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I'm trying to find a justification to the sea level on Earth decreasing by a large amount (in the range of 1000 / 2000 m).

The change would begin during the Modern Era or in the future, and last long enough to allow for new political entities to be established on the newly exposed lands. The interested area doesn't need to be the entire surface of the planet, but it should be extensive enough to allow for said political entities to form.

I imagine for example that something could cause, over a long span of time, the amount of water evaporating to be constantly bigger than the amount of water returned to the ground as rain, snow, sleet. But this requires an explanation of its own.

What kind of event, natural or caused by human actions either deliberate or accidental, could lead to this phenomenon (feel free to use my prompt in the paragraph above, or give a completely different reason)? What sort of timeline would it require?

If this is not possible without large amounts of handwaving with Earth, please use an Earth-like planet as similar as possible to ours.

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    $\begingroup$ My first question on this site might not be a duplicate if this, but is definitely related. $\endgroup$
    – TheDyingOfLight
    Jul 7, 2019 at 6:22
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    $\begingroup$ I have to add... xkcd what if: what-if.xkcd.com/53 $\endgroup$
    – Lupus
    Jul 7, 2019 at 14:10
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    $\begingroup$ @MicheleC -- You only get one question. Please delete the timeline query. If you want to know that as well, you can always ask a follow-up question! $\endgroup$
    – elemtilas
    Jul 7, 2019 at 23:51
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    $\begingroup$ Have you ever read Rainbow Mars by Larry Niven? (Even saying that much is probably a bit of a spoiler, but... yeah. It's relevant to this discussion.) $\endgroup$
    – Mason Wheeler
    Jul 8, 2019 at 14:56
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    $\begingroup$ @elemtilas Asking what timeline the event would require is not a separate question, but just specifying what information the answer should contain. The timeline is directly tied to the solution so it's part of the same question/answer. $\endgroup$
    – Prime624
    Jul 8, 2019 at 17:23

14 Answers 14

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The thing about water is that it is pretty much indestructible. If you don't want it in the sea you need to put it somewhere else.

The usual reason for lowered sea levels is glaciation.

For example, at present Earth is in an ice age, with a lot of water trapped as permanent ice; as a consequence, sea levels are some 90 meters lower that what is usual in geological time. During the Last Glacial Maximum, the sea level was even lower, some 125 meters below the present level. That was enough to make Great Britain a peninsula of Europe and to link Asia and North America by a land bridge.

If you want even lower sea levels you need a colder and longer Glacial Maximum. Unfortunately, this has severe side effects. Not only is a lot of land covered by ice, but glacial periods are cold, obviously, and when it's cold the air has much lower ability to store water vapor; as a consequence, the climate is drier in glacial periods, with extensive deserts and lower rain overall.

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    $\begingroup$ There's a feedback mechanism that stops the sea level from dropping indefinitely. Once the oceans freeze over (which seems to have happened at least once about 600 million years ago, see Snowball Earth en.wikipedia.org/wiki/Snowball_Earth) evaporation pretty much stops and snowfall ceases. It's unclear just how far sea level falls -- certainly more than the fall during the recent ice ages. $\endgroup$
    – Mark Olson
    Jul 7, 2019 at 14:11
  • $\begingroup$ @MarkOlson: Brrrr.... $\endgroup$
    – AlexP
    Jul 7, 2019 at 14:12
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    $\begingroup$ It's long underwear time! $\endgroup$
    – Mark Olson
    Jul 7, 2019 at 14:14
  • $\begingroup$ To reduce sea levels as required by glaciation would require glaciers to be on the order of 10 miles thick. This is not just improbable, it is impossible -- the weight of a very thick glacier would cause it to flow quickly into the sea. $\endgroup$
    – Gary Walker
    Jul 8, 2019 at 0:25
  • $\begingroup$ @gary that's called an ice sheet, they have existed many times. $\endgroup$
    – John
    Nov 17 at 18:04
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The dynamo feeding the magnetic field of the planet has stopped working.

Lacking the shielding effect of the magnetic field, solar wind peels off our atmosphere. First it just impacts the water present as vapor, but as soon as the pressure drops, the oceans start boiling off, losing even more water.

The decrease in the sea level is just an intermediate step to the extinction of the life we know on our planet.

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    $\begingroup$ That works, though only on a hundred million to billion-year time scale. $\endgroup$
    – Mark Olson
    Jul 7, 2019 at 14:04
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    $\begingroup$ Even without the magnetic field failing in a few hundred million years when the sun warms enough to tip the earth into a runaway 'wet' greenhouse mode after the troposphere becomes saturated with water it will overflow into the stratosphere where UV breakdown will be able to slowly destroy it (once freed the hydrogen is light enough to quickly escape the atmosphere). That's still going to be operating on geological timescales and will probably render the Earth uninhabitable in the process. $\endgroup$
    – Dan Is Fiddling By Firelight
    Jul 8, 2019 at 14:54
  • $\begingroup$ Stripping atmosphere from the planet, and thereby water, could be augmented with 'unfriendly high-energy events' to reduce timescale. - ie, "New advanced reactor design in geo-sync orbit 'did weird things' and had an energy output several orders of magnitude higher than expected...' and 'the bad things happened'..." $\endgroup$
    – TheLuckless
    Jul 8, 2019 at 20:56
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Atlantropa was a project from a German architect who planned to build a huge dam at Gibraltar, to lower the Mediterranean Sea by about 200 meters, and produce massive amounts of electricity. I guess you could re-work the project to lower the sea by thousands of meters (the mediterranean is over 5000m deep at its deepest). That would of course have major consequences on the environment, and such water would have to go else where (the other oceans, raising them).

Another idea would be to fit that water elsewhere. You could imagine an immense cave network inside the planet, that suddenly gets opened (maybe by oil drilling or a big earthquake on the ocean floor). Water would begin pouring in, and if the caves are wide and deep enough, you could have the sea level fall several hundred or thousand of meters (speed would depend on how big the opening is).

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    $\begingroup$ The Mediterranean has a strong water deficit -- evaporation extracts far more water than what the Nile and the Rhone bring in. Some water comes from the Black Sea (which has an excedent), but most comes from the Atlantic. If the Gibraltar is dammed, the Mediterranean will dry up almost completely in the blink of a geological eye. The Bosporus and the Dardanelles will turn into roaring rapids, and the Nile will excavate a thousand meters deep canyon right through Cairo... $\endgroup$
    – AlexP
    Jul 7, 2019 at 14:15
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    $\begingroup$ Giant cave was my thought. Tough for the Pellucidarians, though. $\endgroup$
    – Willk
    Jul 7, 2019 at 20:21
  • $\begingroup$ FYI: More discussion around this seems to have appeared on HackerNews recently: news.ycombinator.com/item?id=20380250 $\endgroup$
    – JohnLBevan
    Jul 8, 2019 at 10:59
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    $\begingroup$ If you're going to lower the sea level by a thousand meters, you're going to need to pile the water up on the land to a depth of almost 2000 meters. $\endgroup$
    – Mark
    Jul 8, 2019 at 20:48
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    $\begingroup$ +1 Came here too late! You don't actually need to lower the level of all the sea. It is theorized that this happened naturally in the Mediterranean at some point in geological history. See the Zanclean flood. (You might want to use it to improve your answer, if you think it fits) $\endgroup$
    – Rafael
    Jul 8, 2019 at 21:55
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Oh, the humanity.

A great invention, disposable soluble wipes, for smooth wiping after big business were all the rage over the planet. It was theorised that they would decay when exposed to uv radiation, so nobody paid much heed to their effects, and everybody started using them instead of the traditional toilet paper.

Sewage treatment plants didn't heed the microparticles of the wet wipes because it would be too costly and it got dumped into the natural water ways along with the rest of the treated sewage water.

Over years, all these micro particles had gathered in the great plastic garbage patch and some chemical reaction occurred with the plastic and the massive amount of micro particles creating a form of sponge that incorporated water in the supporting structure as well as trapping it in the micro chambers, that would reinforce and build further upon itself as more plastic and wet wipes were used. The chemical make up of the sponge would split the water into hydrogen and oxygen very efficiently when hit with sunlight, causing the hydrogen to escape earth's atmosphere over the years.

People noticed the sea levels receding, but the cause was hard to determine because the ocean is like really really big, and nobody saw the big sponge because it was hidden by the big garbage patch people tried to hard to ignore.

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    $\begingroup$ A catalyst capable of efficiently splitting water with sunlight would be the holy grail of many researchers, and the scales involved are probably too big for it to work out - but it absolutely deserves a +1 for how well it works as dark humour! $\endgroup$
    – Eth
    Jul 8, 2019 at 17:17
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    $\begingroup$ Back of the envelope calculation: Turning enough water into it's gaseous components to lower the ocean by 1,000 meters would double the Oxygen in the atmosphere. $\endgroup$
    – Muuski
    Jul 8, 2019 at 18:18
  • $\begingroup$ @Muuski exiting times with all the forest fires! $\endgroup$
    – Tschallacka
    Jul 8, 2019 at 20:40
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Maybe the formation of a subterranean ocean. An Earthquake broke some geologic formation and the water of the ocean flowed inside. The kind of ocean scientists presume existed on Mars. This way you have a fast decrease of the sea level with unprecedented consequences.

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    $\begingroup$ This would however mean that there would need to be a kilometre thick cavern below all of Earth's surface, on average. There's no material that could form such a massive space. The only real option would either be some sort of porous rock that isn't already water saturated (but requiring even more thickness of the corresponding rock, which is essentially impsosible) or some sort of chemical change (e.g. water forming new minerals as it flows underground; which is also tricky, since those minerals are already well hydrated). $\endgroup$
    – Luaan
    Jul 8, 2019 at 16:07
  • $\begingroup$ if you filled every cave, lake, and aquifer on earth it would not drop the sea by even a fraction of the desired amount. $\endgroup$
    – John
    Nov 17 at 17:44
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Hydrate cycle

The water could be bound chemically.

Now that's a bit difficult, since water is pretty much the lowest-energy configuration that Hydrogen and Oxygen can have.
It can go between the atoms of some crystals though. So if you have a massive layer of dehydrated rock somewhere, that gets uncovered en masse, then it could start soaking up lots of water.

It's not a very realistic scenario:

  • To bind 1 m³ of water, you need at least the same volume of rock, and possibly more. https://en.wikipedia.org/wiki/Water_of_crystallization has a nice list of chemical compounds; you'd have to look up the densities of the hydrate and the anhydrate, get the weight relations by adding up atomic weights, and with that you could calculation the volume expansion.
  • The anhydrite would have to change from "disconnected from water" to "connected to water" so that it would start soaking up water. The faster this process is, the faster the sea level will drop.

The best I can think of is some tectonic cycle: * The crust is composed of a hydrate that forms a supercontinent. * The hydrate gets pulled under and heated, driving the water out of the crystal, raising the sea levels considerably. * After a while, the anhydrate resurfaces and starts soaking up water. Now what I don't know to make sure that the hydrate/anhydrate gets pulled under at the same time. Density differences could help make it clump better, and maybe help form a supercontinent, but I do not understand tectonics well enough to make even a hypothesis.

Anyway, this would be on geological scales. I am not sure whether that's any better for your plotline than a planet drying up through solar wind.

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Rather more pulp than many of the other suggestions and I did wonder whether it's a useful suggestion given your examples, but have you considered the case of someone stealing it?

Of course, to make that big a difference the operation would need to be truly massive. Maybe they have drilled a hole through the bottom of the ocean into a huge cave network which is now slowly flooding, or perhaps they're shipping it into space via some sort of space elevator?

The advantage with this answer is that the timeline is however fast or slow you want it, limited by the technology or means of the person or peoples relocating the water. Presumably they also have some use for it, which might be relevant...

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  • $\begingroup$ A sufficiently advanced civilization could use all that water to power their massive fusion engines as they ship Earth to a different star system. Regardless of the method, though, I fear liberating such massive amounts of water over human timescales would fry the planet anyway (compare with massive asteroid impacts that didn't even make a dent in the oceans, yet deposited enough energy in the system to cause mass extinctions). $\endgroup$
    – Luaan
    Jul 8, 2019 at 16:11
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    $\begingroup$ @Luaan Definitely pulpy, but a sufficiently advanced sufficiently advanced civilisation may have children who want to prank the local mudball while being careful not to fry it, the way other children hose an anthill with water just to see what happens... $\endgroup$
    – Eth
    Jul 8, 2019 at 17:19
  • $\begingroup$ This is one of the motivations for the alien invasion in V (from the early 1980s). $\endgroup$
    – Eric Towers
    Jul 8, 2019 at 19:28
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I love all the answers especially the wet wipes one, but I can't find one that covers water actually escaping from the planet other than by the death of the core. So what factors might cause it to be lost into space from a living planet? An overheating atmosphere? Warm air rises, and with more evaporation there would be more steam at higher levels in the atmosphere. What could push it out into space? Electricity. I would imagine that Sprites (powerful upward thrusts of lightning) would have the effect of pushing atmosphere, mostly hydrogen, into space. This is a likely cause of planets losing their atmosphere, and with it, their water. The time scale with severe sudden warming could be as little as a hundred years?

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    $\begingroup$ no one gets notifications about new answers (unless they favourtie the question) so I have added this answer as an edit to your post. Worth noting for future reference :) $\endgroup$
    – Gamora
    Jul 8, 2019 at 13:56
  • $\begingroup$ A moon swinging by way to close, creating clouds that are lost to the moon? $\endgroup$
    – Pica
    Nov 24 at 13:47
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Catastrophic cosmic radiation event

A super nova or a gamma ray burst can strip the atmosphere from a planet, pretty quickly.

You'd need to have a composition in the planet that has very little hydrogen, so the atmosphere that forms afterwards does not form too much water.

This answer might be useful for a plotline where people visit a planet and find that the shoreline was some 1000 meters higher than today.

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I have a crazy thought which may open up a giant can of worms for you, but here it goes: There is a giant continent on your world which sits mostly below sea level, but which has high ground along the perimeter which keeps the sea from flooding in. Perhaps some of the land on this continent sits very far below sea level, while other parts are at or near sea level. If an earthquake, asteroid strike, or natural erosion were to suddenly breach the perimeter of this continent, the sea would come flooding in, widening the breach with its flow until equilibrium is reached with the sea around it. Not only could you have your global average sea level drop dramatically as a result of this (reshaping every coast line in the world), you would now have your largest continent suddenly turned into a massive atoll with the potential for scattered islands throughout.

Massive numbers of people would be displaced or killed as entire countries are potentially buried under this new sea. Cities which were once landlocked hilltop fortresses would suddenly become island nations. Port cities would find themselves landlocked and miles away from the nearest water.

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  • $\begingroup$ There's a few big problems with the idea - how it forms without already being full of water (a newly formed mountain range could lock the water in the continent, but how would it dry up?), rain (where does rainwater go, if you're below sea level?) and erosion (exactly how long does this multi-kilometre tall "wall" around the continent survive against the rain and ocean?). It would work great as a more local story, though - especially in the inverted scenario (water flooding out of a previously locked-in reservoir). $\endgroup$
    – Luaan
    Jul 8, 2019 at 16:19
  • $\begingroup$ I suppose there are lots of ways parts of this continent could form, and if they all happened at once you would end up with a continent which has an average elevation below sea level. For example, if the continent sits on it's own tectonic plate it may have a massive mountain range around the outside as a result of other tectonic plates crushing against it. If this happened while this plate was on the inland portion of a pangea-like supercontinent you could wind up isolating dry land this way. $\endgroup$
    – Brendon Dugan
    Jul 8, 2019 at 16:51
  • $\begingroup$ there are quite a few places in the world that are dry despite being below sea level. The area around the dead sea is one of the more famous ones. It is 430m below sea level. (The dead sea itself is water, but the area around it is dry and still below sea level.) $\endgroup$
    – craq
    Jul 8, 2019 at 21:18
  • $\begingroup$ look up the zanclean flood, when the Mediterranean sea flooded. still only dropped sea level a few meters. $\endgroup$
    – John
    Nov 17 at 18:58
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The problem with water, is that you pretty much have to destroy it if you don't want it raining, seeping or channelling back into the oceans.

So let's do that.

First, an assumption: that humans have perfected the nuclear fusion reactor - not just for fusing hydrogen, but for fusing any element. We can fuse all of the elements up to iron and still release energy. So our approach has the handy side-effect of releasing a lot of energy, which we are going to need.

The first step is to gather up water and split it into hydrogen and oxygen, using plain old electrolysis. We're then going to burn the oxygen in a fusion reactor. Not an easy task, given the extraordinarily high temperatures and densities that are required to achieve this in stars - we're talking much, much higher than the kind of fusion reactors that we already struggle to build today. But hey, according to our assumption we've managed to solve that engineering problem.

We could perhaps let the hydrogen go and hope it escapes the atmosphere, or we could burn that too. Since 88.8% of the mass of water is the oxygen, we'll assume that if we can solve the oxygen disposal problem, we can solve the hydrogen disposal problem too.

OK, so each O+O fusion reaction releases 9.593 MeV of energy, and gives us a silicon atom. Cool! We can use that silicon atom to bind some more of the oxygen into quartz. This provides a shelf-stable product that can be used to build more land, or just pile it up into a new mountain somewhere out of the way.

How much oxygen do we need to burn to achieve our goal? Let's assume we only need to burn 50% of the oceans to achieve the desired sea level drop. That's about half a quintillion metric tonnes of oxygen - or about 10^45 atoms worth, according to some rough assumptions about the ocean's mass that I found. But I only spent about 20 seconds googling, so double check this before proceeding.

Did I mention that not only will we need an advanced oxygen-burning fusion reactor, but it will need to be a really, really big one? Or perhaps a number of smaller ones. But at any rate, we've got a lot of fusing to do. The great thing about fusion is that you get energy out of it, so you don't have to worry about how you're going to power all of the operational concerns like gathering up the water, moving it around, splitting it, and so forth. You can definitely assume we'll have enough energy.

But might we have too much?

If we burn all this oxygen, we'll release about 10^34 joules. That's approximately equal to the total energy output of the sun each year. This would definitely set the entire planet on fire, which probably meets the goal of creating "newly exposed lands", though the "new political entities" would have to be of a hardy variety.

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A nuclear winter would drop the global average temperature.

You know how the sea levels are going up because global warming is causing polar ice to melt? Global cooling would have the opposite effect, causing more sea water to become polar ice.

In fact, that happened during each glaciation the Earth has gone through.

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  • $\begingroup$ Warth? Earth? Just checking... $\endgroup$
    – Cyn
    Jul 7, 2019 at 16:47
  • $\begingroup$ @Cyn I don't use spellcheck :/ $\endgroup$
    – The Square-Cube Law
    Jul 7, 2019 at 16:53
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    $\begingroup$ Without spellcheck, you'd all think I was completely illiterate. $\endgroup$
    – Cyn
    Jul 7, 2019 at 16:54
  • $\begingroup$ The last ice age dropped the sea level by 125 meters. To get a thousand meters, you're looking at Snowball Earth levels of ice age. $\endgroup$
    – Mark
    Jul 8, 2019 at 20:50
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If it's acceptable to raise the ground level rather than lowering the sea level, you could consider an earthquake or a series of earthquakes. This is the scientifically accepted explanation for marine fossils found on top of mountains. In the more spectacular earthquakes, the ground level can shift very quickly by a few hundred metres.

Of course, this would be a local phenomenon, not global.

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Volcanoes on the ocean floor or subduction during motion of plate tectonics. The ocean water goes through phases: evaporation, condensation & preciptation. This would cause the phenomena of unusual lower tides or sea levels. Also they call it Snowball Earth when the ocean water becomes ice which would freeze the ocean making the tides & sea levels appear lower.

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  • $\begingroup$ You're not going to get a thousand meters of sea level decline this way. $\endgroup$
    – Mark
    Jul 8, 2019 at 20:49

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