I'm convinced we need zero emissions aircraft. Efficiency improvements alone are going to suffer from the Jevons Paradox. In fact, that "paradox" could help exp

I'm convinced we need zero emissions aircraft. Efficiency improvements alone are going to suffer from the Jevons Paradox. In fact, that "paradox" could help explain why air travel emissions have increased so much even as aircraft have gotten much more efficient. https://en.wikipedia.org/wiki/Jevons_paradox https://en.wikipedia.org/wiki/Jevons_paradox So to avoid the Jevons Paradox, we need actually zero emissions aircraft. Battery-electric or possibly hydrogen are the main contenders, here. Battery-electric in the near term is limited to about 1000km, maybe 2000km range. That's technically enough to do transatlantic crossings if you stop by Greenland and Iceland or possibly the Azores. And you could do transpacific by following the Aleutians. Pretty much the hardest place to visit is actually Hawaii. But for domestic flights, battery-electric is actually a superior option to high speed rail in the US. Typical HSR routes in the US are only about 300 miles or so, well within range of existing electric battery tech that could be adapted to airplanes, and even a fully subsonic, Mach 0.5 propeller-drive electric airplane would be much faster than even the fastest practical HSR. And operating at high altitude (with far lower air density) would mean that the electric aircraft would be more efficient than a hypothetical Mach 0.5 high speed train. But all of this will require probably non-traditional aircraft makers. The duopoly is culturally unprepared for the kind of radical engineering necessary to make a 1000km battery electric aircraft feasible (almost sailplane-like efficiency, combined with very high battery mass fraction, combined with extremely efficient battery packaging, state of the art structural mass, and transition to higher performance chemistry where possible). Liquid hydrogen is also a possibility for electric air travel. Compressed room temperature hydrogen is hardly more energy-dense than battery-electric, though, so you're going to have to go at least cryogenic, if not liquid, for hydrogen storage. Liquid hydrogen wouldn't be too bad in this application since the time of storage wouldn't be too long, and the scale would be large enough (if used in a widebody aircraft) that boil-off may not be too much of a problem (besides, you need a constant feed of hydrogen anyway). But a big problem is the fact that anthropogenic high altitude water vapor is maybe even worse (by a factor of 2) for climate change than CO2, so you'd probably want to condense the output. That means using fuel cells and then storing the condensed water on board and doing occasional low altitude dumps (it may even be feasible to store the water for the entire trip, dumping only right before you land) or perhaps freezing the condensate into pellets (small enough to not pose a lethal hazard but large enough to not evaporate on the way down) and then dropping them along the way. So long-haul air is a major challenge.