And the problem is?
All life came rather close to extinction during the Carboniferous and Permian periods… when atmospheric CO2 fell to 180 ppm. Had the levels dropped another 30 ppm, plants would have died, followed by the herbivores, then carnivores. “Life” today would NOT include anything more than bacteria.
Global Temperature and Atmospheric CO2 over Geologic Time
Late Carboniferous to Early Permian time (315 mya — 270 mya) is the only time period in the last 600 million years when both atmospheric CO2 and temperatures were as low as they are today (Quaternary Period ).
Temperature after C.R. Scotese http://www.scotese.com/climate.htm
CO2 after R.A. Berner, 2001 (GEOCARB III)
See an estimate of CO2 concentrations over the past 15,000 years, based on plant stomata.