Yes, climate has changed dramatically over geologic time, with global average temperatures way above today's.
*Rate* of change is everything in this discussion.
When climate changes slowly, the biosphere adapts.
When it changes rapidly, there are mass extinctions.
The change we are seeing now is extremely rapid (geologically speaking), and the rate of species loss is unprecedented (due to many human activities, not just burning of fossil fuels).
The list of consequences posted by GUEST ought to be weighed against the consequences of triggering (an already underway?) Anthropocene mass extinction. But it is not just atmospheric carbon dioxide that we should be worried about. It is human overpopulation that is at the root of it all.

(a practicing Earth Scientist)