Government reports on biofuels talk about the “GHG savings” of biofuels made from particular crops (often referred to as biofuel “feedstocks”). This is fundamentally the wrong approach. In the attached paper, Biofuel Payback Periods (pdf), I present a method of assessing the worth of growing biofuel crops in terms of the potential of the land on which they are grown to store carbon if biofuels aren’t grown.
Just considering “GHG savings” is like comparing company profits without taking any account of the company’s size (e.g. the capital employed) – for example, £1m profits in a year might be a good performance for the local filling station but would be a disaster for, say, BP.