As Michael Pascoe reported yesterday, President Bush talked up the benefits of biofuels in his SOTU speech. He sideswiped the climate issue (no emissions cuts), but optimists might consider the gasoline reduction target (20% by 2017) promising on that front. Well, yes and no.

First of all, the transport sector only accounts for about a third of emissions in the US. Second, note we’re talking about a 20% gasoline – not fuel – reduction. The lion’s share in that reduction (75%) is to be achieved by rolling out biofuels, the rest by efficiency gains.

The benefits of biofuels seem obvious – since the carbon dioxide emitted in their combustion logically can’t exceed the amount fixed by the growth of the feedstock, the process can theoretically be made carbon neutral. Your solar power plants are self-producing – all you need do is give them food and water and play them Beethoven in the morning (Mozart in the evening, Tchaikovsky if it’s inclement).

Note theoretically. Let’s focus on the energy question. Say that you’re selling it (energy, not ‘it’), in the form of a given biofuel. To meet the condition of carbon neutrality, let’s say the biofuel you’re selling is used to power the process that generates and distributes it.

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Obviously, you’d need a surplus – a positive energy balance, the bigger the better. If you just broke even, you’d have no product to sell. Like eating celery, it’d be an energetic bust. Nobody in their right mind would bother. Not without peanut butter.

In reality, cheap fossil fuels provide most of the energy input to the production process (including farming, processing and distribution) for ethanol, the US biofuel of choice. This means two things: 1) the process is currently far from carbon neutral, and; 2) depending on the state of subsidies for biofuels, the fact that the energy balance is marginal doesn’t automatically mean you can’t make money. Mmmm, peanut butter.

But from the perspective of both oil dependence and climate change, if the principal energy input to the production process is gasoline, then a biofuel of marginal energy balance is pointless (except perhaps as a make-work program). The linked benchmark questions for biofuels, then, are: do they deliver – and if so, to what extent – positive energy balance, and how do total life cycle emissions per unit energy delivered compare with good old-fashioned gasoline?

The biofuel of choice in the US is ethanol, the feedstock largely corn. Nearly 20% of its heavily subsidised crop goes to ethanol production, the fuel in turn subsidised with a tax credit of 15c/L (not to mention a still standing Reagan-era tariff on Brazilian cane ethanol). Corn is pretty energetically marginal – the most optimistic assessments seem to be around +1.25, with more vehement critics arguing that the return is actually negative.

So what’s the emissions saving? A study published in Science last year (27/1/06) reported that current US corn ethanol production saves – on best estimate – 13% relative to emissions from the energetic equivalent in the current fuel mix. Not great.

It’s pretty clear that at its current stage of development, ethanol production (99% from corn) would struggle severely to reach Bush’s milestone. But proponents say that both energy and emissions equations can be made more favourable in future with more efficient agricultural practices and the development of cellulosic ethanol (to which Bush alluded in his speech). Critics, on the other hand, see profiteers in green camouflage and a potential environmental and food security nightmare. The proverbial swallowing of a spider to catch a fly.

Discussion of which will have to wait until next week.