First off, when comparing biofuel vs petroleum fuel production and the overall impact this has, the life cycle from source to product must be taken into account.
A) green house gas emission (GHG). Oh, I thought, what a doozy, biofuels win hands down. Not so fast my friends, in fact take a step back, if you will and take a gander at the lovely graph in figure 1, care of Dr. Azapagic.
You see the shock factor, "Ethanol from US corn"? - its GHG emissions are 1.5x HIGHER than from a fossil fuel refinery! This is because of nitrous oxide emissions from fertilisers applied to the corn fields. Well its not organically grown, is it?! UK wheat and Brazilian sugar cane do better but its not in the negative. Noooo, you don't get that until you start in on the 2nd generation biofuels (the lignocellulose feeds stocks) and then comes another shocker. Miscanthus, a second generation fibrous grass that has GHG emissons 3.6x higher than a fossil fuel.....arghhhhh! WHY?
B) LUC and Biodiversity See those letters - LUC - land use change. This is the term for changing the use of the land from say forest to corn, or from forest to miscanthus or from farm use to forest - a biofuel feedstock forest. This can have two effects: GHG emissions change and biodiversity changes. Forest land in the UK replaced by miscanthus resulted in the 310 g CO2 eq./MJ (Figure 1). Biodiversity also decreased as a monoculture of miscanthus is obviously less diverse than a forest. However a feedstock forest could be more diverse than a field previously used for growing wheat, so the biodiversity issue works both ways.
C) Water use. The source of biofuel feedstock varies a great deal in the amount of water required to grow it. Feedstock from agricultural waste or forestry waste requires little or no water, whereas energy crops such as miscanthus require more water than arable crops such as corn because of a longer growing season. Water has to be transported from somewhere and this will effect GHG and stress the place from which it was taken. Biorefineries themselves require relatively little water use. So, like biodivesity, water use can have positive or negative impact. Well, maybe not positive but at least, less negative.
D) Other considerations in the bio(fuel) life cycle are environmental impacts such as soil pollution (acidification, human toxicity etc), emissions of sulfur dioxide, nitric oxide, nitrite etc. There are also economic considerations such as feedstock costs, capital cost (the commercial biorefineries using second generation feedstock have to be built - the US Department of Energy has a website showing sites where they are being built) and the cost of the biofuel itself. Then there are social considerations such as jobs and regional development, health issues (e.g. pesticides cause cancer and death, particulate emission from biomass handling affects air quality), human labour rights, land availability and food prices (energy crops might displace food crops and drive up food prices) and affects on future generations.
Curiously, the article does not mention the other hazards involved in the petrolum industry such as the contamination of water ways by fracking or the great environmental disasters caused during drilling for oil, many of which we never even hear about. The chance for that kind of disaster seems much less likely in biofuel production.
Still, there is a GREAT DEAL to consider while we go about the nitty gritty of finding exactly how a cellulase chews ups cellulose. It really IS a BIG picture but I believe the conclusion of the article; if managed correctly, biofuels really could be a very good thing.
To read the article for yourself, go here.