I ’ve said it once, and I’ll say it again here: corn-based ethanol is a loser, a subsidy for farm states and the politicians who represent them. According to David Pimentel, a Cornell University agricultural expert, it’s a net energy loser. Just to power a car for a year takes up land that could feed a family of seven for that same time period.

Right now, we’re blending corn ethanol as an additive into gasoline, and also offering higher concentrations of it for sale as a stand-alone fuel, E85. General Moters has built 3.5 million flex-fuel vehicles capable of running on E85, but most cruise around on gasoline, and there are only 1,900 E85 stations in the U.S.

We should give the corn supply a rest. If there’s hope for ethanol, it’s in making it from the stems and stalks of plants, the embryonic “cellulosic” form of the technology. Unfortunately, right now we have no commercial cellulosic ethanol plants, and the process to make it is still evolving. It is a tough nut to crack, but this plant-based product has many advantages, including an energy content roughly three times that of corn ethanol.

If we took all the corn in the U.S. and made ethanol from it, we’d probably offset only about 10 to 15 percent of our transportation fuel use. But a new report from GM and the Sandia National Laboratories, “Growing Green Energy: The 90 Billion Gallons Study,” concludes that cellulosic ethanol could offset a third of our estimated 180-billion-gallon gasoline and diesel habit in 2030.

We need 90 billion gallons of ethanol to replace 60 billion gallons of gas, because ethanol, sad to say, has only two thirds the energy content of petroleum fuels.

General Motors has an obvious vested interest in cellulosic ethanol, as the leading manufacturer of flex fuel vehicles. It is planning to increase its investment in ethanol-capable vehicles, and has invested in two cellulosic startups.

I want to believe that cellulosic ethanol is in our future, but the rosy “Growing Green Energy” report simply assumes that that we’ll have the necessary breakthroughs in developing the technology.

JIM MOTAVALLI is the former editor of E The Environmental Magazine.