The use of corn ethanol in place of gasoline causes enough carbon emissions from land-use changes to cancel immediate tailpipe benefits, according to research published last week that confirms a controversial earlier study.
The analysis examines a hypothesis by Timothy Searchinger and his co-authors published in 2008 in Science magazine that says using U.S.-grown corn for fuel triggers commodity price changes that ultimately lead to native ecosystems being destroyed, with a high carbon price tag.
The new study, published in the March issue of BioScience, finds much lower emissions from indirect land-use change than the Searchinger paper calculated but still enough to tip the balance away from corn ethanol.
The analysis by Thomas Hertel of Purdue University found that the indirect greenhouse gas releases associated with corn ethanol are 800 grams of carbon dioxide per megajoule, or 27 grams of carbon dioxide per megajoule per year over 30 years of production.
That amount is about a quarter of what Searchinger estimated. "Nonetheless, 800 grams are enough to cancel out the benefits that corn ethanol has on global warming," Hertel wrote.
Hertel said the indirect emissions are in addition to 60 to 65 grams of carbon dioxide equivalent per megajoule in direct emissions from cultivating and processing the fuel. Those emissions are subject to process improvements and farming practice changes that could improve the overall picture, he said.
Hertel concluded corn ethanol has limited potential for use in California's low-carbon fuel standard.
He noted that while both Californian and federal regulators have, like himself, found lower indirect land-use change estimates than Searchinger estimated, the figures have consistently been high enough to make corn ethanol "an unattractive compliance option for mitigating current carbon intensity or meeting fuel-use mandates."
Hertel's analysis uses a different model for the price-mediated market analysis than Searchinger did, relying on a global economic commodity and trade model known as GTAP-BIO, and breaks the globe down into 18 distinct regions.
He also used a different analytical approach than that used by U.S. EPA for its analysis of the renewable fuel standard, a comparative static analysis, and said he welcomes further scientific comparisons of the two strategies.
Click here to read the paper.