Comment on 'carbon intensity of corn ethanol in the United States: State of the science'

Seth A. Spawn-Lee, Tyler J. Lark, Holly K. Gibbs, Richard A. Houghton, Christopher J. Kucharik, Chris Malins, Rylie E.O. Pelton, G. Philip Robertson

Research output: Contribution to journalArticlepeer-review

9 Scopus citations


In their recent contribution, Scully et al (2021 Environ. Res. Lett. 16 043001) review and revise past life cycle assessments of corn-grain ethanol's carbon (C) intensity to suggest that a current 'central best estimate' is considerably less than all prior estimates. Their conclusion emerges from selection and recombination of sector-specific greenhouse gas emission predictions from disparate studies in a way that disproportionately favors small values and optimistic assumptions without rigorous justification nor empirical support. Their revisions most profoundly reduce predicted land use change (LUC) emissions, for which they propose a central estimate that is roughly half the smallest comparable value they review (figure 1). This LUC estimate represents the midpoint of (a) values retained after filtering the predictions of past studies based on a set of unfounded criteria; and (b) a new estimate they generate for domestic (i.e. U.S.) LUC emissions. The filter the authors apply endorses a singular means of LUC assessment which they assert as the 'best practice' despite a recent unacknowledged review (Malins et al 2020 J. Clean. Prod. 258 120716) that shows this method almost certainly underestimates LUC. Moreover, their domestic C intensity estimate surprisingly suggests that cropland expansion newly sequesters soil C, counter to ecological theory and empirical evidence. These issues, among others, prove to grossly underestimate the C intensity of corn-grain ethanol and mischaracterize the state of our science at the risk of perversely affecting policy outcomes.

Original languageEnglish (US)
Article number118001
JournalEnvironmental Research Letters
Issue number11
StatePublished - Nov 2021

Bibliographical note

Funding Information:
Financial support was provided by a National Science Foundation Graduate Research Fellowship under Grant No. DGE-1747503 to SAS, and by the Great Lakes Bioenergy Research Center, U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research (Award DE-SC0018409). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. We are grateful to Luoye Chen, John Field, Rich Plevin, and Stephanie Searle for feedback and suggestions that greatly improved our manuscript.

Publisher Copyright:
© 2021 The Author(s).


  • Corn ethanol life cycle assessment
  • Land intensification
  • Land use change
  • Soil carbon
  • Yield price elasticity


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