A somewhat detailed explanation of how corn ethanol is a waste of energy. The impact on the price of food is obvious, but I keep hearing it said that as much (non-solar) energy goes into ethanol as we get out of it without any description of how this works.
I'm not sure how detailed I'm able to go here, but the basic shape of things is that if you think about a coal power plant you have to recognize that not only does the plant generate energy but it also takes a lot of energy to run the plant. The coal needs to be mined, transported, etc. But coal contains a lot of energy so the whole process is worth doing as an energy-acquisition mechanism. By contrast, planting, raising, and harvesting a field of corn is a relatively labor intensive enterprise. And when you're done, you don't have any ethanol at all -- you just have a bunch of corn. The corn then needs to be transported (and it's bulky) which uses energy, and then turned into ethanol which uses yet more energy. All told, that makes it difficult for corn ethanol to really work as a viable source of net energy.
By contrast, I believe you can just get a lot more ethanol from a ton of sugar cane than you can from a ton of corn. It's just like how sugar cane is a more efficient source of sweetener than is growing fields of corn to make high-fructose corn syrup.
You can find a technical discussion of the energy issues involving several different kinds of crops here. I would say, as a layman, that researchers seem to have obtained a range of results on the question of energy efficiency and that the "right" answer seems to depend both on how you count and and on what the specifics of any given field are. It's also worth considering that chopping down forests to clear land for agriculture can contribute to global warming.
Ultimately, though, rather than "figure this out" what we need to do is price carbon emissions through a cap-and-trade scheme with auctioned permits. As with any other human activity, discerning the overall carbon impact is extremely difficult. For example, maybe corn ethanol by raising grain prices will, over time, make beef substantially more expensive in a way that reduces beef consumption and, in turn, lowers carbon emissions? There's just no real way to figure this stuff out, which is why we should mostly rely on the government to price carbon and let the market adjust accordingly, rather than trying to have the government pick "the best" technology.
Photo by Flickr user Kables used under a Creative Commons license