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Anyone who has studied ice ages would know that during the last two-and-a-half million years of Earth's history, there have been periods where there was enough ice to suck up a lot of water. As a result, sea levels fell by an average of 100 meters. Among the most famous examples of this sort of drop is Beringia, the landmass that connected North America to Asia.

But in the event that a worldbuilder is creating a timeline that parallels the Maastrichtian, the latest Cretaceous period, in which sea levels dropped at a time when the influence of ice was minimal--alpine, at the very most--what else could reduce global sea levels if not ice?

And please, keep it natural--no human or alien intervention.

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The effect you're looking for is plate tectonics. Basically if you can't change how much water is in the container, make the container larger. Tectonics and glaciation are the 2 big effects we've seen historically, tectonics explains the Cretaceous sea levels.

Here's some tectonics that change sea level globally:

  1. Open up an inland low area (that could become a sea) caused by the rifting of continents. (<- this was happening during the Cretaceous). Get a feel for how they break and reassemble by watching videos of it: https://youtu.be/uLahVJNnoZ4. Basically some sort of activity happens in the mantle (sorta unclear why exactly the mantle is hard to study), then the crust thins in an area and slowly gets pulled apart until the land sinks and it's a mid-ocean ridge (careful to balance between ridges initially being high elevation and raising sea level if you start them in the ocean), and finally some sort of land blockage breaks open and the ocean floods into this new seabed. Mid-Africa is a good place to do this right now.

    1. Compress some land (make some mountains). Large (at least when they formed) mountain ranges like the Appalachians and Himalayas are made when one continent smashes into another and condenses land getting it to take up less of your sea bed. When India smashed into Asia to form the Himalayas there was a 10 meter sea level drop globally!

    2. Big cold plates. Turns out we're all on a thin layer of solid rock floating around in more magma than you can possibly imagine. Well new, warm ocean crust is more buoyant than old, cold plates and so the later sinks lower. So if you just need a world with lower sea levels than the present without it actively decreasing much just rearrange some plate boundaries so they're bigger and survive longer before being subducted.

I'm not entirely sure what time ranges you need these on. If it's quick on human time scales probably break open a connection between an empty inland low-lying area and the ocean (The Mediterranean is a good example: https://en.wikipedia.org/wiki/Zanclean_flood).

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  • $\begingroup$ 1) Anywhere between 72 and 56ma. 2) Is it also true that the speed in which a plate collides affects how quickly big mountains form? $\endgroup$ – JohnWDailey Feb 13 at 0:34
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If you want to reduce the sea level, you have to trap that water somewhere.

What are the options?

  1. Ice caps: this is the famous ice age
  2. Evaporate it: would turn Earth into a Venus-like planet due to the greenhouse effect of water vapor
  3. Large inland basins disconnected from the sea: water going there can only evaporate to reenter the water cycle.
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  • $\begingroup$ I was in the middle of drafting a "runaway greenhouse/basins" answer. You beat me by thirty seconds :( $\endgroup$ – Zxyrra Feb 11 at 5:07
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    $\begingroup$ @Zxyrra when a man with a comment meets a man with an answer, the man with the comment is losing his time ;-) $\endgroup$ – L.Dutch - Reinstate Monica Feb 11 at 5:57
  • $\begingroup$ Do you think the thermal expansion of the water in the oceans would counteract the losses due to evaporation to an extent? I'm not sure at what temperature changes one effect outweighs the other $\endgroup$ – PoorCorrelation Feb 12 at 5:33
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In supplement to L. Dutch's excellent answer, there are at least two other ways sea levels could reduce dramatically, albeit on quite a long time scale:

Cooling Earth's interior

The seas on our planet are mainly held up by pressure from below. Water that seeps down through the crust is heated until it reaches boiling and creates steam pressure that prevents more water from seeping down. On planets with a colder core, like Mars, water seeps much further down before equilibrium is reached.

Evaporation into space

If carbon dioxide levels rise to roughly 4 times their current level, the world could get very hot — so hot that all of Earth’s water simply evaporates into space. This 'moist greenhouse' effect, once in place, can't be reversed even by removing all carbon dioxide from the atmosphere (water vapor is an excellent greenhouse gas). It us unclear exactly how long it would tale to lose all our oceans to space, or even a large quantity of them, but I imagine that if will take at least several tens of thousands of years, maybe even millions. Even without excess carbon dioxide, this will happen in a billion years or so when our sun expands and heats the Earth dramatically.

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  • $\begingroup$ The level of carbon dioxide in the 55 million years long Cambrian period was ten times as high as now, and all the water did not evaporate into space. Carbon dioxide levels remained over five times as high as now for 350 million years, from the Cambrian to the Devonian. No water (or at least very little water) was lost over the entire Proterozoic and dPhanerozoic eons combined. $\endgroup$ – AlexP Feb 12 at 11:36
  • $\begingroup$ @AlexP: That is true, but even with that much carbon dioxide, the Earth's mean temperature was only 7C above today's. Hence, evaporation of the seas wasn't nearly as high as it will be if CO2 levels rise today. NOAA researcher Max Popp estimates that if the level of carbon dioxide in the atmosphere reaches 1,520 parts per million, temperatures at the surface of the world would reach 57C - 42 degrees higher than today. That makes a huge difference. popsci.com/how-worlds-oceans-might-eventually-vanish-into-space $\endgroup$ – Klaus Æ. Mogensen Feb 13 at 8:22
  • $\begingroup$ I don't get it. We know that for hundreds of millions of years the level of CO2 was very much higher than now. We also know that during those hundreds of millions of years temperatures did not rise to overwhelming levels; the climate was warmer than today, but not exceedingly so. Shouldn't models strive to match? There must be something fundamental I'm missing. (But then I'm quite ignorant of climatology, and I seldom speak about the subject, except in comments.) $\endgroup$ – AlexP Feb 13 at 9:44
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    $\begingroup$ Cimate is a complex thing. On the longer time scales, changes in planetary dynamics also play a role, e.g. changes in orbital eccentricity or inclination of the poles. There are also other greenhouse gases than CO2, such as methane - maybe levels were lower of those? I can't say for sure which is the case here, but I assume the NOAA researcher knows his field better than we do (unless, of course, you happen top be a practising climate scientist). $\endgroup$ – Klaus Æ. Mogensen Feb 13 at 12:25
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Since the purely physical ways of removing water from the cycle have been covered, i'll try a different approach.
Assume that a creature evolves (let's say it's a plant) that use water to create more complex molecules (as every plant does), but it creates solid matter, that won't be consumed by other creatures. that way, the plants will actually consume the water, and it will stay removed from the cycle.

on our planet and in our time that does not happen. But something similar has happened in the past: The first flora with wood-like structures used carbon dioxide from the atmosphere to create wood (simply speaking). But there were no bacteria around at the time being that could decompose wood. The result was massive layers of carbon-rich material that was just lying around. It's what coal and oil have formed from.

A process creating something similar to oil or plastic, meaning combinations of carbon, hydrogen and oxygen, are easily imaginable. and if that substance was toxic to most bacteria and vegetarians, it would also provide an evolutionary benefit, allowing those plants to spread and thus, over time, remove large amounts of water from the cycle.

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Some form of life has sequestered it away

Pick an area where a large amount of rainwater falls and will ultimately flow back into the sea. Some form of life in that area evolves that uses the component elements of water to create biological structures. This could be the shell of a crustacean or the bark of a tree for example. What matters is that all the hydrogen and oxygen it gets from rainwater is turned into a solid form, on land, that won't naturally decompose back into water.

Over a long timescale the mineral would form huge deposits on land as the sea levels decrease. Similar to how CO2 is actually sequestered by sea creatures and trees.

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A sizeable meteor falls on the ocean.

A lot of water gets ejected into space.

This one has plenty of side effects, but I'm pretty sure the sea will eventually stabilize at a lower level.

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    $\begingroup$ Ummm... i have a huge amount of doubt that this could actually happen ...? $\endgroup$ – Burki Feb 13 at 10:18
  • $\begingroup$ "The sea will eventually stabilize at a lower level": yes, it is possible. "Eventually" means millions of years... And the entire planet will be remodelled in the process, with complete loss of life. $\endgroup$ – AlexP Feb 13 at 13:16

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