Navigating the maize

Update – An updated critique reacting to republished results from USDA is available here:

Earlier in 2017, the United States Department of Agriculture published a study undertaken by the consultants ICF International that included a reassessment of the lifecycle greenhouse gas intensity of corn ethanol. The report concluded that corn ethanol’s greenhouse gas intensity is already lower than the level projected for 2022 by the Environmental Protection Agency in the regulatory impact analysis for the Renewable Fuel Standard 2, a conclusion that has been embraced by advocates for the corn ethanol industry.

In this study, commissioned by the Clean Air Task Force and National Wildlife Federation, we provide a critical review of the ICF report. We find that the report shows a lack of balance, systematically emphasising evidence that could suggest that the performance of corn ethanol is better than previously modeled, while understating or ignoring conflicting evidence. More problematic even than this lack of balance, however, we find that the analysis in the report is riddled with errors of methodology and data, many of them at the most basic level, so as to render some of the results presented essentially meaningless. Given the many issues identified, we conclude that the work presented is wholly inadequate to justify any firm conclusion on whether the corn ethanol emissions estimates made by EPA could or should be revised down.



In the report “Navigating the maize” it is stated that, “The ICF report appears to double count the emissions benefits associated with the production of ethanol co-products”. Following clarification of the ICF approach in subsequent papers, we realise that we had misunderstood the approach used and that this is not the case – co-products are not double counted by ICF.

Our misunderstanding arose from a methodological difference between the ICF analysis and the original EPA regulatory impact assessment that was not clear to us from the original documentation. In the EPA work, agricultural emissions are assessed only on the net increase in corn production to meet the mandate (this is a consequential approach). The effect of distillers’ grains in reducing net corn demand is already included before agricultural emissions are calculated. The ICF reassessment makes reference to the net demand change value from the EPA work, and therefore we had understood that the role of co-products was implicitly included by use of this net demand value. Based on clarifications given in updates to the ICF work  we now understand that ICF in fact analyse emissions for an average corn acre and then apply these based on gross corn demand. The step that makes reference to the EPA net demand value is redundant (this term is effectively added on both the numerator and the denominator of the equation so that it is cancelled out) . Based on this understanding it is indeed appropriate to apply a co-product credit under this methodology. 

Corn on the scales