350

http://www.350.org/

And what does this 350 number even mean?

350 is the number that leading scientists say is the safe upper limit for carbon dioxide—measured in "Parts Per Million" in our atmosphere. 350 PPM—it's the number humanity needs to get back to as soon as possible to avoid runaway climate change.

Where did this 350 number come from?

Dr. James Hansen, of NASA, the United States' space agency, has been researching global warming longer than just about anyone else. He was the first to publicly testify before the U.S. Congress, in June of 1988, that global warming was real. He and his colleagues have used both real-world observation, computer simulation, and mountains of data about ancient climates to calculate what constitutes dangerous quantities of carbon in the atmosphere. The Bush Administration has tried to keep Hansen and his team from speaking publicly, but their analysis has been widely praised by other scientists, and by experts like Nobel Prize winner Al Gore. 

If we're already past 350, are we all doomed?

No. We're like the patient that goes to the doctor and learns he's overweight, or his cholesterol is too high. He doesn't die immediately—but until he changes his lifestyle and gets back down to the safe zone, he's at more risk for heart attack or stroke. The planet is in its danger zone because we've poured too much carbon into the atmosphere, and we're starting to see signs of real trouble: melting ice caps, rapidly spreading drought. We need to scramble back as quickly as we can to safety.