ATMOSPHERIC CARBON DIOXIDE AND RADIOCARBON: II 87 



C0 2 may be several times as abundant as at present. As a second objective, we 

 wish to determine separately the fractions of industrial CO2 taken up by the 

 oceans and the land biota. For our purposes, the land biota is meant to include 

 all land and freshwater plants and their detritus. Its mass on a dry-weight basis 

 we will call the "land biomass." 



To obtain a realistic model of C0 2 air— sea exchange, we find we must 

 include a nonlinear effect related to acidification of the surface layers of the 

 world's oceans. This effect has not been included in previous models. Deep- 

 ocean water has a large capacity for storing COt , and most industrial CO2 will 

 eventually reside there or in the sediments below. The surface water serves as a 

 barrier, however, and, as it absorbs C0 2 , it becomes more acidic and even more 

 of a barrier. 



Direct measurements of C0 2 in the atmosphere and of the radioactive 

 isotope ' 4 C in dated wood and in ocean water help fix parameters in the model. 

 It turns out that the model predictions agree with observations only if the land 

 biota has recently increased in mass. If correct, this finding is significant 

 because direct measurements of communities of land plants are inadequate to 

 determine worldwide trends in biomass and are even inadequate to determine 

 whether the biomass has increased or shrunk in recent years. 



RESERVOIR EXCHANGE MODEL FOR C0 2 



The pools and pathways of the natural carbon cycle are complex and only 

 partially documented; investigators, when formulating mathematical models, are 

 forced to make simplifying assumptions. Some simplifications are readily 

 acceptable on geochemical grounds; for example, modelists usually consider it 

 safe to ignore such inorganic solid-phase reactions as rock weathering and 

 limestone formation when considering short-term atmospheric perturbations 

 because such reactions are relatively slow. Other simplifications are made to 

 reduce mathematical complexity. For example, if the governing equations for 

 interacting carbon pools are solved by analytical techniques, the number of 

 reservoirs that can be considered at one time is limited by how many 

 simultaneous first-order differential equations the modelist is willing to solve. 

 First-order exchange processes are usually assumed so that the equations are 

 linear. 



Using an analytical approach in modeling industrial C0 2 perturbations, 

 Revelle and Suess 2 considered the spatially averaged atmosphere and world 

 oceans as two reservoirs; Bolin and Eriksson modified this model by dividing 

 the oceans into surface and deep water. Plesset and Dugas, including the humus 

 as a fourth reservoir, calculated the decay of excess l C from nuclear 

 explosions. Keeling 5 worked out the solution of a five-reservoir model with two 

 land-biota reservoirs, and Plesset and Latter 6 worked out the solution of a 

 similar six-reservoir model with the atmosphere divided into the stratosphere and 



