I devoted the last post to the question, could we provide biofuels? By that, I mean, is the land available. I cited a paper in which it showed fairly conclusively that growing corn to make fuel is not really the answer, because to get the total US fuel consumption, based on that paper you would need to multiply the total area of existing ground under cultivation in the US by a factor of 17. And you still have to eat. Of course, the US could still function reasonably well while consuming significantly less liquid fuel, but the point remains that we still need liquid fuels. The authors of this paper could also have got this wrong and have made an error in their calculations, but such errors go either way, and as areas get larger, the errors are more likely to be unfavourable than favourable because the transport costs of servicing such large areas have to be taken into account. On the other hand, the area required for obtaining fuels from microalgae is less than five per cent of current area. Again, that is probably an underestimate, although, as I argued, a large amount of microalgae could be obtained from sewage treatment plants, and they are currently in place.
 
One problem with growing algae, however, is you need water, and in some places, water availability is a problem (although not usually for sewage treatment). Water itself is hardly a scarce resource, as anyone who has flown over the Pacific gradually realizes. The argument that it is salty is beside the point as far as algae go because there are numerous algae that grow quite nicely in seawater. One of what I consider to be the least well-recognized biofuel projects from the 1970s energy crisis was carried out by the US navy. What they did was to grow Macrocystis on rafts in deep seawater. The basic problem with seawater far from a shore is that it is surprisingly deficient in a number of nutrients, and this was overcome by raising water from the ocean floor. Macrocystis is one of the fastest growing plants, in fact under a microscope you can watch cell division proceeding regularly. You can also mow it, so frequent replanting is not necessary. The US navy showed this was quite practical, at least in moderately deep water. (You would not want to raise nutrients from the bottom of the Kermadec trench, for example, but there is plenty of ocean that does not go to great depths.)
 
The experiment itself eventually failed and the rafts were lost in a storm, in part possibly because they were firmly anchored and the water-raising pipe could not stand the bending forces. That, however, is no reason to write it off. I know of no new technology that was implemented without improvements on the first efforts at the pilot/demonstration level. The fact is, problems can only be solved once they are recognized, and while storms at sea are reasonably widely appreciated, that does not mean that the first engineering effort to deal with them is going to be the full and final one. Thus the deep pipe does not have to be rigid, and it can be raised free of obstructions. Similarly, the rafts, while some form of anchoring is desirable, do not have to be rigidly anchored. So, why did the US Navy give up? The reasons are not entirely clear to me, but I rather suspect that the fact that oil prices had dropped to the lowest levels ever in real terms may have had something to do with it.