When recording historic levels of CO₂ concentration, records from air bubbles in Antarctic ice cores are regarded as the gold standard for paleo-atmospheric global CO₂ concentrations during past interglacial and glacial periods over the last 800,000 years.  Antarctic ice-core data are openly available from the National Centers for Environmental Information (NCEI) and have been studied extensively. The trapped gas in ice is the most direct measurement of CO₂ concentrations for past atmospheric records, providing a continuous global CO₂ baseline.

Another method of recording CO₂ concentrations is from plant stomata. This method, however, provides indirect proxy measurements consisting of discontinuous records over the Holocene and deglaciation period. The highly variable CO₂ concentrations have high uncertainties in measurements as well as calibration models. Plant stomata studies are mostly from the Northern Hemisphere where local conditions can strongly influence the resulting CO₂ proxy estimates. The uncertainties and shortcomings associated with plant stomata CO₂ reconstructions outweigh using this dataset as a valid quantitative indicator for paleo-atmospheric global CO₂ concentrations.

Read the full report by Renee Hannon here:

Measurement of CO2 Concentrations Through Time