350 is an organization founded by environmental activist and writer Bill McKibben, a professor at Middlebury College. The number 350 is the MAXIMUM ppm (parts per million) that CO2 can be in the air for life to be sustained on planet Earth as we know it. Above that it is unsustainable in the long term, and our earth is currently at the dangerous level of 400 ppm! In the 1800's (before the Industrial Revolution) CO2 was only at 280ppm, and we, the Green Goblins, strive to raise awareness for this issue.
Writing out 350 and photographing or filming 350 is what lots of engaged environmental groups are doing to show their awareness of the problem. Now we just have to sequester that carbon instead of putting it in the air!
Check out their website: 350 Website