Changes in the carbon isotope composition of terrestrial plant tissue (δ13C) are widely cited for evidence of shifts in climate, vegetation, or atmospheric chemistry across a wide range of time scales. A global compilation of δ13C data from fossil leaves and bulk terrestrial organic matter (TOM) spanning the past 30 k.y., however, shows wide variability and no discernable trend. Here we analyze these data in terms of a relative change in net carbon isotope fractionation between the δ13C value of plant tissue and that of atmospheric CO213C = (δ13CCO2 – δ13C)/(1 + δ13C/1000)] and identify a global 2.1‰ shift in leaf and TOM Δ13C that is synchronous with a global rise in pCO2 documented from ice core data. We apply a relationship describing the effect of pCO2 on Δ13C to the global record of Δ13C change documented here to reconstruct pCO2 levels across the past 30 k.y. Our reconstructed pCO2 levels are in excellent agreement with the ice core data and underscore the potential of the global terrestrial δ13C record to serve as an accurate pCO2 proxy.

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