"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, but likely less than that."
Hansen, et al (2008) Target Atmospheric CO2: Where Should Humanity Aim? The Open Atmospheric Science Journal, 2, 217-231 Download (4.0 Mb PDF)

350 is the most important number in the world. It is the safe line for our global climate and a start line for a global movement. Visit www.350.org to take action in your community, engage our world's leaders, and build an international movement to solve the climate crisis. Do whatever it takes. Nothing is more important.