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Suppose that we pare down human consumption to a minimum survivable level across the Earth, and devote the entire surface of the planet (as well as the planet's interior, if needed) to support as many human beings as possible. We ignore the damage we are doing to the environment except to the degree to which it may influence how many people we can fit on Earth. How many people could the Earth possibly sustain, using only technology which will be available in the near-future?

Assumptions as to what will be available in the near future:

  • We have fusion power
  • We do not have a space elevator
  • Growing food in space is prohibitively expensive

We have colonies on nearby planets and on the moon, but transport through space is too expensive for us to use extraterrestrial farms to provide food for our Earthlings. Under these conditions, what is the maximum possible population which the Earth could sustain?

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    $\begingroup$ Sustain for how long? Indefinitely? $\endgroup$ – Samuel Jun 19 '15 at 18:32
  • $\begingroup$ Relevant UN report: un.org/esa/population/publications/wpm/wpm2001.pdf, though other reports draw different conclusions. $\endgroup$ – HDE 226868 Jun 19 '15 at 18:54
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    $\begingroup$ @Samuel 'Indefinitely' is a long time. Let's say 50,000 years? $\endgroup$ – ckersch Jun 19 '15 at 19:32
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    $\begingroup$ @ckersch You should add that to the question then - comments can get buried. $\endgroup$ – Zibbobz Jun 19 '15 at 20:22
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    $\begingroup$ What state does a human have to be in to count as population? For that matter, what counts as human? I'm imagining some sort of giant vat of zygotes with their DNA hacked to never grow beyond a single cell. $\endgroup$ – user2357112 supports Monica Jun 19 '15 at 23:12
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It comes down to some assumptions. Does your civilization have to consider the possibility of war at any point? How fragile a system are you willing to accept? Are the people in your world consciously working to maximize the population?

But lets assume we can ignore some of the practical things and just maximize.

You have fusion power so I'm going to assume you can generate pretty much arbitrary amounts of power which in turn implies fresh water isn't an issue since you can use that energy to purify seawater and create fertilizer.

So you're going to need lots of warehouses like this.

Plants

In optimal conditions it takes about 50 square meters to grow enough food and oxygen for one person. Lets double that to be safe and to make it a round 100.

http://www.tylerdwyer.com/files/OASIS_Final_Report.pdf

A large portion of the land is going to be needed for simply living in, working in, and building fusion reactors on so lets assume no more than 50% of the Earths land area used for food growing.

So I'm going to assume that building too tall on a grand scale is unreasonable so lets assume nothing over 100 meters.

The land area of the earth is about 148,300,000 square km or 148,300,000,000,000 square meters.

assume 50% of the space used in our farming towers for walkways or areas under maintenance.

Assume 100 trays 1 meter apart which gives us 3,707,500,000,000,000 square meters of growing plants to work with.

So using half the worlds land area and almost unlimited energy and perfect coordination and everyone being willing to live really really close you might be able to feed about 37 trillion people an utterly no-frills diet and oxygen.

So much of the worlds water is going to be tied up inside these growing towers that the worlds sealevel will drop significantly and the energy required from your fusion plants will be so huge that it's likely to be heating the planet up via physical heat at an unsustainable rate so you might need the help of your moon colonies to build solar shades.

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    $\begingroup$ Dropping sea levels provides more land to live on, yeah? You could use some of these for cooling, if required. $\endgroup$ – Samuel Jun 19 '15 at 19:42
  • $\begingroup$ "everyone being willing to live really really close" -- not close to the point of physical absurdity, though, more like a hotel. You've used half the space for plants, leaving the other half for humans, so we get 100 cubic meters each. Say a 5x5x2 apartment and as much again in communal space. You could pack people much closer by sticking them in coffins and plugging them into VR most of the time, but past the point where it's 50:50 between humans and plants, each percentage point of human space gets you less than a percentage point more capacity, so it's diminishing returns. $\endgroup$ – Steve Jessop Jun 19 '15 at 19:52
  • $\begingroup$ @SteveJessop Just how many of these VR-ensconced Coffin-bound humans need to be tasked with controlling the VR-drones that maintain the farming tower infrastructure, and how many are free to use their VR-drones to build new levels, new drones, or research cheaper space travel? ;) $\endgroup$ – Ayelis Jun 19 '15 at 20:33
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    $\begingroup$ @Ayelis: task at least twice as many to maintenance as you think you need, since they will inevitably spend at least half their time playing ironic MMORPGs based on The Matrix. $\endgroup$ – Steve Jessop Jun 19 '15 at 20:37
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    $\begingroup$ This assumes we can't stack people and/or crops vertically. This is unrealistic, in my opinion. Assuming 10 feet vertically per person/crop, we could stack ~5000 high, and support 100 quadrillion people. $\endgroup$ – isaacg Jun 20 '15 at 11:24
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Fremlin actually calculated this. I found one copy of his article online: https://web.archive.org/web/20180713203419/http://www.claychipsmith.com/Population.doc

Stage 1: up to 400,000 million in 260 years' time Using existing crop plants and methods it may not be practicable to produce adequate food for more than four doublings of the world population, though the complete elimination of all land wildlife, the agricultural use of roofs over cities and roads, the elimination of meat-eating and the efficient harvesting of sea food might allow two or three further doublings -- say seven in all. That would give us, with the present doubling time of 37 years, 260 years to develop less conventional methods, and would allow the population of the world to increase to about 130 times its present size, or about 400,000 million.

It goes on:

Stage 2: up to 3 million million in 370 years' time The area of ice-free sea is some three times that of land. Photosynthesis by single-celled marine organisms may be more efficient than that of the best land plants. If organisms could be found capable of the theoretical maximum efficiency (8 percent of total solar radiation, according to A. A. Niciporovic) we should gain a factor of three in yield. We could then double our numbers a further three more times if all the wildlife in the sea, too, was removed and replaced by the most useful organisms growing under controlled conditions, with the optimum concentration of carbonates, nitrates and minerals. (Of course a reserve of specimens of potentially useful species could be preserved, perhaps in a dormant state.) Again, for maximum efficiency we must harvest and consume directly the primary photosynthesis organisms, rather than allow the loss of efficiency involved in the food chains leading to such secondary organisms as zooplankton or fish. By this stage, we should have had ten doublings, which at the present rate would take some 370 years, with a final world population of 3 million million. Since the world's surface (land and sea) is 500 million million square meters, each person would have a little over 160 square meters for his maintenance-about a thirtieth of an acre-which does not seem unreasonable by more than a factor of two, so long as no important human activity other than food production takes place on the surface. No serious shortages of important elements need be envisaged so far, though extensive mining operations for phosphates might be needed, and we have not yet approached any real limit.

And the ultimate conclusion (the article continues on, but QoL would deteriorate so much to that point that people would spend their entire lives living in a pod):

Stage 4a: up to 12,000 million million in 800 years' time. Dead end Above two people per square meter, severe refrigeration problems occur. If the oceans were used as a heat sink, their mean temperature would have to rise about 1 °C per year to absorb 500 watts per square meter. This would be all right for the doubling time of 37 years, at the end of which we should have four people per square meter. Half another doubling time could be gained if efficient heat pumps (which, for reasons of thermal efficiency, would require primary energy sources of very high temperature) could be used to bring the ocean to the boil. Two more doublings would be permitted if the oceans were converted into steam, though that would create an atmospheric pressure comparable with the mean ocean bottom pressure at present. Since the resulting steam blanket would also be effectively opaque to all radiation, no further heat sink could be organized and this procedure would therefore seem to lead to a dead end.

On an interesting note, Venus atmospheric pressure is equal to that of Earth's oceans at 1 kilometer.

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With infinite clean energy you can produce food, oxygen, and water in nearly any quantity that you desire.

  1. Energy + salt water $\rightarrow NaCl + H_2O$ (through desalinization)
  2. Energy + $CO_2 + 2H_2O \rightarrow 2O_2 + CH_4$
  3. Energy + human waste + spiralina (algae) + $ H_2O \rightarrow $ food + clean $H_2O + O_2$

If this works, then the limiting factors become personal space and waste heat. Because we can always build vertically if space becomes a problem, I think we run into heat problems before we run out of space.

How much heat?
Assuming Spaceship Earth in which we must set up all life support and the natural world no longer does anything for us, Atomic Rockets says this will consume ~0.36 kW / person in electricity and need to expel ~0.1 kW / person in heat.

Compared to what?
Assumptions

  1. The Earth is in thermal equilibrium
  2. We can only increase the Earth's temp by about 10 C
  3. This equates to about a 15% increase in energy input
  4. The Earth's radiated energy is dominated by Sun shine and equals 173,000 TW of power.
  5. We are allowed to generate 25,950 TW.

This Earth can generate enough fusion power for 259.5 Trillion humans.

Ouch

That seems way too large. This is about 37,000x the number of people on Earth today. This means 500,000 people live in every square km of the Earth.

Looked at another way, if we assume every single person needed $900 sq m$ of living space, then we'd need to cover every square meter of Earth with 458 floors of buildings. At 10 ft per floor, you're talking about buildings covering the whole Earth to a height of 4/5 of a mile or about 4x the tallest buildings currently on the Earth.

Either my math's wwaaayy off, my assumptions are no good, or living space becomes an issue long before heat rejection.

and living space requirements all depend upon how much being around other people bothers you.

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    $\begingroup$ I agree that heat is probably the limiting factor, but you can't just consider the heat generated by humans. All energy generated on Earth either stays on Earth or radiates away. All of this infinite clean energy you are using to sustain the population will eventually wind up as waste heat. $\endgroup$ – Mike Nichols Jun 20 '15 at 17:59
  • $\begingroup$ Some heat sources you need to consider: 1) Food production (waste by lighting, ventilation, water circulation, etc.), 2) Products supporting food production (fertilizers, water purification, hydroponics equipment), 3) Waste processing (household waste, dirt-water treatment, other wastes), 4) Transportation, 5) Entertainment, 6) Energy production. The last point is a factor that you have to multiply with all the other points. If you consider all those, you'll need much, much more than just 0.36kW/person. $\endgroup$ – cmaster - reinstate monica Dec 24 '18 at 21:29
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I will try to give an answer, without citing sources and by reading the question in ways that may make my answer to be a bit off-topic. But that's me!

At the current population numbers we should be all able to live like kings - literally speaking. The only reason we don't, is that we have no leaders that are capable of cooperating and making strong friendships with other nations instead of going to war with them, being able to understand the planet's ecosystem and work with it instead of against it, being able to put order into mega-corporate and banking chaos, and also our population is raised with pretty nasty principles (cultivated by our ill-minded leaders again).

So, what's the max number of people our planet can hold without us starving to death? I would say it is about 100 times the current population and without your futuristic assumptions, provided that we fix our attitude. Most people count the land available for producing food, but they tend to forget that most of the planet's surface is oceans which too produce food, while they are brutally harvested and polluted and destroyed for no reason. If there were regulations in place that made it possible for the fish populations to grow in numbers and preserve the oceans clean from pollutants, GMO that eliminate well functioning organisms and other threats, the food coming from the oceans could make a huge difference (I will provide a link to a paper here, although I have not read it yet, but I imagine it shares some of my views: http://bioscience.oxfordjournals.org/content/59/11/967.full). The amount of material, food and energy waste today is unthinkable. The U.S. seems to be the king of wasting resources, but I can assure you, all countries do it in an extreme manner. In summary, our house building ways are terrible, our power production, storage and distribution ways are terrible, our way of life is organized in a terrible and inefficient manner. We live like we have unlimited and free energy already! Surely, future technology could help, but (1) the issue is that we currently don't need that and (2) history teaches that technology rarely do that, especially in our case, since the problem is not technological.

But I think I should divert your question to another path. It's surely in our nature to dream and hope. But I think we should all dream and hope about different things than we currently do. The enemy of humanity is not the supposed lack of resources, energy, living space or food. The enemy always lies within us. It's our choices, our desires, our ideologies, our way of life, our habits, our desire to enjoy ourselves and our very dreams which are corrupt. Those are that always destroy us throughout history and they will destroy us in the future.

But if you desperately need some other threat to humanity that does not emanate from humanity itself, that would be our decaying genome. Contrary to what Dawkings wants you to believe, humanity and the animal/plant kingdom are fading fast genetically. Despite the fact that a vast percentage of this problem is again attributed to ourselves and our choices (one can say so much on this topic...), avoiding this is not possible. And if one dreams of us understating and fixing our genome, good luck with that... Even the simplest things in the inner workings of bacteria baffles scientists for decades. And if understanding those systems in their entirety seems impossible, fixing them is simply not doable. Our best bet for living some extra years as a species is keeping our population big enough and divergent enough and procreate before our 20-ies with one and only mate.

Earth is our home. Our only home. Resource-hungry NASA may manage to plant some toxic GMO plant on another planet making humanity believe that we may soon get a new, fresh, clean and unpolluted home. But that will never become our home and if it does it would only be a misfortune for us. Our home is one and our future is limited. Come to terms with that and let's make the time we have matter.

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  • $\begingroup$ Amazing answer! Especially nowadays, where questioning NASA and the rest of the modern priesthoods results in highly undesirable outcome. Believing that technology solves any of humanity's real problems requires some huge amount of ignorance on history and its teachings. You are my hero! Don't mind the down-voting zombies. $\endgroup$ – user2173353 Oct 29 '15 at 11:36
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The peak populations on Earth could be more than people think. IF this was done to merely maximize population then the Earth could possibly be maximized to many thousand (or even more) times our current population.

First: The Space and Utilities. --Humans have been building skyscrapers for centuries so for space we can just go up once we have used all the space on land. This will allow also for easier energy capture via solar panels for example. There would be no need for travel so making the earth basically one large skyscraper could allow it to tower thousands of feet. We can also utilize the ocean space as well. With the ocean being 95% unknown there is probably a mass amount of space to be utilized. If we lowered sea level by extracting sea water from the oceans and then recycled the water that could not only create more space but also increase resources. This extra space where the sea once was could be used to create nuclear energy which has the one of the highest energy/waste ratios.

Second: Population Needs. --Although different people consume different amounts of food/water this can be standardized. There are 2 ways. One is genetic modification. Using gene modification scientists could make people smaller/shorter which would not only increase the space available but also the food/water needed. Genetics could also be used to allow parents to have maximum offspring using an artificial womb nursing thousands of children at once. Second is hormone modification. Even if the population was not genetically standardized it could be stopped growing by stoping growth hormone in a person before they get passed the minimum required to live and reproduce. These smaller people will consume less allowing for less space for growing food would be need per person.

Third: Food Selection. --Water could be obtained from the extracted ocean water which would supply generations of populations and don't forget the ice caps. GMOs (genetically modified organisms) could also be used to make food denser with certain nutrients and to allow them to produce the nutrients necessary only. These GMO's would take up much less space and could even be modified for reasons other than food.

Forth: Oxygen Dependance. --Oxygen will be need by this smaller population still even though it will need less per person. Oxygen could be extracted from the ice caps which have oxygen trapped inside and even the ocean water if worst comes worst. With this population being housed in this "Earth Skyscraper" the roof could be utilized for plant GMO's that could produced the max oxygen per area and be nourished using the population wastes (CO2, fecal matter, etc). This symbiotic relationship can maximize the capacity of Earth's population.

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There are currently over 7 Billion humans inhabiting the Earth and that value is increasing exponentially, its estimated that by the end of the century the human population will reach numbers of over 10 billion but scientists are unsure if even this is sustainable.

Lets start with food; If you were to take all the land area being used for modern livestock and put it in one place, it would take up all of Africa (7.5 billion Acres), in fact even Africa would not be large enough as it would require 8-9 billion acres. Doing the same for crop production would take up an area the size of South America (4.4 billion Acres) but after a while the soil in these areas will degrade until fertility decreases and it erodes, rendering it useless for growing crops and not only that but scientists are unsure if farmers can even maintain their current cop yields due to changes in the environment.

See, the global average of carbon dioxide emissions per person is about 5 metric tons per year but for Americans that averages closer to 17 metric tons per year. Remember this is per person so if we have about 3 billion more people living on this planet and they all adopt a life style similar to an American, the effects of global warming will be worsened even more, in fact it's projected that the temperature of the Earth will increase by 2 degrees Celsius by the end of the 21st century and with that increase comes more extreme weather conditions which would inevitably disrupt food production even more. This can be partially remedied by forcing a everyone on Earth into a purely vegetarian diet and reclaiming the land to grow more crops instead of raising livestock but the crops won't be enough to feed an infinite number of people in fact some scientists predicted 10 billion people and more than likely most people will not voluntarily switch to a vegetation diet. After all, bacon is a beautiful thing.

But what about the available water? Humans are currently using up to 30% of Earth's accessible water supply with the rest being used for agriculture and the water available to some people in countries life Ethiopia, Cambodia and Haiti isn't even clean enough to drink. So the amount reaching humans would have to increase with the increasing population so to answer the original question, Earth may be able to hold up to 10 billion people living decent lives and may be able to hold more, but the quality of those people's lives (past the 10 billion mark) would be much lower than they are right now.

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  • $\begingroup$ Oh, 10 billion is perfectly sustainable. Just erase meat from the list of available foods... I think there was actually an island somewhere where the people once decided to get rid of all their pigs: They came to the conclusion that it was just not worth it. They were able to sustain much more people on their small island if they didn't waste their precious food by "processing" it into meat first. $\endgroup$ – cmaster - reinstate monica Dec 24 '18 at 21:33

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