I am more optimistic than you are on the future of commercial air travel. Jet fuel is basically 10-carbon alkanes. A big enough offshore wind farm would be able to brute-force a CO₂ and seawater to methanol conversion, and organic chemistry should get you the rest of the way.

From that point, which you can reach with off-the-shelf technology, improvements are basically a question of cost. Sustainable air travel using only off-the-shelf technology will not be cheap, but it performs a service of a nature that means it won't need to be.

Now, what would probably need to happen is a closure of most airports and concentration of the network into very large hubs, serving everything in a thousand-km radius, with rail links taking over the role of short-stop feeder. There is no technological reason that Europe needs two hundred airports if air travel is transcontinental-only. Ten should more than suffice.

- Jake

Friends come and go. Enemies accumulate.