What are the risks, sustainability speaking, of halophytes as biofuel?

Question

Halophytes that can be grown in deserts have been researched for biofuel production, among other applications, in aviation. Such biofuels would not compete with food production and not deplete fresh water resources, which sounds like a highly beneficial situation. On the other hand, deserts have an important role in ecology and climate. I would imagine changing large surface areas of desert into croplands would lower the planetary albedo and therefore accelerate heating (although I think it would be small compared to equivalent fossil fuel burning), and Saharan dust provides essential nutrients to the Amazon rainforest.

What are the potential downsides, from a sustainability point of view, of large-scale halophyte biofuel production? Is it really the revolution that Boeing and co. want us to believe? Or is there some major catch that the articles are not addressing, that would make this possibility a lot less promising than we might hope?

Answer 1

US airlines alone use about 18 billion gallons/430 million barrels of fuel a year (worldwide? Link is not clear on exactly what they include), which at 6.9 lbs/gallon is about 124 billion pounds or 56Tg. From your link above, "Salicornia produced an average annual crop of 1.7 kilograms per square meter of total biomass and 0.2 kilogram per square meter of oilseed". Assuming that all energy inputs come from the non-seed biomass and a conversion efficiency of 100% for the oil seeds (since I have no idea), 56Tg at 200g/m2 means 278G m^2 which is 278,000 square kilometres compared to 3 million square kilometres of total farmland. So in the best case the USA could get it's airline fuel from desert covering about 1/10th of it's current farmed area.

In reality those ratios will be worse, possibly much worse. There is some discussion here with on-farm consumption numbers anywhere from 1% to 30% (the 1% number is speculative).

Air transport is about 12% of the total in the USA, so you could divide the numbers above by 12% and say that the USA would need at least 2,500,000 square kilometres of farmed desert for its transport needs which is roughly the same as the current farmed area (note that on-farm use is implicitly included here, but not indirect inputs like fertiliser).

I expect the major risk is that biofuels will turn out to be a diversionary tactic used by industries that can't otherwise become sustainable. By pointing at small-scale sustainable biofuel production they can keep burning fossil fuels while they work on the trials, rather than working to become sustainable. That's why it's important to look at what would happen if the current systems were scaled up to provide the complete requirements.

Answer 2

This is an incomplete answer, as I can't go into the ecological value of a patch of desert. But as always you need to take a look at the inputs and outputs of a process. First, you need a lot of infrastructure in the desert:

• Tanks and greenhouses for the plants
• pipes to pump the seawaer there
• roads and other infrastructure
• processing equipment for the plants

Now, what will be the inputs?

• Seawater, consisting of salt and water
• Air, sunlight
• fertilizer
• Diesel or similiar, for transportation and possibly for energy on site

The outputs:

• Oil
• other plant matter, containing organic carbon and nutrients
• water evaporation, if you don't build greenhouses
• salty brine (the plants will bind water, leading to a higher concentration of salt)

Now, in the worst case, you bring in chemical fertilizer and water, and produce oil, heaps of rotting plants and a salty desert around your greenhouses, because you have to dump your brine somewhere. You mopve a lot of moisture into the desert, changing the local microclimate. In the best case, you use the non-oily plant matter for biogas and fertilizer, and use sunlight and waste heat to further produce usable salt from your brine. Excess plant matter is used to create topsoil, thus fighting desertification. Either way, you need to build artificial infrastructure with embedded energy and maintain it. Farming is always large scale, so you'll need large scale ponds and possibly large greenhouses. You will certainly change a part of the desert. The questions to ask, then, are: Can this be a change for the better, in some way? Am I willing to expand the effort in energy and money to actually control this change I will make?