Globe cooled when CO2 was Eight Times current concentration.

In 2007, James Hansen et al, in a paper

Target atmospheric CO2: Where should humanity aim? wrote

 If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO₂  will need to be reduced from its current 385 ppm to at most 350 ppm. 

Journal reference:	Open Atmos. Sci. J. (2008), vol. 2, pp. 217-231
DOI:	10.2174/1874282300802010217

Oh, NO! The planet is doomed if atmospheric CO2 rises above the 350ppm mark. Tipping point alert!

The alarmist organisation 350.org led by Bill McKibbin says:

Right now we're at 400 ppm, and we're adding 2 ppm of carbon dioxide to the atmosphere every year. Unless we are able to rapidly turn that around and return to below 350 ppm this century, we risk triggering tipping points and irreversible impacts that could send climate change spinning truly beyond our control.

Source : Nature Communications
To find appropriate species, we choose two regions with similar climate, one from a simulation
 with today’s climatic and CO2forcing and another from an Ordovician simulation with a
high atmospheric CO2 value of 24 PAL. 

AND YET, in a paper published this month, we learn that in the Late Ordovician (450 Ma) CO2