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As mentioned, not all the CO2 emissions remain in the 



atmosphere. Some are removed by green plants, and some are 



absorbed by the oceans. Unfortunately, knowledge of these 



percentages is uncertain because the movement of carbon between 



various "storage compartments" in the earth (the oceans, 



biosphere, and the atmosphere) is only partially understood. 



Two assumptions were made for predicting the fraction airborne: 



o For the high scenario , a model of the carbon cycle 

 developed at Oak Ridge National Laboratories (ORNL) 

 was used.f/ The model represents, at a very high 

 level of aggregation, many of the physical and 

 chemical mechanisms that regulate the exchanges of 

 carbon between the various natural compartments that 

 absorb and release it. The fraction airborne increased 

 over time from 60 to 80 percent. 



o For the low scenario , the percentage of carbon dioxide 

 remaining in the atmosphere was assumed to be equal to 

 its historical average of 53 percent. V This assump- 

 tion is conservative for several reasons. Like a 

 sponge, the upper layers of the oceans have a limited 

 absorption capacity.^/ Thus, as the emissions of 

 CO2 increase, the percentage taken up by the oceans 

 should decrease. Also, warming of the oceans and the 

 resulting changes in circulation could lower the capacity 

 of the oceans to absorb CC>2._ 



o For the mid-range scenario , the ORNL model was used. 



High and low assumptions about CO2 emissions and the 

 fraction airborne were used to generate high and low estimates 

 of the yearly increase in atmospheric CO2. Under the low scen- 

 ario, atmospheric concentrations of CO2 will double by 2085, 

 for the high scenario, by 2055. 



