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Apologies for asking a silly question like this. But I want to offer some points which I could not counter, as follows:

  1. Ice is less dense than water, that is why it floats. For the same unit mass, if density is lowered, the volume increases, thus ice ends up having more volume/consuming more space.
  2. AFAIK, the polar ice on Earth does not originate from other planets.
  3. Above points conclude that, if polar glaciers continue to melt (very unfortunate) the resulting water is denser than the ice so it would occupy less volume per unit mass and as the polar glaciers were originally from Earth, there is no new volume addition which can explain the rising sea levels.

So how does glacial melting cause global water levels to rise? Are there other factors which amplify this, like maybe global polymer waste dumping into the oceans?

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    $\begingroup$ This wasn't mentioned in your question but it's worth noting that aside from glacier melt another significant cause of rising sea levels is the thermal expansion of water. Water takes up more volume as it warms past its freezing point. $\endgroup$ Aug 8 at 8:56
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    $\begingroup$ you might want to read this research.csiro.au/slrwavescoast/sea-level/sea-level-change/… to get a better understanding about this topic. $\endgroup$ Aug 8 at 10:45
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Arctic ice, around the north pole floats on top of water. When it melts it does not add to sea level rises and likewise for other ice on water, as illustrated in this video and this video.

Ice on land is a different matter. Ice on Greenland, Antarctic land & glaciers around the world will add to sea level rise because any melt water will eventually end up in the oceans. It's like adding water to a partially filled glass of water.

The thickness of the Greenland Ice Sheet is more than 2 km and it is of the order of 2900 km long and 1000 km wide. The amount of ice in Greenland is 2,850,000 cubic kilometres. If this alone were to melt it would increase sea levels by 7 m. The island of Greenland cannot contain all the melt water and it will end up in the sea.

The Antarctic ice sheet contains 26.5 million cubic kilometres.

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    $\begingroup$ @lousycoder that question sort of breaks down because the continents have moved a lot since then so sea level is not completely comparable. for instance most hf the middle part of the US was under a sea which went away as the land was uplifted when north America ran over the east pacific plate. $\endgroup$
    – John
    Aug 7 at 20:49
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    $\begingroup$ @lousycoder: As John has mentioned in his comment, the continents have always been in a constant state of movement as explained by Continental Drift. This video is good illustration of what is thought to have happened. Because of continental drift & the appearance of disappearance of some land masses it is difficult to determine what sea levels were millions of years ago. $\endgroup$
    – Fred
    Aug 8 at 4:19
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    $\begingroup$ @lousycoder Another point worth thinking about: Let's say, for arguments sake, that we are just going back to sea levels that Earth had before (which is not true, because you can't compare sea levels when all the continents looked completely different back then). This doesn't mean it's not a problem, it still means that a huge amount of people live where there is going to be ocean and we built a lot of expensive stuff on land that is going to be underwater. $\endgroup$
    – Nobody
    Aug 8 at 11:09
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    $\begingroup$ The second sentence of your answer is not really true. Melting ice floating on salt water increases the water level! Ice contains almost no salt (~0.8% Arctic, 0.0% Antarctic). The resulting water body (molten ice and sea water) is slightly less salty (=less dense) but has (almost) the same mass. Result: larger volume > sea level rise. If all floating ice melts the sea level would rise by ~ 2 inches/5.5 cm! $\endgroup$
    – klanomath
    Aug 8 at 15:35
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    $\begingroup$ @Nobody: Going back to sea levels we had before? When's "before"? If we go back say 20K years (last Ice Age), the continents were pretty much where they are now, but sea level was quite a bit lower. There are largish areas of land that used to be inhabited by humans, but are now under water. Doggerland is perhaps the best known: en.wikipedia.org/wiki/Doggerland $\endgroup$
    – jamesqf
    Aug 8 at 16:48
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Because a lot of that ice is currently sitting on land. When it melts, it will go into the ocean. From the National Snow and Ice Data Center Page (NSIDC), Facts about glaciers:

Presently, 10 percent of land area on Earth is covered with glacial ice, including glaciers, ice caps, and the ice sheets of Greenland and Antarctica. Glacierized areas cover over 15 million square kilometers (5.8 million square miles).

Glaciers store about 69 percent of the world's fresh water.

If all land ice melted, sea level would rise approximately 70 meters (230 feet) worldwide.

Also, just because melting sea ice doesn't directly contribute to rising sea levels, doesn't mean it isn't still a factor. From another NSIDC article, What are the impacts of Arctic sea ice loss?:

Sea ice loss contributes to Arctic amplification

Light-colored surfaces, like sea ice, have high albedo, meaning they reflect most of the sunlight that reaches them. Dark surfaces, like the surrounding ocean water, have low albedo.

At the height of summer, when the Sun shines relentlessly on the Arctic Ocean, exposed ocean water absorbs nine times as much solar radiation, if not more, than sea ice does. Before ice can form again in the fall, the ocean must release the absorbed heat into the atmosphere. The loss of sea ice warms the Arctic, contributing to a phenomenon known as “Arctic amplification” where the Arctic warms at a faster rate than lower latitudes. [...]

Does sea ice loss raise sea levels?

Just as sea ice absorbs wave energy and reduces wave action along shorelines, sea ice reduces wave action on ice, namely ice shelves (thick slabs of ice attached to coastlines that float over the ocean surface) and water-terminating glaciers. Increased wave action caused by sea ice retreat can flex and bend these ice bodies, increasing the possibility of retreat. Glaciers that have lost their ice fronts tend to flow faster, and because this process introduces a body of ice into the ocean that was not there previously, it raises sea level.

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    $\begingroup$ The phenomenon of absorption of solar radiation by sea water was not in the sight, thanks for bringing that to the view. As the deeper levels of water don't receive much sunlight, isn't it possible that the top levels of water release the absorbed solar heat to the bottom layers, as there is plenty of space there, and water to water dissipation might be more uniform and easier than water to atmosphere dissipation. $\endgroup$
    – lousycoder
    Aug 7 at 18:54
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    $\begingroup$ @lousycoder I think the point is that the water at the surface has to cool back off to re-freeze -- where that heat goes (up or down) isn't much of an issue. And ocean heating itself causes sea level rise. $\endgroup$
    – LShaver
    Aug 7 at 19:58
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    $\begingroup$ @lousycoder I think maybe the source is not quite clear if you don't already know part of it: Normally, Arctic sea ice melts every summer and refreezes every winter. That's not a problem. The problem is that there is a vicious circle: If you thaw more ice than normal in summer, then in winter proportionally less ice will freeze again. But come next spring, there is less sea ice and this causes even more sea ice than normal to melt during the summer. So it's possibly an example of runaway change that, once started by external factors, can't be stopped by just removing those factors. $\endgroup$
    – Nobody
    Aug 8 at 11:18
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Loss of ice mass floating in oceans has negligible contribution to sea level rise. Loss of ice mass over land does contribute to sea level rise.

It is the net balance (the difference) between flow of ice and meltwater into the oceans and the amount of snowfall in the catchment that determines the overall change to a glacier's ice mass and that determines if or how much a glacier's ice contributes to sea level rise.

Most glaciers, ice caps and ice sheets in the world are currently losing ice mass.

Picture of glacial ice entering ocean - contributing to ocean volume at that point, to melt later as floating ice -

glacial ice entering ocean

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Your question mixes a few independent issues. Two of the statements are undisputed:

  • Yes, melt water has a smaller volume than the ice from which it comes. But obviously, that (smaller) volume of melt water still adds to the sea water and hence lets sea levels rise if the melted ice was on land before.

  • Melting ice that is swimming does indeed not contribute much to the sea level rise because its pressure displaces almost as much water as it will add to the sea when it is melted.

The conclusions you draw seem muddled, but here is a take on them:

  • The question whether the land ice comes from outer space is irrelevant with respect to future sea level rise: It would be the same, no matter where it comes from. The question is only interesting with respect to the past: If the current land ice is from Earth, then the sea level at times without glacial ice must have been correspondingly higher because the sea is the main depot of water on Earth. Indeed, the sea level has changed hundreds of meters, while glacial ice accumulated or melted.

  • For what it's worth, current research indicates that a substantial fraction of Earth's water (and hence of the glacial ice) may be from outer space1 but as I said, it would raise sea levels no more and no less than "domestic" ice.


1 Well, Earth itself is from outer space, so to speak, because it is accumulated interplanetary matter from the accretion disk of the Sun. What the theory says is that the original composition of Earth's material did not have very much water in it because it was too hot for water to condense; and that oxidation of hydrogen over geological time frames does not suffice to produce the amounts we see today. Since many asteroids contain significant amounts of water they are one of the suggested sources for the water on Earth. So the suggestion is that water was added later in Earth's geological history.

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Thermal expansion of the ocean is responsible for about half the sea level rise:

First, warmer water expands, and this "thermal expansion" of the ocean has contributed about half of the 2.8 inches (7 centimeters) of global mean sea level rise we've seen over the last 25 years, Nerem said. Second, melting land ice flows into the ocean, also increasing sea level across the globe.

See: https://climate.nasa.gov/news/2680/new-study-finds-sea-level-rise-accelerating/

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A few of us here have given eminently reasonable answers already, but I think I know where your confusion comes from. The argument "ice is less dense than water hence it must result in less volume if they melt in water" only works if the ice you are talking about is completely submerged in water. Recall the images of icebergs you've seen -- none is completely submerged in water.

Of course, the argument being flawed doesn't mean the conclusion cannot happen to be correct. So does the water level increase, decrease or remain the same, if a floating iceberg melts?

This question requires some physics of the buoyancy force. Since the buoyancy force provided by water to the iceberg is equal to the force of gravity on the displaced water, F_buoyant=ρVg, where V is the displaced volume of water, and also equal to the force of gravity on the iceberg since it's floating. This means, the mass of the displaced water when the iceberg is floating, should be exactly the same as the mass of the iceberg itself. It follows that if icebergs were the same density as the sea water they float on, then the water level would remain exactly the same. You can verify this by experimenting with a glass of water and floating ice cubes. But salty water is of higher density than fresh water, which is what icebergs become when they melt. Hence the water level would increase if icebergs melt. This can also be empirically verified.

This however is only one of the reasons that the water level rises. As many others have pointed out, much of the glacier is not floating on water -- they are on land. Their melting and pouring into the ocean would obviously increase the water level. Thermal expansion goes the same direction.

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  • $\begingroup$ Icebergs are coming from glaciers. Sea ice comes from freezing ocean water. Icebergs due to their origin clearly consist of fresh water, but for sea ice this is less trivial. This answer would be more complete if it pointed out the difference, and why sea ice is also essentially freshwater, despite forming due to ocean water freezing. $\endgroup$
    – gerrit
    Aug 10 at 13:41
  • $\begingroup$ If sea water is denser than ice, it means that it requires less volume/space, thus the water level rising due to the iceberg would be proportionally lesser (although not zero). $\endgroup$
    – lousycoder
    Aug 12 at 19:14

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