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as discussed above. 



To generate the GISS temperature-time curves, it is first 

 necessary to estimate an atmospheric C02~time curve, since this is 

 the main driving function for the GISS model. The most straight- 

 forward approach is to apply a constant retention factor to the 

 time trend of CO2 emissions generated by the IEA model. In other 

 words, the first step in this approach appears identical to the 

 other methodological approach. In fact, a modification was 

 employed for these analyses: changing retention factors (from 

 about 0.6 in 1980 to about 0.8 in 2100) were used to generate 

 the preliminary atmospheric C02~time curves. 



The GISS model was then run with high and low temperature/ 

 CO2 sensitivity (T e ) values, with and without trace gases, to 

 produce a family of curves of sea-surface temperature versus time. 

 (Values for solar luminosity, volcanic activity, and heat diffu- 

 sivity were specified in each scenario.) These curves are repre- 

 sented by a quadratic equation of temperature versus time with 

 parameters a and b. The parameter a is the temperature difference 

 between 2100 and 1980; b is the temperature difference between 

 2040 and 1980 divided by a. These parameters are then used to 

 calculate coefficients for the quadratic equation T = ct + dt 2 + 293, 

 where T is the temperature rise after 1980 (°K), and t is the number 

 of years after 1980. 



