We can still "have it all" -- flight, meat, etc., if the right incentives exist; this too is more notion than solution but is the only possible path.
We can make "sustainable" fuel, and cattle can be fed seaweed and restricted to existing range lands (i.e., don't destroy more forests).
A key problem is that this issue and associated solutions have been artfully politicized by their incumbent stakeholders so a significant portion of the population will resist these efforts purely out of spite.
A dangerous idea, there are fewer cattle than humans on Earth, even if we add sheep, goats and pigs, there are still more humans.
I'm being a little glib in that comment, but the sentiment is I think true and I wish they'd be honest about it. If they stated plainly that "we think in order to achieve what we believe are the required reductions in xyz, we have to decrease the human population by X% so less is consumed" I would actually respect them more. Instead the de-growth mentality is rhetorized up to look nice, but again it's just window dressing.
And there are natural fertilizer paths that aren't cattle.
You've constructed a strawman.
You don't even understand. The methane's mostly for the hydrogen, used to make ammonia or some other nitrogen compound. There are other ways of making this, Birkeland Eyde, for instance, but there like one third as efficient, at best. This means people starve. You might make the case that we could efficiently use livestock manure as a substitute (it's the green solution), but not if you're going to turn this into Planet Vegan. Given the necessity of animal manure, it's difficult to think that "get rid of all the livestock" stems from anything other than bizarre vegan tendencies.
If you absolutely have to get rid of the livestock, so be it, but then you really do need the fossil fuels. Pick, it's one or the other as far as I can tell.
> And there are natural fertilizer paths that aren't cattle.
Nope. You've got livestock (hogs, poultry, etc), or you've got fossil-fuel-based Haber-Bosch. I'm not even really aware of any science fiction concepts "other paths".
Additionally, pulse crop rotation is increasingly being used to provide some free nitrogen to the soil.
Bioreactors with Azotobacter vinelandii are showing promise in providing nitrogen fixing in a reactor.
And like, industrial composting produces fertilizers with plenty of nitrogen. Soybeans, in particular, need no nitrogen inputs and produce plenty of both useful food and nitrogen containing plant matter.
Our wastewater also contains tons of nitrogen, both from biosolids and from agricultural runoff. These are difficult to get at safely, admittedly, but not impossible.
If you pause to think about it, all that manure needs to come from feedstock plus other inputs (water, etc). If you diverted that same feedstock into producing fertilizer (in the case of soybeans) or not producing at all (in the case of dent corn) you'd have plenty of material and a reduced demand. Manure doesn't just magically come from no inputs.
The Haber-Bosch process needs hydrogen, as you said in your first paragraph. Hydrogen can be made directly from water by electrolysis with renewable or nuclear electricity. This was already done on an industrial scale in the 20th century, with hydroelectric power used to produce hydrogen for ammonia:
"1921–2021: A Century of Renewable Ammonia Synthesis"
https://www.mdpi.com/2673-4079/3/2/11
That's how to use the Haber-Bosch process without fossil fuels or livestock.
Nice. You've just made one of the most energy-intensive necessary industrial processes even more energy intensive. Even while you're doing your best to demolish whatever baseload power we might still have.
> This was already done on an industrial scale in the 20th century, with hydroelectric power used to produce hydrogen for ammonia:
Got any spare hydroelectric? Any more rivers you want to dam up? Maybe you can decommission a few more nuke plants and replace them with brown coal.
I'm almost positive I wouldn't make the cut, the top .003% has gotta be a higher bar than me to clear, hah. But still fun to think about.
I guess that might hint at a fly in the ointment to drive the plot...