Back in, oh I dunno, 2007 or 2008, I was taking a class on Portuguese at a local community college, and in that class was a guy who worked for one of the major investment banks, I forget which. He was learning Portuguese because his job was to go to Brazil to negotiate long term land leases with the government on behalf of big oil.
The scam was that the oil company would buy a long-term lease on unusable land "to prevent development," and for that they could get a carbon credit to sell. Of course, nobody was ever going to develop that land, and there was never going to be any carbon emissions from that development, so absolutely no emissions were prevented, and absolutely no additional carbon was captured, but big oil still got a nice juicy carbon credit to sell on the carbon offset markets.