The most important number on the planet is 350, as in 350 parts carbon dioxide for every million parts of the Earth’s atmosphere. The number comes from a 2008 scientific paper by James Hansen and nine other scientists in three countries:

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.

CO2Now.org | Humanity’s Target for Atmospheric CO2

Here is the way that acclaimed author Bill McKibben introduced the most important number on December 28, 2007, the first media article about “350”:

...what may turn out to be the most crucial development went largely unnoticed. It happened at an academic conclave in San Francisco. A NASA scientist named James Hansen offered a simple, straightforward and mind-blowing bottom line for the planet: 350, as in parts per million carbon dioxide in the atmosphere. It's a number that may make what happened in Washington and Bali seem quaint and nearly irrelevant. It's the number that may define our future.

Washington Post | Remember This: 350 Parts Per Million

Five months later, Bill McKibben went on to dub 350 as the most important number on Earth:

All of a sudden it isn't morning in America, it's dusk on planet Earth. There's a number -- a new number -- that makes this point most powerfully. It may now be the most important number on Earth: 350. As in parts per million of carbon dioxide in the atmosphere.

LA Times | Civilization’s last chance