Since this is an international agreement, you'd have to tax ALL carbon producers around the world. How you divide that tax revenue between the different countries would then be a political game but much simpler than creating a whole new market for carbon credits. Again, tax the carbon at the source and it won't matter weather it's refined, burned directly, or turned into plastic products, carbon out of the ground will all be taxed at the same rate to discourage it's unearthing. You say I'm oversimplifying a complex issue, but I'd argue that others are making it a complex issue when in fact it's conceptually very simple.