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.The article sparked a spirited debate on our web site, and it also sparked a larger debate between forward-looking energy planners and those who would rather preserve the status quo. The duo went on to publish a detailed study in the journal Energy Policy that also called out numbers for a U.S. strategy.