[shiroiushi]

>We have enough space to give every human (not just every family - every individual human person) a large house on a big yard. This wouldn't even cover the earth - we could fit all that in just Ontario.

These kinds of calculations aren't very useful IMO, because they ignore the massive amount of overhead space needed to support a human life in a modern society: roads, train stations, airports (I guess not necessary if everyone lives in Ontario...), schools, water treatment plants, electric power generation facilities, farmland, commercial buildings, office buildings, warehouses, factories, landfills/garbage processing, the list goes on. Look at any modern city, including the dense & walkable ones: the amount of total space for housing, while significant, isn't that much of the total land area of the city, and that's ignoring all the stuff outside the city needed to make the city work (especially agricultural land, but also power generation, and other dirty industries that are either well outside cities, or on their periphery).

Also, at a very minimum, humans need food and water to survive. You're not going to grow enough food for your family in your yard (and certainly not year-round). And freshwater resources are scarce in many places. Ontario cannot grow crops year-round, and only has so much water.

[FredPret]

> ignore the massive amount of overhead space

I allowed for some overhead, but even ifyou double or quadruple the space requirement, it's abundantly clear that space isn't an issue.

> Ontario cannot grow crops year-round,

It can, and it does. There are greenhouses and hydroponics - we would never spontaneously concentrate ourselves like this but if we did mass hydroponics and vertical farming would suddenly become much more economical.

Also, in this scenario we're free to move our ultra-dense spot anywhere on the planet, I just like Ontario.

> and only has so much water.

Ontario happens to have an absolute crapton of water - 20% of the world total. But that doesn't matter. Water goes in a cycle. If you have a large starting volume, the only limiting factors are energy and money to purify used water.

[shiroiushi]

>I allowed for some overhead, but even ifyou double or quadruple the space requirement, it's abundantly clear that space isn't an issue.

I think you'd have to more than quadruple the space, but sure, I do agree humans don't absolutely need to be spread across the world. Still, I think you're discounting things like resource extraction (mines), but I guess since people don't normally live in such places we can ignore those.

Personally I have wondered sometimes what the world would look like if almost everyone concentrated into cities laid out like Tokyo (i.e. very dense, and not very car-friendly). There'd probably be a lot of abandoned places.

>There are greenhouses and hydroponics - we would never spontaneously concentrate ourselves like this but if we did mass hydroponics and vertical farming would suddenly become much more economical.

Forgive me for not knowing that much about agriculture, but are greenhouses and hydroponics really viable for growing enough food for all of humanity? (Of course, they wouldn't be much help for growing livestock or seafood.) And has anyone really shown vertical farming to be viable? It looks like a nice idea in theory but seems to require advances in robotics to be economically feasible.

>Ontario happens to have an absolute crapton of water - 20% of the world total. But that doesn't matter. Water goes in a cycle.

Ontario might be OK here, but a lot of cities really do face freshwater shortages, for instance in the American southwest and Los Angeles. Of course, it'd help if people didn't use so much water for their lawns...