National Geographic: This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge. Imagine that tomorrow, the whole world will stop building things that burn fossil fuels–cars and planes, power plants, and housing tracts. This might sound like an environmentalist’s dream–or a CEO’s nightmare–but it’s a serious question addressed by a new study published in the September 10 issue of Science. If we were to go on using the things we already …