Which sort of begs the question:
If not temperature fluctuations, then what did cause all those prehistoric CO2 increases? And subsequent decreases?
Actually, the chart would seem to indicate that temperature influences CO2 concentrations, not the other way around. So it would seem that higher temperatures allow higher CO2 concentrations. It may also be that higher average temperatures support larger populations of animals and humans, all of which emit more CO2, but the population increase lags behind the temperature increase by a few years. As temperatures cool down, more deaths of the weak and infirm occur, lowering the populations and decreasing the CO2 output.