re: CO2 feedback mechanisms
I remember reading somewhere that back in the Coal Age O2 levels in the atmosphere were so high due to explosive forest growth that it facilitated the evolution of enormous insects like dragonflies with meter wingspans and foot long cockroaches which simply cannot survive in our atmosphere today. The high O2 regime led to massive forest fires that eventually reached an equilibrium more like what we have today, not to mention less outrageous bugs.