Since these emissions are not known with any degree of accuracy during the period for which accurate observations of atmospheric CO2 are available (1958–1979), we know only approximately the ratio between the net increase of CO2 in the atmosphere and the total man-induced emissions. However, at least 50 percent of the emissions and perhaps more than 70 percent have been transferred into other natural reservoirs for carbon. We need to consider three possible sinks for this transfer:
The remaining forests of the world (because of more effective carbon assimilation as a result of higher CO2 levels in the atmosphere);
The surface and intermediate waters of the oceans (above about 1000 m);
The deep sea (below about 1000 m).
The distribution of past emissions of CO2 between these sinks is not entirely clear. On the basis of the radiocarbon concentration in the deep sea, it has been concluded that only a rather small part of the emissions so far have been transferred into the deep sea. However, the proper role of the deep sea as a potential sink for fossil-fuel CO2 has not been accurately assessed. As indicated in Section 3.3 on the oceans, theoretical estimates of mass transfer from the mixed layer into the intermediate waters indicate that this part of the ocean may have been a more important sink for carbon dioxide emitted into the atmosphere than has so far been considered. This conclusion is also in accord with observations of the penetration of radioactive trace substances produced by nuclear-weapons testing into the intermediate waters. Whether some increase of carbon in the remaining world forests has occurred is not known.
Our limited knowledge of the basic features of the carbon cycle means that projections of future increases of CO2 in the atmosphere as a result of fossil-fuel emissions are uncertain. It has been customary to assume to begin with that about 50 percent of the emissions will stay in the atmosphere. The possibility that the intermediate waters of the oceans, and maybe also the deep sea, are in more rapid contact with the atmosphere may reduce this figure to 40 percent, perhaps even to a somewhat smaller figure. On the other hand, a continuing reduction of the world forests will further add to any increase due to fossil-fuel combustion. The ability of the oceans to serve as a sink for CO2 emissions to the atmosphere is reduced as the concentrations increase because of the chemical characteristics of the carbonate system of the sea.
If all the fossil-fuel reserves were used for combustion, the airborne fraction would increase considerably above the values of 30 to 50 percent mentioned above. Global fossil-fuel resources contain at least 5000×109 tons of carbon, of which oil and gas together represent about 10 percent. The