Studies of past climate changes suggest the land and oceans start releasing more CO2 than they absorb as the planet warms. (newscientist.com)
Studies of past climates usually point to the high end of this range, as do climate models. (newscientist.com)
Geological evidence from studies of past climate change now suggests that if longer term factors are taken into account, such as the decay of large ice sheets and the operation of the full carbon cycle, the sensitivity of the Earth to a doubling of CO2 could be double that predicted by most climate models. (reddit.com)