It will take decades for the transition to renewable energy to play out, and it may never phase out fossil fuels completely. Natural gas, for instance, will be important even late in the century. But with each passing prediction cycle, forecasters are more optimistic about how much global energy will come from renewable sources and how quickly. The rate of consumption for renewable energy is currently growing three times faster than the overall demand for energy. A world in which renewable sources account for, say, one-third of the total energy consumed is now entirely plausible, even likely, within the next two or three decades. The shift is already underway, and it will be no less transformative than the transitions from wood to coal and from coal to oil before it....