In its calm and measured way, the Congressional Budget Office (CBO) just delivered a blistering assessment of the environmental value of corn-based ethanol. The CBO had been charged by Congress to calculate just what the public is getting for its investment in ethanol production: specifically, the $0.45/gallon tax credit that gasoline blenders get for mixing ethanol into the fuel supply. In 2009, 10.8 billion gallons of corn ethanol got used in such a manner, costing the federal Treasury $5.16 billion in reduced tax revenue. What did we get for that fat wad of cash, in environmental terms? The question is...