14

I often hear people claim "global warming will cause the ice-caps to melt, and that will cause the water levels in the ocean to rise, and that will cause major world-wide flooding."

Now, ignoring the question "are ice cap melting", I am still wondering how it is possible for melting ice-caps to cause water levels to rise.

Here is why I am skeptical:
    According to the law of displacement, the volume of an immersed object will be exactly equal to the volume of the displaced fluid. Therefor, if an icecap is floating in water, the displacement of the water would be based on the volume of the icecap, not it's shape, and the levels should be the same regardless of whether or not it is melted.

In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced (the volume of the immersed object will be exactly equal to the volume of the displaced fluid).
[....]
In the case of an object that floats, the amount of fluid displaced will be equal in weight to the displacing object.

The icecap will have the same weight, regardless of whether it is in ice form, or liquid form.

So how is it that people claim that melting ice caps can cause floods? Shouldn't water level stay exactly the same?

Icecaps includes polar ice, and smaller icecaps.

Community
  • 1
Ephraim
  • 1,828
  • 4
  • 19
  • 32
  • @kotekzot `In the case of an object that floats, the amount of fluid displaced will be equal in weight to the displacing object.` - The icecap will have the same weight, regardless of whether it is in ice form, or liquid form. – Ephraim May 03 '12 at 20:41
  • 12
    There's land at the South Pole, so there's a lot of ice that isn't floating on water - http://en.wikipedia.org/wiki/Antarctica – Tom77 May 03 '12 at 20:56
  • 2
    According to Wikipedia (http://en.wikipedia.org/wiki/Current_sea_level_rise) it seems one is worried about land-based ice (Greenland and Antarctica). – UncleBens May 03 '12 at 21:02
  • 3
    In fact, most of the ice is not floating, but land-bound. Melt it, and it now quickly becomes seawater. –  May 03 '12 at 21:45
  • 8
    Ice caps, by [definition](http://en.wikipedia.org/wiki/Ice_cap) are not floating. – Flimzy May 04 '12 at 03:23
  • @Ephraim -- I rolled back the edit to the title because it substantially changes the question which already had a great answer by Rory. – Russell Steen May 04 '12 at 23:44
  • Ice is less denser than water, so when ice melts the volume of water produced would be greater than the volume of ice. Furthermore, some portion of ice floats outside the water. Thus polar ice melting should lead to increase in sea water level. – apoorv020 May 07 '12 at 09:40
  • Equality in weight and equality in volume are two different things. – Christian May 07 '12 at 10:34
  • @apoorv your comment makes many false assertions. volume of water would be less but mass would be the same; floating ice that melts will have negligible (basically 0) effect on sea level. You can easily test this with a glass of ice water – David LeBauer May 08 '12 at 06:59
  • @Ephraim Read http://en.wikipedia.org/wiki/Archimedes%27_principle. For a floating object the water displaced is based on the weight of the object not the volume (otherwise ships would sink ...). It still works out tho because 1kg ice displaces 1kg water which is the same volume as the 1kg water the ice melts down to (ignoring volume changes based on temperature changes). This is only the case if all the Ice is floating which it isnt see answers. – Stefan Oct 08 '12 at 18:20

3 Answers3

29

Antarctica contains 70% of the world's ice and the vast majority of this is on land, so there is no displacement confusion - if it melts it will pour off the land into the sea, thus raising the sea level.

The thickness of ice on the Antarctic continent (from this paper at RD Springer) averaged 2.2km in the East Antarctic Ice Sheet!

From The Physics Factbook (which is based on USGS ice caps area and thickness data) the maximum sea level rise potential globally is 73.44 m:

Geographic region: Antarctica

  • Volume: 30,109,800 km3
  • Maximum sea level rise potential: 73.44 m
  • Area: 13,586,400 km2"
Rory Alsop
  • 6,237
  • 3
  • 35
  • 49
  • 2
    Aren't all the continents floating on liquid rock? Are fluid dynamics of lava different than water? – user1873 May 04 '12 at 01:44
  • 4
    @user1873 Yes but (1) The density of the liquid rock is much high than that of water so you get less motion (2) to first order it would only be Antartica and Greenland that rose on that account and (3) the mantle is visco-elastic rather than properly fluid and the relaxation times can be pretty long on the human scale. – dmckee --- ex-moderator kitten May 04 '12 at 02:00
  • 4
    In addition to Rory Alsop's excellent point about Antarctica, water expands as it is heated. Even if the ice was all floating, global warming would still cause the sea level to rise. – DJClayworth May 04 '12 at 03:30
  • 1
    Hmmm... @DJClayworth Pure water actually contacts a little between 0 and 4 degrees C. Don't know if sea water has a similar region. – dmckee --- ex-moderator kitten May 04 '12 at 03:31
  • It would be much better to find a paper that actually states this fact, instead of inferring the answer and then having discussions on "what-ifs" in the comments. It shouldn't be too hard! :-) – Sklivvz May 04 '12 at 08:29
  • The physics factbook does have this data in it, along with the volume of water - I just read the wrong row initially :-) – Rory Alsop May 04 '12 at 08:40
  • 7
    73m did not match my intuition at all. Surely there's not that much ice. So, I checked the reference. The ice caps are up to 5km thick? *FIVE KILOMETRES*. Staggering! – Oddthinking May 04 '12 at 08:50
  • 3
    @Oddthinking: So much for intuition `:-)` – Hendrik Vogt May 04 '12 at 09:48
  • @user1873 - When you get to really high pressure the rules change. – Chad May 04 '12 at 19:30
  • @Oddthinking: *"FIVE KILOMETRES."* Yep. The Ice Cube neutrino detector is a cubic kilometer of instrumented ice 2.5 km under the surface right next to South Pole Station. – dmckee --- ex-moderator kitten May 04 '12 at 22:06
  • @HendrikVogt: I wanted to post a comment defending the use of intuition in the way I did: to guide the fact-checking to the most promising areas first. But then I asked myself did I have proof of this? No, only my intuition. Dammit! – Oddthinking May 06 '12 at 01:12
  • @Oddthinking: I think the point with intuition is that you have to know yourself well enough to be able to tell when you can (somewhat) rely on it. And then you have to be incredibly honest with yourself. Not always easy! – Hendrik Vogt May 06 '12 at 06:45
  • How much ice is supported by Greenland? I bet that has a pretty large volume too. – JasonR May 11 '12 at 16:56
  • @Brightblades - from that same physics factbook link: Greenland has 2,600,000 km3 – Rory Alsop May 12 '12 at 14:27
  • So another 9M potential there (roughly)? So rounding off, 80M increase (over 260 feet). Yeah, future generations will have a heck of a time with that. – JasonR May 14 '12 at 11:54
  • 1
    Warming only causes melting if water is just below 0°C. If the water is already liquid, warming causes no additional melting. If the ice is far below 0°C, warming causes no additional melting. At the South Pole, where temperatures peak at about -30°C, and sometimes drop below the freezing point of CO2, the ice will, practically speaking, never melt. – Dave Burton Jun 05 '13 at 20:22
  • @DaveBurton Right, but most of Antarctica is not anywhere near that cold. – Lennart Regebro Sep 12 '13 at 14:08
  • Northern Europe is still rebounding, presumably from the weight of the ice during the last glacial maximum 20K years ago. Currently land is rising at a rate of 1 cm per year or less. Sea level 20K years ago was 120 meters below the current sea level. Some say that current glacial melting is just a continuation of the glacial retreat starting 20K years ago. – GEdgar Nov 03 '15 at 16:30
  • @RoryAlsop The 73.44m in this answer is wrong. Should be 58.3m http://www.antarcticglaciers.org/glaciers-and-climate/estimating-glacier-contribution-to-sea-level-rise/ Also, much of the ice in Antarctica is below sea level. Melting of ice that is below sea level makes a negative contribution to sea level. – DavePhD Jan 29 '19 at 19:48
  • Dave - the source used focuses on the land based ice, as the vast majority of ice in the Antarctic is on land, as opposed to the Arctic. – Rory Alsop Jan 30 '19 at 05:11
11

Yes, they are already doing so

Here is what the IPCC has to say about this:

During recent years (1993–2003), for which the observing system is much better, thermal expansion and melting of land ice each account for about half of the observed sea level rise, although there is some uncertainty in the estimates.

— FAQ 5.1 Is Sea Level Rising?

In other words, as the ocean warms up it expands and as the land ice melts, it pours into the ocean. At the moment the contributions are 50% each.

The following graph shows the past/present/future of sea levels according to the IPCC:

sea rise

—Time series of global mean sea level (deviation from the 1980-1999 mean) in the past and as projected for the future.

Sklivvz
  • 78,578
  • 29
  • 321
  • 428
  • 3
    In other words, according to IPCC melting ice caps will be responsible for about **4 to 8 inches** of sea level rise by 2100? – vartec May 04 '12 at 09:16
  • 1
    @vartec Did you do the conversion wrong? [200 mm in in](http://www.google.com/search?q=200+mm+in+in) ~= 8 in, [500 mm in in](http://www.google.com/search?q=500+mm+in+in) ~= 20 in. That's about a foot in 2100. – Tacroy May 04 '12 at 16:42
  • @Tacroy but only 50% would be due to the melting ice caps. – Sklivvz May 04 '12 at 19:00
  • 1
    @Sklivvz Ohhh right, I missed the due to "melting ice caps" bit. You can't really extend that out, though - the percentage contribution of expanding water vs melting icecaps will change over time. – Tacroy May 04 '12 at 20:34
  • Only about 50% of sea level rise in the open ocean is due to melting ice, the rest is from thermal expansion. But that's untrue at the coasts. At the coasts, thermal expansion does not affect sea level significantly, because when water in the upper layer of the ocean warms and expands it rises up IN PLACE, like a very stubby iceberg. Its displacement is measured in units of mass, and doesn't change. It gets deeper where it warms, by a (small) percentage of its previous depth. A percentage of zero is zero, so at the coasts, where the depth is near zero, the rise is negligible. – Dave Burton Jun 05 '13 at 20:32
  • 1
    @DaveBurton: Are you sure about that? Consider a simplified situation: a flat wide vessel containing some liquid. If the liquid in the center is less dense, will it bulge above the average surface level, or will it flow until the surface is flat again? Icebergs bulge above the surface because they're rigid; liquid water, of whatever density, is not. – Keith Thompson Sep 12 '13 at 22:19
  • FYI: your answer seems to be contradicted by the latest research as far as Antarctica's contribution: http://www.ingentaconnect.com/content/igsoc/jog/pre-prints/content-ings_jog_15j071 . I'm not certain what publishing stage this is in at the moment, that link was a pre-print. – user5341 Nov 02 '15 at 19:09
  • Also, less related, there's this post, but it merely mentions research without indicating what the publication being mentioned is: http://wattsupwiththat.com/2015/10/30/greenland-ice-melt-due-to-global-warming-found-not-so-bad-after-all/ . I'm unsure if there's a separate S.SE question related to greenland – user5341 Nov 02 '15 at 19:11
3

While the melting of Arctic sea ice may have little direct effect on sea level rise, it will substantially reduce the albedo of the Arctic ocean as sunlight will be absorbed by the dark ocean water, rather than being reflected back into space from the bright ice. This is likely to result in a general warming of the Arctic region, which is likely in turn to result in melting of the Greenland ice sheet, which would result in a very substantial rise in global sea levels over the course of a thousand years or so. This means there is a good reason to be concerned about the loss of summer (when albedo really matters) Arctic sea ice from a sea level rise perspective. This is known as ice-albedo feedback, there is a basic discussion on Wikipedia, a slightly more detailed explanation is described here by the NSIDC, a relevant section of the IPCC WG1 report is here and here.