Evidence from polar ice cores for the increase in atmospheric CO2 in the past two centuries

Precise and continuous measurements of atmospheric CO2 concentration were first begun in 1958 and show a clear increase from 315 parts per million by volume (p.p.m.v.)1 then to 345 p.p.m.v. now. A detailed knowledge of the CO2 increase since preindustrial time is a prerequisite for understanding several aspects of the role of CO2, such as the contribution of biomass burning to the CO2 increase and the sensitivity of climate to the CO2 concentration in the atmosphere. Estimates of the preindustrial CO2 concentration are in the range 250–290 p.p.m.v. (ref. 2), but the precise level then and the time dependence of the increase to the present levels remain obscure. The most reliable assessment of the ancient atmospheric CO2 concentration is derived from measurements of air occluded in ice cores. An ice core from Siple Station (West Antartica) that allows determination of the enclosed gas concentration with very good time resolution has recently become available. We report here measurements of this core which now allow us to trace the development of the atmospheric CO2 from a period overlapping the Mauna Loa record back over the past two centuries.