Ethanol's has had more peaks and valleys than Britney Spears' career and the debate its carbon count still hasn't been cleared up. However, it still enjoys popularity as a non-foreign fuel source, most notably as a plank in President Obama's energy plan.

A couple weeks ago, I wrote about a study saying that 1/3 of U.S. gas could be replaced by corn and cellulosic ethanol. Now, new research out of Carnegie Mellon University (CMU) finds that the CO2 penalty from transporting E85 country-wide may offset any and all emissions or economic gains.

Carnegie Mellon's paper, published in Environmental Science and Technology, finds that emissions from the transport of ethanol may be up to twice that of previous estimates; 95% of those GHG's would result from gas combustion needed to get the stuff from factory to fuel station.

This analysis also brings bad news on a economic front. With transport costs an order of magnitude greater than for gasoline, ethanol's market advantage may dry up as well.

CMU's researchers argue that most ethanol life-cycle analysis studies look largely at emissions from feedstock cultivation and ethanol production, not transport. Further, the studies that do include transport use unrealistic assumptions. For example, the U.S. government's cellulosics scenario assumes no out-of-state ethanol supplies for California, even though that state consumes 23% of all ethanol produced.

The authors believe that a regional approach might combat this emissions problem: "The analysis emphasizes the importance of regional alternative fuel strategies. Ethanol should be pursued in areas near feedstock production, while different transportation fuel alternatives, such as plug-in hybrids, should be explored in other areas remote from feedstock sources."

The bottom line is that the ethanol picture is complex and painting with broad policy strokes often brings unintended consequences.

The clean energy waters are murky with politics, money, and the unknown. I think the good news is that each bit of research on ethanol (and other renewables topics) brings the debate into sharper focus, reducing the unknown, and putting us in a better position to make the right energy choices.