In the perfect world of economic liberals, every commodity has its price. Limited supply makes goods more expensive and vice versa. That's how markets work—at least in theory. In practice, things often look different, and this is especially true when it comes to emissions trading, a business subject to a very different mechanism: laws dictated by the European Union. Economists have generally praised the trading scheme as a nearly ideal instrument for reducing harmful carbon dioxide emissions. … But for the last half year, prices for CO2 certificates have dropped almost continuously, decreasing by about half, to around €8 ($10.60)...