content: Question: could periods of previous high levels of Co2 in the atmosphere produce this same effect?   Carbon dating is only good to about 50,000 years ago. Beyond that we use other radioisotopes. CO2 levels in the last 50k years only rose above 250ppm after the industrial revolution. Even if they were, however, it wouldn't matter. What's important is where the carbon came from. Atmospheric CO2 would not have come from burning fossil fuels in the past, so you don't have the problem of releasing "old" carbon, even if CO2 levels were abnormally high.