The Kyoto protocol to fight climate change expires in 2012. The shape of a successor treaty is still in doubt, but one aspect seems certain: carbon trading will play a major role. A Financial Times investigation today reveals that carbon markets leave much room for unverifiable manipulation. Taxes are better, partly because they are less vulnerable to such improprieties.

Climate change poses a classic spill-over problem: individuals do not suffer the full burden of producing carbon dioxide, but society does. To equate the private cost to the higher social cost, governments can create markets for carbon, by using tradeable permits, or impose a tax.