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 critical, because that long-entrenched tax break is set to expire later this year — and the ethanol industry is scrambling to extend it, with the full support of the Obama administration, Associated Press reports.

As the CBO report makes clear, the environmental case for the tax break is bankrupt. The reports runs 28 pages, but I can boil it down to two points.

1. Subsidizing corn-based ethanol is an mind-numbingly expensive way to reduce greenhouse gas emissions.

2. Corn-based ethanol is really just a clever way to convert natural gas and coal into car fuel. 

Overall, the CBO’s assessments could be summarized like so: The environmentbl benefits of propping up corn-based ethanol production are scant at best — and extremely expensive.