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END NOTES TO APPENDIX B 



The rationale for coupling the GISS and ORNL models in this 

 fashion is based on (a) the sensitivity of many C0 2 uptake 

 and release equations to temperature in the ORNL model, and 

 (b) the more sophisticated treatment of heat flows and 

 temperature changes in response to CO2 increases in the GISS 

 model. An alternative approach would be to replace the simple 

 temperature equations in the ORNL model with the GISS equa- 

 tions, in essence combining the two models. However, the 

 ORNL model is much more expensive to run than the GISS model. 

 Thus, the coupled approach employed here is a cost-effective 

 way to upgrade the treatment of temperature in the carbon 

 cycle model. 



To illustrate the difference in results between the two basic 

 analytical approaches (fixed-retention factor with the GISS 

 model v. coupled ORNL-GISS modeling), we have listed 

 estimated atmospheric CO2 levels and sea level rise from 

 1980 to 2100 in the following table for each approach. 

 Two sets of results are listed for the coupled ORNL-GISS 

 modeling results — the initial outputs from the ORNL and 

 the final outputs from the GISS model. As shown, the 

 coupled ORNL-GISS approach produces higher estimates than 

 the fixed-retention-factor approach. This is reflected 

 in the retention factor inferred from the ORNL-GISS modeling, 

 which varies from 0.53 in 1980 to 0.72 in 2100. 



A second observation concerns differences between the two 

 sets of ORNL-GISS outputs. The initial ORNL CO2 (or sea 

 level rise) time curve is higher than the final iteration 

 from GISS, reflecting the fact that the ORNL results repre- 

 sent one of the four extreme temperature-versus-t ime curves. 

 The final iteration from GISS is a C0 2 time curve interpolated 

 among the extremes, as discussed in the section on coupling 

 the ORNL and GISS models. 



