26 MACHTA 



Mauna Loa concentration of about 322 ppM in 1970 as the base. The 

 concentration of C0 2 will be about 385 ppM in the year 2000 and was about 

 290 ppM at the beginning of the industrial area. This latter value is 

 approximately the correct order when compared to the poorly measured data in 

 the latter part of the 19th century. 



The partitioning of fossil-fuel C0 2 molecules among the three reservoirs 

 shows that, as of 1970, about 65% of the fossil-fuel CO2 remained airborne; half 

 of the remainder appears in the oceans, mostly in the mixed layer, and the other 

 half in the biosphere, almost entirely in the long-term land biosphere. This 

 model assigns a surprisingly large fraction of the fossil-fuel C0 2 to the biosphere 

 relative to the oceans. During the period 1958 to 1970, the average airborne 

 fraction of the fossil-fuel C0 2 was between 60 and 65%, which is about 10% 

 more than is suggested by the Mauna Loa and other stations. 



The following are some of the special details of the model. First of all, using 

 the crude estimates provided by George Woodwell, we have assumed that the 

 increase in photosynthesis amounts to 5% for each 10% increase in the carbon 

 dioxide content of the troposphere. However, this is applicable to only one-half 

 of the land biosphere since the other half is assumed nutrient or water limited. 

 This factor (0.5 X 0.5 = 0.25) allows for an increase in the C0 2 content of the 

 biosphere; without it a steady state would exist in the biospheric carbon 

 irrespective of the atmospheric growth. Second is a buffering effect of the 

 oceans. Suppose that the exchangeable carbon mass in the mixed layer (isolated 

 from the deep ocean) equals that of the air. Then for every 11 units of carbon 

 dioxide added to the atmosphere, 10 remain airborne and only 1 goes into the 

 ocean instead of the 50—50 split expected by their carbon masses. However, the 

 1 C0 2 is added in exceedingly low quantities, so that its buffering effect is 

 unimportant and has been neglected. 



SENSITIVITY TESTS 



Sensitivity tests have been performed on various doubtful components of the 

 model. In these tests, however, we must remember that whatever model is 

 chosen must account for the full decrease in the l C0 2 evident in the 

 nontesting period of Fig. 3. Should the biosphere be eliminated, for example, 

 then the oceans must compensate for its omission. Later, when the model is 

 applied to the fossil-fuel C0 2 , the oceans, in our example, will partially fulfill 

 the role of the neglected biosphere. 



In the sensitivity tests the biosphere has been omitted, the NPP doubled, the 

 long-term biosphere altered from a 40- to a 20-year return time, and the growth 

 limitation reduced to 0.05 rather than 0.25. In the oceans the buffering factor 

 has been reduced to 6 and increased to 14 rather than the 10 in the model, and 

 the deep oceans are assumed to be only 400 rather than 1600 years old. The 

 changes in the forecast of the C0 2 concentration in the year 2000 generally lie 



