Tyler Lark, a geographer at the University of Wisconsin-Madison, grew up among farms, working on a neighbour’s dairy, vaguely aware of the tension between clearing land to grow food and preserving nature.

As an engineering student working on water projects in Haiti, he saw an extreme version of that conflict: forests cleared for firewood or to grow crops, producing soil erosion, environmental denudation and worsening poverty. “I think it was that experience that told me, ‘Hey, land use is important,’” he says.

He decided to study how farmers transform landscapes through their collective decisions to plough up grasslands, clear trees or drain wetlands – decisions that lie at the heart of some of the planet’s greatest environmental challenges, and also provoke controversy.

Lark carries professional scars from recently stumbling into one of the fiercest of these fights: the debate over growing crops that are used to make fuel for cars and trucks.

About 15 years ago, government incentives helped to launch a biofuel boom in the United States. Ethanol factories now consume about 130 million metric tonnes 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 kilometres of land. In addition, more than four million metric tonnes 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.

Lark and a group of colleagues recently jolted this debate back to life. In a February 2022 study, they concluded that the law that unleashed the ethanol boom persuaded farmers to plant corn on millions of acres of land that would otherwise have remained grassland. Environmentalists had long feared that biofuel production could lead to deforestation abroad. This research findings showed a similar phenomenon happening within the United States.

That land conversion, the scientists concluded, would have released large amounts of carbon dioxide and other greenhouse gases into the air and makes ethanol fuel every bit as bad for the climate as the gasoline it’s intended to replace.

Farmers and biofuel trade groups lashed out against these findings – and against Lark himself. A biofuel industry association demanded that he and one of his co-authors be blackballed from a government expert review panel on renewable fuels.

The dispute came at a moment when world events laid bare the trade-offs of biofuels. Less than two weeks after Lark’s paper appeared, Russia invaded Ukraine, provoking a spike in prices for both food and fuel, which already had been scarce and expensive because of the pandemic.

Biofuel supporters have called for incentives to blend more ethanol into gasoline in order to bring down gasoline prices. Anti-hunger advocates are demanding less biofuel production, to free up land to grow more food. And natural ecosystems continue to disappear.

The lesser prairie chicken needs large grassland areas to thrive. Government programmes aimed at protecting and expanding such grasslands have helped the species to survive.

As the controversy roils on, a more technical debate among scientists and economists is simmering out of public view: How reliable are the economic models used to evaluate biofuels anyway? Their users defend them; others disagree.

“The results coming out of these models are driven more by assumptions than by actual information,” says Stephanie Searle, an ecologist specialising in biofuel sustainability at the International Council on Clean Transportation (ICCT). She and others say that one influential model, in particular, adopts assumptions that whitewash the fuels’ environmental risks.

America’s biofuel boom launched in 2005 as Congress passed a law that created the Renewable Fuel Standard (RFS), which required sharp increases in the use of biofuels over the following decade. Congress increased those biofuel targets in 2007.

Fuel companies could satisfy the law by mixing more ethanol into gasoline, or by supplementing standard diesel fuel with a version of diesel made from plant oil or animal fat.

The law rested on a foundation of mixed goals. Farmers wanted new markets for their crops. Others hoped that biofuels could be a homegrown, cleaner alternative to foreign oil. Biofuels were supposed to cut greenhouse gas emissions because the carbon contained in them is recycled: It had previously been captured from the air by growing the corn or soybeans to begin with. And even though the factories that turn corn into ethanol require lots of energy and typically burn fossil fuels, it was assumed there would still be a net climate benefit.

At the time, “you could easily envision an incredibly optimistic view” of the future, says Sivan Kartha, an environmental scientist with the Stockholm Environment Institute. Bioenergy supporters promised fuels made from plants that were similar to those in native ecosystems, delivering the environmental benefits of grasslands, for instance, while simultaneously replacing fossil fuels.

Yet Kartha could also imagine a darker future, with profit-driven plantations of biofuel crops displacing native forests. He urged caution in an article published in the Annual Review of Environment and Resources in 2007.

“Bioenergy has the potential to contribute to sustainable development,” he wrote. But “the fulfillment of this potential cannot be presumed.”

Ethanol production in the United States rose sharply from 2005 to 2012 in order to meet targets set by the Renewable Fuel Standard (RFS). Most ethanol is blended into gasoline, so when consumers abruptly stopped driving in the early stages of the Covid pandemic, ethanol use dropped as well.

As US ethanol production headed toward the RFS-mandated goal of roughly 15 billion gallons a year, scientists grew increasingly worried that the appetite for biofuel, added to rising demand for food, could consume vast amounts of land.

“It got us thinking about what the consequences might be, for the climate,” says Jason Hill, an environmental scientist at the University of Minnesota. In 2010, Hill and co-authors wrote in the Annual Review of Ecology, Evolution, and Systematics that “the largest ecological impact of biofuel production may well come from … land-use change.”

• A Knowable Magazine report