Assuming a technologically advanced civilization that has grown sustainably from the resources of its own planet (and maybe "a few" additional resources from its own solar system), what would be the maximum population the planet could host in a stable ecosystem for millions of years?

We have enough time and refined technology to increase our population, completely re-engineering a new ecology with the most efficient oxygen producing plants / cells / organic microchips, wastewater treatment, growing genetically engineered food from any suitable energy source, have cheap fusion energy and whatever other non-hyper-fantasy-technology we could realistically imagine (i.e. no Star Trek replicator scenario).

Or in other words: Imagine a highly efficient CELSS (Closed ecological life-support system) of Earth's size. How many people could live there, taking into consideration the abundancy of natural resources?

Edit: Maybe I should explain the background of my thoughts and be more explicit about the constraints that I see.

I try to imagine where Earth and mankind could develop in the next 10k, 100k or 1M years (hard to set a timeframe for such a development). I assume mankind learns from the ecological mistakes we are currently doing, but still will have a tendency to grow as technological progress allows it. Also the desired standard of living will presumably grow.

So everyone wants to live comfortably. We could define limiting factors for a comfortable life and calculate different scenarios, depending on what people still would consider comfortable. Let's think about space and resources.

For the space factor we need maybe at least 50 m² for privacy/living space, plus space for community life, recreation, work, transit. Makes 200, 500, 5000 or whatever m² per person.

Yes, we have fusion energy, but no arbitrary conversion of matter. Some technologies might still need rare earths, precious metals or other elements. We probably can't build everything just from carbon and silica since we might want to make use of what physics has to offer.

An approach to estimate that resource factor could be to consider the abundancy of several technologically relevant elements on earth or the solar system today and define a usage per person based on todays usage divided by a "technology efficiency" factor of our future Earth of 10, 100, 1000 or whatever.

Oh, and of course we need water to drink and maybe to shower. Considering all of Earth's water, how many people could live from it if one person needs 100, 1000, 10000 l of water in circulation?

Really ultimate limits could be waste heat and gravity like @Ryan_L and @Chickens pointed out.

So what are the limits for which scenario?

Did I miss other relevant constraints in my considerations?

  • $\begingroup$ It depends entirely on technology level. Given the technology level you suggest is not one that exists in real life, I suspect any possible answer will be a matter of opinion. At our current level of technology, the Earth can comfortably sustain about 3.8 billion people. $\endgroup$ – Arkenstein XII Aug 29 '19 at 22:50
  • $\begingroup$ Ok, I've re-read the question, presumably if you have cheap fusion, then you can use all the materials of the solar system to construct layer-upon-layer of civilizations on the Earth, that is until the Earth's mass generates a gravitational field prohibitively strong for the human race to survive, so, the question becomes, what are the biological parameters of your race? What biological engineering do you posses. Frankly the answer would be trillions, what are the constraints to this number? You seem to offer none, can you edit your question to clarify? VTC unclear what you are asking. $\endgroup$ – Tantalus' touch. Aug 30 '19 at 2:36
  • $\begingroup$ Suppose there is a base on the Moon or Mars that grows into a city and is totally self supporting with no resources from Earth and closed cycle ecology. What effect would the base's garbage, which is totally recycled there, have on Earth? None. The base would have zero effect on Earth's ecosystem. and it is possible to build closed cycle moon bases on Earth which are equally disconnected from Earth's ecosystem. So countless billions and maybe trillions of humans could live on Earth and the rest of the solar system with little or no inter action with Earth's ecology. $\endgroup$ – M. A. Golding Aug 30 '19 at 16:54
  • $\begingroup$ Given the Sun as an ultimate sustainable power source, divide that by the about 250 W average power a human needs for sustenance and technology. Raise that to 500 W to cover for more advanced technology. Then SunPower/PowerPerPerson will give you the maximum sustainable population. Space is not a problem (you can both build up, and deep down). Of course, space harvesting of solar energy or drastic diminishing of human power requirements might increase the figure. $\endgroup$ – LSerni Sep 1 '19 at 12:24
  • $\begingroup$ I've edited my question. I'd vote to reopen it if I could :) $\endgroup$ – ascripter Sep 6 '19 at 13:58

It depends on your definition of sustainability. If you mean it in the sense that the natural ecology remains in balance, the answer is probably less than we have now. I think I saw someone cite 3.8 billion once, but it's anyone's guess really. I personally think that number is a little low, because current production expects growth, not stasis. You don't need to cut down as many trees if there are no more new families that need houses, for instance. The resources we gather today aren't used today.

If you mean the society is sustainable in that it avoids boom and bust cycles, i.e. growing too fast followed by famine, your real problem isn't a lack of resources. Your real problem is waste heat. Given fusion or beamed solar power, there is easily enough material on Earth alone to sustain a population of a hundred billion or more, assuming you don't mind wrecking the biosphere in the process. But all these people want computers and indoor lighting and air conditioning and who knows what else. All these things produce waste heat as they run, and so do the factories building them and the fusion plants powering them. This heat has to go somewhere, and the Earth doesn't radiate heat away all that quickly. Eventually you'll cook your people.

It also depends on how long you want these people to last. Eventually the sun will burn out. If they survive that, eventually they will run out of hydrogen to put in their fusion reactors. If they survive that, eventually they will run out of matter to put into their black hole generators. Nothing is sustainable forever.

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There's always "room for one more", but why would you want to push the envelope?

The real question should be: given a world with a long-term stable population, is there any justification or advantage for this population to be greater than 1 billion?

Larger populations mean less room to accommodate emergencies and even long term environmental changes. Maintaining a maximum population would mean that even a small unusual change could be catastrophic.

And on the other hand I can't think of any way in which a world of 2 billion people would be significantly better than a world of 1 billion.

But perhaps others can.

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50 billion people

The Netherlands is one of the densest populated countries on the Earth, yet also a net exporter of food (and I believe that the farming methods used are, in the main, sustainable). Nor does the Netherlands seem overpopulated; the cities are low and open, and there is lots of canals and forests.

It is not hard to imagine that with advanced technology, all land on the Earth, barring Antarctica, could achieve a similar state of sustainable farming, open lands, and no great population pressure. Even mountains, tundra, and deserts could be covered in hothouses or planted with crops genetically modified to extreme climates.

The population density of the Netherlands is 417.6 per square km. The world's total land area, minus Antarctica, is 135 mill. square km. This gives us 56 billion people.

Parts of Greenland, Siberia, and Canada may be just as inhospotable as Antarctica, but on the other hands, there will be regions capable of holding even more people in a sustainable way than the Netherlands by adding urban farming and cutting down on non-food farming such as tobacco and growing high-nutrition crops and keeping high-nutrition livestock. Note that quite a bit of livestock will be needed to provide grazing and natural fertilizer, since artificial fertilizer is highly unsustainable. Other technology might include precision fertilization and mechanical weed-killing by solar-powered robots. Ocean farming is also underutilized today.

Given all this, I think a maximum sustainable population of roughly 50 billion is realistic with advanced technology- The number could very well be far higher if we add fusion power, which allows extensive vertical farming, but I want to stick to a low, conservative number.

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