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Thread: Sea level Rise isn't distributed evenly

  1. #1 Sea level Rise isn't distributed evenly 
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    I found the study below really interesting because it indicated that gravitational effect of melting Antarctica ice is enough to effect the distribution of the melt water. Stong enough to actually lower sea level near Antarctica and result in 25% increase in the rise of seas of the Northern Hemisphere. I never would have guessed the effect could be this great.

    (This isn't meant to be a global warming thread).

    Here’s the story
    ---

    Geophysicists predict amplification of sea-level rise in North America following collapse of West Antarctic Ice Sheet
    Thurs. Feb. 5/09
    By Sean Bettam

    University of Toronto geophysicists have shown that should the West Antarctic Ice Sheet collapse and melt in a warming world – as many scientists are concerned it will – it is the coastlines of North America and of nations in the southern Indian Ocean that will face the greatest threats from rising sea levels.

    “There is widespread concern that the West Antarctic Ice Sheet may be prone to collapse, resulting in a rise in global sea levels,” says geophysicist Jerry X. Mitrovica, who, along with physics graduate student Natalya Gomez and Oregon State University geoscientist Peter Clark, are the authors of a new study to be published in the February 6 issue of Science magazine. “We’ve been able to calculate that not only will the rise in sea levels at most coastal sites be significantly higher than previously expected, but that the sea-level change will be highly variable around the globe,” adds Gomez.

    “Scientists are particularly worried about the ice sheet because it is largely marine-based, which means that the bedrock underneath most of the ice sits under sea level,” says Mitrovica, director of the Earth System Evolution Program at the Canadian Institute for Advanced Research. “The West Antarctic is fringed by ice shelves which act to stabilize the ice sheet – these shelves are sensitive to global warming, and if they break up, the ice sheet will have a lot less impediment to collapse.” This concern was reinforced further in a recent study led by Eric Steig of the University of Washington that showed that the entire region is indeed warming.

    “The typical estimate of the sea-level change is five metres, a value arrived at by taking the total volume of the West Antarctic Ice Sheet, converting it to water and spreading it evenly across the oceans, says Mitrovica. “However, this estimate is far too simplified because it ignores three significant effects:

    1. when an ice sheet melts, its gravitational pull on the ocean is reduced and water moves away from it. The net effect is that the sea level actually falls within 2,000 km of a melting ice sheet, and rises progressively further away from it. If the West Antarctic Ice Sheet collapses, sea level will fall close to the Antarctic and will rise much more than the expected estimate in the northern hemisphere because of this gravitational effect;

    2. the depression in the Antarctic bedrock that currently sits under the weight of the ice sheet will become filled with water if the ice sheet collapses. However, the size of this hole will shrink as the region rebounds after the ice disappears, pushing some of the water out into the ocean, and this effect will further contribute to the sea-level rise;

    3. the melting of the West Antarctic Ice Sheet will actually cause the Earth’s rotation axis to shift rather dramatically – approximately 500 metres from its present position if the entire ice sheet melts. This shift will move water from the southern Atlantic and Pacific oceans northward toward North America and into the southern Indian Ocean.

    “The net effect of all of these processes is that if the West Antarctic Ice Sheet collapses, the rise in sea levels around many coastal regions will be as much as 25 per cent more than expected, for a total of between six and seven metres if the whole ice sheet melts,” says Mitrovica. “That’s a lot of additional water, particularly around such highly populated areas as Washington, D.C., New York City, and the California coastline.” Digital animation of what various sea-level rise scenarios might look like for up to six metres is at www.cresis.ku.edu/research/data/sea_level_rise.

    “There is still some important debate as to how much ice would actually disappear if the West Antarctic Ice sheet collapses – some fraction of the ice sheet may remain quite stable,” he says. “But, whatever happens, our work shows that the sea-level rise that would occur at many populated coastal sites would be much larger than one would estimate by simply distributing the meltwater evenly. Any careful assessment of the sea-level hazard associated with the loss of major ice reservoirs must, of course, account for the sea-level fingerprint of other sources of meltwater, namely Greenland, the East Antarctic and mountain glaciers. The most important lesson is that scientists and policy makers should focus on projections that avoid simplistic assumptions.”

    The findings are presented in a paper titled “The Sea Level Fingerprint of West Antarctic Collapse”, which is to be published in the February 6 issue of Science. The research was funded with support from the Canadian Institute for Advanced Research, the Natural Sciences and Engineering Research Council of Canada, the John Simon Guggenheim Memorial Foundation and the US National Sciences Foundation.

    View an illustration of the processes contributing to the geometry of sea-level change following the potential collapse of the West Antarctic Ice Sheet.







    http://www.artsci.utoronto.ca/main/c...ctic-ice-sheet


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  3. #2  
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    I was wondering if the effect of weight on crust is selectively ignored here, and pressure distribution completely forgotten. The Earth's crust floats on a bounded fluid volume so if one area rises others must sink. As well I imagine that water shifted off Antarctica would apply more pressure to the ocean floors.


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  4. #3  
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    Quote Originally Posted by Pong
    I was wondering if the effect of weight on crust is selectively ignored here, and pressure distribution completely forgotten. The Earth's crust floats on a bounded fluid volume so if one area rises others must sink. As well I imagine that water shifted off Antarctica would apply more pressure to the ocean floors.
    Think they're referring to the isostatic compensation here: "2. the depression in the Antarctic bedrock that currently sits under the weight of the ice sheet will become filled with water if the ice sheet collapses. However, the size of this hole will shrink as the region rebounds after the ice disappears, pushing some of the water out into the ocean, and this effect will further contribute to the sea-level rise;"
    Is that what you had in mind?
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    Quote Originally Posted by whatever sensationalistic reporter wrote this bit of junk science jive
    “The net effect of all of these processes is that if the West Antarctic Ice Sheet collapses, the rise in sea levels around many coastal regions will be as much as 25 per cent more than expected, for a total of between six and seven metres if the whole ice sheet melts,” says Mitrovica. “That’s a lot of additional water, particularly around such highly populated areas as Washington, D.C., New York City, and the California coastline.”
    Carl Rove! You magnificent bastard! I knew you'd find a way to punish those pinkos who live in the Blue States! :-D Hell, man, those girly men can't even build dikes. Please say hello to W for me and tell him to go pop some popcorn; the dunce and donkey show has just begun. :-D

    Note: I tried to respond point by point to the article posted by Linx_Fox but explaining anything to a True Believer is like unchurning milk. :-D
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  6. #5  
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    This is quite fascinating. Not much to discuss here, but I do wonder about their methods. It is can be reading tiem nao!
    Om mani padme hum

    "In dishonorable things we are not bound to obey any man." - The Book of the Courtier [1561], pg 99 (144 in pdf)
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  7. #6  
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    Quote Originally Posted by Lynx_Fox
    "2. the depression in the Antarctic bedrock that currently sits under the weight of the ice sheet will become filled with water if the ice sheet collapses. However, the size of this hole will shrink as the region rebounds after the ice disappears, pushing some of the water out into the ocean, and this effect will further contribute to the sea-level rise;"


    Is that what you had in mind?
    Yes. They foresee this Antarctic hole shrinking/plate rebounding, and good job they realized that would happen, but then they ignore that same principle holds true for all the Earth's crust. They seem to have forgotten that plates sink under weight just as well as they rise when weight is removed. Well, that's common sense isn't it?

    Apart from the distribution of water, we have magma to distribute also. Magma isn't created or expanded to allow a local plate rebound. Rather the rebound happens because liquid magma, seeking pressure equilibrium, flows in from elsewhere. From where?

    I have this vision of our slightly crinkled Earth suddenly scrunching a bit at the seams.


    PS: I am pretty confident of some sea level increase, and the bit on tidal effect of ice mass really blows my mind without reservations.
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  8. #7  
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    I think you make a good point. As the bedrock rises a lot, so the seabed should fall by a little. But the rising of the bedrock also exposes more land and reduces the total area of the oceans, so even as the seabed falls the sea level could still rise. How would it all balance out, and did they address these issues? Anyone have a Science subscription?
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  9. #8  
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    Quote Originally Posted by Lynx_Fox
    (This isn't meant to be a global warming thread).
    You coulda fooled, me, Fox. Your cited artricle uses all the Global Warming buzz words and portends a doomed Earth drowned in semantical hyperbole.

    Quote Originally Posted by The News Reporter
    [in part]
    “...The typical estimate of the sea-level change is five metres, a value arrived at by taking the total volume of the West Antarctic Ice Sheet, converting it to water and spreading it evenly across the oceans, says Mitrovica. “However, this estimate is far too simplified because it ignores three significant effects:


    1. when an ice sheet melts, its gravitational pull on the ocean is reduced and water moves away from it. The net effect is that the sea level actually falls within 2,000 km of a melting ice sheet, and rises progressively further away from it. If the West Antarctic Ice Sheet collapses, sea level will fall close to the Antarctic and will rise much more than the expected estimate in the northern hemisphere because of this gravitational effect;

    Yeahbut, goodbuddy, the five meters you are talking about refers to the dissolution of the entire West Artarctic Ice sheet, not just the collapse of the Ice shelves that are said to be breaking loose. These perimeter shelves are much less than one percent of the West Artarctic ice mass and should they all melt the world's seas would be but a silly centimeter higher. Whoop-de-do.

    2. the depression in the Antarctic bedrock that currently sits under the weight of the ice sheet will become filled with water if the ice sheet collapses. However, the size of this hole will shrink as the region rebounds after the ice disappears, pushing some of the water out into the ocean, and this effect will further contribute to the sea-level rise;

    Yeahbuddy! Rock substrate rebounding takes hundreds, maybe thousands, of years. Update me in 3008, will ya?

    3. the melting of the West Antarctic Ice Sheet will actually cause the Earth’s rotation axis to shift rather dramatically – approximately 500 metres from its present position if the entire ice sheet melts. This shift will move water from the southern Atlantic and Pacific oceans northward toward North America and into the southern Indian Ocean.

    What? A shift of five hundred meters of the Earth's rotational axis will spill water from the Southern Hemisphere to the Northern?
    Cool; Transspherical currents. What ever happened to the Coriolis Effect?



    “The net effect of all of these processes is that if the West Antarctic Ice Sheet collapses, the rise in sea levels around many coastal regions will be as much as 25 per cent more than expected, for a total of between six and seven metres if the whole ice sheet melts,” says Mitrovic."

    [altogether now...]

    Ooooh...
    I was born at night one morn when the whistle rang boom-boom.
    I can bake a snake I can boil a cake when the mudpies are in bloom.
    We know that camels sail the sea and we get honey from the flea
    And every horse can climb a tree...on the road to Mandalay.
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  10. #9  
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    Yeahbut, goodbuddy, the five meters you are talking about refers to the dissolution of the entire West Artarctic Ice sheet, not just the collapse of the Ice shelves that are said to be breaking loose.
    The article discusses the entire melting as a hypothetical not a forecast.

    2. the depression in the Antarctic bedrock that currently sits under the weight of the ice sheet will become filled with water if the ice sheet collapses. However, the size of this hole will shrink as the region rebounds after the ice disappears, pushing some of the water out into the ocean, and this effect will further contribute to the sea-level rise;

    Yeahbuddy! Rock substrate rebounding takes hundreds, maybe thousands, of years. Update me in 3008, will ya?
    The initial elastic isostatic adjustments happen much faster, in Patagonia there are vertical rise of 2cm/year being observed now where there's been glacial melt in the past century.

    What? A shift of five hundred meters of the Earth's rotational axis will spill water from the Southern Hemisphere to the Northern?
    Cool; Transspherical currents. What ever happened to the Coriolis Effect?
    What's confusing you here? The physics seems pretty straight forward--mass of the earth is redistributed from the gigantic ice patch in one location to the around the world. Coriolis would have no effect. It would also slow down the rotation--just a tiny bit.
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    Quote Originally Posted by Fox
    What's confusing you here? The physics seems pretty straight forward--mass of the earth is redistributed from the gigantic ice patch in one location to the around the world. Coriolis would have no effect. It would also slow down the rotation--just a tiny bit.
    Think, Fox.

    A steel ball measuring fifty feet in diameter rotating at 1000 rotations a minute becomes a desired spot for a humingbird to sit and rest for a minute.

    What! The spinning steel ball is thrown out of kilter?

    Geez.
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  12. #11  
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    Quote Originally Posted by milum

    Think, Fox.

    A steel ball measuring fifty feet in diameter rotating at 1000 rotations a minute becomes a desired spot for a humingbird to sit and rest for a minute.

    What! The spinning steel ball is thrown out of kilter?

    Geez.
    What?

    Specifically what don't you understand, or doubt?
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  13. #12  
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    milum, surely you're not likening the earth to a steel ball ?
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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    Quote Originally Posted by marnixR
    milum, surely you're not likening the earth to a steel ball ?
    In my circles we call my example a parallel rendering to promote understanding.
    The mass of the West Antartic Ice Sheet, even if relocated en masse, is unlikely to shift the Earth's axis of rotation to a measurable extent. Do the math. You know, relative mass and inertia.

    Good Golly, yall! The entire mass of all the Ice on Earth is less than 1/3,344.000 of the swinging spinning Earth as a whole. A fly on an elephant's butt.
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  15. #14  
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    Quote Originally Posted by milum
    Good Golly, yall! The entire mass of all the Ice on Earth is less than 1/3,344.000 of the swinging spinning Earth as a whole. A fly on an elephant's butt.
    And 2 meter of water depth is but 1/6,371,000 of the radius. That 500m change in center of gravity is about 1/10000 of the radius. Relatively tiny fractional changes for the planet--sure; but plenty large enough to result in big changes relative to our scale, especially if you live in Miami.
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    Quote Originally Posted by Lynx_Fox

    (This isn't meant to be a global warming thread).



    Geophysicists predict amplification of sea-level rise in North America following collapse of West Antarctic Ice Sheet
    Thurs. Feb. 5/09

    University of Toronto geophysicists have shown that should the West Antarctic Ice Sheet collapse and melt in a warming world – as many scientists are concerned it will – it is the coastlines of North America and of nations in the southern Indian Ocean that will face the [color=Red"greatest threats from rising sea levels[/color].

    “Scientists are particularly worriedabout the ice sheet because it is largely marine-based, which means that the bedrock underneath most of the ice sits under sea level,” says Mitrovica, “The West Antarctic is fringed by ice shelves which act to stabilize the ice sheet – these shelves are sensitive to global warming, and if they break up, the ice sheet will have a lot less impediment to collapse.” This concern was reinforced further in a recent study led by Eric Steig of the University of Washington that showed that the entire region is indeed warming....

    [yada, yada, we all gonna die...]

    “ Mitrovica says, “That’s a lot of additional water, particularly around such highly populated areas as Washington, D.C., New York City, and the California coastline.

    The findings are presented in a paper titled “The Sea Level Fingerprint of West Antarctic Collapse
    Maybe not, Fox...

    The Future of the West Antarctic Ice Sheet
    Exploring ice thickness, melting and global climate change

    By Marc Airhart, The University of Texas at Austin

    ...Satellites have revealed that the ice sheets are thinning and their glacial slide into the sea is speeding up. Ice cores show that at times in the geologic past, Antarctica was ice free. Complicating matters, the West Antarctic Ice Sheet (WAIS), a mass of ice the size of Texas storing enough water to raise global sea level by 5 meters (about 17 feet), is resting on rock below sea level.

    “Not just a bit below sea level, it's 2,000 meters below sea level,” said David Vaughan, a principal investigator with the British Antarctic Survey. “If there was no ice sheet there, this would be deep ocean, deep like the middle of the Atlantic.”

    Antarctic Ice Flow and Sea Level Rise
    The recent series of reports from the Intergovernmental Panel on Climate Change (IPCC) did not include such bold predictions for the possible loss of Antarctic ice. The IPCC's estimate was that Antarctic ice flow would continue at the same rate it did from 1993 to 2003, despite an observed acceleration since then.

    The IPCC's restrained estimate about the ice flow, and its possible contribution to sea level rise, was not, however, a heartening sign. Rather, it reflected the consensus view that changes in the Antarctic have been so rapid, science can not yet account for them.

    “Models used to date do not include . . . the full effects of changes in ice sheet flow, because a basis in published literature is lacking,” stated the reports. “[U]nderstanding of these effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea level rise.”

    The IPCC's mid-range projection is that seas will rise 44 centimeters (17 inches) by the year 2050. That would put 100 million people each year at risk from being displaced from their homes by coastal flooding. If the WAIS were to entirely melt—which most experts doubt will happen in our lifetimes—seas would rise ten times higher.


    Antarctica is Melting Below Sea Level
    A lay person hearing that the melting of the West Antarctic Ice Sheet is speeding up might not be all that surprised given the routine nature over the past few years of news reports describing how the greenhouse effect is warming our atmosphere, speeding the arrival of spring, melting glaciers, and altering plant and animal ranges.

    For Antarctica, the emerging picture is far more complex than the headlines. If the hypotheses of polar experts like Blankenship and Holt are correct, Antarctica might resemble less a block of ice liquefying in a sunny greenhouse than a cog in an intricate Rube Goldberg machine.

    The surface of Antarctica is so cold and the ice so thick that raising the region's air temperature a few degrees is not enough to cause significant melting. Instead, scientists have long suspected that warm water in the Amundsen Sea is flowing up under ice shelves—platforms of floating ice attached to the grounded ice sheet—and melting them from below. This increased melting speeds the flow of grounded ice sheet into the water.

    But it's unlikely these warmer waters result directly from recent climate change. By measuring oxygen content, oceanographers have discovered that the warm water welling up below the glaciers has not been near the sea surface in the past few centuries. In oceanographer's terms, the water is “old.” It is part of a mass known as Circumpolar Deep Water connected to the North Atlantic through the globetrotting ocean conveyor belt. This water has been at depth for too long, scientists believe, for its temperature to reflect recent global warming.

    Polar scientists meeting at the three-day WALSE Workshop knew that explaining this upwelling could go a long way towards predicting the future of the WAIS. Fortunately, the workshop brought together experts in atmosphere, oceans, and ice—all critical players in this story.


    New Hypothesis on Atmospheric Currents
    Adrian Jenkins, a polar researcher from the British Antarctic Survey and WALSE participant, developed a computer model that showed a possible solution.

    Antarctica is encircled by atmospheric currents that largely insulate it from the rest of Earth's climate and keep it colder than it otherwise would be. Jenkins' model showed that these circumpolar currents, sometimes called “Westerlies,” “the Screaming 50s,” or “the Roaring 40s,” actually push surface waters out away from the continent. This results from the Coriolis Force, the byproduct of Earth's rotation that causes cyclonic systems to turn counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. As surface water is pushed away, warm deep water rises to replace it.

    If the atmospheric currents speed up, more water is pulled up. Indeed, observations indicate these atmospheric currents have sped up in recent decades in response to global warming. So increased upwelling seems likely.

    There isn't enough observational data to validate this hypothesis yet. For one thing, sea ice makes it difficult to get there to do the work. Polar experts say repeated missions over several years are necessary to correlate wind speeds with the temperature structure of the water.
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