Just five years ago, biofuels were being touted as a cure for global warming and America's oil addiction. We'll just fill up our cars with liquid corn, the thinking went, and everything will be grand. Yeah. So much for that. In the time since, biofuels have become one of the most maligned clean-energy technologies out there. And not without reason: The U.S. biofuel industry is slated to receive up to $420 billion in subsidies between now and 2022, yet recent studies have suggested that, when you take indirect land-use effects into account, the two biggest biofuels on the block, corn- and soy-based ethanol, create more greenhouse-gas emissions than old-fashioned gasoline. And farm-staters in Congress have reacted to this shocking news by... arguing that we should ignore indirect land-use effects.

So ethanol never lived up to the early hype, and its defenders have acted badly. But a major new study in Science suggests that, despite the broad disillusionment with crop-based ethanol, other types of biofuels can still play a role in the shift away from fossil fuels. The paper was co-authored by eleven leading biofuels experts—some pro, some con—who spent a year debating the subject. When judging whether biofuels are truly sustainable, they write, scientists need to consider all possible impacts on "the efficiency of the global food system, greenhouse-gas emissions, soil fertility, water quality, and biodiversity."

Looking at things through this lens, their conclusion was that biofuels can still be produced in large quantities, without having adverse effects on land use or food prices, but only if they're tightly regulated and come from a few specific sources:

* Perennial plants grown on degraded lands abandoned from agricultural use
* Crop residues
* Sustainably harvested wood and forest residues
* Double crops and mixed cropping systems
* Municipal and industrial wastes

No corn- or soy-based ethanol on that list. (Although no algae, either. Why's that?) Now, some of these technologies aren't commercial yet—no one has quite mastered how to get cellulosic ethanol from crop residue. Still, there's plenty of potential: The authors calculate that the United States alone could get 500 million tons worth of biomass each year from the above sources, without jacking up food prices or indirectly triggering deforestation in the Amazon.

What's that all amount to? Quick back-of-the-envelope calculation: You could get roughly 47 billion gallons of cellulosic ethanol from 500 million tons of biomass, which, if used as a substitute for gasoline, would save about 195 million tons of carbon-dioxide each year—reducing total U.S. emissions about 3 percent from its current level. So, in that scenario, biofuels would play only a modest supporting role, but they would have the advantage of actually being sustainable—as opposed to our the current situation.

Then again, the authors of the Science study are also assuming a future in which biofuels policy is driven by "the best available science, continually updated… used to evaluate the extent to which various biofuels achieve their multiple objectives." Must be some weird strain of scientific gallows humor, because, back on Capitol Hill, we just saw Minnesota Democrat Collin Peterson hold up the House climate bill until he could prevent the EPA from considering the full land-use impact of ethanol. Not exactly a promising sign.

P.S. In comments, r.ennis reminds me that, as we noted a few months ago, it's actually a lot more efficient to burn all that biomass for electricity (say, to power up a plug-in car) than to convert it into liquid fuel and run it through an internal combustion engine. Excellent point.

--Bradford Plumer