Hard thing to discuss without some politics.
I learned of it in the 80's in Environmental Economics. (I have a BA in Economics)
The way it works is, you have an umbrella over a region and you Cap the amount of a pollutant (NO2, SO2, etc) that can be generated in that region. Then the firms (or in school we call them polluters) are given their pollution credits that conform to the Cap. Usually there are automatic reductions to the total credits in 5, 10, 15 years.
The CAP portion causes the long term reduction in the region for that pollutant.
Now comes TRADE. Firms have the option to a) ride it out until they are no longer viable with their level of credits, b) modernize to reduce their emissions to meet the new lower caps, or c) BUY the credits on the market and keep on polluting.
This works because the policy goal of reduction is met regardless of the firms choice. All options result in less pollution.
Sounds great, yah? Well, in Southern California the SCAQMD found out that the firms who stay had a windfall. They were selling credits galore because they were already below all the published limits. More often, the A firms would buy for a while until the supply of credits priced them out. Not a lot of evidence of option B being adopted.
In the end, jobs eventually left but the air got cleaner.
So that's an umbrella over SoCal and the jobs that left are in Phoenix, Tucson, etc. where they could fresh build, move existing equipment and escape the rules.
Now some macaroons think Cap and Trade can work nationwide. How can that be? If the natural outcome of the policy is to push industry out, where will they go?