Do biofuels slow global warming—or accelerate it?

When it comes to greenhouse gases, there are no easy answers at the pump.

It seems like a simple question: are biofuels better for the climate than fossil fuels? One recent study says greenhouse gas emissions from corn-based ethanol are 40% lower than those from gasoline—not bad. But another new study says ethanol's total emissions can be as much as 50% higher than those from the gasoline it replaces—not good. And an informal ES&T survey of more than half a dozen scientists who study the issue drew a resounding "it depends."