¹³C/¹²C ratios in tree rings and the transfer of biospheric carbon to the atmosphere

The δ 13 C records of 11 Pacific coastal trees have been measured. Relationships between δ 13 C, ring area, and ring width were investigated. Three of the six trees with records longer than 200 years yield a significant (2σ level or better) correlation between δ 13 C and ring area. The correlation is most pronounced for Sequoia giganteum , where the variability in ring area accounts for 60% of the δ 13 C variance. The environmental factors influencing the isotopic composition of trees are many. A major portion of the pre‐1850 variability in the δ 13 C record is most likely caused by variable isotope fractionation of the trees. Only about one quarter of the δ 13 C variance in the average record can be removed by normalizing on ring area. The remaining δ 13 C variability results in a standard deviation of 16 ppm in model‐calculated preindustrial CO 2 levels. This is an upper limit for atmospheric CO 2 variability induced by changes in the size of the biospheric reservoir. Average preindustrial CO 2 levels are estimated to be in the 260–290 ppm range. The cumulative amount of biospheric carbon released to the atmosphere between 1600 and 1975 is approximately 150±100 gigatons C.