ATMOSPHERIC CARBON DIOXIDE AND 

 RADIOCARBON IN THE NATURAL CARBON 

 CYCLE: I. QUANTITATIVE DEDUCTIONS 

 FROM RECORDS AT MAUNA LOA 

 OBSERVATORY AND AT THE SOUTH POLE 



CARL A. EKDAHL and CHARLES D. KEELING 



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



La Jolla, California 



ABSTRACT 



The concentration of atmospheric C0 2 in the Northern Hemisphere increased by 6.8 ppM 

 from 1959 to 1969, according to a long series of measurements at the Mauna Loa 

 Observatory, Hawaii. In the same period the Southern Hemisphere concentration increased 

 by 6.7 ppM, according to an even longer series of measurements of South Polar air. These 

 increases, when averaged, correspond to 2.34% of the presumed preindustrial C0 2 

 concentration of 290 ppM and imply that approximately 49% of the contemporary 

 production of C0 2 from combustion of fossil fuel and kilning of limestone remained 

 airborne. The approximately 51% not remaining airborne was evidently incorporated into 

 the oceans and land biota. To learn more about this uptake, we have taken into account 

 atmospheric radiocarbon ( 14 C) variations as recorded in tree rings, and to interpret this 

 record we have examined a series of geochemical models of the reservoirs into which C0 2 

 and ' 4 C can mix. 



We established the essential character of these models by evaluating an analytically 

 derived atmospheric transfer function for a wide range of linear-model parameters. For a 

 purely oscillatory ' 4 C source, this function gives the attenuation and phase shift of ' 4 C0 2 

 in the lower atmosphere; for a purely exponentially increasing C0 2 source, it gives the 

 fraction of C0 2 remaining airborne or in the lower atmosphere. In both cases the source is 

 assumed to have operated until initial transient variations have died out. For reasonable 

 reservoir sizes and transfer times, the stratospheric (heliomagnetic) variation in ' 4 C0 2 , as 

 estimated from direct observations of cosmic rays and sunspot numbers, is strongly 

 attenuated in the lower atmosphere, whereas the predicted airborne fraction agrees with the 

 earlier implied value of 49% (42% for the lower atmosphere) only if the carbon pool of the 

 land biota grows appreciably. These models, with nonlinear corrections, are used in the 

 paper following (part II of this study) to predict year-by-year simultaneous variations in 

 atmospheric C0 2 and ' 4 C0 2 . 



INTRODUCTION 



A substantial portion of the carbon dioxide (COt ) produced since the beginning 

 of the industrial era by the combustion of fossil fuel (coal, petroleum, and 



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