Here under the harsh blue lights, in a cavernous ballroom of the Walter E. Washington Convention Center in Washington, D.C., people are celebrating the clean-energy revolution with their hips. It’s April 16, 2011, the second night of Power Shift, a biennial climate conference best described as a sort of TED summit for young climate activists. Whether you’re a Greenpeace organizer or a recruiter for an environmental master’s program, Power Shift is the place to trade ideas and phone numbers. Many of the attendees are undergrads, few are older than 30, and all are shaking it to a soundtrack of electro-pop and Ryan Adams. A lot of them wear their beliefs on their chests. One T-shirt reads iMATTER. Some are more colloquial: FUCK COAL, FUCK STRIP MINING.

Stage lights cue the next speaker. He’s big—35 pounds heavier than the first time I saw him talk, about a year ago—and he wears an odd piece of flair: a small orange scarf. His shaved head gleams under the lights. The crowd erupts; he smiles and speaks softly.

“Thank you for that very nice welcome,” he says. “I wish I could give you an equally nice speech. I wish I could say something really nice today that would make everyone feel really good. But sometimes the truth isn’t very nice, and it needs to be said anyway. … The truth that our movement has not been willing to talk about is that it’s probably too late for any amount of emissions reductions to prevent the collapse of our industrial civilization.”

Silence. The revolution has been paused.

Tim DeChristopher, 30, is one of Power Shift’s keynote speakers, following the author Bill McKibben; Lisa Jackson, the head of the Environmental Protection Agency; Van Jones, President Obama’s deposed green-jobs guru, now a senior fellow at the Center for American Progress; Josh Fox, the director of the Emmy-winning documentary Gasland, an investigation into the impacts of natural-gas drilling; and Al Gore. DeChristopher has not written an...