Adds material to the CO2 lifecycle that was not there earlier? Isn't it just rereleasing CO2 that was in the atmosphere earlier?
Humans, and much of the life we see around us, evolved in the subsequent periods of higher oxygen and depleted CO2. As we reverse that effect, we return the atmosphere nearer to how it once was - when humans and such weren't around. The planet and its life will be fine. Us and the multiple species on which we depend may well not be.
By burning it we are releasing carbon that has been stored for millennia and accumulated over millennia. It's introduced as though it were new into our carbon cycle.
I don't know the facts here, but I'm pretty sure carbon can slowly be added from other sources. So sure, that CO2 might have been in the atmosphere before, but then we added more CO2 as well.
The point being, the CO2 we're digging up wasn't in our atmosphere before. The atmosphere that we and all the animal and plants have been adapted too. Sure, there's a time when the CO2 concentration (and even O2) was wildly different. But then there were other animals living on earth, that was adapted to those conditions. Slow change is fine, rapid change is disastrous.
Yes, and the Arctic was once tropical.