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I have been toying with the idea that all oceans are gone. Underground water still exists but the Earth has essentially turned into the planet from Dune.

My characters all live / survive out where the oceans once were as it's the closest source for water. The Earth is now just one big desert.

My question is, instead of 100% of all surface water gone, is 90% more feasible? I plan on having larger civilisations, mining towns etc. I want water to be a rare commodity, but not so rare that only 5 people can live off it.

As the majority of the story takes place out in these vast oceans of desert, there are going to be some large swathes of distance in-between continents / countries. Initially people were going to get around on horse back, but there's too much distance for a horse to travel, then I considered intercontinental railroads but that is taking technology in a direction I hadn't planned on going. So, if I leave a small portion of the ocean, travel by boat could seem plausible.

There are a number of issues with eliminating our planet's oceans...

Our planet's gravity holds onto our water pretty tightly. The energies needed to evacuate several oceans worth of water out into space would probably also take our atmosphere along for the ride. You could send the water deeper into the planet's crust, but that leads to other issues...

50-85% of our oxygen comes from phytoplankton which live in the oceans near the surface. Eliminate the oceans (or even just poison them badly enough) and that oxygen goes bye-bye. Similarly, if the water submerges deeper into the crust, the percentage that remains near the surface will decrease greatly, leaving less room for the phytoplankton. So no matter where the water goes, oxygen will be in short supply after it is gone.

To combat the oxygen loss, you could evolve the phytoplankton to no longer require an aquatic environment. They could continue doing their job for us, living off of the nutrient rich silt and rotting fish corpses which cover the newly exposed ocean floor. That would give you a green and smelly desert, but at least your characters would still be able to breathe.

There would be other issues such as climate change and storm intensification. We might even discover that the former ocean waves served a hidden function, absorbing momentum from the winds which would otherwise rise to constant storm force everywhere. Your ocean-less planet is quickly transforming into a literal hell on Earth.

Earth would undoubtedly survive without its oceans, but its current infestation of organic life would probably not remain upon it for long.

The single biggest issue you'll face is the loss of thermal mass. One of the key reasons that deserts are so hot during the day, so cold at night is that there's no water that soaks up the heat and releases it slowly through the night. In your world, this is now the norm so your days will be extremely hot, and nights extremely cold.

Within the bounds of habitability? With underground homes, perhaps. I wouldn't want to be living on the surface though.

Water in such an environment is very precious, and your inhabitants will most likely have mechanisms similar to those described in Dune for rendering water from the dead and other biological material for re-use. Their Stillsuits would also be a likely invention as you wouldn't want to waste sweat or urine because of the water content.

Of bigger concern would be the impact on vegetation, both marine and land based. The real issue here is that trees (which you'll still need to generate oxygen, especially now the phytoplankton is gone) need lots of water to grow, and they expect to live in an environment where they regularly get rainfall. You've also got the food issue as crops need water. Meat is definitely out because you won't have enough water for grass, and the amount of water required to 'raise' a meal of red meat by comparison to a vegetable based meal is massive.

Even with 10% of the previous water levels, I don't see you rebuilding large cities of any kind because the amount of water required in most mining and industrial applications is prohibitive and the infrastructure required to do so requires seeding from an industrial complex probably from before the water loss.

I'd see society degenerating into small clans, fiercely protecting what water holdings and crops they have the manpower to defend. Clan structures don't scale well, the lack of water also makes trade next to impossible because of the increased cost of 'shipping' and as such, societies won't reach the critical mass of size where there is sufficient food and protection in place to support a small core of researchers or scientists tasked with making things better.