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I made these sketches to envision a world where food is grown in buildings and people can live in forests covered lands instead of the land being used for agriculture. But how big would such a city need to be? Let's say the skyscrapers are about the size as they are in New York.

Description of first sketch: What you are seeing in the distance is not a city. Or at least, there are no people living there. That’s where food is grown. People are living in the forests again.

Description of second sketch: Each high-rise has a variety of species that are grown in vertical gardens to ensure biodiversity on each floor. These fully automated buildings yield plenty of crops that no other land is needed for agriculture.

People in forests Crops in buildings

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    $\begingroup$ Not every user can read infographics. It would be much better if you put those lines in actual text $\endgroup$
    – L.Dutch
    Dec 21, 2021 at 14:53
  • $\begingroup$ What would help too - do you also mean with the population of N/Y City (circa 9 Million)? Maybe the state population's not that different anyhow, but could you clarify? $\endgroup$ Dec 21, 2021 at 14:55
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    $\begingroup$ would this future population have a different diet than today. Also, diets from around the world do significantly impact the amount of land necessary to support a population. $\endgroup$
    – Sonvar
    Dec 21, 2021 at 15:06
  • $\begingroup$ The sun light which falls on an area is the sun light which falls on an area. You cannot use it more than once. Vertical agriculture needs some sort of artificial light source; and anyway, even if you hand wave the source of light, you still have the issue that people, on the average, greatly prefer living in houses and not in the forest. You know, having a roof over your head, having a source of heat, having a source of light at night, and so on. $\endgroup$
    – AlexP
    Dec 21, 2021 at 15:11
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    $\begingroup$ @AlexP i believe OP meant log cabins, small farmsteads (only without the farm, apparently), hamlets and suchlike, and not in the raw wilderness. your points, though, are very valid. unless unlimited power(tm) is available, agri-skyscrapers are unfeasible. and unless building space is very limited, agri-skyscrapers are utterly pointless IMHO. (besides that, i kinda like the idea, though) $\endgroup$ Dec 21, 2021 at 16:46

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Let's throw some math around, shall we?

The average area of soil needed to feed a person for a year seems to be about 370 square meters. Let's round that up to a generous 500 to ensure a varied diet and safety against crop failure.

The average floor space of a skyscraper seems to be about 125 000 square meters

Which means that one Agri-Scraper would feed 250 people on average with current technology.

Let us very generously assume we have unlimited energy, high-efficiency grow-lights and irrigation, sustainable high-tech fertilisers - which are absolutely required to make this even work - and bump that number up to 1000.

New York City (metropolitan area) has 20 million inhabitants - which means you'd need 20 000 Agri-Scrapers. New York currently has ~300. So...

Your Agri-York would be huge

Even when we're a bit more optimistic and say we don't need all the smaller building for logistics, storage, etc., and assume the Agri-Scrapers only make up 10% of the agricultural area available, you'd end up with roughly 6 times the current footprint of new york.

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    $\begingroup$ But that's still a lot smaller than the amount of farmland that feeds New York currently $\endgroup$ Dec 22, 2021 at 0:00
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    $\begingroup$ It's a good answer but you haven't allowed for the greater space efficiency of vertical farming where the idealised theoretical vertical farm skyscraper can feed 50,000 people and you'd only need 400 for your model of New York $\endgroup$
    – Separatrix
    Dec 22, 2021 at 8:35
  • $\begingroup$ I agree with @Separatrix - skyscraper has a lot of cubage. I think you failed to account for that. If the building is devoted solely to plant cultivation, you need to, at the very least, quadruple the available area, because you don't need 4 meter high floors... $\endgroup$
    – AcePL
    Dec 22, 2021 at 12:42
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    $\begingroup$ Not to mention the amount of forest you would need surrounding Agri-York to effectively house those 20 million people. It would be utterly impractical for any of them to visit the food in town when they're hundreds of kilometres away in the forest... with no roads, etc. Assuming you had a population density of 50/km2 (similar to Madagascar) you would need 400,000km2 of land for your residents. You'd have a bunch of food skyscrapers surrounded by a forest the size of Iraq where people actually lived. $\endgroup$
    – J...
    Dec 22, 2021 at 14:04
  • $\begingroup$ Note that your 370m^2 per person quote is for a strictly vegetarian diet that is like 90% potatoes. That is pretty much what people in Europe did around say 1700 but I'm not sure that a modern or near future society would be happy with that. The rounding to 500m^2 doesn't help much here. Even if you stay vegetarian but want a variety closer to what a modern day vegetarian eats you should probably multiply by 10 or so. For a non-vegetarian diet multiply by 10 again. $\endgroup$
    – quarague
    Dec 22, 2021 at 19:59
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Here is a frame challenge.

Why would someone build agricultural skyscrapers instead of one agricultureal skyscrapter?

Why build a lot of tall buildings side by side when you can build one giant tall building that is many miles wide?

Natural sunlight would be insufficient anyway, no matter how the buildings were shaped to let it in. So artificial lighting would be needed anyway, using electricity generated by wind farms, solar engery, fission power plants, fusion power generators, etc. at the site of the vast farm or far away and transmitted to it by electric wires.

And why would people prefer to live in the forests in log cabins or whatever instead of in habitation sections of the vast city farm?

So people love nature and love living amid nature. Other people hate nature and would attack and destroy nature if they were close to it, so they have to live away from nature to protect nature from them. And probably the vast majority of people would prefer to live apart from nature in vast enclosed habitats.

Probably most people would prefer to live in vast enclosed habitats like moon bases or space stations on Earth, with totally enclosed ecosystems recycling air, water and food. Once such habitats are constructed and perfected, they will be far moe comfortable than living in tiny individual houses surrounded by the great outdoors.

The air will always be the right temperature, it will always be free of artificial pollution and natural irritants like allergy causing pollen and dust.

And people living in enclosed habitats with total recycling will have no effect on the surrounding natural habitat. They will take nothing from it and put nothing in it.

If you love nature, and want what is good for nature, you will want to remove human interactions with nature. If you love humans, you won't to remove them by exterminating them, so you will want to isolate them from nature as much as possible to minimize their interactions with nature.

There have been a number of questions about the acreage needed to support a specific number of humans using agriculture, hydroculture, and aeroculture.

See my answer to this question:

How can Dwarves produce honey underground?

Giving Tolkien Architecture a Reality Check: Dwarvish Kingdoms

And see this question:

How many people can you feed per square-kilometer of farmland?

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    $\begingroup$ "skyscrapter" Is that where the fertilizer for the agricultural skyscraper is produced? $\endgroup$
    – Glen Yates
    Dec 22, 2021 at 16:18
  • $\begingroup$ @GlenYates, good one! But likely any scraps from waste would be composted within each building to help provide heat in the winter. It also produces some CO2, which plants use to grow. And the methane could be accumulated for power and heat production, and capturing it within the building that uses it would prevent the need of piping between buildings. Animal wastes would have to be collected outside the building. Using human feces in agriculture is a biological hazard. It would have to be processed first, which would reduce the positive impact of using it in the first place. $\endgroup$ Dec 22, 2021 at 17:53
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Eloi:

Given energy and population, you city size depends on the tech level and population. Assuming a post-scarcity society that simply wants to live in the forest and nature, hydroponics can supply abundant food. There isn't an equation to calculate food production in this fashion per person, but it would require advanced tech to set up and maintain.

But why no do one better and eliminate the city all together? The same hydroponics, built underground (along with any needed industry) mean there is only small surface distribution nodes where forest dwellers go to pick up goods. The City size is ZERO. In this scenario, your Morlocks are automated systems to support the populace. It's an idyllic paradise (unless your Morlocks are less than altruistic).

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  • $\begingroup$ "But why no do one better and eliminate the city all together?" Because high rises are cheaper to build and operate; also less damaging the environment in almost all stages of their life. $\endgroup$ Dec 22, 2021 at 0:23
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    $\begingroup$ @AdrianColomitchi Not sure about the environmental aspects, long term. It's post-scarcity, so I'm assuming money isn't a factor. I know construction for underground isn't great, But I was putting ascetics first, assuming the goal was idyllic paradise. I like the tie-in to a degenerate post-scarcity society that comes with the underground. $\endgroup$
    – DWKraus
    Dec 22, 2021 at 1:12
  • $\begingroup$ "Not sure about the environmental aspects, long term" if considering only air availability and temperature regulation (which are ongoing energetic and environ penalties to it). "I like the tie-in to a degenerate post-scarcity society that comes with the underground" fair enough, de gustibus non disputandum. $\endgroup$ Dec 22, 2021 at 1:36
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Your idea won't work, unless you have same magic, infinite and free power source for your vertical farms.

To grow plants you need a certain amount of energy that your plants can use. In current farms this energy comes as free sunlight. By moving your farmland indoors you need artificial lights, and they need energy. Because solar panels are only ~20% efficient, each square meter of indoor farmland now needs an additional 5 square meters of land covered in solar panels to deliver the same energy as artificial light to grow your plants.

Source: https://www.youtube.com/watch?v=ISAKc9gpGjw

Same goes for your water supply. If you concentrate your farming area, you still need the same amount of water, and this means you must extract a lot water from an area much larger than your vertical farms footprint. Sure you could catch and recycle a lot of the evaporated water inside your farms, but you still need a large reservoir near your city for dry summer months.

This still won't work, because you have no supply chains set up to move goods, service crews, fertilizer, seeds, spare parts, construction workers etc. in and out of your vertical farm district.

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