Fossil fuels supply about 80% of the energy in corn ethanol
Corn ethanol seems like a renewable energy source — a source that cannot run out — because corn can be planted again and again, and the energy in corn comes from the sun, not a limited supply of coal or oil in the ground. Furthermore, since corn takes carbon dioxide from the air as it grows, returning that CO2 to the atmosphere by burning corn ethanol seems like it should cause no net greenhouse pollution.
But a closer look reveals a murkier picture.
That is because growing and harvesting corn takes a lot of energy — not just for farm machinery, but also to make the fertilizer corn crops depend on. It also takes energy to process corn into ethanol, and to transport the product to market. Most of this energy currently comes from fossil fuels like coal, oil and natural gas. In 2006, Alex Farrell and several colleagues published a paper in the journal Science carefully analyzing earlier studies about the fossil fuel that goes into making corn ethanol. They found that when you include this energy, 74-95% of the energy content in corn ethanol comes from fossil fuels. In other words, there is only a 5-26% renewable energy “profit.”[[Farrell, Alexander E., Richard J. Plevin, Brian T. Turner, Andrew D. Jones, Michael O'Hare, and Daniel M. Kammen. “Ethanol Can Contribute to Energy and Environmental Goals.” (Abstract) Science 311, no. 5760 (January 27, 2006): 506-508.]]
Another analysis, published in 2006 by Jason Hill and his colleagues, found that about 25% of the energy in corn-ethanol represents renewable profit[[Hill, Jason, Erik Nelson, David Tilman, Stephen Polasky, and Douglas Tiffany. “Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels.” (Abstract) Proceedings of the National Academy of Sciences 103, no. 30 (July 25, 2006): 11206-11210.]]. In 2007, a widely-cited computer model known as GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation) was updated to reflect the latest understanding of the process used to create corn-based ethanol. The result in this analysis: approximately 23% of the energy in corn ethanol is renewable energy.[[“Argonne Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) Model.”]] Considering all of these analyses, a rough estimate would be that corn ethanol as produced today comes 20% from renewable energy and 80% from fossil fuels.
While this would still appear to offer a net benefit from the perspective of carbon emissions into the atmosphere, changes in farming practices at home and abroad may mean that corn ethanol use and production cause more net emissions than plain gasoline.