Tradable emissions permits are a popular, but inferior, way to tackle global warming THE pressure for political action on climate change has never looked stronger. Even George Bush has now joined the leaders of other rich countries in their quest to negotiate a successor regime to the Kyoto protocol, the treaty on curbing greenhouse gases that expires in 2012. Too bad, then, that politicians seem set on a second-best route to a greener world. That is the path of cap-and-trade, where the quantity of emissions is limited (the cap) and the right to emit is distributed through a system of tradable permits. The original Kyoto treaty set up such a mechanism and its signatories are keen to expand it. The main market-based alternative-a carbon tax