Why Bill McKibben thinks solar energy could save the world

For decades Bill McKibben has been warning the world about the risks of climate change. But his latest book is surprisingly hopeful, even if he does think it's too late to save the world from climate change. "Here Comes The Sun" documents the remarkable growth of solar power — and the dramatic drop in its cost. He joins us for a conversation about where the solar revolution is going — and what keeps him hopeful about the future of humanity.