DOE-backed study questions life-cycle analyses of carbon footprint

Source: Amanda Peterka, E&E reporter • Posted: Monday, September 30, 2013

A new study expresses doubts about the ability of today’s life-cycle analyses to capture the full greenhouse gas impacts of biofuels.

Analyses assume enough carbon dioxide is taken up by feedstock plants during their life on the ground to offset emissions released during biofuel production and car use on the road. But the University of Michigan study says current life-cycle analyses — including those used by U.S. EPA to regulate renewable fuels — are “inherently flawed” because they assume that biofuel feedstocks absorb more carbon than whatever else may be growing on the ground.

“Plants used to make biofuels do not remove any additional carbon dioxide just because they are used to make fuel as opposed to, say, corn flakes,” said John DeCicco, author of the report and a professor who specializes in transportation systems and greenhouse gas emissions.

The study was published this month in the journal Climatic Change and supported through the U.S.-China Clean Energy Research Center Clean Vehicle Consortium, a partnership of national labs, universities and companies. It was funded through the Department of Energy.

Biofuels, according to the study, fall under a category of emissions reduction techniques that seek to remove carbon dioxide from one location in order to balance emissions released in other locations where fuels are burned.

But biofuels will mitigate CO2 emissions relative to producing petroleum-based gasoline only if growing crops for energy removes more of the heat-trapping gas from the air than growing crops for food or other purposes, DeCicco says. That’s because tailpipe emissions of fuels containing ethanol or biodiesel are roughly equal to petroleum-based fuels, and biofuel production processes release CO2.

“If there is any climate benefit to biofuels, it occurs only if harvesting the source crops causes a greater net removal of carbon dioxide from the air than would otherwise have occurred,” DeCicco said.

Over the past few years, there have been several studies assessing the impacts of biofuels on CO2 emissions using different models, and most have come up with different numbers

According to DOE, life-cycle analyses have found that corn-based ethanol production and use reduces greenhouse gas emissions by up to 52 percent, and cellulosic ethanol use drops emissions by as much as 86 percent. Based on life-cycle analyses, EPA requires that ethanol reduce emissions by 20 percent and cellulosic ethanol by 60 percent in order to be included for credit in the renewable fuel standard.

A recent study published in Yale University’s Journal of Industrial Ecology found that greenhouse gas emissions from ethanol production at modern facilities saw reductions as high as 59 percent compared with gasoline.

Current life-cycle analyses, though, are limited in scope and do not fully account for the emissions from all the steps of biofuels production and use, the University of Michigan study concludes. It adds that such analyses also make too many assumptions about factors such as future biomass yields, sequestration, market behaviors and time horizons that are almost impossible to actually measure on the ground.

Policymakers should instead rely on models that are more global and take into account climatic, biogeochemical and economic systems, the study says. In terms of reducing CO2 emissions, people would be better off doubling their efforts at reforestation rather than emphasizing biofuels, according to DeCicco.

“Although biofuels can play a mitigation role when certain conditions are met,” the study says, “de-emphasizing biofuel production in favor of terrestrial carbon management may offer more immediate and effective ways to counterbalance the CO2 emitted when using carbon-based liquid fuels of any origin.”