We hear a lot about climate change and what we need to do to reduce carbon emissions and so preserve the environment.

One suggestion is that we should replace fossil fuel consumption with biofuel consumption, moving from burning coal and oil to burning ethanol and biodiesel.

Biofuels are liquid fuels made by fermenting plant material such as corn and rapeseed. Although they release carbon when burned, this is carbon aborbed by the plants when they’re grown. Fossil fuels, on the other hand, release into the atmosphere carbon which would otherwise be stored in coal and oil desposits underground. The impact on the environment of burning biofuels is therefore less than the impact of burning fossil fuels.

Or is it? Critics of biofuels point out that to fully understand their environmental impact we need to think more carefully than this.

The process of growing the crops used to make biofuels can be polluting, as can the process of converting the crops into fuel. Areas of rainforest, which act as a carbon sink, are being destroyed to make room to grow biofuel crops; whatever carbon would have been absorbed by the rainforest but now isn’t is a carbon cost of biofuel production.

It may well be that all things considered, biofuels are greener than fossil fuels, at least when they are produced in the right way. What we can’t do, though, is generalise from biofuels being greener than fossil fuels in one way (whatever carbon is emitted into the atmostphere when biofuels are burned is first absorbed when the biofuel crops are grown) to the conclusion that they are greener than fossil fuels full stop.