You might be thinking about ethanol again today since it is being hotly pursued by power and money, even more so than yesterday. Tiffany Groode sets us straight.

Using a technique called life cycle analysis, she looked at energy consumption and greenhouse gas emissions associated with all the steps in making and using ethanol, from growing the crop to converting it into ethanol. She limited energy sources to fossil fuels. Finally, she accounted for the different energy contents of gasoline and ethanol. Pure ethanol carries 30 percent less energy per gallon, so more is needed to travel a given distance.

While most studies follow those guidelines, Groode added one more feature: She incorporated the uncertainty associated with the values of many of the inputs. Following a methodology developed by recent MIT graduate Jeremy Johnson (Ph.D. 2006), she used not just one value for each key variable (such as the amount of fertilizer required), but rather a range of values along with the probability that each of those values would occur. In a single analysis, her model runs thousands of times with varying input values, generating a range of results, some more probable than others.

Based on her "most likely" outcomes, she concluded that traveling a kilometer using ethanol does indeed consume more energy than traveling the same distance using gasoline. However, further analyses showed that several factors can easily change the outcome, rendering corn-based ethanol a "greener" fuel.

One such factor is the much-debated co-product credit. When corn is converted into ethanol, the material that remains is a high-protein animal feed. One assumption is that the availability of that feed will enable traditional feed manufacturers to produce less, saving energy; ethanol producers should therefore get to subtract those energy savings from their energy consumption. When Groode put co-product credits into her calculations, ethanol's life-cycle energy use became lower than gasoline's. . .

"The results show that everybody is basically correct," she said. "The energy balance is so close that the outcome depends on exactly how you define the problem."

Well, but the energy value of the co-product diminishes as the distance from ethanol plant to feed yards grows. This too needs a probability assessment. Still, everyone should be happy since you are right no matter what you think. Perfect.