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version of the ORNL represented this temperature-C02 relationship 

 by a simple exponential equation, rather than through a series of 

 heat-flux equations. This portion of the model was replaced by 

 the relatively sophisticated heat-flux relationships in the 

 GISS model, by coupling the two models (see the discussion under 

 "Coupling the GISS and ORNL Models"). 



The model inputs are estimates of fossil-fuel carbon emis- 

 sions on a yearly or longer time period basis from 1980 until 

 some future year. (The IEA model estimates are on a 25-year 

 basis. Estimates were provided on a 5-year basis by interpola- 

 tion of the 25-year estimate.) Outputs are atmospheric CO2 for 

 5-year intervals. 



The model simulates historical patterns of carbon cycling 

 from 1740 to 1980 on an annual basis. The resulting time trend 

 in CO2 was modified slightly so that the concentration in 1980 

 was 339 ppm. This corresponds to the level estimated by the GISS 

 model in 1980, given estimates of fossil fuel carbon emissions in 

 1975 (the starting point of the IEA model) used in this study. 

 The model was then run to produce estimates of atmospheric CO2 

 from 1980 to 2100. 

 Reasonableness of the Assumptions 



The fraction of carbon emissions retained in the atmosphere 

 is the product of a complex biogeochemical cycle that evolves 

 over time. Evidence indicates that as the atmospheric concentration 



