maximum conceivable amount of future releases from the land biosphere due to deforestation and other changes in land use is of the order of 500×109 tons. An emission of 5000×109 tons of carbon as CO2 (i.e., about eight times the pre-industrial amount of CO2 in the atmosphere) during the next few centuries probably would lead to four to six times higher CO2 concentration than at present, i.e., 1300–2000 ppm. In view of the huge amounts involved, it seems unlikely that increases in carbon stored in the terrestrial biosphere could reduce these values substantially.
Decline of CO2 levels in the atmosphere will take centuries because of the slow turnover of the deep sea. However, as the more CO2-rich waters reach the calcium carbonate deposits on the continental slopes, dissolution may increase the capacity of the oceans to absorb CO2. Since this process fundamentally depends on the rate with which ocean water can get in contact with the bottom sediments, it is not likely to proceed quickly, although our knowledge is inadequate to assess the role of this process more than qualitatively at present.
Considering the uncertainties, it would appear that a doubling of atmospheric carbon dioxide will occur by about 2030 if the use of fossil fuels continues to grow at a rate of about 4 percent per year, as was the case until a few years ago. If the growth rate were 2 percent, the time for doubling would be delayed by 15 to 20 years, while a constant use of fossil fuels at today’s levels shifts the time for doubling well into the twenty-second century.
There are considerable uncertainties about the future changes of atmospheric CO2 concentrations due to burning of fossil fuels. It appears, in particular, that the role of intermediate waters as a sink for CO2 needs careful consideration. Predictions of CO2 changes on time scales of 50 to 100 years may be significantly influenced by the results of such studies. However, considerable changes of atmospheric CO2 levels will certainly occur as a result of continuing use of fossil fuels. This conclusion is a sufficient basis for the following discussion of possible climatic changes.