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In a scifi universe I'm currently writing up, a colony was established on a weird world. strangely, it's a habitable planet with an extremely shallow sea, with an average depth of 4 meters, excluding the occasional 2 km deep rift valley. The planet is also dotted with chains of islands, mostly small piles of sand, the largest island is about the size of Earth's Madagascar Island. Volcanism is rare despite the evidence of a magnetosphere and tectonic activity.

But what would the climate of a world that is nearly all shallow ocean be like? How would the currents work? would it be a sunny tropical paradise or a stormy hell hole? And would it be a good place to put up a beach resort for human colonists?

Specific characteristics of said planet

  • 90% the size of the earth
  • Gravity of .95 gs
  • the surface is nearly 99% ocean
  • ~1 Au from its parent star
  • said parent star is a little dimmer than our own Sol
  • deep open oceans are rare on this planet, with most parts going no deeper than 20 meters
  • habitable, complex multicellular life is present, and an atmosphere like Earth's is also present
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  • $\begingroup$ I'm sure I'd heard this question before, but I can't find any duplicates. Possibly related, though: Can an Earth-like world of shallow, global, seas be stable over geological time?, Archipelago worlds build up atmospheric oxygen faster? $\endgroup$ Jan 12, 2023 at 15:22
  • $\begingroup$ Putting a little bit more thought into this... where does your sand come from? You don't really have an equivalent of Earth's sand-forming environments. $\endgroup$ Jan 12, 2023 at 15:52
  • $\begingroup$ @StarfishPrime a mix of rock broken apart by tides, coral/snail analogs being broken apart, reefs, and a "Mud" mix made from decaying organic compounds and fine silt, that's how the sand is formed. I didn't put it in the initial question because I thought it wasn't that important of information. $\endgroup$ Jan 12, 2023 at 15:57
  • $\begingroup$ So not sand, then. Silt might work, but you do end up with mudworld… $\endgroup$ Jan 12, 2023 at 16:03
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    $\begingroup$ Fair warning: climate is incredibly complex and asking on behalf of an entire planet is a pretty tall order that often violates the Help Center's book rule (see help center). That means you're likely to receive very generalized answers. Can you explain why you're asking the question? What kinds of effects are you looking for or what type of information do you want? At a guess, your world will have a lot of clouds. $\endgroup$
    – JBH
    Jan 12, 2023 at 16:30

2 Answers 2

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The climate on your world would, to a good approximation, be similar to that on Earth. The oceanic currents would be different, but in broad terms its an Earth analog, Earth is after all 70% ocean. o

Obviously the effects of land masses / mountains would be missing so just extrapolate Earth's oceanic climate around the entire planet. Might as well take the Pacific Ocean as a starting point as it more or less covers half the planet anyway.

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  • $\begingroup$ Kinda my thoughts too. Presumably take "Pacific Island-chain weather" and assume that's everywhere. Problem is I'm not sure if making the oceans 20m deep would change that. Are hurricanes worse because the planet is mostly ocean, or non-existent because the oceans are mostly 20m deep? $\endgroup$
    – JamieB
    Jan 12, 2023 at 19:02
  • $\begingroup$ I think the 20m deep oceans are probably deep enough to mimic our oceans for hurricane purposes. I doubt that water below 20m has any major effect on a passing hurricane on Earth as convection from such a depth takes time. The main players are surface temperature and atmospheric effects. That said its hard to be certain. timehttps://www.weather.gov/source/zhu/ZHU_Training_Page/tropical_stuff/hurricane_anatomy/hurricane_anatomy.html $\endgroup$
    – Slarty
    Jan 13, 2023 at 19:58
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I think planets with (carbon-based) life but shallow oceans make for an interesting combination. Organic matter and nutrients from dead matter in our world descend deep into the ocean through the process of marine snow. To the best of my knowledge this explains why on our planet the CO$_2$ concentration has been able to get so low (100's of ppms in stead of 10's of percents): the carbon was fixed from the atmosphere by photosynthesizing cyanobacteria and then fell to the ocean floor by virtue of the overdensity of the dead organic matter. On your planet, there will be an abundant ecosystem on the ocean floor efficiently recycling (burning) whatever is deposited there and so the atmospherical $CO_2$ will likely stay very high. So the greenhouse effect is going to be very strong here (but if the star is sufficiently dim there may not be a problem) and the atmosphere not exactly breathable for humans.

Of course you might circumvent that predicament by having a weird primordial chemical composition for your planet (very scarce C), but you're probably finetuning then.

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