Bill McKibben
It began with a simple number: 350. That's the level, in parts per million, of carbon dioxide in the atmosphere that some scientists believe is the only safe amount for the climate. (We're currently at 392 ppm, and headed upwards fast.) McKibben — a longtime environment writer — founded the group 350.org in 2007 to focus attention on climate change. It hasn't managed to change policy yet, but in 2011 McKibben achieved his biggest concrete victory yet, marshaling popular resistance to the proposed Keystone XL pipeline, which would carry crude from Canadian oil sands to the U.S. Oil sands have a bigger carbon footprint than conventional crude, and McKibben and his allies argued that beginning importation into the U.S. would mean game over for the climate. Analysts considered the approval of the pipeline a certainty, but weeks of protests in front of the White House helped change President Obama's mind-delaying a decision on Keystone XL until 2013.