We have already blown past the CO2 levels of the Holocene maximum, so looking back to it as a stopping point isn't helpful. The Cretaceous had CO2 levels aroun

We have already blown past the CO2 levels of the Holocene maximum, so looking back to it as a stopping point isn't helpful. The Cretaceous had CO2 levels around 1000ppm so we aren't there yet. We are currently at around 418ppm and rising about 2ppm per year (but that rate is increasing). If we don't change our ways soon this scenario becomes increasingly likely. Also, there is an alternate theory that Humans finally developed agriculture because the climate change had made their previous hunter-gatherer lifestyle untenable and they were forced to improvise. Many species of animal died out.