I'm thinking we each make our own little ecosystem, maybe a half an acre would be enough. Maybe make it double walled, just in case, and then say screw it to everyone else as climate change and various other big things occur on the outside.
You might be interested in Biosphere 2 [1], a 3 acre hermetically sealed dome stucture designed to house about 8 people with an ecosystem to provide them with everything they need to survive.
They didn't quite get oxygen and food production to the required level, but if you add another acre or so it should work. With renewed interest in moon and mars colonies somebody is bound to revive that line of research.
The first experiment was hampered by them not realising the cement needed CO2 to cure, to be honest I would have halted the experiment to find out what the issue was then restarted it. Although they might not have realised this at the time, but then they weren't really testing a proper closed system.
It would have been useful if they were able to test a closed system without the cement sucking up the CO2.
On the restarting, I completely agree. You'd need to restart the whole thing multiple times as your knowledge increased. Maybe somebody else is doing large scale closed system human habitats, but I haven't heard of any.
edit: A bit of googling led me to the Russian one and some books on the problem generally. It sounds like a great speciality but probably hard to get a gig in.
If I had a multi-decade old terrarium, I'd be darn tempted to think of sensors you could stick in there and see if there's some sort of cycle going on.
If you want similar projects that weren't influenced by Hippy culture to try and emulate a complex system that nobody really understands you should look into the stuff the Soviet Union did. For example https://en.wikipedia.org/wiki/BIOS-3 is much more interesting imho. Looking into the minimum number of species you need for sustainability and not being afraid of using technology to augment the system is the better way for reaching truly closed systems.
I've a vague fantasy about returning to the remote Scottish island where I was born, which is pretty windswept with almost no trees, and building a house and garden inside a large Solar Dome, growing trees and plants that otherwise wouldn't survive in such a hostile environment. Apparently it has been done to some extent[0]. Side note - they discovered in the Biosphere 2 that trees need some wind, because the stress helps form reaction wood to strengthen the tree[1].
What if the optics get damaged? What lasing medium are you using and what if it leaks?
For long-term defense systems, I'd try to go as low-tech as possible. Every bit of technology of the past 300 years is tied to a rather large manufacturing & supply chain. If your turret has μC in it, where will you get a new one if the marauders happen to score a lucky shot?
You can reshape the question into how small could you make a microcontroller fabricator which can produce all of the electronics within itself? One square mile to one square foot - it seems likely that the best possible is within that range. How well could it be optimized?
It would get too hot in most places, I think. Glass, especially if double walled, stops the heat getting out, but most of the heat from the sun will be able to get in (as infra-red). So you'd need supporting systems (i.e. air conditioning) outside.
AFAIK, at a certain depth below ground-level, the temperature is constant all-year-round. Something like that in two meters the ground is constantly around 14°C or something like that.
...so basically all you'd need to do is just dig a hole, put a bunch of metal pipes inside, seal it all up and run cooling water through and couple it to a closed-loop temperature control and you're done.
The idea is from an article from overclockers.com from the mid-2000s, about a dude who built his own water-cooling system and wanted a more efficient radiator. Not sure if I can find it again...
