Let me start with the following principle: “Energy is the only universal currency: One of its many forms must be transformed to get anything done.” Economies are just intricate systems set up to do those transformations, and all economically significant energy conversions have (often highly undesirable) environmental impacts. Consequently, as far as the biosphere is concerned, the best anthropogenic energy conversions are those that never take place: No emissions of gases (be they greenhouse or acidifying), no generation of solid or liquid wastes, no destruction of ecosystems. The best way to do this has been to convert energies with higher efficiencies: Without their widespread adoption (be it in large