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How many people can live comfortably on planet Earth, assuming that humanity reaches Kardashev scale Type I level?

This implies:

  • Energy is cheap, and available in an almost unlimited quantity, compared to current level.
  • Sea water can be desalinated and pumped to anywhere, a technology that is already available, but its usage is currently limited by the high energy consumption.
  • Artificial sunlight can be created anywhere. This technology is also available even today.
  • Food, advanced building materials, and almost everything else, can be synthesized from basic molecules and atoms.
  • Transmutation of elements, e.g. making gold from lead, is possible, although not on a mass scale, because this requires enormous amounts of energy, that is beyond the reach of even a Type I civilization.

Energy can come from

  • The Sun, as originally thought by Kardashev,
  • or from fusion reactors, here on Earth,
  • or even from advanced fission reactors, using all the uranium, thorium, and other heavy elements available on Earth, that can be converted to fissile fuel.
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    $\begingroup$ More detail required. First, what does living "comfortably" mean - how much area per person? Second, what about all non-human life - is it acceptable to wipe out everything that isn't being grown as food? Third, does food need to be grown on Earth or can it be grown in orbit? Depending on the answers, this can become a simple math problem or completely unknowable. $\endgroup$
    – KerrAvon2055
    Dec 30, 2019 at 11:05
  • $\begingroup$ 10B is the uppermost limit based on omnivorous diet and available spaces, at type 1 we can still squeeze in a bit more I'll bet the happiness will drop exponentially per head. $\endgroup$
    – user6760
    Dec 30, 2019 at 11:52
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    $\begingroup$ An analysis of the upper limit for a human population on planet Earth was done by Professor Fremlin back in the 1970's. It was published in New Scientist. The real limit is amount of heat produced by the population. $\endgroup$
    – a4android
    Dec 30, 2019 at 12:03
  • $\begingroup$ @KerrAvon2055, people do not need to grow food in this scenario - they just compose it from their .. anything. And the area is not a big ptoblem since this civilization can build upward and downward (think of glacial like megacities). And most limiting factor in this condition is not the area, but energy - 10^16 - 10^17 Wt. We are 0.15% of that now $\endgroup$
    – ksbes
    Dec 30, 2019 at 12:18
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    $\begingroup$ @KerrAvon2055, I leave it to the answerer to define "comfortably". A reasonably minimum is the size of a prison cell, an average, the way you live today, the maximum is the way you want to live, if you had a little more money. $\endgroup$
    – pintergabor
    Dec 30, 2019 at 12:37

5 Answers 5

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Your most obvious limiting factor here will become energy since pretty much all of your farming needs to be brought inside once you get to such a scale.

Using indoor farming you need at least 110 watts/m^2 and the average person consumes about 270m^2 (when using areoponics). This means that you each person needs 29,700 watts to have enough to eat, and if we use the USA as a standard for "comfortable living" we also need to add ~1400 watts per person to power things like lights, microwaves, transportation, computers, etc. This puts the total power consumption per capita at 31,100 watts.

Since a Type I civilization can harness 1.74×10^17 watts of power, that means you cap out at about 5.6 trillion people, but future tech lighting and GMOs might be able to push this number even higher without having to sacrifice modern food diversity.

This introduces two other limiting factors which are building materials and global warming.

Materials:

Since Earth has a total land mass of 510.1 trillion m^2, this means that if everyone lives on Earth, you'll have a total population density of 1 person per 91 square meters. Between recreation, industry, residential space and infrastructure, your average urban resident needs about 200m^2 worth of space in addition to the farming totaling ~470m^2. If you only build on land, this means that doing this will result in an average building height per square meter of about 17.2 stories. Since about 50% of any city is open spaces (roads, parks, etc.), this means that your average occupied square meter of the planet will have a 34 story building on it.

Making reasonably luxurious buildings that tall might be doable with some nice future tech materials, but not on that scale. The entire Earth's crust does not have enough of certain materials to sustain ideal levels of comfort; so, you can assume that you'll need to do a lot of space mining to get all the copper, lithium, etc such a civilization would need, but it may be doable if your tech allows for it.

Heat:

Once you start building all these mega structures and roads and parks, there is no way to actually get 100% power efficiency from just solar. Even if future tech solar is close to 100% efficient, and you put it over every building and roadway completely blocking out the sun to all of humanity, there is still that 70% of the world that is oceans. One way or another, you'll need to use something other than sunlight for most of your power. Let's say you do 30% solar and 70% super clean fusion reactors. All those reactors are generating massive amounts of heat. As far as the people on the surface of the world are concerned, this is no different than turning up the sun by 70%. YIKES!

This means that your Type I civilization will need to be able to radiate out all of that heat. The best way to do that may be to reverse global warming and build up the world to have an opaque atmosphere that reflects more sunlight, but this reduces the effectiveness of your solar power. Any way you cut it, 1.74×10^17W is the theoretical hard cap on how much power we can make without cooking the world, and that is assuming all other environmental factors can be 100% controlled for.

In reality though, solar will never be 100% efficient and the atmosphere will never be made 100% opaque; so, it's probably best to just say your civilization maxes out on the solar power of all available land mass (5.22×10^16) at about 25% efficiency (1.305×10^16), and your population caps at about 420 Billion. A Type I civilization probably can not exist on a single planet, instead, most of your civilization's power will actually be produced/expended in space. Anything beyond that is going to be VERY hard to keep cool without some pretty hand wavy mega-structures

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  • $\begingroup$ The farming numbers don't translate. Indoor farming is faaar more efficient per unit volume than outdoor volume. Indoor farming doesn't have cloudy days or that period of low light intensity called night or bad weather or Winter or pests or weeds or.... -- The point being that everything is perfectly optimized for maximum plant growth. $\endgroup$
    – SurpriseDog
    Dec 30, 2019 at 17:16
  • $\begingroup$ @SurprisedDog 110 watts/m^2 was based on indoor farming statistics. From what I can find it seems that most outdoor agricultural areas average at least 300 watts/m^2 during their growing seasons. $\endgroup$
    – Nosajimiki
    Dec 30, 2019 at 19:04
  • $\begingroup$ Your second link (zebu) does not explain how it arrived at the total solar energy amount. The figure it uses (84 TW) seems way too low. $\endgroup$
    – Alexander
    Dec 30, 2019 at 23:31
  • $\begingroup$ I've found other sources all saying similar watts per square meter including NASA. It probably sounds wrong because that page has been cited on WB.SE so many times before, but the math all agrees that ~84 TW is what actually hits the Earth through the atmosphere. $\endgroup$
    – Nosajimiki
    Jan 1, 2020 at 17:19
  • $\begingroup$ Hold on, you think that 1.74×10^17 W is incorrect, and 84×10^17 W is correct? The former figure is cited widely, for example in here: Solar energy Wiki. Quickly checking the math: Earth radius is about 6400 km, so its cross-section is about 1.2×10^14 m2. Sun energy received by 1 m2 is over 10^3 W, so the total should be > 10^17 W $\endgroup$
    – Alexander
    Jan 6, 2020 at 17:28
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Maybe NOT As Many as Folks Think

While material limitations are an important consideration for maximal possible population, I think physical matter is not your limiting factor here.

Your scenario basically makes the First World look like a Fourth World backwater. Given the technology level and the free energy, even our own Third & Fourth Worlds will disappear as near instantaneous material prosperity reaches every corner of the planet: every person on Earth will be a citizen of an Ultra-First World country.

When we look at present world data, wealthy countries where the entire population lives a very cushy life (i.e., the First World) tend to decrease in population. Particularly as said population attains high levels of education.

Once poor countries where subsistence living requires a large workforce (i.e., big families) are brought into the wealthy rank, their large families will go away just as they did in America and Japan and Western Europe. There may be some instability for a while as food and medication and technology put paid to disease, hunger and child mortality.

No longer being required to work the whole day for mere subsistence, people will be able to pursue educational opportunities and will fill economic niches they could never even dream of. With all that education, worldliness and free time, population growth will level off and then drop.

Assuming the situation remains stable, I'd suspect the population could eventually shrink back to the point where a few billion or even less can live extremely comfortably in your scenario.

Your limiting factor here is simple material laziness, which tends to drive population down rather than up.

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    $\begingroup$ I could easily see a future, where VR improves to the point of being indistinguishable from the real thing and robots automate away all of the work that needs doing. What happens next? Everyone retreats into their own hedonistic fantasy universe, of course, and no one bothers raising kids. We're already heading that direction with the birth rate in 1st world countries falling decade over decade. $\endgroup$
    – SurpriseDog
    Dec 30, 2019 at 23:13
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    $\begingroup$ @ArcanistLupus - Fourth World refers to peoples who are excluded, for whatever reason, from global society. These could be the truly dirt poor of a Third World country; they could be purposefully sequestered & secluded peoples of any higher level country. And kind of interstellar society makes even the most highly advanced country on Earth an excluded & secluded enclave, deprived of contact and intercourse with galactic society. $\endgroup$
    – elemtilas
    Dec 31, 2019 at 22:15
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    $\begingroup$ I would caution against assuming that "first world" implies "plunging birthrates". There is a huge confounding variable in the form of feminism. Is the birthrate plunging because the standard of living is improving? Or because women aren't second class citizens anymore? (Not that we've achieved equality, by any means, but there's worlds of difference in most places between today and decades past.) $\endgroup$
    – Ton Day
    Jan 1, 2020 at 3:16
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    $\begingroup$ My point was the drop in birth rates is very recent and appears (to me, anyway) to also be correlated to the social changes brought about by women's lib, whereas standards of living have been rising for a lot longer. I'm not a social scientist, so this leaves it unclear to me which (if either, or maybe both, possibly among other factors) are responsible for the asymmetric decline in birth rates seen in parts of the world today. $\endgroup$
    – Ton Day
    Jan 1, 2020 at 3:59
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    $\begingroup$ @DaytonWilliams you are right that it is caused by social change, but not so much about women's rights. Children used to be valuable assets that would make their parents money as farmhands, factory workers, or apprentices. Child labor laws, social security, and education now mean that children are just massive liabilities who never make any money for their parents and rarely continue the family trade or support their parents in old age. Parents in more liberal countries wonder if they can afford to have kids, but in less developed countries they wonder if they can afford not to. $\endgroup$
    – Nosajimiki
    Jan 3, 2020 at 16:15
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Hard to answer, it depends on how much damage you are willing to do to the place

Keep Earth earth-like

Assuming we don't want to engage in major mega-engineering and keep humans human-like, waste heat is the limit. This means everything depends on efficiency. Paradoxically embracing the greenhouse effect would increase the maximum population as the efficiency of an object radiating away heat rises with the fourth power of temperature. Keeping the place at a cozy 49 C average temperature, just short of a runaway greenhouse effect should be optimal. The population and industry should be spread evenly over the surface. Sea-cities would be a necessity. I can't give you any population values, as this would depend on how much waste heat one person produces.

I can, however, paint you a picture of this Earth. keep in mind that this is one possible scenario and that things would look radically different if some of my ideas about the available technology are wrong. Between the Earth and the Sun and on the Lagrange points gigantic solar arrays have been places, producing energy for Earth and shading it from the heat of the Sun. The planet is evenly covered in black high rise cities, the oceans deep blue, dotted with these black settlements and the continents covered in black radiators. Earth is a black-blue marble, as cloud formation has been cut down, as it decreases the efficiency of Earth as a black body radiator. Nature only exist in enclosed habitats. Food is produced in bioreactors and factory farms.

Such an Earth could probably support tens or hundreds of billions.

Matrioshka-World / Shellworld

This assumes you are willing to put layer over layer and slowly burry the original surface. The technology you need for this is the dynamic orbital ring. The first link leads to an explanation of the concept from the Orions Arm project, this video explores the concept a bit further. How many layers you put up depends on how much space you need. The important thing is that you can run coolant pipes from the lowest level up to the surface. Then you transport the coolant up to a ring either short of the Moons sphere of influence or if you can get rid of the moon at the edge of the Earth's Hill Sphere. Up there you install liquid droplet radiators to cool the place.

Such a place might support many trillions. Gravity in the outer shells probably becomes the limiting factor at some point.

Dissasamble the Place

Now, you asked about "on planet Earth", but I'll assume that Earth isn't more than the sum of its molecules. Meaning we turn the entire planet into spinning habitats. Assuming we need ten tons of material per square meter, this will give us 5.972 × 10^17 km2 of habitable area. A bit more than one billion times the surface area of Earth or six billion times the habitable surface area of Earth. Waste heat won't be an issue since we can simply spread out the habitat swarm far enough. Here material becomes the bottleneck.

Computronium

We've already true the Earth into a habitat swarm, but this of cause still assumes we are dealing with biological baseline humans. Here we push the idea of "living" and "human" a bit further. Assuming we can upload a human mind onto a computer, all we need is hardware and energy. To maximize population we should run the entire thing as cold as possible, near Landauer's Limit. This meant that a single human thought might take millennia, but that's beside the point of this question.

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Interesting question.

Since the element you need from the most, oxygen, is as 'thedyingoflight' stated the most common element on earth, I'll take carbon as the most limiting factor to create/grow humans. I don't know anything about the rarer components of the human body and don't have the motivation to go through a checklist. I'll assume that it needs so little of those per person that we don't need to think about them anyway.

If we take the estimations of this documentation and are really optimistic when it comes to the future of 'carbon mining', we could use a little over 7.7842628*10^19 kg of carbon to transform into humans.

A grown up human body weighs on average about 62 kg today. Let's say they'll bring the average weight down to 60 kg. That's 10.8 kg carbon per human.

This means we could create up to about 7.1296296296*10^18 humans. That's such a high number, that I doubt carbon could be the most limiting factor, but here we are, fuck it...

Now we'll have to estimate what percentage of that carbon will be rather used for production, housing and of course nourishment than for creating a sea made out of flesh. Now the broad/hard stuff gets important, especially what kind of food they use and how long it takes to create food out of feces.

If we expect such things to matter, we're probably already talking about a lot more humans than are able to live comfortably on earth. I'd say 3*10^18 humans not living in constant pain feels somewhat plausible to me, if you have enough energy to build a city built from all of earths sediments. The usage of space will be a big problem with that.

Earth's surface is about 5.101*10^14 m^2. Yikes. It means that we'd have to build one giant building with a little over 100'000 floors to give everyone 17 square meters to live on. The building would have a height of about 190 km, if the floors had an average height of 170 cm. This would be very hard to supply with energy, food and tech and even harder to keep from imploding. You'd have to build 'very' light, which ironically would most likely need a shitton of carbon. They'd also have to walk around with small gas bottles which they'd have to refuel every few seconds...

-I guess my 3 quintillion humans would rather live in caves, naked and sore, but alive. The only thing they'd have left is love. It's cheap, it's easy and yes we want there to be as many of us as possible, because why not?

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    $\begingroup$ More than enough oxygen in the oceans? The ammount of oxygen in the oceans is puny. Oxygen is the most common element on Earth. Almost everything you call rock contains oxygen. $\endgroup$
    – TheDyingOfLight
    Dec 31, 2019 at 14:54
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Ten to hundred trillion. Assuming population density somewhat below modern cities worldwide.

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