The study, based on US Department of Agriculture crop-production data, shows that during the period when US biofuel production rapidly ramped up, the increased carbon dioxide uptake by the crops was only enough to offset 37% of the CO₂ emissions due to biofuel combustion over the period 2005-2013.

The use of biofuels to displace petroleum has been driven by public policies, including subsidies but most compellingly by regulations, notably the US Renewable Fuel Standard (RFS) and California Low-Carbon Fuel Standard (LCFS). Policy rationales include agribusiness income, energy security, oil depletion and greenhouse gas (GHG) mitigation.

The environmental justification rests on the assumption that, as renewable alternatives to fossil fuels, biofuels are inherently carbon neutral because the CO₂ released when they are burned is derived from CO₂ uptake during feedstock growth. That convention is premised on globally complete carbon accounting in which biogenic emissions are not counted in energy sectors when carbon stock changes are counted in land-use sectors. This assumption has been used in cap-and-trade programs and carbon taxes as promulgated to date, which address only fossil-derived CO₂ emissions. However, errors arise when bioenergy is treated as carbon neutral in national and subnational policies, which do not impose globally coherent accounting that tracks all carbon stock changes. The carbon neutrality assumption is also embedded in lifecycle analysis (LCA), which traditionally focused only on production-related GHG emissions within a fuel’s supply chain.

… Thus, although it was proposed as an objective way to compare fuels, LCA has become a form of scenario analysis. However, it is inferior in this regard to integrated assessment modeling (IAM), which uses a biogeochemically and economically coherent analytic framework that LCA lacks. Moreover, as a static framework, it fails to reflect the stock-and-flow dynamics that are fundamental to bioenergy systems. Indeed, policy applications of LCA raise serious questions regarding the limitations of the method. Given such concerns, it is useful to analyze the situation by a method other than LCA.

—DeCicco et al.

The researchers applied Annual Basis Carbon accounting to investigate the changes in carbon flows directly associated with a vehicle-fuel system. Unlike LCA or other forms of carbon accounting used for climate policy to date, ABC does not treat biofuels as inherently carbon neutral; it tallies CO₂ emissions on the basis of chemistry in the specific locations where they occur.

ABC accounting reflects the stock-and-flow nature of the carbon cycle,— i.e., that changes in the atmospheric stock depend on both inflows and outflows. LCA, on the other hand, focuses only on inflows (GHGs discharged into the atmosphere).

The system in the study was defined to include motor fuel consumption, fuel processing operations and resource inputs, including cropland for biofuel feedstocks.

Thus, instead of modeling the emissions, lead author Professor John DeCicco and his colleagues analyzed real-world data on crop production, biofuel production, fossil fuel production and vehicle emissions—without presuming that that biofuels are carbon neutral.

This is the first study to carefully examine the carbon on farmland when biofuels are grown, instead of just making assumptions about it. When you look at what’s actually happening on the land, you find that not enough carbon is being removed from the atmosphere to balance what's coming out of the tailpipe.

When it comes to the emissions that cause global warming, it turns out that biofuels are worse than gasoline. So the underpinnings of policies used to promote biofuels for reasons of climate have now been proven to be scientifically incorrect. Policymakers should reconsider their support for biofuels. This issue has been debated for many years. What’s new here is that hard data, straight from America’s croplands, now confirm the worst fears about the harm that biofuels do to the planet.

—John DeCicco

DeCicco’s co-authors include current and former students at the U-M School of Natural Resources and Environment and the U-M Program in the Environment, as well as a postdoctoral researcher at the Energy Institute. They are Danielle Yuqiao Liu, Joonghyeok Heo, Rashmi Krishnan, Angelika Kurthen and Louise Wang.

Some funding for the study was provided by the American Petroleum Institute.

Resources

• DeCicco, J.M., Liu, D.Y., Heo, J. et al. (2016) Climatic Change doi: 10.1007/s10584-016-1764-4