ideal cities: how many inhabitants?

We try to answer here to the question 3 from http://ihp-lx2.ethz.ch/energy21/ws14/ubg01/ubg01.pdf

Assuming 1000 m2 needed to produce food for one person, sets a limit on the density of inhabitants, that must be lower than 1000 peoples per km2. That is coherent with this document on "ruralization":

http://www.unicamp.br/fea/ortega/energy/Folke.pdf






What is intended here is the size in terms of inhabitants, while the radius is considered the "minimal exploitation land" to produce food. Today this ends up to be anywhere in the world (like using African land to feed Europe) thanks to the globalization. What if this is not the case? Let us imagine that all the food has to travel by land, disentangling by available transports.


-- Backpack on foot --

1) In case of a circular city of 5km of radius (~78km2), where all the food must be produced in the land inside, the maximum allowed population will be of the order of ~80 thousand peoples. By comparing to the cities in the wiki

http://en.wikipedia.org/wiki/List_of_largest_cities_throughout_history

this is indeed the case of Babylon and Thebes up to more than 1000 years BC.
According to wiki, Babylon was 9km2 (http://en.wikipedia.org/wiki/Babylon), but elsewhere the walls are of a size not so clear http://www.globalsecurity.org/military/world/iraq/babylon-walls.htm going up to 60-80 km of circumference.
If the density was actually larger, that is probably due to the help of animals (see later).

2) Let us now imagine that the circular city of 5 km radius is full of concrete (just to simplify, still some small amount of food could be produced on roofs, etc). Since peoples at the border of the circle could go 5km toward outside to collect food, we can imagine a city fed by a circular crown around the center, reaching 10 km. This works only if everybody walks the maximum to feed itself: even if he could find food much closer, this is left for the peoples in the middle. In an imaginary city organized in this "egalitarian" way, the land should be enough to feed 235 thousand peoples.

In principle, provided the maximum density is respected, an additional external crown could be added to feed people in the first crown, and so on. But this has to face the limitation of building houses, streets, services, etc for the peoples that would live in the first crown, that is also where the food is produced for the peoples in the inner circle. And this neglect the reality of a geography that could be quite complex with mountains, lakes, etc. Anyway, various settlements like those "crowned cities" could be the upper limit for a "foot-only" agglomerate.

3) To note that if we imagine that the inner circle is not actually filled of concrete, but still being able to host and contemporaneously feed the initial 78k peoples, they would sum up to ~300k peoples (like Alexandria, in Egypt), half of them living in the circle, but only one quarter comfortably sitting on their food.

-- Horses city --

To feed an horse I assume are needed 6000m2. But on the same pasture it can be raised a cow, and 2-3 sheep. Given the additional milk and the meat, I assume that this land produces food for 4 peoples (instead of 6). The horse is assumed to carry a chariot with the food for 4 peoples. In summary 1ha would feed 8 peoples, instead of 10 in a foot city.
Then a 15 km radius city could contain ~120k horses (plus cows, sheep) and 560k peoples (of which 80k sitting on their food in the inner 5km circle). First cities of this size are Alexandria and Rome 100 BC.

-- Bikes city --

Let us assume that peoples riding bikes eat the same (i.e. they require the same land) than peoples going by foot: 15km radius --> ~700km2 --> ~700k peoples, but probably those peoples could be more hungry than in the previous case. Factories for bikes, extraction of metal, wood/coal burning for factories have to be created somehow, and environment could be not nice around, and then bikes should be produced from enough far outside the city. Beijing reach this size around the XVI-XVII century (not yet thanks to the bikes, still to be invented, but more probably to the exploitation of the empire farmers!).

-- Truck city --
To feed a car/truck oil fields are needed somewhere, and the transportation to bring to the city. Energy is required to refine, construct trucks etc. Similarly than for bikes that should happen enough far away.
In a 50 km radius then could fit ~8M peoples, and we are reaching here the size of London/New York in the full running industrial era.

-- Conclusions --
The current megacities need fossil fuels to work, and the peoples living inside so big cities can do because they exploit land somewhere else. In many modern countries the amount of farmers is of the order 1:100, and they can produce so much food only thanks to fossil fuels.

According to the last IPCC report fossil fuels should be phases out by 2100 http://www.bbc.com/news/science-environment-29855884
What will be the destiny of the cities in this scenario knowing the boundaries?