Carbon offsets — and emissions-trading schemes, their industrial-scale siblings — are the environmental version of subprime mortgages.

They both started from some admirable premises. Developing countries like China and India need to be recruited into the fight against greenhouse gases. And markets are a better mechanism for change than command and control. But when those big ideas collide with the real world, the result is hand-waving at best, outright scams at worst. Moreover, they give the illusion that something constructive is being done.