This number in parts per million (ppm) is the current measure of carbon dioxide in the atmosphere.

What's important to know, is that 350 ppm is widely believed by scientists to be the safe upper limit for carbon dioxide in our atmposphere. When atmospheric concentrations of carbon dioxide are above 350 ppm global warming would be dangerously out of control, according to NASA scientist Jim Hansen. In this paper, Hansen and his team assert the following:

If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm.

Currently, our magic number for earth is rising about 2 ppm annually.

For more information, visit 350.org's science page.