Elevated Eocene atmospheric CO2 and its subsequent decline

Quantification of the atmospheric concentration of CO2 (CO2]atm) during warm periods of Earth's history is important because burning of fossil fuels may produce future CO2]atm approaching 1000 parts per million by volume (ppm). The early Eocene (~56 to 49 million years ago) had the highest prolonged global temperatures of the past 65 million years. High Eocene CO2]atm is established from sodium carbonate minerals formed in saline lakes and preserved in the Green River Formation, western United States. Coprecipitation of nahcolite (NaHCO3) and halite (NaCl) from surface waters in contact with the atmosphere indicates CO2]atm > 1125 ppm (four times preindustrial concentrations), which confirms that high CO2]atm coincided with Eocene warmth.