Larry Elliot, the Guardian's economics editor, is commissioned by Alan Rusbridger to do something no economist has yet managed; to figure out the maths of keeping it in the ground. What would happen to the global economy if we turned away from fossil fuels? Would we need to accept a voluntary global recession on a scale never before witnessed? Would we have to turn away from the idea of growth altogether? Larry gets out his pencil, undaunted, and starts doing the sums.