The following excerpts are from Target Atmospheric CO2: Where Should Humanity Aim?, published in the Open Atmosphere Science Journal by James Hansen et al in 2008. 

Excerpt:

A CO2 amount of order 450 ppm or larger, if long maintained, would push Earth toward the ice-free state. Although ocean and ice sheet inertia limit the rate of climate change, such a CO2 level likely would cause the passing of climate tipping points and initiate dynamic responses that could be out of humanity’s control.