Let's stretch our imaginations for a moment and assume that the left is right that global warming will bring apocalyptic warming by the end of the century and that the only way to save the planet from extinction is to stop using fossil fuels right now. That will be a spectacular disruption to world economic prosperity, because cheap fossil fuels account for about two-thirds of all electric power generation and at least 80 percent of transportation fuel. But if we did stop using fossil fuels, what would make the most sense as a mass-scale substitute to coal, natural gas and...