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In my part of this (Earth-like) world, there is a stable community of completely isolated pre-industrial humans, numbering perhaps 1,800.

They live at an altitude of (at least) 25,000 feet (7620 meters) in a very high inaccessible valley, possibly adjoining the caldera of an extinct volcano.

What geological/ecological conditions would need to prevail in order to support a thriving community?

Stipulations:

  • No goats. Yes to liquid water and thriving plants.

  • If Reinhold Messner can climb all 14 peaks over 8,000 without oxygen, and do Everest 8,848 m (29,029 ft) twice in two years without it, once solo, then a civilisation can happen at that height.

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    $\begingroup$ "Yes to liquid water and thriving plants" - that's the big problem. Is the magic acceptable? $\endgroup$ – Alexander Jan 28 at 18:12
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    $\begingroup$ @SJuan76 The people themselves, like geese, can thrive at high altitude, they are of hardy stock. $\endgroup$ – Agrajag Jan 28 at 18:12
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    $\begingroup$ I think your stipulation about Reinhold Messner might be irritating people? Realistically, there's a difference between a trained athlete spending a brief time at that altitude, and a civilisation existing there. $\endgroup$ – Arkenstein XII Jan 28 at 19:08
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    $\begingroup$ Personally, I don't have a problem with the idea that the population has adapted to high altitude. Hence, I haven't given the question a downvote. :) $\endgroup$ – Arkenstein XII Jan 28 at 19:23
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    $\begingroup$ Have a look at Brandon Sanderson's The Stormlight Archive series. In the second book, they rediscover the long-lost ancient city of Urithiru, and a lot of the third book is spent trying to figure out how in the world the ancients actually lived there. It's located extremely high up in the mountains, high enough that the air is too thin for crops to grow and it's too cold for people to live in comfort. The best anyone can tell is that the ancients had some sort of magic that increased air pressure and temperature that none of the characters can figure out how to reactivate. $\endgroup$ – Mason Wheeler Jan 29 at 16:41

11 Answers 11

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If Reinhold Messner can climb all 14 peeks over 8,000 without oxygen, and do Everest twice in two years without it, then a civilisation can happen at that height.

That is false. Messner wasn't depending on running water or forageable food at those heights: he was bringing it all with him. He wasn't staying any appreciable time at those altitudes, either. There's an astounding difference between one incredibly well prepared guy momentarily scaling all those heights and a community enjoying day-to-day existence at those heights.

Please read this wiki article. The highest permanent human settlement is 5,100 m and the tolerance is 5,950.

Therefore, we already have a problem. Your Earth-like planet... isn't.

  • There's not enough atmosphere (especially oxygen and carbon-dioxide, which plants need) at those altitudes. The atmosphere would need to be thickened.

  • The tree line is approximately 3,050 meters (10,000 ft). The highest altitude moss (thought to be the highest altitude plant life) grows at 6,480 meters (21,260 ft). Animals come close, the Yak (with two coats of fur!) can make it up to 6,100 m (20,000 ft). But nothing lives at 25,000 ft without help.

What, then, can be done?

  1. You can increase the density or mass of the planet such that it retains a larger atmosphere. Gravity decreases as you increase altitude... but not that much. So you'll likely have a high-G race. You're also going to have a higher percentage of low-altitude deserts as a thicker atmosphere will increase sea-level temperatures. This is the only natural solution I can think of and the consequences are very high (which is to be expected). Note also (thanks to Arkenstein XII), that as the mass of a planet increases, the tendency is for mountains to be lower, reducing the plausibility of the scenario.

  2. You can bring man-made structures into play: plumbing low-altitude atmosphere to your settlement. You'd be moving a LOT of air, but it wouldn't necessarily create wind (just volume). It would dissipate quickly, meaning your settlement would have defined limits, and if the surrounding climate pushed the wind speed up, it would be easily blown away. You could do the same for water with heating at the top of the pipe. However, none of this would allow plants to grow — too cold.

  3. Underground, in a dome, sealed away from the environment (L.Dutch's answer).

  4. Seriously change your race such that it's not human anymore. Capable of living at very low atmospheric pressures (would die at sea level), capable of horrendously high UV (might even need it for photosynthesis), capable of living off of lichen and eating snow. Very happy at very cold temperatures (like the Yak, maybe two coats of fur... again, would die at sea level). Etc.

The more earth-like your planet, the less possible your goal.

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    $\begingroup$ This is a pretty good answer overall. +1. One additional point that I'd like to suggest is that if the mass of the planet is greater, that will have an effect of the isostatic equilibrium of continental crust such that mountains will generally be lower. $\endgroup$ – Arkenstein XII Jan 28 at 19:42
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    $\begingroup$ @ArkensteinXII, that's a very good point. I'll add it. $\endgroup$ – JBH Jan 28 at 19:52
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    $\begingroup$ @JBH Fair enough, I'd like to hear their disputations, but 'till they're published... I'm learning huge amounts asking this question, so glad I did. *Subsequent comment, that's the idea, a variously jumbled mosaic of facts and ideas has been presented, I'm delighted and still left unsatisfied - I'd like to ask the question again more clearley. I'd suspect this is a common experience. $\endgroup$ – Agrajag Jan 28 at 20:03
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    $\begingroup$ ^ I know, I just find it amusing is all, it's the equivalent of telling someone who asked how can we make more millionaires (when they really meant more rich people) "double everyone's wages & devalue the currency so a dollar is worth the equivalent of what 50 cents is now" & having them think it'll do what they want :) $\endgroup$ – Pelinore Jan 28 at 23:19
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    $\begingroup$ Wait - high-G race would produce shorter beings. Getting outside natural resources like air. Living underground. Inside mountains. Change them to not be humans... You're talking about dwarves! $\endgroup$ – VLAZ Jan 29 at 12:23
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Liquid water at that height is going to be a challenge: at the Equator the limit for perennial snows is 5000 meter above sea level, so you see that there will be no liquid water in the open.

You could go around this if you have some sort of heat source, which can provide local warmer conditions. You mention an extinct volcano, so it might be possible to have some geothermal water source, creating warm oasis in an otherwise barren and frozen land.

The lack of a sufficiently dense atmosphere is the second hurdle, both for plants and animals (humans included). At that height you are at the border of the death zone due to the severe scarcity of oxygen, therefore the only solution I see as viable is to have some sort of sealed cave system where a higher atmospheric pressure can be maintained. Gases might be supplied by volcanic activity, mostly CO2 which is then converted in O2 by the plants.

But a cave has a lack of light. You might handwave a bit around this, giving a ice dome sealing the volume you are interested. The ice being transparent would let some light in, and would seal the gases from a quick escape to the outside. Volcanic heat could explain why the ice has a dome shape.

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  • $\begingroup$ Geothermal water is good - warm oasis - cave, ice walls, oxygen from plants of some type. Excellent. Now you've gotten me thinking - they would have originaly ascended through a now-collapsed part of the cave system and become isolated there. $\endgroup$ – Agrajag Jan 28 at 18:19
  • $\begingroup$ How thick is this ice dome? can't find any detail on ice but I do know 200 meters is about the limit for any significant light in water. $\endgroup$ – Pelinore Jan 28 at 18:48
  • $\begingroup$ @Pelinore I figure that I can say that the first up in the Highholm, would have found a comparativley small area with light, but can, over time have expanded using artificial means - glass/ice, maybe both. Given sufficient starting resources before being cut off, well, we'll see. $\endgroup$ – Agrajag Jan 28 at 19:00
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    $\begingroup$ Forget photosynthesis. It might be easier to handwave some oxygen-producing bacteria that live off the geothermal heat. Cyanobacteria won't do, unfortunately, as they need light. $\endgroup$ – John Dvorak Jan 28 at 19:03
  • $\begingroup$ Ice caves are they permanently trapped inside this dome or will they have airlocks (a bit more than I'd expect from a pre-industrial community) so they can wander around outside if they want? $\endgroup$ – Pelinore Jan 28 at 23:55
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I believe this is not possible for a pre-industrial society. The highest permanently inhabited 'settlement' is indeed at around 5,200 m. But from my experiences in the Himalayas (husband of a doctor who worked at high altitude settlements there), the highest altitude villages are usually considered to be seasonal and the inhabitants need to descend to below around 4,500 m at intervals, in order to 'stay healthy'. Furthermore, in such communities, pregnant women almost always drop to still lower altitudes (from memory below around 4000 m) to give birth as babies do not 'thrive' above that altitude.

I guess that over a very long time period people could evolve/adapt to those conditions. But after several thousand years of adaptation, populations in the Himalayas and Andes are still more than 2000 m below your target altitude.

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    $\begingroup$ I get what you say about altitude, this is where I failed in asking the question - I should have specified an already adapted biology, when I thought it was implicit in the question - it's clear that I should have made it explicit. $\endgroup$ – Agrajag Jan 28 at 21:03
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    $\begingroup$ @FaySuggers, the Tibetan peoples are adapted -- there are some fairly significant physiological differences between them and lowlanders. If they can't thrive at 7500 meters, then it's likely that humans in general can't. $\endgroup$ – Mark Jan 29 at 23:45
  • $\begingroup$ @Mark I'll bet that 15 thousand years ago, you'd have said that the existance of the Tibetans were improbable, hell, so might I have. I need to reframe the question to focus in on the author's intentions. $\endgroup$ – Agrajag Jan 29 at 23:54
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Lobuche

This is Lobuche. It's located at 16,207' in elevation in the Khumbu region of Nepal. It's the last stop before the Kumbu glacier on your way to Everest Base Camp.

As you can see, there is no real vegetation. Moss grows here. Some small ground covering plants. Most plants stop growing around 15,000' rocky tundra begins to give way to snow fields. Moss will keep growing up to 21,000', but that's the upper limit of where you will find vegetation.

Some mammals like yaks can be found up to 20,000'. In fact, they don't do well below 10,000'. Snow leopards live between 10,000-17,000'. Rodents like the Pika do not generally go above 14,000'.

Nepalese and Tibetians live and thrive at around 15,000' and it's possible to be born, live and die in this area and never go lower. However, it should be mentioned that they have a genetic mutation that allows them to thrive at high altitude that most of humanity lacks. Oxygen is only 60% of what you would find at sea level. Villages like Naamche Bazaar are at 11,000' look like any remote village. You could live here after acclimatizing and carry on with a normal, if not treeless life.

The biggest problem with your group is the elevation. At 25,000' they are clearly above the high altitude habitats where humans can survive.

The biggest problem is that oxygen levels drop as you rise in altitude. At sea level, the usable oxygen is about 20%. At Naamche Bazaar, the effective oxygen level is 13.7%. At Lobouche, it drops to 11.4% and at 25,000' where your people will be living, the oxygen level is at 8%. You're at the upper limits of extreme altitude, above food sources and in a zone where long-term survival is pretty remote because of the caloric and water intake needed to sustain your current weight. You need a lot of calories to maintain the body. There is also an issue with altitude sickness that starts with people above 8,000'. Later in life, Sir Edmund Hillary, the first documented human to climb Everest, damaged his health and was unable to go above 11,000' without becoming very ill.

I actually talked with him about this very subject on a ridge overlooking Naamche Bazaar many years ago. He was there dedicating a school and we attempted to get him to come with us to Lobuche. His spirit was willing, but he was already at the upper limits of where he could function without getting altitude sickness.

It's remotely possible for your group to exist at 25,000', but to make this story realistic, you are going to need a solution to food, water and how they maintain pretty good health. One thing is for sure, few people lowland are going to bother them.

Good luck.

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    $\begingroup$ "Rodents like the Pika" Are they bright yellow with an affinity for electricity? $\endgroup$ – Mason Wheeler Jan 29 at 16:37
  • $\begingroup$ @MasonWheeler Peekachoo! - en.wikipedia.org/wiki/Pika $\endgroup$ – gwally Jan 29 at 19:27
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Double the relative amount of oxygen and carbon dioxide in the atmosphere.

Presumably, this still falls within the realm of "Earth-like".

As already noted, entire populations of humans have already adapted to live at ~15,000 feet, and trees grow up 10,000+ ft where atmospheric pressure is about 69.6 kPa. The altitude you want is 25,000 ft, which has an average pressure of 37.6 kPa.

Let's focus on the altitude trees thrive at: 10,000 ft.

The problem with high-altitude life, as everybody else has mentioned, is that the air density is too low to support life; the air density at 37.6 kPa is too low for large organisms to live. In particular, the problem isn't low air density overall. Humans need enough oxygen and plants need enough carbon dioxide to live, so it's the density of these two gases we care about. So if you can alter the composition of the atmosphere a little bit, you can achieve the same density of these two gases on Earth at higher altitudes on your Earth-like planet, while still keeping atmospheric density about the same.

Using a little ideal gas law, for any gas in a mixture, when we try to achieve the same density at two different pressures, we get:

$$ \chi_2 = \chi_1\frac{P_{T1}}{P_{T2}} $$

where $\chi_1$ and $P_{T1}$ are the molar ratio and total pressure at the higher pressure, and $\chi_2$ and $P_{T2}$ are the molar ratio and total pressure at the lower pressure, respectively.

The molar ratio of the three gasses of interest in Earth's atmosphere are ~78% nitrogen, 21% oxygen, and .03% carbon dioxide.

When we stuff the numbers into this equation, we get a new atmosphere which is about 61% nitrogen, 39% oxygen, and 0.06% carbon dioxide, and the atmospheric pressure at 25,000 ft is still the same as Earth: 37.6 kPa.

This calculation assumes that the temperature is the same at both altitudes, but we would of course expect a lower temperature at higher altitudes. If we drop that assumption, the equation becomes:

$$ \chi_2 = \chi_1\frac{P_{T1}T_2}{P_{T2}T_1} $$

which means decreasing the temperature at the higher altitude actually makes it easier to achieve our desired oxygen and carbon dioxide densities, so you don't actually need to double the amounts of these gases.

Dealing with the cold

Incidentally, the first equation above also enforces the same temperature at both altitudes, so doubling both gases will also keep the temperature at 25,000 ft on your Earth-like planet the same as 10,000 ft on Earth.

You can also use the second equation to play with the temperature at your higher altitude to achieve some desired combination of temperature at 25,000 ft and sea-level conditions. Sea-level conditions follow from your new relative amount of oxygen and carbon dioxide. (See side effects below.)

Resulting ecology

The primary factor we've changed is the altitude range that large mammals and plants can survive and thrive at. This means the ecosystem should be similar to those seen in mountainous environments near 10,000 ft on Earth.

An important caveat here is that land life evolved from ocean life, and there's obviously a large distance between oceans and 10,000+ ft mountain ranges. Thus, whatever early amphibious life evolved would need to deal with much higher oxygen levels at sea level.

Side effects

Just as the temperature on your new planet is equal to the temperature on Earth at a lower altitude, this means the sea-level conditions of your planet will also be some combination of higher pressure and hotter than on Earth. This is because increasing the relative amount of oxygen and carbon dioxide will make the atmosphere more dense, overall.

This means the oceans will necessarily be hotter, as well. The effects of this can be complicated, but it would be easier for life to emerge due to greater energy abundance.

The planet as a whole is almost identical to Earth

The above approach doesn't change the mass or size of the planet, so it's gravity remains the same.

Average sea level is almost completely unrelated to atmosphere composition, so you can have the same land and ocean topography.

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what conditions are needed to support a human community?

Bearing in mind the criteria that you stipulate.

  • a stable community of completely isolated pre-industrial humans
  • They live at an altitude of (at least) 25,000 feet (7620 meters)

The conditions needed will be at least one of the following three options.

  1. A lower altitude or higher atmospheric density so they're effectively living at lower altitude.

  2. A significantly higher knowledge of & ability with scientific principles than would normally be associated with a pre-industrial society, to provide artificial conditions to live & grow food in.

  3. To not be isolated from lower altitude settlements that can provide them with food & other supplies, because nothing grows naturally at that altitude.

So in short, you're going to have to give a little on at least one of your chosen criteria.

If you use option 3 you need a reason other communities supply them, if there's no commodity found exclusively in the mountains they can trade for supplies some sort of religious community like Buddhist monks might fit the bill, but I suspect isolation is the last criteria you want to surrender.

Perhaps the easiest way around things is to say that they live in a valley at a comparable altitude to the highest known permanent human settlements but that the only access to it is through a pass (or route) that climbs to the altitude you want.

Some significant altitudes & the effects of altitude on humans.

Armstrong limit altitude 60,000 to 62,000 feet.

Death zone altitude 26,247 feet.

The altitude you want 25,000 feet.

World’s highest plants 20,177 feet.

Highest altitude humans can survive 19,520 feet.

Highest permanent settlement in the world 16,700 feet.

Snow line altitude 15,000 feet.

Organisms at high altitude The Himalayan pika lives at altitudes up to 13,800 feet.

Treeline altitude 10,000 to 13,000 feet.

Effects of high altitude on humans

Taking those details into account perhaps the highest plausible altitude for your community is 19,520 feet & they will need considerable acreage per person to sustain themselves on plants that can grow at that altitude as those plants grow extremely slowly (in part due to the cold), that they might have domesticated Yak & Pika adapted to the altitude for meat, wool & milk etc seems reasonable.

So in summary, perhaps set your community in a high valley or plateau in the mountains with an average altitude of around 19,000 feet that can only be accessed from high passes through the mountains with altitudes of around 25,000 feet at their highest points?

At that altitude the things that are going to form the base of your food chain are lichens, mosses & cushion plants, nothing else grows at that height & you'll have no trees for wood.

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    $\begingroup$ The altitude of the snow line is very dependant on climate. At the moment, my house in Germany (at less than 3000 feet) is above the snow line, and my son has been above the snow line in midsummer at sea-level - but that was in Antarctica. $\endgroup$ – Martin Bonner Jan 29 at 13:36
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    $\begingroup$ @MartinBonner : I know, gotta stick it somewhere though, from the link "The interplay of altitude and latitude affects the precise placement of the snow line at a particular location. At or near the equator, it is typically situated at approximately 4,500 meters (or about 15,000 feet) above sea level." $\endgroup$ – Pelinore Jan 29 at 16:10
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The highest settlement on Earth is supposed to be at 16,830 feet or 5,130 meters, which is only about 0.6732 of your altitude of 25,000 feet or 7620 meters. https://en.wikipedia.org/wiki/List_of_highest_cities1

I doubt that there could be a human settlement at 25,000 feet, 1.48 times the record, on Earth without advanced technology like in an Antarctic base or a Moon base. Thus conditions on your Earth-like planet should have to be less than 100 percent Earth-like to make it possible.

It would help if that Earth-like planet has an atmosphere significantly denser than Earth's. Thus it is noticeably denser at any altitude than Earth's atmosphere is at the same altitude.

It would also help if the Earth-like planet had a weaker surface gravity than Earth. Thus it would pull down and compress the atmosphere a bit less and the atmosphere would thin out a bit less with increasing altitude.

As a general rule, astronomical objects with lower surface gravity and lower escape velocity are expected to have less dense atmospheres, not more dense atmospheres.

But exceptions are possible, and in fact there are two exceptions in our solar system. Venus has slightly lower surface gravity and escape velocity than Earth, but for various reasons has an atmosphere about 90 times as dense as Earth's. Titan has much lower surface gravity and escape velocity than Earth, but has an atmosphere a bit more dense than Earth's.

Another factor which might help would be to have a higher proportion of oxygen in the atmosphere of this Earth-like planet, so that the thin air in the high valley will have a bit more oxygen in it than Earth's atmosphere at the same overall pressure.

So I imagine your Earth-like planet might have a little more than 90 percent of the surface gravity of Earth, and a little less than 110 percent of Earth's atmospheric density, and a little less than 110 percent of Earth's percentage of oxygen in the atmosphere, and that might be enough to have sufficient atmosphere and oxygen at a height of 25,000 feet or 7,620 meters above the sea level of that world.

And any Earth people who might be in the story might describe the planet as Earth-like, with less than a ten percent difference from Earth conditions.

In most parts of the planet the different conditions will have little effect on the various ecosystems and the differences will only be significant at extreme high altitudes.

But you might want to have someone calculate the atmospheric conditions at 25,000 feet in such a world to make sure that would be enough.

Added 01-29-2019:

If necessary you might have to increase the percentage of oxygen in the atmosphere to much more than 1.1 times that in Earth's atmosphere. Dry air on Earth contains 20.946 percent oxygen by volume.

The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of about 30% around 280 million years ago, significantly higher than today's 21%

https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Third_atmosphere2

Thirty percent oxygen would be about 1.428 times the present concentration of oxygen in Earth's atmosphere.

Since the start of the Cambrian period, atmospheric oxygen concentrations have fluctuated between 15% and 35% of atmospheric volume.[10] The maximum of 35% was reached towards the end of the Carboniferous period (about 300 million years ago), a peak which may have contributed to the large size of insects and amphibians at that time.[9]

https://en.wikipedia.org/wiki/Geological_history_of_oxygen3

Thirty five percent oxygen in the atmosphere would be about 1.666 times as much as the present concentration.

And those examples of actual oxygen levels on Earth hundreds of millions of years ago prove it is possible for an Earth-like planet to have significantly higher percentages of oxygen in its atmosphere.

An otherwise Earth-like planet with 35 or even 30 percent oxygen by volume in the atmosphere would not be described as Earth-like by any visiting Earth people who might be in the story. Humans can suffer from oxygen toxicity when they breath too high a concentration of oxygen, although at 30 percent or 35 percent oxygen the atmosphere would have to have several times the sea level density of Earth's atmosphere for oxygen toxicity to affect humans.

I suspect that if your planet has a significantly higher percentage of oxygen in its atmosphere Earth humans might need to wear respirators at sea level, and find the atmosphere at higher levels invigorating and enjoyable, and have great difficulty breathing in your hidden valley at 25,000 feet and probably need respirators up there, since they wouldn't have the biological adaptations that high altitude humans on Earth have or the natives of the hidden valley have.

Another thing which might make your high valley more habitable would be lakes heated by volcanic heat. The lakes would warm up the air around and there could be aquatic planets and animals in the lakes and many more plants and animals in the valley than would be expected at such altitude.

And possibly there would be slight concentrations of volcanic gases in the atmosphere of the high valley,not enough to be poisonous but enough to increase the concentration of greenhouse gases and raise the temperature.

Of course in such a scenario it would seem probable that sooner or later the valley would be wiped out by a volcanic eruption. But your story might not be affected by what might possibly happen centuries or millennia in the future.

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    $\begingroup$ There are periods in Earth history where oxygen concentration was as high as 35% (Carboniferous Period). That alone could make a huge difference to habitability at high altitude. $\endgroup$ – Arkenstein XII Jan 28 at 19:48
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I cannot comment so i just have to start a new answer. It is specifically about JBHs answer. You are wrong in one point. More mass=higher gravitational force will make the atmosphere more dense but also smaller. So what you need is actually a planet with a lower gravity so it's atmosphere reaches farther out. Also the density of the atmosphere / the pressure does not decrease as fast as on a high gravity planet. Your argument of high gravity for more atmosphere is also not valid. While the planets in our solar system tend to have more atmopshere the more mass they have, there is no physical law that prevents moon from having an atmosphere. The only problem for terraforming moon with an atmosphere is, that you need hilarious amounts of atmosphere gases to reach a point of enough pressure for humans to breath. That is of course because of it's low gravity. So simply spoken you have gravity * amount of gas = air pressure. But also the more gravity you have on your planet the higher the pressure gradient will be - means the pressure will fall much faster. But there is a configuration of planetary mass and atmospheric mass which yields a earth like biosphere and climate on sea level but also provides sufficient breathable atmosphere at 7km altitude.

So contrary to JBHs answer i propose your planet is exactly as i described. Less mass then earth but more atmossphere gas.

Another problem is temperature. But you mentioned yourself maybe those guys live in a vulcano caldera. Maybe a caldera which is still thermal activ. Under perfect conditions there will be a stable mircoclimate in this caldera with enough air pressure to breath and enough heat to live. For plants as well as your people.

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You could make it work by lowering the sea. Earth gravity, mostly Earth-like, with Earth-descended species, but the oceans are either mostly gone, or the average bottom of the ocean is lower, so there's a lower sea level. Lower the sea level by 2-3km, and you can have your isolated tribe at the height above sea level you want. You'd probably want to drop the ocean by more than 2km, because dropping the ocean that much would have atmospheric consequences. Though, as Arkenstein XII stated in a comment, you could also raise the overall O2 concentration.

How could the sea level drop?

  1. It's not actually Earth, it's just mostly Earth-like. It didn't have that much water when we arrived.

  2. Aliens took it.

  3. In an effort to regain farm land, we dredged the seas for all the soil that had eroded there. It was expensive removing the salt from the soil so that it could be used for farming, but we wanted the salt anyway. After we'd recovered all the soil that we'd lost within the past couple thousand years... we just kept going. We made arable land enough to cover the deserts of the US, the Middle East, and Asia. We made arable land for Africa. And any new land made accessible by lowering the ocean floor. And in addition to removing all that dirt from the ocean floor, we also pumped out water and desalinized it enough to irrigate all of that land, and provide swimming pools and artificial lakes for all.

  4. Humanity has been using hydrogen/oxygen fuel for their rockets so much, the planet has run low on water. Obviously, when these rockets are firing directly at the Earth, no real loss of water happens, because we recover them. But after humanity reached the point of colonizing other worlds, reaching the asteroid belt, and traveling to the stars, their travels more and more had their rocket thrusters pointing sufficiently away from Earth that the water exhaust was not recovered.

  5. The people of Earth in the early 21st century were concerned about the perils of global warming. While their concern was real, and it was difficult for them to get traction with enough of the rich and powerful to have any effect, in the end, they were successful. Too successful. The Earth is now in another ice age, even worse than the ones before humanity became the dominant species.

  6. Fearing the rising oceans due to global warming, the governments of earth started building super-massive reservoirs, to hold all of the excess water. And then some of them had the idea of controlling the population by controlling enough of the water, so they built bigger reservoirs.

  7. We colonized Mars in a major way. One of the things we needed to do to accomplish that was fix Mars' lack of water. It had to come from somewhere, and we had plenty at home.

  8. Several of Earth's governments made massive underground building projects, to provide enough housing for Earth's trillions while still recovering some farm land. And this worked fairly well, for a long time. But we built too close to the oceans, and an earthquake caused massive leaks in the walls of our underground homes. The earthquake also knocked out our power, so we were defenseless against it, and 90% of everyone drowned.

  9. Geologic disaster of some kind. Maybe a mad scientist did it?

It's late, I'm going to bed, most of these ideas are crazy, but they're just to give an idea of the sorts of things that could be done. They could, for what it's worth, work together. The average depth of the oceans is shallow enough you would probably need to do something about it, rather than simply removing water.

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    $\begingroup$ That... was an excellent idea! Change the reference point! (In reality, dropping the sea level would probably pull the sea-level atmospheric pressure down to the new level, meaning that you still have the same problem - it's just a bigger deal to get to the top of Everest. But the idea... oh, the idea....) $\endgroup$ – JBH Jan 29 at 7:13
  • $\begingroup$ Gravity is based on proximity to the center of mass. By bringing the sea level down, without bringing the average distance of dry land down, the air is closer to the center of mass and pulled more. I'm really not sure how much more compressed the air would be by the added gravity, but it would be some. But if the total amount of air remained constant, ... I'm not a physicist, I haven't done all the math, but it feels like the air pressure at a given distance from the center of the planet shouldn't change a huge amount. $\endgroup$ – Ed Grimm Jan 29 at 23:26
  • $\begingroup$ And when you're writing sci fi, you're not exactly appealing to the most sophisticated physicist. You're just trying to get something that's believable if people don't think too hard about it. Oh, and just came up with another one: additional air, possibly harvested from Venus to try to get its run-away greenhouse effect under control. $\endgroup$ – Ed Grimm Jan 29 at 23:31
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All current answers are missing a major element. They all focus on how humans could survive there. What they (and you) are missing is a deeper question.

Why are they there in the first place?

Humans certainly have colonised a wide variety of landscapes. However in all cases there have been good reasons for humans to move into those landscapes, by expansion from existing inhabited areas. Whilst settlers would need different specialised skills to survive in, say, the heat of California and the cold of Alaska, you can see a clear gradation of those skills in people living between the two.

Even with that though, populations don't expand into places which are too challenging to survive. You don't just need to be able to survive sitting still, you need to be able to carry out farming, raising a family and so on. Where there may be good reasons to use risky areas, such as transhumance moving herds to higher pastures in the Alps, only the herders travel and the rest of the population stays in a "safer" area.

The only reason to migrate to a more challenging area would be if there was no choice; or if the area itself changes over time and there is no way to leave. Penguins in the Antarctic for instance are trapped by their location, and have evolved survival strategies as the climate in their location changed over time. If there is a way to leave a less-favourable location, then that location will usually be abandoned pretty quickly (or perhaps more accurately, populations started by individuals who leave will almost immediately become larger and more successful than the original population).

So, with that in mind, why are these people up there? Was there some event which rendered the lowlands uninhabitable or poisonous? How is the valley inaccessible, given that humans are naturally well-suited to climbing, and various areas of the world have high paths which are routinely travelled by the local tribes' children? With people trapped in the valley, have they adapted to existence there over millennia as geological events have pushed the mountains higher?

Lots of questions, because the answers are all going to have to be things you work into your worldbuilding and plot!

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  • $\begingroup$ Hmm.. Maybe the mountains around the valley are active volcanoes, and the people living in the valley are trapped in just as much as the people outside the valley are kept out? $\endgroup$ – Ed Grimm Jan 29 at 23:35
  • $\begingroup$ @EdGrimm Possibly, although then you'd need to explain how they've managed to not be toast over the previous few million years, each time one of the volcanos cooks off a bit more enthusiastically than normal. And even then, most active volcanos spend most of the time not doing anything life-threatening. Etna is the most active volcano in Europe, but it's a major hiking area and even has a cable car up it! I know the "secret valley" trope has a long history, but the OP needs to do better than those Victorian pulps to be convincing. $\endgroup$ – Graham Jan 29 at 23:50
  • $\begingroup$ The secret valley trope is so popular because before flight, it happened on occasion. But since we can fly over anything, not so much these days. A lot of it is not the fact that the mountain is impassable, but that it's really dangerous to pass it, so why bother without good reason? If you're a tribe that's being hunted, that's a good reason. If you're the group that was hunting them, and you just wanted them off your land, they've disappeared, why bother hunting them in dangerous places where they'll not survive long. Thanks for explaining what you asked me to, btw. $\endgroup$ – Ed Grimm Jan 30 at 0:29
  • $\begingroup$ @EdGrimm I'm not convinced it even really happened back then TBH. Maybe places looked remote to a pith-helmetted Victorian, but it doesn't mean the people never came out. Tribal wars make sense on a scale of years or even decades, but not for millions of years. Anyway, it's in the OP's hands now - if they want to use the secret valley trope then they need to work out a solution, and something more convincing than Conan Doyle or Rider Haggard. $\endgroup$ – Graham Jan 30 at 0:50
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Earth-like is of course a relative adjective. It would be easier to have a thriving biome if the world in question is one with greater than earth-normal atmospheric pressure at sea-level. Quite apart from things needing to breathe you're going to need to have rain clouds high up to provide your bowl with hydration.

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