350 Is The Upper Limit "If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on earth is adapted... CO2 will need to be reduced from its current 385 ppm to at most 350 ppm." Jim Hansen, NASA
Great video, takes about 10 minutes to watch, includes actual photographs, taken through an electron microscope, at a resolution of one atom. If you’re short on time, start the video around 7:30.
But you really ought to take ten and watch the whole thing. This is some truly amazing stuff.