The global headquarters of the international climate justice campaign 350.org is located on the fourteenth floor of a random building in downtown Oakland, California. Though “global headquarters” might be over-stating things: The offices consist of three rooms with worn carpeting and a collection of reclaimed desks arranged in a Tetris-like pattern. When I visited on a sunny afternoon in mid-September, the place was strangely silent given that the campaigners were just weeks from another worldwide demonstration demanding sharp greenhouse gas reductions. The organizers had already registered 2,700 events in 100 countries scheduled for 10-10-10; by the time the date arrived, they would clock in about 7,300 actions across the globe. Yet the office had none of the war-room frenzy one associates with a political operation in the lead-up to election day. The young and stylish – if rumpled – campaigners were at their desks quietly sending out emails, zipping instant messages, updating blogs, and posting Twitter updates. To my disappointment, there was no map on the wall full of pins marking confirmed actions. No one was on the phone shouting something like, “Get me Bogota!” The only sound was the click of keyboards.

“I wish we could still campaign like that,” Jamie Henn, one of the 350’s organizers said wryly as we sat down at the beat-up wood table that doubles as conference and lunch space. Henn has a disarrayed shock of red hair and dark-frame glasses, which makes him look like the lost member of some art-rock band. He is also, true to type, scary-smart. A second after joking about old-school campaigning, he was holding forth on the state of the environmental movement and theories of social change. “There’s been a sense that’s been missing from the movement about what we are really up against. People use the metaphor a lot of World War II and a World War II-like mobilization. That mobilization didn’t happen because people suddenly got really excited about manufacturing. That mobilization happened because there was a real threat that people felt very personally.”

Well said, I thought. And even more impressive given the fact that Henn, like the rest of the 350.org organizers, is 26 years old. With the exception of a couple contractors hired for the weeks before 10-10-10, the entire 350 staff is half the age of the campaign’s spokesman and (unpaid) figurehead, author-activist Bill McKibben. The folks who brought you what CNN called “the biggest demonstration in history” weren’t old enough to cast a vote in 2000.

The 350 campaigners’ age might make their accomplishments extraordinary, but among environmental organizations their youth isn’t unique. For the first time in a generation, a number of significant green groups are led by people under 40. Phil Radford, who took over Greenpeace USA last year, is 34 years old, as is Phaedra Ellis-Lamkins at Green for All. Erich Pica, the new executive director of Friends of the Earth USA, is 36. Becky Tarbotton, a 38-year-old Canadian, has been tapped to lead Rainforest Action Network. In the clearest sign of a generational shift in leadership, last spring the board of the Sierra Club – the oldest and largest US environmental organization – picked 39-year-old Mike Brune to head the century-old group.

The transfer in leadership away from the Baby Boomers who built today’s environmentalism comes at trying moment for US greens. Despite some minor victories, 2010 has been an annus horribilis for environmentalists. The Senate defeat of even weak legislation to cap greenhouse gas emissions was a body blow. Perhaps more demoralizing was the BP disaster in the Gulf of Mexico: The worst oil spill in US history occurred last summer and Washington, at least, just shrugged.