Hydrogen sounds like a great idea for decarbonization until you get around to asking, “wait, where do we get the hydrogen from?” and realize that it’s incredibly energy intensive and the most popular process releases a lot of CO2 directly.
Hydrogen is an energy storage, like a battery, so of course it requires a lot of energy to produce, that’s the energy that you get back when consuming it (minus inefficiency losses of course).
The advantage of hydrogen over fossil fuels is that it can be produced from renewable energy, while fossil fuels cannot.
There’s a comment on another post with this article doing the math on this, and it seems like the net emissions (when you account for efficiencies) actually favour steam-reforming + fuel cells.
Last time I checked, CO2 released at that altitude has 3x the effect on radiative forcing, so it’s good that we’re not dumping it up there. I know water is also a greenhouse gas, but I expect the residence time to be substantially lower than for CO2. So it would be a net positive as long as we’re emitting on the ground the same amount of CO2 as emitted up there (we’re probably emitting more, but probably not 3x more and it would be easier to capture at the exhaust than from up there)
PS: more on radiative forcing factors here https://sustainable.stanford.edu/sites/g/files/sbiybj26701/files/media/file/s3-radiative-forcing-rfi-memo_public.pdf
