Global warming can only be arrested by putting less…a lot less…so called “greenhouse gases” (mostly CO2 from burning fossil fuels) into the air, taking CO2 already released out, or – a longshot and controversial approach – making the planet more reflective. (This post is one in an ongoing series you can review in the Energy and Environment thread.) Among the current favorite ways to do the first is to substitute biofuels for fossil fuels, especially in cars and trucks, an approach that mostly (now) means making more ethanol, in turn mostly (now) from corn.
Unfortunately, “ethanol” denotes a specific chemical whose manufacture can produce a wide range of environmental benefits, all the way from “no better than gasoline” to “pretty darn good”. It depends on what crop you start with, how you grew the crop (tillage, fertilization, etc.), how far you had to haul it, what you distilled the fermented mash with (coal? not green! sugar cane stalks (bagasse)? verdissimo!), and so on. Unless ethanol can be distinguished at the point of wholesale or retail purchase and ‘graded’ on this dimension of environmental merit, however defined, it will be impossible to make important greenhouse gas gains with biofuels, whether we wield the tools of regulation, public education, or taxes and subsidies.
My colleagues and I have studied this issue at some length for the Natural Resources Defense Council, finding that an environmental index for biofuels is a practical policy goal and that it can be implemented in several different ways. The paper is here.