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of CO2 increases, the fraction retained in the atmosphere 

 will also increase. Consequently, using a constant fraction 

 airborne is a very conservative assumption. 



The number chosen for the low estimate was 53%, the best 

 estimate of the historical fraction airborne.^/ A lower number 

 could have been chosen, based on the work of Woodwell et al., 

 which states that past estimates of the biosphere's contribution 

 were too low and thus caused fraction airborne estimates to be 

 too high. ~JJ 



Lugo and other biologists, along with Broecker and other 

 oceanographers , however, have challenged the Woodwell estimates.^/ 

 Broecker has shown that the oceans do not have the physical 

 capacity to absorb CO2 contributions from the biosphere as quickly 

 as Woodwell had originally estimated, and Lugo has shown that 

 Woodwell underestimated tropical forest regrowth, thereby over- 

 estimating net forest contributions. 



Although it is a strong challenge to the Woodwell estimate, 

 the rebuttal is not absolutely conclusive. Nevertheless, because 

 all carbon cycle models show fraction airborne increasing over 

 time, use of 53 percent probably constitutes a very conservative 

 estimate of retained CO2 emissions. The ORNL model modifies 

 the fraction airborne over time by taking into account water 

 heating and changes in ocean chemistry as well as temperature- 

 sensitive changes in carbon flows among all compartments. 



