Remember the diaper wars of the 1980s? If your memory goes back that far, you may recall the battle over whether cloth diapers or disposables were worse for the environment. Most people assumed that Huggies and the like were eco-villains because they caused so much solid waste and were made with petroleum-based plastic. But Pampers partisans pointed out that cloth diapers require loads of hot water to wash (using fossil fuels and thus causing emissions of greenhouse gases) and loads of gasoline to pick up and deliver to the households using a diaper service. At the end of the day, it was basically a draw, coming down to whether you viewed landfills or global warming as a worse problem.

The current war over biofuels, such as ethanol from corn and sugarcane, has the same flavor. Boosters call biofuels eco-saviors; detractors say they will eat up vast amounts of farmland and natural habitats, not to mention drive up food prices (as the burgeoning use of corn for ethanol has in the U.S.) and yield almost no reductions in greenhouse-gas emissions anyway. But as Jörn P. W. Scharlemann and William F. Laurance of the Smithsonian Tropical Research Institute in Panama note in the current issue of Science, whether biofuels are a net good or a net evil depends on what aspects of the environment you care about.

Compared to petroleum, they note, “nearly all biofuels diminish greenhouse-gas emissions.” But switchgrass is a much better feedstock than corn or soy, and the exact greenhouse balance sheet depends on local growing conditions (are you going to use a lot of petroleum-based fertilizer?). More central, they argue, the “focus on greenhouse gases and energy use is too narrow. The arguments that support one biofuel crop over another can easily change when one considers their full environmental effects.”

For instance, sugarcane such as that used in Brazil to make ethanol doesn’t look as green if, to grow it, you raze carbon-rich tropical forests to make the sugarcane fields, causing vast greenhouse-gas emissions. And if you care about not only greenhouse emissions but also about biodiversity and soil protection, then turning jungles into sugarcane fields looks even worse. And are you planning to use nitrogen fertilizers, as corn and rapeseed require? In that case, you’ll be emitting a lot of nitrous oxide, a greenhouse gas that also destroys stratospheric ozone.

A study done for the Swiss government and summarized by Scharlemann and Laurance compared gasoline, diesel, and natural gas with 26 biofuels, assessing the total environmental impact of each. Of the 26, 21 reduce greenhouse-gas emissions by more than 30 percent relative to gasoline. But 12 of 26—including corn ethanol, Brazilian sugarcane ethanol and soy diesel, and Malaysian palm-oil diesel—have worse environmental impacts than fossil fuels. The best biofuels are produced from organic material that isn’t specially grown for that purpose, such as biowaste or recycled cooking oil, or ethanol from grass or wood.

Things get even more complicated if you factor in things like U.S. government subsidies for corn-based ethanol production. That is making many American farmers shift from soy to corn, which in turn is driving up global soy prices. Result: greater incentives to destroy the Amazon rainforest and Brazilian tropical savannas for soy farms. “Multibillion-dollar subsidies for U.S. corn production appear to be a perverse incentive from a rational cost-benefit perspective,” conclude the scientists. But then, who ever said U.S. environmental policy was rational?

All in all, a welcome reminder that weighing enviro-costs and benefits of biofuels is no easier than the diaper decision.