I was introduced to emissions trading way back in law school nearly 30 years ago by Prof. Jim Krier, who was a big fan of it because he wanted markets to carry the load of distributing emissions.  My thought at the time was that, while it may have looked good in an ivory tower, there was no way it wouldn't turn into a broker's shell game.  The trading units would end up sliced and diced in wholly arbitrary ways designed to line the brokers' pockets.