[darksaints]

Fuel cells are a possibility. There is some pretty high power densities (2.5kw/kg for SOFCs, 10kw/kg for less efficient PEMs). Electric motors are now reaching 15kw/kg. For comparison, a GE90 is about 10kw/kg, and older turbofans down to 2kw/kg. Running on liquid hydrogen presents some problems that are definitely solvable, but SOFCs can run on pretty much any hydrocarbon fuels at extremely high efficiencies (50-60%, 80% with combined cycles, compared to the ~45% that jet engines get).

I actually think the future may be a lot more simple than we're making it out to be. With some slight modifications to design, we could run existing turbofans on liquid hydrogen fuel. The biggest objection that most people have is the storage of hydrogen...but liquid hydrogen doesn't actually need to be stored in cryo tanks as long as the fuel is being used at faster rates than it evaporates. That's not a problem with jet engine consumption rates...low tech insulation may be sufficient.

[avmich]

There reportedly is an additional threat to the climate coming from inversion traces (modifies enough of Earth albedo?), so if we're delivering water to high altitudes, it's still not good.

Electrical motors would solve this; if we'd be able to avoid dispersing water from fuel cells up high, that could work.

LH2 has storage - and safety - issues still not solved. Toyota Mirai approaches it from a different angle, with some other shortcomings.

[WalterBright]

Burning kerosene also delivers water to high altitudes - the result is contrails.