Every time the PPM count has gone over 400 (and it has many times in the past), the temperatures have been significantly HIGHER than they have been for most of the evolution of modern man. When the dinosaurs died out, the CO2 levels were calculated at 1200 PPM, and the global temperature was 7 C hotter than it is now (which means there were little or no ice caps or glaciers, and Antarctica was green).
That's not the environment in which we evolved and are adapted to live.
Moreover, the current increase in CO2 can't be explained by natural causes, nor solar orbits, nor catastrophic meteor impacts (as they were in the past). There is a direct correlation between the human use of fossil fuels and the concurrent rise of atmospheric CO2 in modern times.
Finally, what happened in the past doesn't make a damn bit of difference with what's happening today. There have been many causes for increases in CO2 in the past. None of those causes are why it's happening NOW. It's irrelevant for anyone to mention the past rises. Climate scientists have repeatedly mentioned them in the past, as have politicians (deniers, all, by the way).
So, what your completely irrelevant wall of text says is squat. It's irrelevant to the situation today. It's been pointed out by scientists, and politicians (the ones who don't hide from reality) that the climate has changed many times on Earth in the past for a multitude of reasons. This time, the reason is US. We're doing the changes to ourselves, and we know this because the other causes aren't happening (yet - if we reach a tipping point to start a natural feedback loop, such as if the methane hydrates start melting, that would completely take away our ability to try to reduce the CO2 concentrations in the air).