The big behemoths in
the sea level picture are the two ice sheets, in Antarctica, and in Greenland.
Here is something of a cross-section of the Antarctic ice sheet.
Here's the east Antarctic ice sheet,
the TransAntarctic Mountains, and the west Antarctic ice sheet.
One thing that you can see is that
the land, where that the ice is sitting on,
is actually lower than sea level throughout much of
the western part of the ice sheet.
You may wonder how you could do such a thing like that.
How do you stick ice onto a piece of land that is underneath a whole bunch of water?
It turns out that the land was above the surface
of the water when it formed, but this process of isostatic adjustment,
the weight of the ice, gradually over millions of years, pushed the
land down and so now the ice is extending down below sea level.
It's grounded below the ocean.
This leads to the possibility that if
it starts to melt back a little bit, the grounding line
gets deeper and deeper as you melt it back,
and that makes it easier and easier for
the ice to float up off the ground.
Once it floats up it can flow in
one of these things which is called an ice shelf.
Ice shelves can be hundreds of meters thick.
They're not like sea ice which is just a few meters thick.
They're like giant cliffs of ice.
They are floating in the water.
An ice shelf when it melts doesn't cause the sea
level to rise, because it was floating already.
But the ice shelf can act to pen in
the outward flow of ice from the ice sheet itself.
When these ice shelves crumble and collapse, as we have
seen them doing, in the last few years, they can allow the ice behind it to flow
more quickly into the ocean.
The Greenland Ice Sheet, in contrast, is not grounded below sea
level, but it is extra-vulnerable because it's warmer at the surface.
In Antarctica, the surface of the ice is always
pretty much below freezing, and so the only way to
melt the Antarctic ice sheet is to flow it
off into the water and melt it in the water.
Maybe let it drift away up into warmer climates.