People often ask why we should worry about increasing CO2 concentration when the world was full of life back in the time of the dinosaurs when it was much higher. But what is important is not the level itself but its rate of change.
In Mesozoic times the CO2 level was maybe ten times higher than the “historical” level of the million or so years during which we and our current environment evolved, and so back then the world was about 6 to 10 degrees warmer. And of course we (and an ecosystem to support us) could certainly have survived under those conditions. But what the ecosystem (and so many of us) may not be able to survive is a sudden transition to those temperatures – which forces the habitable zones for various species to move faster than the species themselves (especially plants) can move to keep up.