If humanity wishes to preserve a planet similar to that on which civilization developed, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm.-- from Target Atmospheric CO2: Where Should Humanity Aim? by Hansen et al. It's a draft only, subject to corrections and criticisms.
P.S. 19 March Andy Revkin blogs it here.