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I am writing a small piece of fiction for a game. I wanted to describe it as much as possible. The setting is Earth, more than a milenia after an event of cataclysmic proportions that made the planet almost uninhabitable. The result is a planet mostly toxic, in which people need some sort of breathing apparatus to survive(am still going to work on that) and with something as close as to 20-40% of all its surface water. Would be that believable? Or for most people it would seem like a impossible condition for sustaining life, even on a ridiculous small amount? Also, if possible what would be the smaller percentage that I could use and still keep it believable? I really want to me it a hell on Earth.

Most of the water was evaporated into space, part of it went underground due to a massive damage to the planet's crust and a tiny amount is still trapped in the most higher mountains and in the poles as ice and snow. Big animals like elephants, and whales doesn't exist anymore, most of the creatures wandering Earth now are less dense and some have way more volume and less density, some even like balloons of gas. The underground, in other way, and in old places covered from the sun and from the dry there is a lot of moisture and it helps the overgrowth of fungus.

It's important to note that the creatures in this setting aren't realistic, and as knowledge of how they came to Earth is unknown for the survivors, this topic is rarely raised. Most of them are highly toxic and poisonous to humans like nothing on the old Earth.

The world is most likely a big desert. Much like mars. But on the deepest locations of the old oceans is where there is still water and a green forest circles it all. The idea goes like this: imgur.

I don't need equations and such things. I don't really want it to be hard fiction. I just don't want people thinking that it is all wrong at the first moment they read it.

Thanks, and sorry for the English, I'm Brazilian and I'm still working on making English my second language.

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    $\begingroup$ Hello and welcome to Worldbuilding.SE. Your English is pretty good, so don't worry, we've seen worse (there may be edits for the typos, but it happens to everyone) :o) Maybe you could add some information about the tech level of your survivors? $\endgroup$ – Alexander von Wernherr Feb 17 '17 at 12:32
  • $\begingroup$ Is the water gone forever (i.e. evaporated into space) or simply trapped in non liquid form? Also, what is the life level you want to sustain? small microbial colonies need far less water than elephants and a jungle... $\endgroup$ – L.Dutch Feb 17 '17 at 12:59
  • $\begingroup$ Could you add a few other bits of information: -** Where is the water lost from?** (ie are lakes untouched but the oceans are lower. - What level of detail would you like in an ideal answer? (on worldbuilding we vary from "yes this is believable, the water cycle should still work" to "Here is the water cycle and how yours would change..."(usually with references and equations)). $\endgroup$ – Lio Elbammalf Feb 17 '17 at 13:14
  • $\begingroup$ Most of the water was evaporated into space, part of it went underground due to a massive damage to the planet's crust and a tiny amount is still trapped in the most higher mountains and in the poles as ice and snow. Big animals like elephants, and whales doesn't exist anymore, most of the creatures wandering Earth now are less dense and some have way more volume and less density, some even like balloons of gas. The underground, in other way, and in old places covered from the sun and from the dry there is a lot of moisture and it helps the overgrowth of fungus. $\endgroup$ – Victor Matheus Feb 17 '17 at 13:15
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    $\begingroup$ @VictorMatheus you will do better to include those infos and picture into your post rather than place it here into comments :) $\endgroup$ – Antoine Hejlík Feb 17 '17 at 13:33
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Before I answer your question, welcome to World Building. This community is related to Science Fiction in many ways, but here we answer and discuss questions related to building an environment according to your requirements.

Your world focuses on an alternate Earth, where Earth has gone through some major changes from what it is now in real world. In particular, you ask:

1- ... with something as close as to 20-40% of all its surface water. Would be that believable?

If that is believable or not, depends on the type and severity of the disaster which fell upon Earth. If the type of disaster is correct, then it will definitely be believable.

For example, if Earth was hit by a series of extremely large (~15 km diameter) asteroids, that would release immense amount of heat, equal to millions of atomic bomb explosions. The temperature required to break up water into hydrogen and oxygen is about 2000 °C. In case of a mega disaster, a large amount of Earth's water will decompose into hydrogen and oxygen. Hydrogen will rise up in the atmosphere and will slowly be lost into space (hydrogen often escapes into space due to solar wind, from the atmosphere of low gravity planets like Earth). This will leave behind an Earth which is completely different from the Earth we know. A lot of water would be lost, but the atmosphere would be extremely rich in oxygen. I have not done any calculations about it, but my crude guess is that 90% of the atmosphere (by mass) would be oxygen. Also remember, that a disaster of this scale will wipe out everything other than extremophile bacteria. There will be nothing larger than amoeba left alive in the oceans or on the land. The climate would undergo extreme changes and the whole planet will probably undergo an ice age for tens of millions of years. Only the people up in the satellites would survive such a disaster. All the others (even the last man on Earth) would die within one year after the first 15 km asteroid falls.

Another, less damaging method of removing surface water could be by some sort of new bacteria. The bacteria breaks down water into oxygen and hydrogen, and then uses hydrogen in respiration. This bacteria thrives in the oceans and rivers/lakes and begins to decompose water into hydrogen and oxygen in large quantities. Scientists program a virus to kill that bacteria only. After many decades, all the bacteria of that kind are killed, but they have already decomposed 60% of Earth's water. This will again leave a planet very very different than what it is today. Almost all of the humans and other creatures will die. The only humans surviving the global change will be living deep in shelters with artificial environment.

So yes. If you plan your disaster nicely, it will be believable. Heck! If movies like Looper and Matrix can be believable, your idea will be believed too.

2- Or for most people it would seem like a impossible condition for sustaining life, even on a ridiculous small amount?

Yes, most of the humans with our current body structure will die. Very few can survive, but they will be living under hellish conditions. Seasons will be extremely severe and wind storms will be far more powerful than anything we know today. Usually the wind and dust storms will blow over areas as large as whole continents.

3- Also, if possible what would be the smaller percentage that I could use and still keep it believable?

You could go on and remove all the water from your world if you want. Most of the things in science fiction are not possible under current scientific knowledge, that is why it is fiction.

Just keep in mind that all life forms on our planet require water for survival. If people and animals don't get drinkable water, they die. If plants don't get enough water in the soil, they die. If plants die, animals die because there are no plants to eat. If there is no rain (due to too less surface water), many bad things will happen to the large scale climate and a lot of plants and animals will go extinct. If you only want to keep a few people in far off places to survive in artificial conditions, you could go on and remove 85% of all water on the planet. Earth will become hellish and most animals will die. Most of the plants will die too. The surviving people will need to have their facilities located near the small seas and convert seawater into freshwater to survive. They will have to keep fish and cattle in small areas and grow crops too. It will be possible with advanced technology, but it will only support small groups of people, located far away from one another, near seas and rivers.

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  • $\begingroup$ Thanks, by the way I loved your approach with the water breaker bacteria. It's a hell of an idea. But I need to keep the asteroid plot, because there will be fictious minerals that arrived in Earth on them and those are also important to the plot. Also, the craters can be a nice place to have oceans, etc... $\endgroup$ – Victor Matheus Feb 18 '17 at 7:56
  • $\begingroup$ Just keep in mind that with the asteroid plot, you are going to end up with a planet which would lose all of its local fauna and flora and there would be no advanced life forms outside the protection of the facilities. Earth would be almost as dead and hostile to life as Venus. Oh, the horror! $\endgroup$ – Youstay Igo Feb 18 '17 at 8:01
  • $\begingroup$ Well, the game is meant to be a storydriven topdown 2D hardcore survival. The survivors begin in a shelter and for some reason have to travel through the desert surface until another shelter far far away. It is not combat focused, even though combat will be somewhat present. The player will have to follow strict paths in the desert in the form of stics in the ground with a rope in it in order to not get lost. And he will have the liberty to go off the "road" and loot and scavenge, but will have to be fast and cautious not to lose himself because of thirsty hunger and temperature as well as O2. $\endgroup$ – Victor Matheus Feb 18 '17 at 15:36
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Even at 20% water surface, your planet is more believable, hydrologically than Arrakis. Go for it!

I'm no climatologist, but even with 20% surface water, you'd be in a more realistic range than Frank Herbert's famous (and Hugo winning) desert planet Dune, or more formally https://en.wikipedia.org/wiki/Arrakis
Suggest you read that (if for good notions on arid-planet survival tech) and for some amazing world building.

Surface water in the warmest areas will contribute more to your planet's water cycle. I'll presume that your planet has enough of a water cycle to have something like Hadley cells; that might help you rough out your large-scale climate regions: https://en.wikipedia.org/wiki/Hadley_cell
I suspect you'll want an equatorial ocean or several significant equatorial seas. Is the ground water close enough to the surface to extract and use? If so, competition for still-farmable places is a great plot element.

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  • $\begingroup$ This is basicly what I was looking for, thanks! In fact the setting is Earth, only somewhere near the year of 3500, 1500 years after this cataclysmic event. As the people there are just the descendants of the survivors, the events that lead to the current state of the planet aren't much big of a deal, even thought it is very important to the lore itself. $\endgroup$ – Victor Matheus Feb 17 '17 at 17:23

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