Something most cap and trade proponents don’t seem to get: The cap isn’t really a cap at all.
While the cap in cap-and-trade is entirely virtual, realized only through pricing or administrative actions like penalties, the concrete image of a placing a physical object like a cap over emissions reassures that there is a limit to emissions somewhere. Yet without both a price for emissions and some pretty daunting penalties, the cap hovers in the air without any foundation at all; it remains simply a well-meaning “climate pledge.”
There is no more certainty in emission levels from a cap and trade system then from a carbon tax, unless one implements draconian penalties for emitting without a permit. Given that no one is willing to do such a thing the cap becomes little more than a well-meaning climate pledge. It certainly isn’t a cap, at least not by any normal definition of the term.