First of all, it's not necessarily tax we are talking about but price. You can create a price by various methods, either through a tax or through a constraint. If you constrain carbon through a cap or a regulated target, the price will go up because carbon becomes a commodity. So you don't necessarily have to tax carbon to raise the price of it.
Some of the studies that have been done, looking at $30-a-tonne pricing, show that you would get a whole bunch of efficiency measures, a whole bunch of industrial reductions, with $30 per tonne. You increase the price, through whatever method, and you make additional measures, additional reduction opportunities, more cost-effective, more viable.