A Harvard University study finds that a rise of atmospheric CO2 of about 100 PPM occurred coincident with the end of the last ice age.
100 PPM is also the amount of increase experienced in the last 100 years due to human burning of fossil fuels.
At the end of the last ice age, CO2 rose from about 180 parts per million (ppm) in the atmosphere to about 260; and today we're at 392," explained lead researcher Dr Jeremy Shakun.
"So, in the last 100 years we've gone up about 100 ppm - about the same as at the end of the last ice age, which I think puts it into perspective because it's not a small amount. Rising CO2 at the end of the ice age had a huge effect on global climate.
BBC
So if 180 → 260 ends an ice age, what does 290 → 392 do? And we are not stable at 392; we're on our way to God only knows what level (500 ppm? 700 ppm?).
We are conducting an experiment on the climate using the entire planet as the laboratory. In a reversal of the Heisenberg Uncertainty Principle, we must perforce observe the results of the experiment, because we are part of it, and our observations have not yet resulted in any change to the trajectory of C02.