Because converting grass into biofuel crops might be swapping one environmental problem for another.

There’s biofuel in them thar prairies!

You’ve got it, Yosemite Sam. A biofuel rush has officially swept the U.S., with farmers planting corn, soy, sugarcane and other plants for use in fuel. Their crops help meet the EPA requirement that almost all gasoline sold in the U.S. contain at least 10 percent corn ethanol, the most common biofuel. As early as the 1970s, scientists touted biofuels as a clean, renewable energy source. And since a cornfield can’t burst and pollute surrounding habitats like an underwater oil well might, some conservationists endorsed them as environmentally friendly.

But recent data have put this onetime green energy panacea under scrutiny. Demand for corn ethanol helped triple the price of corn from 2005 to 2011. And studies have shown that corn, canola and other biofuel crops with high nitrogen requirements produce a net increase in greenhouse gas emissions.

Such fast rates of grassland conversion haven’t been seen since the 1930s.

Now we might have another reason to second-guess biofuels. Between 2006 and 2011, farmers converted more than 1.3 million acres of Great Plains grasslands into corn and soybean fields, according to a report by South Dakota State University ecologists Christopher Wright and Michael Wimberly in the Proceedings of the National Academy of Sciences. It’s a rate that keeps pace with deforestation rates in Brazil, Malaysia and Indonesia. Imagine 1.3 million football fields, or an area roughly the size of Delaware.

Wright and Wimberly have put numbers on what had been only anecdotal evidence of “the worst-kept secret in the Northern Plains.”

“We’re not anti-farmer,” Wright says. “It’s just an important story that needs to get out there.”

Amber waves of grain…

When the Natural Agricultural Statistics Service released satellite images showing land coverage in the U.S. each year from 2006 to 2011, Wright and Wimberly weren’t sure what they’d find. They used their grant from the Department of Energy to study biofuel sources, and focused on the Western Corn Belt (WCB), a region that includes North Dakota, South Dakota, Nebraska, Minnesota and Iowa.

Farmers shoulder the risk, thanks to subsidized crop insurance and tantalizingly high prices.

As they combed through the data, their jaws dropped. “We were surprised at how much [grass conversion] there was and how quickly it was happening,” Wright says. Such fast rates haven’t been seen since the 1930s.

Wright and Wimberly saw an explosion of cropland even along the western edge of the WBC — a region prone to drought. But farmers shoulder the risk, thanks to federally subsidized crop insurance, not to mention tantalizingly high corn and soy prices.

North American grasslands, or prairie, is an ecosystem that once covered a wide swath of the continent, from the Rocky Mountains east to Texas. It’s responsible for the productive soils still found in the region. Grasslands trap floodwaters, absorb toxins from the soil and serve as crucial breeding grounds for ducks and other birds.

Crop plants don’t hold carbon in their soil as well as native grasses, research shows. All plants “inhale” carbon dioxide, a greenhouse gas, and “exhale” life-giving oxygen, collectively acting like the “lungs” of the global ecosystem. But grassland loss — similar to rainforest deforestation — reduces that lung capacity. What’s more, tilling the soil for planting stimulates bacteria to release carbon dioxide. And the birds that once nested in protective tall grasses now have populations that are plummeting faster than any other group of birds in North America. Today, Wright and Wimberly are also looking at how grassland loss affects bird migration.

Did the scientists get it right?

But the satellite images used in the study are “fraught with issues” — namely, they aren’t hi-res enough to distinguish crop switching from grassland loss, according to Benjamin Rashford, a professor of agriculture and applied economics at the University of Wyoming.

The researchers admit they struggled to distinguish croplands from grasslands using the satellite data. For example, hay grows naturally in grasslands but can also be planted for pasture, so the appearance of corn in a region once covered in hay might represent crop switching — not grassland loss.

Biofuel advocates also argue that although they aren’t quite “carbon neutral,” biofuels still come out on top. They emit less carbon dioxide than traditional fuels, and the plants that serve as raw material do absorb some carbon dioxide from the atmosphere.

Who are you going to believe — a satellite … or farmers who are on the land and know what’s happening on it?

Geoff Cooper, senior vice president of research and analysis for the Renewable Fuels Association, an ethanol industry trade group, wonders why Wright and Wimberly didn’t verify the accuracy of the satellite data by visiting the farmland themselves. “Who are you going to believe — a satellite that can’t tell the difference between a prairie and a hayfield, or farmers who are on the land and know what’s happening on it?”

But Rashford notes that Wright and Wimberly used statistics to account for their uncertainty. “So while their specific numbers may be a little off, their general conclusions are very robust,” he says.

Solutions, missed opportunities, and a bit of hope

So what can be done to reverse or slow grassland loss? Johann Walker, Director of Conservation Programs in the Dakotas and Montana, points out that the Sodsaver and Swampbuster provisions in the most recent Farm Bill helped slow loss rates by preventing farmers who drain wetlands or farm erosion-prone lands without a conservation plan from qualifying for Farm Bill programs. Wright also suggests granting cattle producers a premium for grass-fed beef.

Scientists are also working to develop cellulosic ethanol — biofuel made from leftover husks, stems and other inedible plant parts from all sorts of crops. Cooper says the economic recession discouraged investment in scaling up this alternate technology. But with the economy picking up, four U.S. facilities — in Iowa and Kansas — could start commercial production of cellulosic ethanol in the next few months.

But the long-awaited arrival of cellulosic ethanol might be too little, too late. In 2012, the biofuel industry produced about 13 billion gallons of corn ethanol — enough to satisfy the demand for the 10-percent blend most vehicles burn. Yet demand for gasoline has actually fallen. The market doesn’t have room for any more ethanol, period — cellulosic or otherwise.

There’s no clear-cut solution, but Wright and Wimberly’s study has started a conversation. “It’s kind of blown my mind that so many people are interested in the topic,” Wright says. ”People are recognizing there is an issue, and there’s potential to amelioriate land cover change.”

Our ecosystems are vastly interconnected, so clearing any existing habitat — whether grasslands or rainforests — often brings unintended consequences. As the biofuel rush and deforestation continue, studies like Wright and Wimberly’s keep land cover change on the map — so our Earth’s lungs can breathe a little easier.