And depending on which process and which study you personally ascribe to, the answer on how "carbon clean" ethanol looks depends. In most debates centering on corn fermentation, for example, the studies cite a range from say, 20 to 30% less carbon intensive than gasoline, to 20 or 30% more. This begs one very big question in my mind, what's the difference? How does the same ethanol in my car have a possible carbon footprint range that wide?
The true answer lies in the ground we walk on. When I started to read a few of the studies and articles about them, an interesting fact emerges, the difference depends in large part on which land gets counted. Most of ethanol's carbon footprint falls into one of several categories, in roughly ascending order (depending on the source and process), the fuel used to make it, the fuel used to grow or transport the feedstock, the carbon content of the fuel itself, and the lost carbon not sequestered in the vegetation that would have been on the land used to grow the feedstock.
The last one, land use change, is the bugaboo. For example, if you assume that all the land used to produce the ethanol feedstock is already in production, you tend to find a carbon footprint at the low end of the range, since there is little net reduction in the carbon sink, and ethanol looks pretty good. If you assume that all the land used to produce the ethanol feedstock came from forests that had been chopped down, or marginal land that produces very low yields, you tend to find a carbon footprint at the high end of the range, and ethanol looks bad. Thought about another way, ethanol made from corn or sugar that displaces human or animal food production is likely to be relatively greenhouse gas friendly compared to ethanol made from corn or sugar that comes from new land put into production just for ethanol. The same logic applies to cellulosic ethanol sources, though not quite to the same degree. Interesting conundrum.
As usual, the devil's in the details, and people tend to use the case that best addresses their agenda, and apply it to all ethanol as a whole. Personally, I'm buying all my ethanol from land that is already in production, so my carbon footprint must be good. The rest of you can buy the OTHER ethanol with all the bad carbon footprint.
Neal Dikeman is a founding partner at Jane Capital Partners LLC, a boutique merchant bank advising strategic investors and startups in cleantech. He is founding contributor of Cleantech Blog, a Contributing Editor to Alt Energy Stocks, Chairman of Cleantech.org, and a blogger for CNET's Greentech blog.