Since the 19th century, use of chemical methods has provided reliable atmospheric CO2 gas analyses results that have been obtained predominantly from the northern hemisphere. These direct chemical analyses results provide information on past atmospheric CO2 concentrations in addition to the modern direct atmospheric CO2 measurements since 1958 and the indirect reconstructions of past atmospheric CO2 from ice cores. Comprehensive literature indicates that the chemical methods have provided a systematic accuracy within ± 3 Vol% since 1857.