Total CO2 exposure levels in a naturally enriched site (Lajatico, Italy) were reconstructed using radiocarbon analysis by accelerator mass spectrometry combined with dendrochronological analysis on wood cores extracted from trees grown in the fossil CO2 source proximity. Over 3 decades (1964-1998), the data show a mean CO2 concentration in the atmosphere of 650 ppm, about twice the current concentration in atmosphere, with a maximum around 1980. © 2005 by the Arizona Board of Regents on behalf of the University of Arizona.