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.
But Greenland, it gets warm enough to
melt at the surface, especially around the edges.
And, what seems to be happening, is that meltwater from the surface is able
to penetrate through the ice sheet and run down, and lubricate the bed.
And so, there are fluctuations in the flow velocity of
the ice according to the seasonal cycle.
They flow faster in the summer when there's
more of this meltwater than there is in the winter.
Without this mechanism, if the only way to get heat from the
surface to the bed is by diffusion,
by conduction of heat, it would take a very long time for a warming
at the surface to translate into faster flow of the ice.
But with this meltwater mechanism the ice is
able to respond and flow much more quickly than it would otherwise.
It's still a mystery how this meltwater
is able to get through all of this ice.
This is supposed to be subfreezing through here, but it's observed happening.
Here you can see a photograph of what's called a moulin, where the
water is running and just going down the drain to the bed.
They can trace the water as it goes in here and comes out there, but
mechanistically it's difficult to understand how this works.
Ice sheets are very complicated.
The Greenland ice sheet in particular is very complicated.
You have these regions of very fast flow, called ice streams.
There seems to be some a positive
feedback mechanism that if the ice starts to flow quickly enough
the deformation crumbles the ice or melts it to lubricate the bed,
and allows it to keep going quickly,
so there are places that ice streams are
going very quickly, and then suddenly stop,
and then start again, it's very unpredictable and quirky.
The ice sheet can
be pinned by local islands and rocks and things like that.
Lateral stress can hold the ice from moving.
it'd be much easier if we could just model it
in a one-dimensional sense, so the models that you'll be
playing with in the lab are just one-dimensional models, but
those aren't really good enough to capture everything that's going on.
Then you have this ice shelf here
which can also act to buttress the ice
slowing down its rate of flow into the ocean.
After the ice shelf on the Jakobshavn
glacier in in Greenland collapsed.
It was observed that the flow coming down
the mountain into the sea about doubled.
All these different pieces have to be put together
to predict how an ice sheet will respond to changes in climate.