The real challenge is: How do you make low-carbon energy really cheap—as cheap as coal is? I think the reality is that we are not going to get beyond a fossil-fuel economy, and I don’t think we are going to impose big costs on the fossil-fuel economy either in the  or in foreign developing countries like China, until the alternatives become a lot cheaper. I think while it is conceivable to have a carbon tax in the U.S., it will never be high enough to make fossil fuels as expensive as clean energy technologies are today.