the oceans are able to take up. The impacts of deforestation on CO2 levels are not well established, and it is not even reliably known if the biosphere is currently increasing or decreasing. These deficiencies will need to be overcome if the impacts of man’s input to the carbon cycle are to be evaluated . 2.3.3 Projected Future CO2 Levels The future concentration of CO2 in the atmosphere is a function of two factors: the growth of global fossil fuel utilization and the fraction of fossil fuel CO2 input that remains in the atmosphere. If energy policies similar to those of the past 25 years prevail in the near future, the world consumption of fossil fuels may continue to increase by 4% per year for at least a few decades. It seems unlikely, in any case, that the growth of global fossil fuel utilization will be substantially slowed before the year 2000. The fraction of fuel-derived CO2 remaining airborne has probably been fairly constant during the past, with short-term variations. It is probably reasonable to assume that up to the year 2000 the airborne fraction will remain constant at around 50%. However, significant increases or decreases in the biosphere could change this. Numerous projections of global atmospheric CO2 levels have been made up to the year 2000.^0,58,59 The projections generally fall in the range of 375 to 400 ppm as compared to the current concentration of about 330 ppm. Beyond 2000, projections become much more difficult to make. Man’s input will depend on energy policies and the emergence of nonfossil energy technologies. It is important to be able to project CO2 levels for the 21st century, because by that time the levels are expected to be extremely high and possibly significant with regard to global climate. An important parameter in making such estimates is the total fossil fuel reserves that can produce CO2. This has been estimated as being between 5 and 15 times the preindustrial amount of CO2.60“62 Other important parameters are the rate at which fossil fuel resources are exploited, and as previously mentioned, whether or not the airborne fraction of CO2 will change appreciably in the future.
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