New results of radiocarbon concentration in tree rings from the City of Valladolid (Spain) covering a growth period of 22 year have been measured using an AMS. Samples were taken using a hollow drill from a living tree, and α-cellulose was extracted from each of annual rings (early and late wood separately). The set of data shows lower radiocarbon concentration than that reported for “clean air” at the reference station, indicating a remarkable input of “dead” CO 2 of fossil fuel origin. Using data of carbon dioxide and 14C concentrations from Schauinsland, the corresponding summer and winter values of the fossil component ( c f) in carbon dioxide were calculated for the City of Valladolid. By fitting exponential and linear functions to the experimental data, the exchange time was calculated, and the expected future 14C concentration in the atmosphere was estimated.