Sources say the subsection on biofuels was a particularly divisive part of the IPCC report, with optimistic scientists and renewable fuel-focused countries pushing to highlight the benefits of replacing petroleum fuels with plant-based alternatives, and others pushing to include the negative aspects.

The result was a scientific decision to agree to disagree, said Helmut Haberl, one of the lead authors of the chapter on agriculture, forests and land use and a professor in the Institute of Social Ecology at Alpen Adria University in Austria.

“There’s substantial disagreement on possible future yields of energy crops,” he said, as well as questions on the consequences of growing biofuels on water supply. “The opinions at the moment do not converge.”

Emissions from secondary effects

The change in phrase was made to avoid a potential misunderstanding, said Pete Smith, the lead coordinating author for the agriculture, forestry and land-use chapter. “Indirect” emissions often refer to gases released far from the source, like nitrous oxide that is emitted downstream from the fertilizer from which it originates. It could also mean emissions that are not directly accounted for in a sector.

Smith, a professor of soils and global change at the University of Aberdeen in Scotland, said the biofuels paragraph makes a balanced case, acknowledging both the benefits and the concerns.

“There are two very different camps: There’s the camp that thinks that bioenergy is going to save the planet, and there’s the camp that thinks we shouldn’t do bioenergy under any circumstances,” he said. “The literature on these indirect impacts of bioenergy is still very divided.”

The idea of ILUC became widespread after the publication of a paper on the topic in 2008 in the journalScience. U.S. EPA and the California Air Resources Board incorporate ILUC to determine the greenhouse gas-cutting benefits of biofuels in the federal renewable fuel standard and the low-carbon fuel standard, respectively.

While biofuels from crops, grasses, wood, agricultural residues and other materials emit less carbon than fossil fuels over a crop-to-vehicle life cycle, recent studies have questioned the availability of material to make fuels on a large scale. The IPCC’s Working Group II report on climate risks and impacts acknowledged the limitations of biofuels (ClimateWire, March 31).

However, models to measure ILUC remain uncertain and inconsistent, and the biofuel industry has called ILUC an unproven theory.

“Even if we are getting better at characterizing the process, there’s not full consensus on calculating the boundaries to account for these emissions,” Haberl said.

The Brazilian Sugarcane Industry Association (UNICA), the trade group for sugar-cane ethanol from Brazil, criticized the IPCC for raising alarm on biofuels in the Working Group II report published on March 31.

The IPCC took an “undisciplined speculative tone,” wrote Geraldine Kutas, the Brussels-based head of international affairs for UNICA, in a blog post.

Row over ranking low-carbon fuels

Brazil has curbed its deforestation rate while growing the sugar-cane industry, said Kutas. As cattle raising in the country has intensified, meaning there are more cows on less land, those acres are taken up by sugar cane growers. Sugar cane provide a better carbon sink than pasture, leading to a net decrease in emissions.

Although Working Group II wrote that Brazilian biofuels expansion could impinge on sensitive ecosystems, the report released yesterday specifically includes sugar cane as an option with low life-cycle emissions.

The authors included specific examples — sugar cane, the grass Miscanthus, fast-growing tree species and biomass residues — in order to provided clarity.

The agriculture, forestry and land-use chapter received close to 4,000 comments, Haberl said. They will be published on the Working Group III website tomorrow, along with the technical report.

Even if fuels from agricultural waste, wood, grasses and household trash are the greenest transportation option available, manufacturers have yet to produce them with any commercial success.

“Clearly, there’s still a lot of discussion and debate around just how big this opportunity is,” said Chris Malins, the fuels program lead at the International Council on Clean Transportation. However, Malins is confident that there will be increasing consensus among scientists on which opportunities are good.

“There are certainly cases where we have a very clear sense that we’re delivering generally sustainable fuels,” he said.

Mandates like the U.S. renewable fuel standard, which calls for production of 36 billion gallons per year by 2022, are useful in promoting low-carbon fuels, like waste vegetable oil for biodiesel, said Malins. In the United States, the volume of biodiesel from recycled feedstocks like used vegetable oil has nearly doubled since 2011, according to the Energy Information Administration.

“We’re confident that has been delivering significant carbon savings and taking advantage of an otherwise underutilized resource,” he said.