About 15 years ago, government incentives helped to launch a biofuel boom in the United States. Ethanol factories now consume about 130 million metric tons of corn every year. It’s about a third of the country’s total corn harvest, and growing that corn requires more than 100,000 square kilometers of land.

In addition, more than 4 million metric tons of soybean oil is turned into diesel fuel annually, and that number is growing fast.

Scientists have long warned that biofuel production on this scale involves costs: It claims land that otherwise could grow food or, alternatively, grass and trees that capture carbon from the air and provide a home for birds and other wildlife.

But government agencies, relying on the results of economic models, concluded that those costs would be modest, and that replacing gasoline with ethanol or biodiesel would help to meet greenhouse gas reduction goals.