ESS101C
Prof. Jin-Yi Yu
CO2: Past and Future
qCO2 levels were last at the 2xCO2 value near 7 Myr ago.
qCO2 levels were last at the 4xCO2 value at least before 50 Myr ago and possible since the Cretaceous greenhouse world of 100 Myr ago.
(from
Earth’s Climate: Past and Future)