The Δ13C records of 11 Pacific coastal trees have been measured. Relationships between Δ13C, ring area, and ring width were investigated. Three of the six trees with records longer than 200 years yield a significant (2&sgr; level or better) correlation between Δ13C and ring area. The correlation is most pronounced for Sequola giganteum, where the variability in ring area accounts for 60% of the Δ13C variance. The environmental factors influencing the isotopic composition of trees are many. A major portion of the pre-1850 variability in the Δ13C record is most likely caused by variable isotope fractionation of the trees. Only about one quarter of the Δ13C variance in the average record can be removed by normalizing on ring area. The remaining Δ13C variability results in a standard deviation of 16 ppm in model-calculated preindustrial CO2 levels. This is an upper limit for atmospheric CO2 variability induced by changes in the size of the biospheric reservoir. Average preindustrial CO2 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.<