The significance of a 400 year (1550--1950 AD) 13C/12C chronology from a juniper from the Sinai Peninsula has been reassessed. A calibration of the isotope data covering the period 1740--1950 was carried out using the 13C/12C of CO2 trapped in ice cores and a carbon isotope fractionation model. The calibration indicated a surprising constancy of Ci/Ca (concentration of CO2 in the leaf intercellular space to that in the atmosphere) during this time period. Using this procedure of calibration, when the pre-1740 tree ring record was examined in terms of the 13C/12C of the atmospheric CO2, significant variations were indicated. These variations, comparable to that imposed by anthropogenic causes occurred between 1700 and 1850 AD. Obviously, such variations in the 13C/12C of atmospheric carbondioxide is unlikely, but rather these should reflect variations in the Ci/Ca ratio. Interestingly, the atmospheric 13C/12C variations deduced from this tree ring record parallels the 14C variations in the atmosphere during the same period. Since the 14C variations (at least prior to 1850 AD) have been linked to solar activities and by implication to climate, covariation between 13C/12C and 14C supports the hypothesis that the marked shifts in 13C/12C prior to 1850 resulted from a climate-induced perturbation in the Ci/Ca ratio. The combined calibration procedure and comparison with 14C provides a more useful approach to interpret 13C/12C time series in trees. ¿ American Geophysical Union 1996