The most straightforward path would be if we could bring the cost of solar electric and wind down by another factor of say, three, and then have some miraculous storage solution, so that not only over the 24-hour day but over long periods of time where the wind doesn't blow, you have reliable energy. That's a path. But energy storage is hard. That's not a guaranteed path.

Living on $6 a day means you have a refrigerator, a TV, a cell phone, your children can go to school. That's not possible on $1 a day.

So we do software for watches, for phones, for TV sets, for cars. And some of these take a long time to catch on.

Depending on how quickly you get ocean rise, you have people who live in river deltas [at risk].

I think it makes sense to believe in God, but exactly what decision in your life you make differently because of it, I don't know.

Taking a look back, one big reqret is, I left Harvard with no real awareness of the awful inequities in the world. The appalling disparities of health and wealth and opportunity that condemned millions of people to the lives of despair. I learned a lot here at Harvard about new ideas and economics, and politics. I got great exposure to the advances being made in the sciences. But humanities greatest advances are not in its discoveries, but in how those discoveries are applied to reduce inequity.

I don't have any TVs with their over-the-air receivers connected in my house. But when I'm in a hotel room or other places that have a TV, then I turn it on and flip the channels just like everybody else. I'm not immune to the lures of television. I just try to stay away from it because I like to read.