Re: Emissions
When I ran the numbers a while back, water vapor has about 100 times the greenhouse effect of CO2
* CO2 is only about 0.04% of the atmosphere and is relatively stable, where water varies greatly and can be 1% OR MORE
* CO2 has a tiny absorption spectrum for IR radiation within the band of energies corresponding to temperatures found on earth. Water has an OBSERVABLY SIGNIFICANT IR absorption spectrum, and makes a HUGE difference on earth surface temperatures at night.
And so on.
Worrying about CO2 emissions as compared to WATER VAPOR is like picking up pennies and ignoring $100 bills
(then again I wouldn't worry about either - we cannot control water vapor and C02 pales by comarison, drowned out by the chaos that water adds to the normal weather cycles)
besides, the volume of hydrogen and the containers it would need for an aircraft would make it LESS FUEL EFFICIENT (based on cargo weight and humans on board) than using regular hydrocarbon jet fuel. Until we master hydrogen fusion in an aircraft engine, this will CONTINUE to be the case, because of the laws of physics and the properties of materials. If, however, a clever hydrate-based storage method is developed that can make this "no longer true", then I'll be wrong and you can laugh at me. Until then, hydrogen fueled planes are NOT practical.
The problem isn't the mass of the fuel, it's the container volume. Designing container volume of about 4 times (from an online source) the size of equivalent jet fuel tanks that ALSO have cryogenic insulation and some additional things to keep the H2 both liquid AND pressurized, and that's just the beginning.
The Saturn V first stage and Musk's Falcon rockets use Kerosene-like fuel because of physical tank size and atmospheric resistance. If you can make a taller rocket that has no air resistance, like in space, hydrogen makes more sense for mass-to-thrust. But the tanks still have to be 4 times bigger, so you lose something in the materials used to construct the rocket, not to mention the cryogenics.