That's hardly relevant. A single big farmer with modern equipment can farm a lot of land, and they make a big chunk of the rural population. And that equipment burns a lot of oil. But if people de-urbanized they wouldn't all suddenly become big farmers, if for no other reason than for lack of land. Dig up some statistics on carbon footprint of third world sustenance farmers and compare that to city dwellers instead.
There's more to environmental damage than carbon footprint, though.
Part of the conceptual problems that worry me is that most low-carbon agriculture methods (that I'm aware of, anyway) tends toward requiring a good deal of
space -- and arable land is, indeed, limited. One of the numbers I've never seen crunched is if it's even
possible to support our current population with subsistence farming techniques. Even if modern techniques have a higher carbon footprint, it's certainly more productive per square foot. The primary questions there, I'd say, is A) What's the ft
2 per person needed by low carbon agriculture and B) Does that much arable land exist? With partial tertiary questions being if that much land will exist long enough to start reducing carbon issues and what sort of damage utilizing that land is going to do (Does it mean clear-cutting more forest? Does it mean driving certain animal and plant species to extinction?). If we end up having to decimate our own population several times over and
still end up wrecking the biosphere, then it's likely not a viable solution (though certain utilizations of it may be
part of the solutions.).
There's also the flat fact that even subsistence farming does a certain degree of damage. It's still going to be killing off local species (crop killers of various sorts, both plant and animal) -- our species has been an extinction event and environmental hazard since
before we started low-carbon agriculture, and that won't change if we return to it. It's also still going to be consuming resources of varying sorts, even if the carbon use is reduced.
Basically, there's going to be negative aspects to low-carbon farming and related means of supporting populations, and likely more than seems immediately evident.
Especially if it's going to support a population in the billions. My big question to what you're supporting, DJ, is if it's actually going to be less damaging (I'd guess
yes, but when you're dealing with scales like the topic in question is, a lot of non-obvious shit goes down) than what we're doing now and, just as importantly, if it's not, where are the gains occurring? What's being traded out for those gains?
And all that's just a sort of representative sample of the sort of issues that pop up when you're actually contemplating the problem of something as freakishly wide scale as actually supporting our entire population on subsistence farming. Someone actually familiar with agriculture methodlogy and the general logistics involved would probably have dozens more. Doesn't mean it's not a good thing to ask and study, but it
is a bloody huge subject, and I'll admit I have trouble swallowing any solution that's, well, simple. Without a great deal of support, anyway. Because simple solutions often have very,
very complicated consequences
... I should probably actually look into this stuff more, really. I'm actually aware of a few academic sources I could probably tap for information, but... bleh. Effort, ha.