The world could need four to five times as much land for biofuels as it uses today.
A major factor you don’t mention is the energy density of the fuels you are discussing. Aviation jet fuel or kerosene has an energy density of 43MJ/l which is much higher than ethanol’s 28MJ/l. This means if a turbofan jet engine could run on ethanol (because of lack of lubricants, turbo jet engines cannot use more than 40% ethanol or the engines will seize) it would need 65% more fuel by weight, either reducing range or increasing prices (to make room for extra fuel storage). This means your simple comparisons are not very accurate. Also battery powered passenger flight is now only possible for very short distances with few passengers, again a consequence of the mass of the batteries needed.
Isn't there also a cost/price dimension to explore? Higher costs of renewable fuel sources should increase airfares, and so reduce demand for air travel. We find an equilibrium where air travel is limited by allowable carbon emissions. One reason air travel has grown so much is that flyers did not pay the externality cost of emissions. Recognizing that cost will limit demand.
Hannah, there is no reason to assume that the biofuels required to "recarbonize" the global aviation industry must be obtained from farmland. That assumption skews your entire study into ignoring the prime global potential for sustainable liquid fuels.
In reality, prior to the era of subsidized fossil gas (aka "natural gas") the Standard Industry Practice for the production of the versatile liquid fuel methanol [CH3OH] was via feedstock from Native Coppice Forestry for charcoal production, whose hydrocarbon offgasses were reacted to produce methanol. As a leading hydrogen carrier methanol is not only a prime candidate for the feedstock of so-called Sustainable Aviation Fuel (aka synthetic kerosene) it will also serve to some extent as an easily tanked aviation fuel for the Reformer Methanol Fuel Cells that are now in commercial production in Europe, USA, & China.
Since Native Coppice Forestry has traditionally thrived on poor hill lands, its present expansion would logically pose no threat to farmlands. You may recall the 2012 joint study by WRI & WFN of the available land area suitable for afforestation - without taking farmland, native forest or special ecosystems - which identified 1.6Gha.s globally. This is about 152% of the entire global area of arable farmland.
In terms of the methanol yield available from biomass feedstock, the US NREL gave some useful findings (see: https://www.nrel.gov/docs/legosti/old/5570r2.pdf ).
"Estimates of the [USA] biomass resource available for U.S. fuels production average 2.45 billion metric tons per year. One ton of feedstock can be converted to 721 liters (186 gallons) of methanol. As a renewable resource, biomass represents a potentially inexhaustible feedstock supply for methanol production."
With methanol weighing 0.791 kg /litre, the NREL finding indicates a potential output - just from extant US biomass resources - of :- 0.721 m3 x 0.791 t x 2.45Gt feedstock = 1.397Gt /yr methanol output. As the fuel has an LHV of 19.9 GJ /t, the finding points to an energy value of 27.805 EJ in that output. Given that US biomass resources are a small fraction of what 1.6 Gha.s of Native Coppice Forestry could produce, let alone accounting global sustainable biomass resources overall, there is plainly no justification for presenting biofuels as necessarily posing a threat to agriculture.
Is it possible that you have overlooked the opportunities for co-production and integration. What is needed is a carbon source to convert into biofuels - this can come from many bio-based systems - so to assume that all the feedstock comes from "cropping" as we think of it today is all a bit silly. I suspect we can make a lot of progress without having to use a land area the size of India.
This analysis assumes that crop-based biofuels are the only path forward for long-haul aviation. Is there a reason you don't mention alternatives such as electrofuels, fuels from waste biomass, or offsetting aviation emissions using some form of carbon removal (e.g. DAC)? Do you see these approaches as nonviable?
The impression I've gathered is that the ideal amount of land that should be dedicated to biofuel crops, whether for aviation or otherwise, is zero. As you note, the land requirements are enormous (thus displacing food crops, natural forests and other habitat, etc.). There are other substantial impacts, such as water and fertilizer use. And alternatives do exist; they present enormous scaling challenges, but at least in principle those challenges can be overcome; given the land use issues, is it possible even in principle to imagine a future where biofuel crops are a good idea?
(I attempted to summarize the issues around crop biofuels back in May: https://climateer.substack.com/p/biomass-overview.)
Apologies if this is outside the scope of this article, but what do you think of the potential of other sources of biofuels besides corn to bring down the price? I know algal biofuels never really took off, but there's still work being done on them to make them economically competitive with fossil fuels. Also, they avoid the land use and water use problems you mention.
Thanks for this post.
It's clear that the world needs alternative ways to fuel aviation than allocating large amounts of cropland. The sustainable aviation fuel effort seems ripe for acceleration via an "Advance Market Commitment" mechanism that actively pulls new technology forward.
A connected world is good, for individuals, economies, and global cultures (and subcultures!). We've got to figure out how to this without monocropping 19% of the world's farmland.
Getting through the long dark months of winter used to seem impossible for wind and solar. Then as they scaled up, costs went down. They’re now ¼ the cost of nuclear (Lazard.) Now we can Overbuild renewables to have enough power during winter. But this means we have enormous excess power the rest of the year? What are we going to do with maybe 2 or 3 TIMES MORE power than we need the other 9 or 10 months of the year? The possibilities are amazing. With super-abundant super-cheap electricity we could Gasify all municipal household waste into jet-fuel and building materials. https://eclipsenow.wordpress.com/gasification/
We could split water for abundant hydrogen for industrial heating and smelting uses. We could filter some CO2 out of the ocean (where it’s 100 times more concentrated than in air) and mix that with hydrogen to make all the ‘green’ jet-fuel we could want. Desert countries could desalinate heaps of water. No wonder Tony Seba calls it ‘Super-Power’. https://youtu.be/fsnkPLkf1ao