But markets normally work fine in many other areas – why not in this case?
Because markets are used to balance supply and demand. In this case there is nothing to balance – it isn’t important that we reduce the emissions to exactly 80% and not to 85% or 70%. Preferably the emissions should have been reduced to zero 20 years ago. The reason we have not done so already is that reducing emissions is costly – in money and in inconvenience. By agreeing upon a tax rate for emissions we would be agreeing upon how much we are willing to spend on reductions instead – which is the important part! And a tax system is simpler, more predictable and easier to implement than a Cap-and-Trade system.