Books on the global energy system are replete with utopian visions: the nuclear future, the solar future, the efficiency future, the hydrogen future, and the “small-scale energy technologies are beautiful” future. All too often, these visions are described in detail, but the path for getting there is left vague. This reminds me of the cartoon of two mathematicians at the blackboard gazing in satisfaction at a complex set of equations on the left, the simple elegant solution on the right, and an incomprehensible jumble of equations and symbols in the middle to link both sides, all of which are crossed out except for the statement “somewhere about here a miracle happens.”

I too have presented a vision of a future, desirable energy system. In developing this vision, however, I have taken into account key real-world constraints on the potential for shifting away from current trends. These include the strong penchant for humans to use substantially more energy as population and wealth increases, public attitudes to extreme event risks that are especially challenging for nuclear power, wide-ranging concerns for the geopolitical risks associated with oil import dependence or the global spread of nuclear weapons, the difficulties of attaining a rapid scale-up of modern renewables-based technologies, and the path dependence advantages of fossil fuels. In combining these constraints in a choice evaluation that involves both prescription and prediction, I have outlined the energy forms, technologies, costs and international developments for achieving a more sustainable energy path.