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Is an ocean all one level?

  1. May 11, 2009 #1
    My friend and I are arguing about sea levels in relation to global warming. I argue that if one island is experiencing an increase in ocean levels then the surrounding ones should likewise.

    Apart from tidal effects, atmospheric low/high pressures, contained inlets etc, do not all bodies of water reach the same level regardless of the size or shape of the vessel?
  2. jcsd
  3. May 11, 2009 #2


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    It sort of depends where you are measuring from!
    The Earth bulges out around the equator so sea level at the equator is 100km further from the centre than the arctic. You normally compare sea level to an average shape of the Earth (geoid).
    But the sea level does differ by upto a metre because of different density/temperature/salinity etc.

    It also varies from country-country depending on what historical level they measure from. Famously there was a freeway built from Austria/Switzerland that didn't line up because of this
  4. May 11, 2009 #3
    O.K. I see how it is relative from the point one is measuring from so lets use the island as a benchmark.
    To me an island is a giant dipstick giving you the oil level of the ocean at that location. If there are several other islands close by then surely a change in the oil level of one would be reflected in the others. Are there natural phenomena that can keep an ocean at various levels?
    Also I don't understand how differences in temperature/density/salinity can change the level of water in one localized area with respect to the surrounding area. example: if I heat one end of my swimming pool with my giant magma plasma gun I wouldn't see a bulge in the water the overall water depth would increase, would it not? Same with adding salt or dropping in a large object.
    Wow would it suck being the engineer on that bridge!
  5. May 12, 2009 #4

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    Not all islands are created equal. For example, except for Banaba, most of the Kiribati is at most 6 meters or so above sea level. The Carribean isles, even the small ones, will not suffer Kiribati's fate because they are mountainous (or at least hilly).
  6. May 12, 2009 #5


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    Yes. Currents, for example, can push large masses of water around to make a substantial difference in sea level on opposite sides of an ocean.

    When you are talking about the global warming effect, the changes in sea level are measured in millimeters per year. (About 3.2 mm/year over recent decades)

    In the short term, this change is overwelmed by regional differences. For example, the El Nino, La Nina cycle involves changes in mean sea level at the Pacific Island of Guam of several hundred millimeters, between a La Nina year and an El Nino year. There's a large shift in the mass of water between the East and West of the Pacific.

    Also, just to confound your measurements, individual islands can move up and down themselves. The surface of the Earth is not stable on these scales, and in some places the rate at which land itself is rising or falling is larger than the rate at which sea level is rising as the oceans heat up with global warming.

    Cheers -- sylas
  7. May 12, 2009 #6
    Thanks guys I think I've caught the flaw in my reasoning. I'm thinking in terms of a closed system. Thinking that the laws governing a small vessel should scale up equally but not taking into account all the other factors that come into play when the scale gets that large.
    Humble pie yummy, yummy!
    Thanks again and great site.
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