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West Antarctica is something that glaciologists are really worried about in the long term. The West Antarctic Ice Sheet's bed slopes downward as you go inland,

West Antarctica is something that glaciologists are really worried about in the long term. The West Antarctic Ice Sheet's bed slopes downward as you go inland, especially around Amundsen Sea where Thwaites Glacier is. This reverse slope means that if the grounding line (where the ice begins to float) retreats back, it is in deeper water. This causes the ice flux to increase, making it retreat backwards into deeper water still, etc. This is called the marine ice sheet instability. There's no mechanism known thus far that would countervail it, so it is something that should be taken seriously. Antarctica is too cold to melt much from surface warming anytime soon. Ocean warming and changing currents are a different story, though.

Simon_says 8y ago
I'm sorry, but could you break this down more for stupid people? What exactly are glaciologists afraid will happen and why is it bad?
- lenticular 8y ago
  Ice sheets, of which the West Antartic, East Antarctic, and Greenland ice sheets are the only current examples, are basically like very large domes of ice, on the order of 1000 kilometers wide and 1-3 kilometers high. Unlike mountain glaciers, they completely cover the underlying topography. They flow outwards under their own weight into the ocean. In Antarctica, it is cold enough that melting at the surface is negligible. This means that almost all snow that falls ultimately must flow into the ocean. At some point at the edge, the ice loses contact with the ground and floats. This is the grounding line. For a constant ice flow velocity, the amount of ice that gets spit out into the ocean is proportional to how thick the ice is at the grounding line. Now, suppose that the slope of the bed gets deeper as you go from the grounding line into the interior of the ice sheet. Then, if the grounding line retreats, it's in deeper water. So, the ice flux increases. The increasing ice flux means that the ice sheet thins near the edge. Since it's thinner, it can't stay grounded in as deep of water. Imagine a big chunk of ice in a puddle of water that sits at the bottom of a puddle, versus a little piece that floats. Now, since it can't float in as deep of water, the grounding line retreats because bouyancy is pulling up on the edge. This makes it go into deeper water, ice flux increases more, etc. This could cause a collapse of the ice sheet, resulting in a few meters of rise. The timescale that this could happen is very poorly constrained (decades? centuries? a couple millenia?). It may have already irreversibly begun, or maybe not.
  - function_seven 8y ago
    I still had trouble visualizing what you're describing. This is a typical cross-section[1]. To make it match what you're describing, I'd draw a slope from the grounding line and downward to the left, right? (Image depicts a flat bed underneath the ice, instead of a slope downward as you go inward) [1] http://cdn.antarcticglaciers.org/wp-content/uploads/2014/09/Fig-2.-Grounding-line.png http://cdn.antarcticglaciers.org/wp-content/uploads/2014/09/... EDIT: I think this image is a better representation: http://cdn.antarcticglaciers.org/wp-content/uploads/2014/05/PIG.ai_.jpg http://cdn.antarcticglaciers.org/wp-content/uploads/2014/05/...
    - dsfyu404ed 8y ago
      Flip the terrain illustration in that picture left to right and make it steeper. The terrain that this particular glacier is a bowl that is well below sea level. The more the glacier retreats the deeper the water gets (because the terrain is deeper), the more of the glacier can be floated, the faster the glacier can melt. Imagine a pool that slopes from 3ft to 8ft full of ice melting. The thin part melts first, lifts up, water gets a little further underneath lifting the next bit of ice and melting it faster but because the pool gets deeper as you go the more ice can be lifted up and broken off so it melts faster as it goes. Edit: yes, your second image is spot on.
      - deleted 8y ago
        [deleted]
      - Gibbon1 8y ago
        First image alludes to something. Near freezing slightly warmer water is denser. So convection is reversed. Unless I'm mistaken.
        function_seven 8y ago
        Both images show that convection pattern, but only the second image illustrates what parent was referring to, with the ground level dropping inward. As the groundline recedes, the surface area of ice exposed to the water increases, resulting in a positive feedback loop and further accelerating the melt rate.
- pboutros 8y ago
  They're noting that the way that West Antartica is laid out encourages a powerful feedback loop with negative effects. The more it melts, the more it gets into warmer water, which accelerates the melting.
- mixedmath 8y ago
  I know it sounds sensationalist, but the article "The Doomsday Glacier" [1] in Rolling Stone is a nice, understandable article on the feedback mechanism in Thwaites glacier. (They call it Doomsday because they consider the worst scenario, when the inflow of warm water causes mass melting, and Thwaites and the ice held back by Thwaites leads to 10 feet of ocean rise. This 10 feet is also mentioned in the main article). [1]: https://www.rollingstone.com/politics/politics-features/the-doomsday-glacier-113792/ https://www.rollingstone.com/politics/politics-features/the-...
  - gdubs 8y ago
    That was quite a read, thanks for sharing.
- baking 8y ago
  Most glaciers have friction points that keep the ice from sliding down too fast, to resist the enormous pressure of the built up ice sheets flowing downward from higher up. A typical glacier might be more narrow than it is long so the sides will be a source of friction. The ice will usually be in contact with the ground beneath it, causing even more friction. And there will frequently be islands or undersea ridges that can cause floating ice sheets to back up and keep the glacier from moving forward. The worry is that in some cases melting glaciers may be released from these friction points causing an accelerated movement and rapid calving at the ice front. This will cause an even faster increase in sea level on top of the gradual thermal ocean expansion and freshwater runoff of meting ice that we are already experiencing. An example of this is the Larson ice shelves on the Antarctic Peninsula that have had a series of collapses already. But luckily the Larson ice shelves do not have a huge sheet of ice behind them since the peninsula is rather narrow. Thwaites Glacier has been identified as a potential "perfect storm" for global warming caused sea level rise. It has a wide (90-100 mile) ice front and and there is no narrowing or potential bottle-neck from side to side. It has an enormous ice mass behind it reaching to the center of West Antarctica. The main friction point currently holding it back is the grounding line where the ice meets the sea floor. Between current grounding line and the center of West Antarctica there only seems to be one other ridge, known as the "Ghost Ridge" which is one of the subjects of the current expedition to determine if it is a true bedrock ridge or just silt and sediment deposited by the glacier. If the basin does in fact extend all the way back, the concern is that warm ocean water can work it's way under the ice and cause melting from below. As the ice melts it will float allow more water underneath. Meanwhile the ice front will calve and expose an even steeper front that will become unstable due to the height of the ice front. The icebergs will be washed out with the tide causing the process to continue. Here is the most recent talk by Richard Alley I've found on the subject: https://www.youtube.com/watch?v=WE9Gqy8Yy9w https://www.youtube.com/watch?v=WE9Gqy8Yy9w
  - baking 8y ago
    Here's an older talk by Richard Alley with much more detail on Thwaites Glacier (including simple demo simulations of what happens over time): https://www.youtube.com/watch?v=o4oMsfa_30Q https://www.youtube.com/watch?v=o4oMsfa_30Q
- seagullwindow 8y ago
  https://upload.wikimedia.org/wikipedia/commons/e/e9/West_Antarctic_Collapse.ogv https://upload.wikimedia.org/wikipedia/commons/e/e9/West_Ant...

Simon_says 8y ago

I'm sorry, but could you break this down more for stupid people? What exactly are glaciologists afraid will happen and why is it bad?

lenticular 8y ago
Ice sheets, of which the West Antartic, East Antarctic, and Greenland ice sheets are the only current examples, are basically like very large domes of ice, on the order of 1000 kilometers wide and 1-3 kilometers high. Unlike mountain glaciers, they completely cover the underlying topography. They flow outwards under their own weight into the ocean. In Antarctica, it is cold enough that melting at the surface is negligible. This means that almost all snow that falls ultimately must flow into the ocean. At some point at the edge, the ice loses contact with the ground and floats. This is the grounding line. For a constant ice flow velocity, the amount of ice that gets spit out into the ocean is proportional to how thick the ice is at the grounding line. Now, suppose that the slope of the bed gets deeper as you go from the grounding line into the interior of the ice sheet. Then, if the grounding line retreats, it's in deeper water. So, the ice flux increases. The increasing ice flux means that the ice sheet thins near the edge. Since it's thinner, it can't stay grounded in as deep of water. Imagine a big chunk of ice in a puddle of water that sits at the bottom of a puddle, versus a little piece that floats. Now, since it can't float in as deep of water, the grounding line retreats because bouyancy is pulling up on the edge. This makes it go into deeper water, ice flux increases more, etc. This could cause a collapse of the ice sheet, resulting in a few meters of rise. The timescale that this could happen is very poorly constrained (decades? centuries? a couple millenia?). It may have already irreversibly begun, or maybe not.
- function_seven 8y ago
  I still had trouble visualizing what you're describing. This is a typical cross-section[1]. To make it match what you're describing, I'd draw a slope from the grounding line and downward to the left, right? (Image depicts a flat bed underneath the ice, instead of a slope downward as you go inward) [1] http://cdn.antarcticglaciers.org/wp-content/uploads/2014/09/Fig-2.-Grounding-line.png http://cdn.antarcticglaciers.org/wp-content/uploads/2014/09/... EDIT: I think this image is a better representation: http://cdn.antarcticglaciers.org/wp-content/uploads/2014/05/PIG.ai_.jpg http://cdn.antarcticglaciers.org/wp-content/uploads/2014/05/...
  - dsfyu404ed 8y ago
    Flip the terrain illustration in that picture left to right and make it steeper. The terrain that this particular glacier is a bowl that is well below sea level. The more the glacier retreats the deeper the water gets (because the terrain is deeper), the more of the glacier can be floated, the faster the glacier can melt. Imagine a pool that slopes from 3ft to 8ft full of ice melting. The thin part melts first, lifts up, water gets a little further underneath lifting the next bit of ice and melting it faster but because the pool gets deeper as you go the more ice can be lifted up and broken off so it melts faster as it goes. Edit: yes, your second image is spot on.
    - deleted 8y ago
      [deleted]
    - Gibbon1 8y ago
      First image alludes to something. Near freezing slightly warmer water is denser. So convection is reversed. Unless I'm mistaken.
      - function_seven 8y ago
        Both images show that convection pattern, but only the second image illustrates what parent was referring to, with the ground level dropping inward. As the groundline recedes, the surface area of ice exposed to the water increases, resulting in a positive feedback loop and further accelerating the melt rate.
pboutros 8y ago
They're noting that the way that West Antartica is laid out encourages a powerful feedback loop with negative effects. The more it melts, the more it gets into warmer water, which accelerates the melting.
mixedmath 8y ago
I know it sounds sensationalist, but the article "The Doomsday Glacier" [1] in Rolling Stone is a nice, understandable article on the feedback mechanism in Thwaites glacier. (They call it Doomsday because they consider the worst scenario, when the inflow of warm water causes mass melting, and Thwaites and the ice held back by Thwaites leads to 10 feet of ocean rise. This 10 feet is also mentioned in the main article). [1]: https://www.rollingstone.com/politics/politics-features/the-doomsday-glacier-113792/ https://www.rollingstone.com/politics/politics-features/the-...
- gdubs 8y ago
  That was quite a read, thanks for sharing.
baking 8y ago
Most glaciers have friction points that keep the ice from sliding down too fast, to resist the enormous pressure of the built up ice sheets flowing downward from higher up. A typical glacier might be more narrow than it is long so the sides will be a source of friction. The ice will usually be in contact with the ground beneath it, causing even more friction. And there will frequently be islands or undersea ridges that can cause floating ice sheets to back up and keep the glacier from moving forward. The worry is that in some cases melting glaciers may be released from these friction points causing an accelerated movement and rapid calving at the ice front. This will cause an even faster increase in sea level on top of the gradual thermal ocean expansion and freshwater runoff of meting ice that we are already experiencing. An example of this is the Larson ice shelves on the Antarctic Peninsula that have had a series of collapses already. But luckily the Larson ice shelves do not have a huge sheet of ice behind them since the peninsula is rather narrow. Thwaites Glacier has been identified as a potential "perfect storm" for global warming caused sea level rise. It has a wide (90-100 mile) ice front and and there is no narrowing or potential bottle-neck from side to side. It has an enormous ice mass behind it reaching to the center of West Antarctica. The main friction point currently holding it back is the grounding line where the ice meets the sea floor. Between current grounding line and the center of West Antarctica there only seems to be one other ridge, known as the "Ghost Ridge" which is one of the subjects of the current expedition to determine if it is a true bedrock ridge or just silt and sediment deposited by the glacier. If the basin does in fact extend all the way back, the concern is that warm ocean water can work it's way under the ice and cause melting from below. As the ice melts it will float allow more water underneath. Meanwhile the ice front will calve and expose an even steeper front that will become unstable due to the height of the ice front. The icebergs will be washed out with the tide causing the process to continue. Here is the most recent talk by Richard Alley I've found on the subject: https://www.youtube.com/watch?v=WE9Gqy8Yy9w https://www.youtube.com/watch?v=WE9Gqy8Yy9w
- baking 8y ago
  Here's an older talk by Richard Alley with much more detail on Thwaites Glacier (including simple demo simulations of what happens over time): https://www.youtube.com/watch?v=o4oMsfa_30Q https://www.youtube.com/watch?v=o4oMsfa_30Q
seagullwindow 8y ago
https://upload.wikimedia.org/wikipedia/commons/e/e9/West_Antarctic_Collapse.ogv https://upload.wikimedia.org/wikipedia/commons/e/e9/West_Ant...