OUR understanding of how future changes in atmospheric carbon-dioxide concentrations will affect the global climate system arises in part from comparing past changes in climate and CO2. To date, these comparisons have come mainly from ice-core data, which show a strong correlation-between past atmospheric CO2 concentration and polar temperature1. Here we present a new method for reconstructing atmospheric CO2 concentration using the C-13/C-12 ratio (deltaC-13) in mosses and sedges in peat. Our method exploits the fact that, unlike sedges and most other plants, mosses do not possess stomata, and are therefore unable to regulate their uptake of CO2 and water. The deltaC-13 of mosses thus depends on both atmospheric CO2 concentration and available water, and the of sedges from the same peat can be used to remove the water signal. The method provides a resolution of about a decade much higher than is possible from ice cores. We present initial results for the past 14,000 years, which show three sharp increases in atmospheric CO2 concentration : at 12,800 years ago, corresponding to an episode of warming in the North Atlantic region, 10,000 years ago, corresponding to the end of the Younger Dryas cold period; and 4,400 years ago, after which time modern climates were established globally.