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The big behemoths in 

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the sea level picture are the two ice 
sheets, in Antarctica, and in Greenland. 

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Here is something of a cross-section of 
the Antarctic ice sheet. 

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Here's the east Antarctic ice sheet, 

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the TransAntarctic Mountains, and the west 
Antarctic ice sheet. 

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One thing that you can see is that 

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the land, where that the ice is 
sitting on,

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is actually lower than sea level 
throughout much of 

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the western part of the ice sheet.

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You may wonder how you could do such a thing like 
that. 

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How do you stick ice onto a piece of land 
that is underneath a whole bunch of water?

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It turns out that the land was above 
the surface 

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of the water when it formed, but this 
process of isostatic adjustment, 

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the weight of the ice, gradually over 
millions of years, pushed the 

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land down and so now the ice is extending 
down below sea level. 

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It's grounded below the ocean. 

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This leads to the possibility that if 

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it starts to melt back a little bit, the
grounding line

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gets deeper and deeper as you melt it back,

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and that makes it easier and 
easier for 

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the ice to float up off the ground. 

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Once it floats up it can flow in 

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one of these things which is called an ice 
shelf. 

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Ice shelves can be hundreds of meters 
thick. 

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They're not like sea ice which is just a 
few meters thick. 

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They're like giant cliffs of ice.

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They are floating in the 
water. 

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An ice shelf when it melts 
doesn't cause the sea 

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level to rise, because it was floating 
already. 

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But the ice shelf can act to
pen in 

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the outward flow of ice from the ice 
sheet itself. 

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When these ice shelves
crumble and collapse, as we have 

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seen them doing, in the last few years,
they can allow the ice behind it to flow 

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more quickly into the ocean. 

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The Greenland Ice Sheet, in contrast, is 
not grounded below sea 

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level, but it is extra-vulnerable because 
it's warmer at the surface. 

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In Antarctica, the surface of the ice is 
always 

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pretty much below freezing, and so the 
only way to 

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melt the Antarctic ice sheet is to flow it 

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off into the water and melt it in the 
water. 

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Maybe let it drift away up into warmer 
climates.