Not all biofuels are "green" according to where the crops from which they are derived are grown. The worst offenders are palm oils which may have ten times the carbon emissions of normal diesel fuel derived from petroleum, if the palm is grown on converted rainforest land. In contrast, when the palm is grown on previously cleared land, its emissions are around one fifth that for conventional petroleum-derived diesel. These conclusions are from a life-cycle analysis of 14 different source-fuels made by a team at M.I.T. which takes account of the carbon emissions incurred in the growing and harvesting of the crop, including land-clearance, its processing and the CO2 that results from final combustion of the fuel. http://pubs.acs.org/doi/abs/10.1021/es102597f?journalCode=esthag
Accordingly, it is suggested that biofuels that depend far less on changes in land use may be the cleanest in terms of carbon emissions, including those from salicornia and algae. The latter particularly require smaller areas of land to produce them than their equivalent quantity derived from land-based crops, and the fertility of that land is not an issue, since they can be grown in tanks or biorectors placed on land of any quality.
Certainly, greenhouse gas emissions are not the only consideration to be borne in mind in choosing a particular biofuel, and there is the matter of production-costs and the feasibility of making the fuel on the large scale, if any significant substitution for petroleum-derived fuels is to prove practical. In the evaluation of diesel from algae given in the M.I.T. paper, is given a considerable range (0.1 - 2.1) as normalised to conventional diesel, i.e. algal fuel is reckoned as between one tenth to more than twice as carbon-emitting as petro-diesel depending on details of the processing. What is interesting is that the conversions of biomass (e.g. switchgrass and salicornia) to synthetic diesel using Fischer-Tropsch (F-T.) and coal to liquids with F.-T. generally appear favourable in comparison with standard petro-diesel production.
I am grateful to Dr Jim Hileman for sending me a copy of the paper.