"One of the reasons I think the population question is important, if we want to be as green as possible, any of our energy that is truly renewable is limited. Solar and wind are intermittent and they're so diffuse, it's difficult to harness them in a significant way. But one thing we could be doing is making it a law (like it is in Israel and Cyprus) to take every building eight stories or under and heat all of the water in those buildings with solar energy. It's absolutely simple and cheap technology."