sorry, what? A tax is simple, like a sales tax or VAT, and there is a strong incentive to collect it at a centralized source that is pretty easy to monitor.

How is that complex vs. creating a new commodity in the form of carbon credits, and checking that the emission is backed up by credits?

At best, cap-and-trade is equivalent to a carbon tax plus some corporate welfare to return the tax to the polluters.

In the most likely scenario, it's an opportunity for rent-seeking politicians to dole out the credits to those who either currently pollute most or just have their ear, for worst current polluters to cash in on opportunity to reduce pollution, for financiers to make big bucks trading credits.

At worst, it's a fraud where people will buy credits from people who don't reduce pollution or would have done so anyway.