Humanity has a golden opportunity to slow climate change enough to save civilization. As the noted environmentalist Bill McKibben points out in his new book, "Here Comes the Sun," solar energy has finally arrived. Yes, we've been here before. If you're of a certain age, you remember the gasoline shortages of the 1970s and the empty promise by environmentalists that solar energy would save the day. We never ran out of gas, but solar energy had nothing to do with it. What's different now?