The "Safety Valve"

My friend M. Yglesias calls me cunning for linking to an article bashing the concept of a "safety valve" in a carbon emissions cap-n-trade system. Just to be clear: I don't have an opinion on whether a safety valve is a good idea or whether it's a way for the stakeholders to avoid acting against self-interest. If the safety valve allows for unlimited additional pollution permits and if the trigger price is too low, then, yes, the cap-n-trade system won't work as well as advertised. But there's a lot of unknown unknowns.