Department of Earth System Science

We assumed that the tree-ring-based ¹³C/¹²C record constructed by Freyer and Belacy (1983) to be representative of the fossil fuel and forest-soil induced ¹³C/¹²C change for atmospheric CO₂. Through the use of a modification of the Oeschger et al. ocean model, we have computed the contribution of the combustion of coal, oil, and natural gas to this observed ¹³C/¹²C change. A large residual remains when the tree-ring-based record is corrected for the contribution of fossil fuel CO₂. A deconvolution was performed on this residual to determine the time history and magnitude of the forest-soil reservoir changes over the past 150 years. Several important conclusions were reached. (1) The magnitude of the integrated CO₂ input from these sources was about 1.6 times that from fossil fuels. (2) The forest-soil contribution reached a broad maximum centered at about 1900. (3) Over the 2 decade period covered by the Mauna Loa atmospheric CO₂content record, the input from forests and soils was about 30% that from fossil fuels. (4) The ¹³C/¹²C trend over the last 20 years was dominated by the input of fossil fuel CO₂. (5) The forest-soil release did not contribute significantly to the secular increase in atmospheric CO₂ observed over the last 20 years. (6) The pre-1850 atmospheric  values must have been in the range 245 to 270×10⁻⁶atmospheres.