Corn ethanol and associated greenhouse gas emissions in the USA: importance of the nitrous oxide emission factor

Abstract

Increasing concern over anthropogenic global warming, finite oil reserves and a desire for energy security have all to helped bring about an increased interest in biofuels worldwide. The US is currently the world’s leading ethanol biofuel producer, with most being derived from a corn (Zea mays L.) feedstock. The net climate forcing benefits of some biofuels have been questioned in recent years, in particular the potential enhancement of nitrous oxide (N2O) emissions during cultivation of their feedstocks. This study aims to determine whether US corn ethanol can reduce net GHG emissions from surface transport relative to gasoline under two N2O emission scenarios: one using the an emission factor for managed soils of 1.3% (scenario A) derived from Intergovernmental Panel on Climate Change default values, and the other using an upper N2O emission factor of 5% (scenario B). It also aimed to determine the GHG reductions that could be achieved relative to future US ethanol production targets under these two scenarios. This study shows that a net reduction in GHG emissions can be achieved under both scenarios, although the emissions reduction benefits of ethanol production are considerably reduced where an emission factor of 5% is applied. Under the latter scenario, new-facility corn ethanol production in the US would fail to meet the US Environmental Protection Agency mandate for a 20% net reduction in GHG emissions relative to gasoline.