Estimates of the atmospheric concentration of CO 2 , [CO 2 ] atm , for the “hothouse” climate of the early Eocene climatic optimum (EECO) vary for different proxies. Extensive beds of the mineral nahcolite (NaHCO 3 ) in evaporite deposits of the Green River Formation, Piceance Creek Basin, Colorado, USA, previously established [CO 2 ] atm for the EECO to be >1125 ppm by volume (ppm). Here, we present experimental data that revise the sodium carbonate mineral equilibria as a function of [CO 2 ] and temperature. Co-precipitation of nahcolite and halite (NaCl) now establishes a well-constrained lower [CO 2 ] atm limit of 680 ppm for the EECO. Paleotemperature estimates from leaf fossils and fluid inclusions in halite suggest an upper limit for [CO 2 ] atm in the EECO from the nahcolite proxy of ∼1260 ppm. These data support a causal connection between elevated [CO 2 ] atm and early Eocene global warmth, but at significantly lower [CO 2 ] atm than previously thought, which suggests that ancient climates on Earth may have been more sensitive to a doubling of [CO 2 ] atm than is currently assumed.