One of the 2007 energy bill’s most ambitious provisions  the ethanol mandate  has turned out to be its most troublesome. The provision would boost ethanol production from 7-plus billion gallons today to 36 billion gallons by 2022. In practical terms, this means doubling the production of corn ethanol until advanced forms of ethanol and other biofuels kick in.

Corn ethanol came under fire earlier this year when evidence mounted that the diversion of cropland from food to fuel had contributed to the spike in worldwide food prices. What is less clear is whether corn ethanol is good or bad for the planet  whether it emits fewer or more greenhouse gas emissions than conventional gasoline. The answer turns on how you measure emissions.

Congress stipulated that ethanol be cleaner than gasoline and handed the job of measuring emissions to the Environmental Protection Agency, which has found itself under ferocious pressure. The ethanol industry wants its product shown in the best possible light. Environmentalists want an honest accounting, which the public deserves but which they do not think an industry-friendly Bush administration is capable of.

The most contentious question involves the emissions caused by direct and indirect changes in land use associated with growing biofuels. Until late last year, corn ethanol had been seen as at least carbon neutral  and thus much cleaner than gasoline  because the greenhouse gases it absorbed while growing canceled out the gases it emitted during combustion. This made it a win-win fuel  even a win-win-win fuel  because it also encouraged the construction of ethanol refineries in the American heartland and eased, to some extent, America’s dependence on imported oil.