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Can humans live on an Earth-like planet where the only difference is that the surface is ~50% water, instead of the ~75% of now?

That is, paleolithic humans...

And the new land area hosts the same ratio of biomes that currently exist on Earth, or whatever it needs to produce a human-livable climate.

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    $\begingroup$ Is your 50% only a surface area thing? Or is it a volume of water thing? Basically do you want your planet to be like Earth where sea level has dropped drastically (so the Atlantic is shallower and you can walk from Spain to Ireland)? Or to be like Earth where the continents are all wider, but the ocean depth is still the same (so Spain and Ireland are bigger and thus closer together, but you'd have to take a boat to get there)? My answers would be different in each of those cases. $\endgroup$ – DrBob Aug 22 '16 at 17:01
  • $\begingroup$ @DrBob - I think I just wanted to know there were any ~50% water-covered area Earth-like planets that humans could live on... $\endgroup$ – Malady Aug 22 '16 at 20:16
  • $\begingroup$ If you have a look where humans live on this planet, i would assume the change you suggest would annoy very many people, but the planet would still be habitable. $\endgroup$ – Burki Aug 24 '16 at 12:37
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Water works as a temperature barrier, especially in the formation and melting of ice. Its also heavily responsible for the redistribution of energy on the planet.

So, compared to our planet, your planet won't be as insulated from warming and cooling trends, and things like a global cooling event (volcano, nuclear winter from asteroid impact etc...) will have an exaggerated effect. Global warming events will also require less time to warm your globe as well.

Less water might make it difficult for thermohaline circulation (great oceanic heat conveyor belt). If this fails to start, then energy isn't able to redistribute around your globe. This leaves your equator overtly hot and polar regions frozen over.

The extremes might make it a bit more difficult, but there is no reason why humans couldn't come about.

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I think this is a "yes, but..." answer.

First off, less water in the oceans means less water in the hydrological cycle. So less rainfall. Earth at the height of the ice age is a good model for this: with lots of water locked up as ice, the climate became drier. Deserts expanded, rainforests contracted. Here's a map of how small the rainforests and how large the deserts were at the glacial maximum.

So your planet will be like an ice-age Earth on steroids! Except without the massive ice caps. If it actually HAS an ice age, it will get drier still.

How hospitable it will be for humans (or any big land animals) will depend on how your continents are arranged. If you have a third less water than Earth, then supercontinents are going to be a lot more common during your planet's history.

The centre of a supercontinent is very arid and uninhabitable by pretty much everything other than microbes. Basically it is such a long distance from the sea (where water gets into the atmosphere) that any clouds have dumped their rain and ceased to exist long before they reach the centre of the continent. Think of the driest deserts on Earth (hot or cold) - those are positively dripping with water compared to the centre of a supercontinent.

Bits of the supercontinent are subject to a mega-monsoon climate, so the biomes there will be unlike anything currently on Earth. All this mega-monsoon data is for Earth at a time it had no ice caps. I've no idea how polar ice sheets would affect this mega-monsoon wind pattern. Here's a paper on the mega-monsoon through geological time.

This article suggests that reptiles and proto-mammals may have lived in separate parts of Pangea because of the weird climate. So again, biomes would be very different from Earth today.

So your humans could live there, but they - and much of the rest of life - will be constrained to the habitable areas. Ironically your planet has more landmass than Earth, but it very likely has less habitable land area than Earth.

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Yes.

Your Earth-like planet would have a lot more land that is like Sub-Saharan Africa. Existence would be difficult, but it'd be more than possible.

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