The Cretaceous is a period in ancient earth that ended about 65 million years ago. The Eocene was a period in Earth's past about 45 million years ago. According to research examining geological sediments, how high did atmospheric concentrations of carbon dioxide reach in these periods?

The atmospheric concentrations of CO2 during Earth's past have always been changing, sometimes slowly, sometimes abruptly. The changes continue to the present day, and will continue in the future. The reasons for the oscillations in the CO2 levels are numerous, but while in the past they have been exclusively from natural causes, in modern times it is also the humans that got involved in it. The Cretaceous period had 1000 ppm of CO2, while the Eocene had 560 ppm of CO2. In the present, the CO2 concentrations are 405 ppm, thus significantly lower than the Cretaceous and Eocene. What can be learned from these two periods though is that such high concentrations of CO2 result in much warmer global climates, with ice sheets lacking, and the temperatures being much more equalized across the latitudes. If the current trend continues, there is every chance that our planet will experience such changes in the near future.