Three times now, Mark Jacobson has gone out on the same limb. In 2009 he and co-author Mark Delucchi published a cover story in Scientific American that showed how the entire world could get all of its energy--fuel as well as electricity--from wind, water and solar sources by 2030. No coal or oil, no nuclear or natural gas. The tale sounded infeasible--except that Jacobson, from Stanford University, and Delucchi, from the University of California, Davis, calculated just how many hydroelectric dams, wave-energy systems, wind turbines, solar power plants and rooftop photovoltaic installations the world would need to run itself completely on renewable energy.