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I mean, could enough of the Earth be flooded enough that most of the surface would be uninhabitable?

Would there be enough water in the glaciers/ice caps etc for this to happen? Could any sources from outside the Earth contribute to this?

I'm looking more for 'realistic' scenarios than, say, aliens or magic...

Edit: as in the parts of the Earth that are currently not already under water...

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    $\begingroup$ Most of Earth's surface is already relatively uninhabitable due to being covered by water. How much more water do you want? $\endgroup$ – ckersch Jun 4 '15 at 20:07
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    $\begingroup$ If you could get some massive amounts of erosion, you could probably just distribute the land evenly enough to submerge it. Not sure how that could be managed, though. $\endgroup$ – DaaaahWhoosh Jun 4 '15 at 20:28
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    $\begingroup$ related: earthscience.stackexchange.com/questions/4720/… $\endgroup$ – plannapus Jun 5 '15 at 7:49
  • $\begingroup$ It could become inhabitable, due to the rise in global temperatures and increase in dramatic weather, but there would still be land. And I'm sure we'd be able to mitigate against the effects. $\endgroup$ – Mikey Jun 5 '15 at 19:35
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A controversial theory suggests that there is a vast quantity of water trapped in the Earth's mantle. If just 1% of the mass of the transition zone of the mantle was composed of water, that would be about 3 times the amount of water in the oceans today. Most was trapped there during the formation of the Earth, while some is pulled down through the subduction zones of the Earth's crust

The hydrological cycle would have to be interrupted somehow to allow that water to percolate out of the mantle. Given the massive pressures and temperatures found at that depth, the water would probably return to the surface in the form of high pressure steam blasted out through volcanos and undersea vents. This injection of high energy steam into the hydrosphere and atmosphere would create rather alarming changes to the global environment, and a lot of water from the oceans would be evaporated along with the new water being introduced as steam, filling the atmosphere with a hot mist which will eventually condense into a massive amount of rain.

One other possible effect of the water being extracted from the mantle is that the natural lubrication effect of water trapped in these rocks will cease, making plate tectonics much more difficult. Plates locked together will have less ability to move or deform to release stress, causing much more massive earthquakes and huge tsunamis in the now global ocean. (Tsunamis breaking over the submerged Rockies or Himalayas will be spectacular).

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    $\begingroup$ Seems worth noting that this is not liquid water, but water molecules contained in mineral crystals. Mostly ringwoodite. Such water is called crystallization water. But, this is an excellent point and provides a feasible source of the additional water required to flood the planet. $\endgroup$ – Samuel Jun 4 '15 at 21:16
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Not easily.

All the ice melting is not enough to flood all the dry land on Earth. National Geographic has some nice maps predicting how that would look.

enter image description here

Magic would be required to flood the entire surface of the Earth. Similar to what happens in the mythological story of Noah, recently a blockbuster movie.

For a science explanation, perhaps the solar system passes through a dense nebula of water ice. It would take hundreds of years for enough water to accumulate, and there would be other more severe problems, so it's not a very dramatic possibility. Thucydides provides an excellent alternative for the source the water, the water of crystallization trapped in some very abundant minerals in the Earth's mantle. If enough of the ringwoodite went through dehydration melting, its water would be released, possibly making its way to the surface.

Otherwise you'll need to level out all the land. If Earth were a smooth sphere, the water would be about a mile deep all around. This is less feasible than aliens hitting us with a planetary super-soaker though.

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    $\begingroup$ Important correction: The story of Noah is not "mythological", and it has nothing to do with the recent movie. The historical account of Noah in Exodus is one of the oldest and best-preserved in all of human history. Studies have shown several well-reasoned theories to explain the way in which the world was flooded. $\endgroup$ – woz Jun 5 '15 at 12:36
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    $\begingroup$ @Woz The story of Noah by its definition is mythological in that there is no documented proof of it happening. Myth: noun A traditional or legendary story, usually concerning some being or hero or event, with or without a determinable basis of fact or a natural explanation, especially one that is concerned with deities or demigods and explains some practice, rite, or phenomenon of nature. There is also nothing in the archaeological record to indicate that a great flood wiped everything out. $\endgroup$ – James Jun 5 '15 at 13:59
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    $\begingroup$ @woz I didn't actually see the film. However, I'm sure it's exactly as historical as the biblical version. I'll bet they even did a bunch of studies to get all the science right. Please don't edit my answer to include your beliefs, they're not facts. ArtOfCode & bowlturner, please don't approve such edits. $\endgroup$ – Samuel Jun 5 '15 at 14:39
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    $\begingroup$ always hated Florida anyway. $\endgroup$ – Serban Tanasa Jun 5 '15 at 14:58
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    $\begingroup$ @James: Not only is there no documented proof that it did happen, there are literally mountains of evidence that it could not possibly have happened as described. So yes, the Biblical account is exactly as mythical as the Norse Ragnarok, the Navajo stories of the First World, the Hindu Kurukshetra War... $\endgroup$ – jamesqf Jun 5 '15 at 19:18
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If all of the ice on Earth melted, the sea level would rise by a bit over 200 feet. While most of the current landmass of Earth is above this level, significant portions of the Earth's population live in coastal areas that are below it. Around a billion people would be displaced if all of the ice melted. If you're curious which parts of the Earth would be most affected, National Geographic created this nifty series of maps showing the new coastlines.

You could bring the new water in from space, of course. In order to cover up more than half of the land area in the US, you'll need to cover the Earth with another 3km of water, roughly. This corresponds to around 1.53 billion cubic kilometers of water. A sphere of this much water would be a bit over 700km in radius, which is bigger than any asteroid in the solar system, but smaller than Pluto. The danger in getting water this way is that anything coming from space will come at a high velocity. Aside from the global flooding, a series of massive tidal waves will probably scour all of the surfaces of the Earth, and debris and steam will block out the sun, probably for several years. Almost all life on Earth will perish. The kinetic energy released will turn to heat, which will melt the ice caps and raise the sea level by an additional 200 feet.

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  • $\begingroup$ The high speed arrival is easily fixed. Launch your water in small chunks (say as ice from Europa, using a linear accelerator). It then aerobrakes in the atmosphere, just as meteors do today. $\endgroup$ – jamesqf Jun 5 '15 at 5:17
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    $\begingroup$ aerobraking causes heating. 1.53 billion GT of water, at 100 km/h has kinetic energy of 276 times the energy of all nuclear weapons ever detonated. Please don't do this. (assuming my friday afternoon maths is correct). It would also transfer momentum. A lot of it. Maybe fire from multiple positions around the solar system? $\endgroup$ – Scott Jun 5 '15 at 6:55
  • $\begingroup$ @Scott: Well, you don't dump it in all at once, but over many years, so the heat has time to dissipate. $\endgroup$ – jamesqf Jun 5 '15 at 19:20
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If you want to avoid magic, maybe try comets. Some undiscovered comets could skim past Earth in a (very) precise manner, losing water. Comets are icy, and if they passed just close enough to aerobrake and continue past Earth, they would lose a lot of water into our atmosphere.

That would require a lot of work and precision, and a lot of small comets, though, so maybe you will need the aliens afterall.

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    $\begingroup$ The ocean masses about 1,400,000,000,000,000,000 metric tons. The largest comet I'm aware of - Halley's - masses about 300,000,000,000 tons. In order to increase the ocean's mass by 1%, you'd have to hit the earth with approximately 50,000 Halley's comets, and that would probably be insufficient to cover all land. It also assumes 100% of the comet's mass transfers to Earth. $\endgroup$ – Dan Smolinske Jun 4 '15 at 20:43
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    $\begingroup$ Yup. It's gonna take aliens to do that. $\endgroup$ – Mikey Jun 4 '15 at 20:49
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    $\begingroup$ @DanSmolinske Scientific notation is your friend. $\endgroup$ – Samuel Jun 4 '15 at 20:55
  • $\begingroup$ @Samuel: I thought about it but wikipedia listed it out that way and I was feeling too lazy to count zeros. $\endgroup$ – Dan Smolinske Jun 4 '15 at 21:01
  • $\begingroup$ @DanSmolinske I don't blame you, it's the same reason I commented; too lazy to count them. Of course, if you read to the end of that sentence you'd see it in scientific notation. $\endgroup$ – Samuel Jun 4 '15 at 21:02
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I assume you mean, "using only water presently on the Earth". If we can import unlimited amounts of water, then of course the answer is "yes".

No one knows exactly how much water is underground, but estimates are that it's a very small amount compared to the amount in the oceans. I came across some numbers years ago, (quoting from my own book here, is that cheating?) 97% of the world's water is in the oceans, 2% in ice caps and glaciers, 1% underground, 0.02% in fresh water rivers and lakes.

So as others have said, melting the ice gaps and bringing up all the underground water wouldn't increase the level of the oceans very much, not enough to cover all the land.

On the other hand, I think the common theory among creationists today is that before Noah's Flood the Earth was more nearly spherical, i.e. the mountains weren't as high nor the ocean trenches as deep. If you think about it for a moment, if the Earth was a perfect sphere, it wouldn't take very much water at all to cover it (relatively, any way). Do the arithmetic and if the Earth was a perfect sphere, the amount of water in the oceans today would cover it to a depth of 1.7 miles.

So the (logically) easiest scenario to flood the Earth is: level the mountains and fill in the ocean trenches. If the highest mountains and the deepest trenches were about 1/4 what they are today, the entire world would be under water.

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Based on some rough calculations, if we wanted to cover all the land on the earth with water up and above the peak of Mt. Everest, we would require at most 200,000 cubic miles of water. Based on Galileo Probe data, Jupiter's moon Europa has about 933,000 cubic miles of water on/beneath its surface. (For reference, a cubic mile of water is about how much flows over Niagara Falls in a month.)

Is it feasible to remove this water from Europa and place it on Earth? Randall Munroe answered a similar question here -> http://what-if.xkcd.com/143/ Using some rough potential energy calculations, we can find that the energy required to lift 200,000 cubic miles of water out of Europa/Jupiter's gravity well would be on the order of 10^20 Joules, or about 10,000 Terrawatt-Hours. For comparison, the entire planetary electric generation of the Earth in 2012 was about 22,000 Terrawatt-Hours. Obviously this is a wildly optimistic back-of-the-envelope calculation, but depending on how much you're willing to suspend disbelief, it could be done by a sufficiently advanced civilization with access to enough power.

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