We should mandate that aviation fuel is 10% carbon neutral. And then steadily up that fraction yearly. How they satisfy that is up to them - synthetic fuels generated using solar/wind, biofuels, or whatever. The point is that we're not going to be able to replace liquid fuels for long-haul aviation, so we both need to factor in the increased cost of carbon-neutral fuels and at the same time provide a market that stimulates investment.

For many routes, if we did this, aircraft would perhaps not be price-competitive with rail. So be it.