The U.N. climate panel concluded last month that carbon emissions should be capped at a trillion tons, a total the world is rapidly approaching. Now comes the hard part: How will we decide how the remaining emissions are apportioned? The latest report from the Intergovernmental Panel on Climate Change (IPCC) for the first time sets a cap on the amount of carbon emissions we can allow into the atmosphere before calling a complete and permanent halt if, that is, we are serious about keeping global warming below two degrees Celsius. That cap was set last month at a trillion tons of carbon. We are something over half way there, with time fast running out.