On the list of paradoxes, not many beat flying around the world to give talks about climate change. I can justify my carbon footprint if I have to--with myself as the main moving part, we've managed to build 350.org into the first big global climate campaign, organizing what CNN has called the "most widespread...political activity in the planet's history." If you want to be active in every country on earth except North Korea, you better be prepared to fly. But I'd rather not justify it, and I'd rather not fly. So in recent years I've learned to tell an increasing number of the people who ask...