks of forests cleared. The various analyses of deforestation are difficult to compare because of definitional differences. Sinks of CO2 are the atmosphere (55%), oceans (30%) and terrestrial biota (15%) . The annual increase of the atmospheric concentration is about 0.5% or 3.6 Gt C y hoch -1. The increase of the human world population correlates well to the atmospheric increase of 1% y hoch -1 of methane (CH4). This indicates that the rise in atmospheric CH4 is most likely related to anthropogenic activities. The present annual release rate is 300 to 700 Tg CH4 y hoch -1 (Tg = terragram; 1 Tg = 10 hoch 12 g) which is a sum of all source estimates. The major individual sources are rice paddies (60-140 Tg), wetlands (40-160 Tg), landfill sites (30- 70 Tg), oceans/freshwater lakes/other biogenic (15-35 Tg), ruminants (66-90 Tg) , termites (2-42 Tg), exploitation of natural gas and coal mining (65-75 Tg), biomass burning (55-100 Tg). The total sink of CH4 includes: reaction with OH radicals in the troposphere (260 Tg y hoch -1), transport to the stratosphere (60 Tg hoch y -1) and oxidation in arid soils (16-48 Tg y hoch -1). Clearly sources and sinks are not balanced in this budget, and the allocation among the individual sources and sinks is still not well known. The strong increase of methane over the past 200 years is primarily caused by increasing emissions and a minor cause is the depletion of OH radicals, which are responsible for oxidation of atmospheric CH4 and carbon monoxide (CO). This OH depletion is caused by increasing CH4 and CO emissions. The major sources of CO are known, but their magnitudes are still uncertain. The background concentration of CO is increasing at a rate of 0.6-1% per year or higher, but estimates are uncertain due to fluctuations of sources and sinks and the relatively short residence time of CO in the atmosphere. Estimates of global emission range between 1270 and 5700 Tg y hoch -1, the major sources being: biomass burning (800 Tg), fossil fuel burning (450 Tg), ...
