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Atmospheric CO2 concentration is hypothesized to influence vegetation distribution via tree-grass competition, with higher CO2 concentrations favouring trees. The stable carbon isotope (delta C-13) signature of vegetation is influenced by the relative importance of C-4 plants (including most tropical grasses) and C-3 plants (including nearly all trees), and the degree of stomatal closure - a response to aridity in C-3 plants. Compound-specific delta C-13 analyses of leaf-wax biomarkers in sediment cores of an offshore South Atlantic transect are used here as a record of vegetation changes in subequatorial Africa. These data suggest a large increase in C3 relative to C4 plant dominance after the Last Glacial Maximum. Using a process-based biogeography model that explicitly 10 simulates 13 C discrimination, we show that climate change alone cannot explain the observed shift in δ13 C values. The physiological effect of increasing CO2 concentration is decisive, altering the C3/ C4 balance and bringing the simulated and observed δ 13C values into line. It is concluded that CO2 concentration itself was a key agent of tropical vegetation 15 change during the last glacial-interglacial transition. Two additional inferences follow. First, long-term variations in terrestrial δ13 C values are not simply a proxy for regional rainfall as has sometimes been assumed. Such interpretations need to be re-examined. Second, rising CO2 concentration today is likely to be influencing tree-grass competition in a similar way, and thus contributing to the “woody thickening” observed in savannas worldwide. This second inference points to the importance of experiments to determine how vegetation composition in savannas is likely to be influenced by the continuing rise in CO2 concentration