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will continue to increase throughout the twentyfirst century in 

 all scenarios. Table B-5 shows the resulting concentrations 

 of these gases in 25-year intervals. 



TABLE B-5 



CONCENTRATIONS Of CHLOROFLUOROCARBONS 

 (parts per billion) 



Year 



1980 20UU 2020 2040 2060 2080 2100 



Low Scenario 



Existing knowledge of the environmental fates of methane 

 and nitrous oxide did not permit us to model the biogeochemical 

 cycles for these gases. Instead, we projected their concentrations 

 directly. For the low scenario, we assumed that the historical 

 trends in the concentrations of methane and nitrous oxide would 

 continue; for the mid-range and high scenarios, we assumed that 

 they would accelerate. The concentration of nitrous oxide was 

 assumed to increase from the 1980 level (0.3 ppm) geometrically 

 by 0.2, 0.45, and 0.7 percent per year for the three scenarios 



