The supposed benefit of biofuel is that, although it still releases carbon dioxide when it burns, that carbon was drawn down from the atmosphere by the plants that make up the fuel rather than being released from oil that was once underground. But growing fuel creates emissions too.

The biggest problem is when land that used to be a carbon sink is plowed up to plant crops, but manufacturing fertilizer is also a major source of emissions, and applying that fertilizer to land also releases greenhouse gasses in the form of nitrous oxide emissions.

In 2010 the Environmental Protection Agency (EPA), which sets the amount of corn ethanol required by the RFS, estimated that by 2022 corn ethanol would have total life-cycle emissions 20 percent lower than gasoline.

But these projections didn’t account for the dramatic effect the RFS would have on land use in the US. “I don’t think people expected as much land to come back into production,” says [researcher Tyler] Lark.

His study found that the RFS increased corn prices by 30 percent and the price of other crops by 20 percent. In response, farmers who previously used their land for cattle grazing or who were involved in conservation schemes started growing crops instead.

All this land-use change has essentially outweighed the reductions in greenhouse gas emissions that come from growing fuel instead of pumping it out of oil wells.

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