A record of atmospheric CO₂ mixing ratios from 1006 A.D. to 1978 A.D. has been produced by analysing the air enclosed in three ice cores from Law Dome, Antarctica. The enclosed air has unparalleled age resolution and extends into recent decades, because of the high rate of snow accumulation at the ice core sites. The CO₂ data overlap with the record from direct atmospheric measurements for up to 20 years. The effects of diffusion in the firn on the CO₂ mixing ratio and age of the ice core air were determined by analyzing air sampled from the surface down to the bubble close-off depth. The uncertainty of the ice core CO₂ mixing ratios is 1.2 ppm (1 σ). Preindustrial CO₂ mixing ratios were in the range 275–284 ppm, with the lower levels during 1550–1800 A.D., probably as a result of colder global climate. Natural CO₂ variations of this magnitude make it inappropriate to refer to a single preindustrial CO₂ level. Major CO₂ growth occurred over the industrial period except during 1935–1945 A.D. when CO₂ mixing ratios stabilized or decreased slightly, probably as a result of natural variations of the carbon cycle on a decadal timescale.