Marine carbonates and organic matter show a sharp increase in their C-13/C-12 isotope ratio at the Cenomanian/Turonian (C/T) boundary(1,2) in the Cretaceous period. This isotopic shift resulted from an increase in the rate of sedimentary burial of C-13-depleted organic carbon in response to the C/T ＇oceanic anoxic event＇(2), The enhanced burial rate should have led to a significant drop in the atmospheric CO2 concentration, which could explain the apparent climate cooling of early Turonian times(2-4). Here we present stable carbon isotope data for specific compounds from terrestrial leaves and marine phytoplankton, and quantify the abruptness and magnitude of the atmospheric CO2 concentration change. Isotope shifts in leaf-wax components extracted from abyssal sediments in the northeastern tropical Atlantic Ocean-the components are wind-delivered from Africa-indicate a sudden change in plant communities of the north African continent. Specifically, the data suggest that plants using the C-3-type photosynthetic pathway were succeeded by plants using the C-4-type pathway. If C-4 plants can outcompete C-3 plants only at atmospheric CO2 concentrations below 500 p.p.m.v. (ref. 5), the observed vegetation change indicates a far larger reduction in C/T CO2 concentration-some 40-80%-than previously suggested(6). The isotopic excursion in the marine phytoplankton compounds is consistent with this estimate. We infer that this dramatic fall in the atmospheric CO2 concentration was abrupt, occurring in just 60,000 years.