CHANGES in atmospheric carbon dioxide concentrations in the past may have caused changes in vegetation type1,2, and it has been suggested2 that the isotopic signature of such vegetation shifts, preserved in palaeosols3, might be used as a proxy for past CO2 variations. But the connection between palaeosol isotopic signatures and atmospheric CO2 concentrations has been difficult to establish, partly because of the unreliability of CO2 proxies and partly because of the difficulty in ruling out other potential causes of vegetation changes, such as climate4. Here we present palaeosol carbon isotope ratios that reveal a shift from C4-dominated grasses to C3-dominated shrubs about 7-9 kyr ago on an alluvial fan system in the Chihuahuan desert, New Mexico. This coincides with a rapid increase in atmospheric CO2 concentration recorded in Antarctic ice cores5-8 and increased aridity recorded by geomorphic reconstructions9-11 and packrat remains12. Palaeosol oxy gen isotope ratios, which depend on temperature and moisture, were relatively constant during the vegetation shift, suggesting that the CO2 change, rather than climate, was the dominant cause. We conclude that the carbon isotope ratios of ancient soils can indeed be used as a proxy for past CO2 changes.