Statements about biofuels being carbon neutral should be taken with a grain of salt. This is according to researchers at the University of Michigan Energy Institute after completing a retrospective, national-scale evaluation of the environmental effect of substituting petroleum fuels with biofuels in the US. America's biofuel use to date has in fact led to a net increase in carbon dioxide emissions, says lead author John DeCicco in Springer's journal Climatic Change.

The use of liquid biofuels in the transport sector has expanded over the past decade in response to policies such as the US Renewable Fuel Standard (RFS) and California's Low-Carbon Fuel Standard (LCFS). These policies are based on the belief that biofuels are inherently carbon neutral, meaning that only production-related greenhouse gas emissions need to be tallied when comparing them to fossil fuels.

This assumption is embedded in the lifecycle analysis modelling approach used to justify and administer such policies. Simply put, because plants absorb carbon dioxide as they grow, crops grown for biofuels should absorb the carbon dioxide that comes from burning the fuels they produce.

Using this approach, it is often found that crop-based biofuels such as corn ethanol and biodiesel offer at least modest net greenhouse gas reductions relative to petroleum fuels.

Field data for assessing the net carbon dioxide emission effect of biofuels has been available since the Renewable Fuel Standard was passed in 2005. DeCicco's team evaluated the data up to 2013, using the Annual Basis Carbon (ABC) accounting method he previously developed.

It takes a circumscribed look at the changes in carbon flows directly associated with a vehicle-fuel system, and does not treat biofuels as inherently carbon neutral.

Instead, the ABC method tallies carbon dioxide emissions on the basis of chemistry in the specific locations where they occur. The system takes into account motor fuel consumption, fuel processing operations and resource inputs, including the use of cropland for biofuel feedstocks. Unlike lifecycle analysis, ABC accounting reflects the stock-and-flow nature of the carbon cycle, recognizing that changes in the atmospheric stock depend on both inflows and outflows.

DeCicco's team found that the gains in carbon dioxide uptake by feedstock, such as corn, were enough to offset biofuel-related biogenic emissions by only 37 percent, rather than 100 percent, during the period 2005 to 2013.

"This shows that biofuel use fell well short of being carbon neutral even before considering process emissions," says DeCicco.

In this regard, the researchers concluded that rising US biofuel use has led to a net increase rather than a net decrease in CO2 emissions. This finding contrasts with those of lifecycle analysis models which indicate that crop-based biofuels such as corn ethanol and soy biodiesel lead to a modest reduction in greenhouse gas emissions.

DeCicco's work demonstrates that it is possible to empirically evaluate the necessary condition for a biofuel to offer carbon dioxide mitigation benefits.

"Doing so provides a bounding result that suggests a need for much greater caution regarding the role of biofuels in climate mitigation," DeCicco concludes.

DeCicco, J.M. et al. (2016). Carbon balance effects of U.S. biofuel production and use, Climatic Change. DOI 10.1007/s10584-016-1764-4