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I'm writing a story that happens on a dry and arid planet that has only a few oasis's where life can thrive. Life on this world either lives in the oases or lurks in the sand waiting for migrating animals to pass by. The inhabitants of the world are also nomadic, spending some time at each location before moving on so as to not burden the land. Though there are exceptions of course, some groups prefer to settle in a single oasis and expand it. However this world wasn't always a desert, otherwise life as we know wouldn't be able to develop. The planet was once covered in oceans and something caused it to dry out.

What could cause oceans to disappear? Either through a cataclysm, though life may not survive or through some long term change in the environment.

I'm not too late in the writing process, so there's wiggle room for explanations. Although it has to allow for life to survive the drying, so no meteors evaporating the oceans or anything that extreme. My first guess was for the seas to get sucked under the planets crust, though I am no seismologist so I don't know the specifics of how that would happen.

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  • $\begingroup$ How fast do you want oceans to disappear? Mars once had oceans. $\endgroup$
    – Alexander
    Aug 24 at 16:31
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    $\begingroup$ (1) Have you heard of a red planet called Mars? It once had an ocean, and now it doesn't. There must have been a point when it was only mostly a desert. (2) The plural of oasis is either oases (for the classically inclined) or oasises (for those deprived of a classical education). A plural in apostrophe ess is (grudgingly) allowed only for bare letters or digits. $\endgroup$
    – AlexP
    Aug 24 at 16:44
  • $\begingroup$ The key issue here seems to be "lose the oceans - whilst hanging on to an atmosphere". Sure mars had oceans, but it lost it's atmos first. $\endgroup$ Aug 24 at 16:51
  • $\begingroup$ @Alexander It may take millennia for a planet to lose its oceans, centuries would be a little too optimistic. Yes, I know Mars had oceans. The question was inspired by Barsoom, which is also a world that lost its oceans. $\endgroup$ Aug 24 at 17:39
  • $\begingroup$ A hot atmosphere could easily cause this if it heated up overtime, and depending on if this planet had former inhabitants, severe global warming could cause the oceans to evaporate. $\endgroup$
    – Jay
    Aug 24 at 17:46
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They are where they have always been, underground.

Most of the worlds water is and has always been deep underground. Oceans of it. Life developed there getting its energy either chemically or geothermally and eventually evolved to colonize the surface. If the surface dwellers ever learn to tap deep, they can bring up more water than they ever dreamed of....and maybe some things they'd rather not dream of.

This solves your "However this world wasn't always a desert, otherwise life as we know wouldn't be able to develop." without having a cataclysm that everything has to survive.

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  • $\begingroup$ Perhaps you should clarify that you are talking about the world in the question and not about the Earth. I had to look it up. The amount of groundwater on Earth is a tiny fraction of what rests in the oceans. See en.wikipedia.org/wiki/Water_distribution_on_Earth $\endgroup$
    – EdvinW
    Aug 26 at 17:00
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Perhaps a meteor strike in the ocean at a point where only a thin layer of rock separated the surface ocean from an enormous void within the crust. If the meteor pierced that rock layer, a great deal of the ocean might follow it into the void, causing shorelines around the world to recede until the void filled up. That wouldn't dry out the planet entirely but it might reduce the percentage of the surface which is covered by water to a small enough value to suit your story.

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    $\begingroup$ A sufficiently large void would collapse long ago, and never have formed. $\endgroup$
    – Stilez
    Aug 25 at 16:54
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Climate change

Given your scenario, letting all the water disappear is unnecessary. Oceans and seas make up for about 96.54% of all water on earths surface. Fresh drinkable water is about 0.76% groundwater and 0.007% lakewater1.

As you can see from the map below, your story might become closer to reality in the next 10 or so years.
map 2030-2039 climate change drought effect
Note: this image was not taken from a scientific article and could be an inacurate artistic representation

From a paper by Yerlikaya B.A., et al. 2020:
"Within the next decade, an estimated 60% or more of the world population will also begin to experience serious water shortages"

What is nice about climate change drought is that: "Groundwater systems generally respond more slowly to climate change than surface water" (Kundzewicz et al. 2007). This could mean that oasis formed from underground water stay longer. And be one of few viable options for some.

reference

Kundzewicz, Z & Mata, Luis & Arnell, Nigel & Doell, Petra & Kabat, Pavel & Jiménez, Blanca & Miller, Kathleen & Oki, Taikan & Şen, Zekai & Shiklomanov, I. (2007). Freshwater Resources and their Management.

Yerlikaya B.A., Ömezli S., Aydoğan N. (2020) Climate Change Forecasting and Modeling for the Year of 2050. In: Fahad S. et al. (eds) Environment, Climate, Plant and Vegetation Growth. Springer, Cham. https://doi-org.vu-nl.idm.oclc.org/10.1007/978-3-030-49732-3_5

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Either the sun bloated, or the earth broke. Don’t get it? Look…

Maybe the sun bloated. You know, it would work if this was a dystopian or science fiction (which it probably is, if I am right). But that presents a problem: You say nomadic people thrive in this world, which means there must be the proper climatic conditions (minus water) to survive, and I don’t think a bloated sun is going to help. Then here comes another theory: the sun did bloat, but then it shrunk yet again. (Okay, okay, I’m sorry)

Then go reverse. The mystery here would be- “A long time ago, there used to be water in the planet, a cold liquid gushing down the barren channels that lay now. What happened to the water?” And then we answer it. A heat wave? That would be the explanation. A massive wave of heat, a natural disaster that struck the entire world, resulting in mass damage, and inevitably, the disappearance of the most valuable asset of life: water. A heat wave that left survivors in only ten thousands, which explains why they are nomads.

Now, you would say: “You dimwit, if the water vaporised, it would take the form of clouds and rain down again!” Well … all right. Then: the water froze. That’s all the explanation that can be. Either the sun bloated and vaporised the entire water content entirely, or the sun turned into a dwarf. Or, due to a temporary disaster, it was predicted that the earth would break loose from its course, and that’s what happened. Then the water froze, clumped in the chilly poles of the earth.

If neither sound plausible, tell you what: this is fiction. Have fun!

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Alien life

The sample retrieval mission from Bennu was a success beyond all expectations. Not merely fossil evidence of life, but life itself, from a barren asteroid in the midst of space!

They cultured it so carefully, with such amazing results. They noticed that oh my, how it hoards water! It has complex silicon chain enzymes that can convert sand to orthosilicate, or any step in between. SiO2 + 2H2O = SiO4(2-) + 4H+, the hydrogen either being tagged onto the orthosilicate or used for more peculiar inorganic biochemistry. It had the ability to set up networks between grains of felsic and ultramafic rock and extract energy from their rearrangement. And naturally, it could also do very well when cultured on a substrate of pure glass.

Wups... did I mention they had it in a glass container?

The irony was that one day Bennu would have struck the Earth anyway, and in all likelihood it would have survived from the fragments and had the same effect. But whatever the reason, this spectacularly well evolved alien microbe has incorporated the water into its silicon-rich spores embedded in the sand and silt and stone of the planet, leaving nothing but salty wastes and vast piles of infectious dust where once the oceans heaved.

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  • $\begingroup$ I’ve seen weird answers before but this is completely unrelated to the question. Well maybe not completely... life without water and whatnot. $\endgroup$ Aug 24 at 20:33
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    $\begingroup$ An orthosilicate of what? It needs to steal cations from some other salts, with the end results of oxygen and/or water being released back. $\endgroup$
    – Alexander
    Aug 24 at 20:36
  • $\begingroup$ @Alexander That's a fair complaint (and I was slapdash writing what happens), but does it have to steal cations? I think it can find metal ions with multiple oxidation states, and use those as counterions for the orthosilicate while using the neutral H for other purposes. $\endgroup$ Aug 24 at 20:43
  • $\begingroup$ @LiveInAmbeR - if you need the Earth's oceans to disappear, you need something capable of reacting them with the Earth's crust to form other minerals. $\endgroup$ Aug 24 at 20:44

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