106 BACASTOW AND KEELING 



an oceanic model with meridional circulation such as proposed by Eriksson 37 



-a q 



and Keeling and Bolin ' might produce a relation between inactive and 

 radioactive carbon in the atmosphere different from that shown in Fig. 5. Thus, 

 without better measurements of the Suess effect and additional model 

 formulations, we cannot demonstrate conclusively that the land biota has 

 increased in mass during the recent past. 



We find in conclusion that, although our calculations lead to some 

 interesting speculations about past variations in ' C and land biomass, we have 

 been unable to determine precisely which reservoir, the oceanic or land plant, 

 removes more industrial C0 2 from the atmosphere. The parameters most critical 

 to this determination, N m0 /N a0 and (3, cannot be fixed within narrow limits on 

 the basis of the data we have used. 



Machta (this volume)' ' further tests the six-reservoir model using more 

 recent ' C data. He does not attempt to fit closely the predicted and observed 

 atmospheric COi increase, but he obviously could do so by adjusting upward the 

 biota growth factor, which he fixes at the value 0.25. He finds that the large 

 observed variations in atmospheric ' C produced by nuclear weapons tests after 

 1954 are best predicted by a model with N m0 /N a0 equal to about 2, in good 

 agreement with our model when adjusted to predict the ' C data before the 

 nuclear era. 



The nuclear era 14 C data used by Machta, although they furnish inde- 

 pendent evidence of the rate of exchange of C0 2 between the atmosphere and 

 the oceans and land biota, are not without their own shortcomings. The useful 

 record is only a few years long, and the weapons produced C unevenly 

 dispersed, even within the atmosphere. The assignment of model parameters is 

 sensitive to unavoidably rough estimates of annual changes in 14 C inventory. 

 Imprecision in the nuclear-era data must produce uncertainty in Machta 's 

 estimate of N m0 /N a0 , but we do not know how much. We suspect that values of 

 N m0 /N a0 as high as our tentative upper limit of 6 will produce acceptable 

 predictions of nuclear-era 14 C variations. If so, the relative importance of the 

 oceans and land biota to industrial COj uptake is still an unsettled question. 



PREFERRED MODELS 



Since we cannot decide from present observational evidence how much 

 increased growth to assign to the land biota, we will examine in detail two 

 possible growth rates. We employ the six-reservoir model and the perturbation 

 coefficients of Eq. C.36 of Appendix C. In the first model we adjust the 

 atmosphere to surface-layer transfer time, T am , and the biota growth factor, j3, 

 to fit precisely: (l)a C0 2 increase in the atmosphere from 1959 to 1969 of 

 2.34% and (2) a C/C ratio of ocean surface water in 1954 relative to standard, 

 R m /R*, of 0.999. The ocean surface layer to atmosphere carbon ratio, 

 N m0 /N a0 , is set equal to 2 and the deep ocean to atmosphere transfer time, 



