Opportunity cost provides the best way to think about the recent decision to reject a new coal mine at Rocky Hill. That’s the central theme of my latest piece in Inside Story and also of my forthcoming book, Economics in Two Lessons. Given a tight budget of the carbon dioxide and other greenhouse gases we can afford to emit while stabilizing the global climate, every new source of emissions comes at the opportunity cost of an existing source. Since phasing out coal is among the cheapest options any new coal mine comes at the expense of faster closure of existing mines. Almost invariably, its more costly to open a new mine, than to maintain production at an existing mine. So we should not start any new mines and, in general, not expand old ones.