Biofuels were once envisioned as the heroes of the new green economy, but just as Anakin Skywalker turned to the dark side and became Darth Vader, the formerly celebrated energy source now plays the part of villain owing to its detrimental effect on food security, the environment and even human rights.

Most green groups slowly changed tack from calling for more public support for biofuel development to demanding its restriction, or even abandonment.

The sharpest shift against biofuels began in 2008 when a pair of full life-cycle studies of first-generation biofuels – such as those derived from sugar cane, palm oil, soy, rapeseed or cereals –found their carbon footprint to be worse than that of fossil fuels. Princeton ecological economics researcher Tim Searchinger led one study that appeared in the U.S. journal Science and the other was by chemical ecologist John Pickett and sponsored by the Royal Society.

The problem, in large part, was a fight between food and fuel. Biofuels pushed food prices up, and simultaneously encouraged conversion of forest and grassland into new cropland. In one of the worst cases, Searchinger and his team found that corn-based ethanol nearly doubled greenhouse gas emissions over a 30-year period.

Both Searchinger and Pickett held out hope for advanced or ‘second-generation’ biofuels – feedstocks such as waste or crop residues that did not compete with existing food production. No one eats corn cobs or wheat stalks, so the same bits of land could produce both food and fuel without any deforestation or spike in food prices.

To varying degrees, policymakers in the United States and European Union have been reluctant to retreat from biofuel subsidies and targets, partially due to pressure from agricultural interests. And, biofuels have been an easy win for politicians in winning support from rural communities, contrasting the controversies which have accompanied so many other climate strategies.

But faced with objections from the European Commission, Brussels has changed its tune. Running a variety of different models, the EU’s Joint Research Centre in 2010 came to conclusions similar to Searchinger and Pickett. In June, European energy ministers acknowledged the problem and dialled back an earlier continent-wide 2020 target for renewable transport fuels from ten to 7 percent.

“The official European Commission position has changed 180 degrees,” Searchinger told Road to Paris, reflecting on how Brussels has gone from refusing to acknowledge his research to embracing it. “While there was real resistance at first, [outgoing EU climate commissioner] Connie Hedegaard recognised that this was a terrible mistake, and the European Parliament had a very good proposal too, but now there’s a battle with the national governments.”

“Once you have a powerful interest benefiting from a government policy, it becomes very hard to dislodge it,” he added.

Here too, the energy ministers alighted upon second-generation biofuels as a solution to the problem, introducing a new sub-target of 0.5 percent for these so-called advanced biofuels. In the U.S., plans are well in hand to expand production capacity for second-generation biofuels by 2022. The U.S. Energy Independence and Security Act aims to achieve production of 61 billion litres per year of cellulosic ethanol, a biofuel made from lignocellulose – essentially those usually inedible structural bits of a plant that make up most of its mass – and 19 billion litres of other advanced biofuels.

It now appears that the use of crop residues can be worse than the fossil fuels they are supposed to replace – although for different reasons than their first-generation forebears. According to a $500,000 study funded by the U.S. Department of Energy published in April in Nature, the removal of residues from cropland can release carbon trapped in the ground – known as “soil carbon” – and produce an overall increase in CO₂ emissions.

Soil carbon losses can occur by wind or water erosion, and by soil respiration, where the carbon is directly oxidized to CO₂. Crop residues have traditionally been left on the field after harvest to prevent this erosion and to maintain soil fertility. Tracking the amount of CO₂ emitted is complicated by the high variability in the amount of soil carbon present. It’s also difficult to measure small changes over the course of a single year and many studies until now have been short-term.

“Methods used to estimate net changes in greenhouse emissions for alternative fuels compared to fossil fuels are highly uncertain,” University of Nebraska-Lincoln agronomist Adam Liska, the lead author of the Department of Energy study, told Road to Paris. “The net impact from many indirect effects is highly uncertain too, and another major issue is the time horizon of calculations that can markedly change results.”

Liska and his colleagues overcame these hurdles by producing a model based on data from 36 field studies in North America, Europe, Africa and Asia. Then, to test the accuracy of the model they compared its results with measurements of a no-till corn field experiment in Nebraska over the course of nine years.

The results were not good for the biofuels industry. They concluded that these second-generation biofuels yield a small amount of energy while oxidizing a large pool of carbon. The CO₂ emissions per unit of energy turned out to be similar to what Searchinger had found in indirect land-use change from first-generation biofuels.

And as with Searchinger’s research, there has been resistance from the sector’s various lobby groups and some U.S. agencies. The Renewable Fuels Association and the Biotechnology Industry Association dismissed the findings, saying that the researchers had assumed in their scenario that 100 percent of residues were removed from fields when actual practice removes much less. National Corn Growers Association president Martin Barbre attacked “researchers in their ivory towers” for being out of touch with farmers on the ground. The Environmental Protection Agency and U.S. Secretary of Agriculture Tom Vilsack took a similar line in efforts to calm markets.

The researchers said that their findings were independent of the amount of residue removed. They do offer biofuels from crop residues one final reprieve, however. Their estimates assumed that farmers would engage in no mitigation efforts in the process of removing crop residues, because this is what they observed happening in the majority of cases. With better crop management practices, such as no-till cover crops or use of animal manure and compost, the numbers might change, they said. But even here, more research is needed to compare the results of different residue removal practices.

Despite the complaints by Vilsack and the lobbyists, the nonpartisan U.S. Congressional Budget Office in June acknowledged the Liska paper (alongside other papers more optimistic about the fuel source), and warned that greenhouse gas emissions reductions would be small or could even increase. The best the CBO could say about the sector was that it faced “significant challenges” while offering “limited potential” – a euthanasia by bureaucratese if there ever was one.

The U.S. departments of energy and agriculture are encouraging farmers to reduce the amount of crop residue that is harvested. Farmers will see conservation subsidies trimmed if excessive residues are removed. Yet the EPA has already scaled back its target for the industry on five occasions due to farmers’ inability to deliver the required volumes. The new requirements on sustainable harvesting of residues will make meeting the target of 61 billion litres per year of cellulosic ethanol even more difficult.

The researchers suggest that a better strategy would involve investigating perennial grasses or forestry waste, which may not be as bad in terms of erosion and loss of soil carbon. However, Liska is quite blunt about the prospects for biofuels from any source: “It’s actually energy efficiency that has the best prospects for reducing fossil fuel use.”

Searchinger, for his part, has all but given up on bioenergy, saying that its use is inefficient compared to other options. In a report last December for the Washington-based World Resources Institute, a climate and energy think tank, Searchinger noted that to produce 20 percent of world energy from biomass, as the International Energy Agency calls for, would require a pool of biomass equivalent not just to the entire global crop production, but the total harvest of grasses, crop residues and trees.

“Why take so much biomass for such little energy? ” said Searchinger. “It’s absolutely more rational to work towards an electrification of the transport fleet. Either we learn to do the electric car thing or we die.”