ATMOSPHERIC CARBON DIOXIDE AND 

 RADIOCARBON IN THE NATURAL CARBON 

 CYCLE: II. CHANGES FROM A. D. 1700 

 TO 2070 AS DEDUCED FROM 

 A GEOCHEMICAL MODEL 



ROBERT BACASTOW and CHARLES D. KEELING 



Scripps Institution of Oceanography, University of California at San Diego, 



La Jolla, California 



ABSTRACT 



A nonlinear geochemical model of the interaction of atmospheric C0 2 with the oceans and 

 land biota has been constructed to predict future changes in atmospheric C0 2 concentration 

 in the next century. If production of C0 2 from fossil fuels continues, the perturbations 

 from preindustrial times may become so large that a linear model is unrealistic, especially 

 because it fails to take into account that ocean surface water will become progressively more 

 acid and less able to absorb each new increment of industrial C0 2 . On the assumption that 

 industrial C0 2 production continues to increase at the rate of the past 20 years and that the 

 ultimate increase in biomass of the land biota is no more than twice the present biomass, the 

 atmospheric C0 2 concentration will reach a value six to eight times the preindustrial value 

 in 100 years. 



When the radiocarbon concentration in dated wood and the recent atmospheric increase 

 in C0 2 are compared in the context of the model, it appears that the land biomass has 

 increased 1 to 3% since the beginning of the industrial era. This calculation takes account of 

 the actual year to year variations in industrial C0 2 production and the heliomagnetic 

 variation in radiocarbon production in the stratosphere. The inferred biomass increase, 

 presumably owing to C0 2 fertilization, is too small to be verified by direct observation and 

 is not considered to be established. On the other hand, the trend in atmospheric C0 2 

 apparently rules out any large recent change in biomass. 



Combustion of fossil fuels (coal, petroleum, and natural gas) is adding 

 increasing amounts of carbon dioxide (C0 2 ) to the atmosphere each year. The 

 fate of this C0 2 attracts interest because a sustained increase might modify the 

 earth's climate through the "greenhouse" effect 1 and because, from the effects 

 of this input, we may learn more about the earth's carbon cycle. 



We have constructed a geochemical model of the natural reservoirs into 

 which industrial CO2 can mix. Our first objective is to predict the C0 2 

 concentration in the atmosphere during the next century, when atmospheric 



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