It is one of the greatest political conjuring tricks ever performed on the general public.
It is the fatal flaw at the heart of both the EU and US climate policies which threatens to make a scientifically robust and fair climate deal in Copenhagen impossible.
It is the reason that when the EU says it is cutting greenhouse gases 20 per cent by 2020 or 30 per cent if there is a global deal in Copenhagen, they are in fact putting a cut of just 10 or 15 per cent on the negotiating table. And it is the reason why when the UK says it is cutting greenhouse gases by 80 per cent by 2050 it is actually committing itself to a cut of only 40 per cent within the UK.
This week "The 300-350 Show" takes a look at the creative accounting mechanism which makes this sleight of hand possible - carbon offsetting under the United Nations Clean Development Mechanism - more commonly known at the CDM.