To partially offset the nonarable areas of the earth, one can, at least for food production, add back in large areas of the sea, and a bit for freshwater bodies. Properly managed aquaculture can produce enormous amounts of protein, as evidenced by either catfish farms, or, even more dramatic (but bonier) the stunning numbers / biomass of Asian Carp now in some of our waterways. (I’ve seen the latter 1st hand - it is “mindblowing”.)
It’s also true that one can look at the Israeli example: Many nonarable areas need not be so. Basically, we just need efficient distribution & usage of existing (and renewable) fresh water sources.
Granted, some fisheries HAVE already been depleted, but a lot of that is poor management. I would contend that aggressive and well run aquaculture programs world-wide could easily feed double the existing population, with no overall negative effect on other food production.
Incidentally, your figure of 1 person per 270 x 270 meter block is “interesting”. That’s 72,900 sq. meters, or a bit over 18 acres per person. If we assume aquaculture can fill in for truly nonarable and uninhabitable areas, and knock off 1/3th for roads, buildings, parks, and such, in arable areas, that leaves us with 12 acres per person.
At least one source states that on average, in the US, one acre can feed one person.
http://www.farmlandlp.com/2012/01/one-acre-feeds-a-person/
So, it would seem we have quite a bit of potential left.
BTW, that article is REALLY interesting, especially the parts about well managed livestock production, and the point that only 10% of US corn production goes to human consumption. Roughly 90 MILLION acres worth goes elsewhere. Yet, we really don’t need to burn food as fuel...
Looking at it another way, I was intrigued to learn that as a rough average, in the US, per year, an acre typically produces around 150 bushels of corn, or 8,400 lbs. of shelled corn. So, the US produces roughly 840 BILLION lbs. of corn each year. That’s approx. 1/3 lb. of shelled corn per person on the planet, 365 days a year.
The WORLD will likely produce around 1 billion metric tons of corn this year, or around 0.86 lbs. per person per day. And that’s just corn. (Most people don’t realize that when it comes to wheat, the US is only about 8% of world production. The EU, China, and India far outproduce us, with Russia somewhat ahead in good years for them.)
Other fun facts:
“A bushel of corn fed to livestock produces 6 pounds of beef, 13 pounds of pork, 20 pounds of chicken, or 28 pounds of catfish.”
http://www2.kenyon.edu/projects/farmschool/food/bushel.htm
So, basically, in one year, 2 acres of land, plus 1 acre of catfish farm, can crank out as much or more catfish than I want to eat* in my entire life, assuming I “only” make it to age 85 or so, and assuming half of the 28 lbs. of catfish mentioned above is waste / discarded.
*I really like properly prepared catfish. :-)
This also assumes the 2008 figure given in this article, of 8000 lbs. of catfish per year per acre, from a commercial catfish farm in Alabama: