Shell Energy ad for shale gas in Atlantic Monthly Magazine backfires

An advertising from Shell Energy in the July-August 2012 issue of the Atlantic Monthly Magazine promotes development of shale gas in the U.S. (from newly developed horizontal shale gas drilling techniques) as follows: 1) Shale gas will help the environment because it produces half as much CO2 emissions per unit of electricity produced compared to coal, and 2) the U.S. has a 250-year supply of this gas.
   The problem with this ad is that it gives you the evidence you need to argue against the environmental merits of shale gas, within the very text of the ad itself.  If the supply is 250 years, and the emissions are 50% of the emissions from coal, then the emissions from using up that gas are the equivalent of burning coal for 125 years.  In 2008, the world emitted 30.3 Gt or gigatonnes (billion metric tons) of CO2 to the atmosphere, in 2010 that figure had increased to 31.8 Gt.  That's a lot of carbon, and the number keeps getting larger each year.  In the face of mounting problems from climate change, the proposed reductions in emissions from the implied proposal of replacing massive use of coal with massive use of shale gas hardly seem worthy of much excitement.