Diminishing Impact of CO2 on climate with Increasing Concentration

So according to some studies from last century it's been known for a long time that with increasing concentration of CO2 in the atmosphere, its impact on the climate diminish exponentially. In other words... going back to pre-industrial levels of CO2 in the atmosphere will only have a minimal impact on global warming. 

Also... if CO2 concentration falls below a certain threshold, plants stop growing which basically means the end for life on earth. Makes sense since The C in CO2 is what plants use to turn light into matter.

According to geological studies CO2 concentration in the distant past has been a lot higher than it is now. To the tune of thousands of parts per million... and evidently there was no runaway climate effect since life flourished during those periods...