00:00Basically, what we're doing is getting some physical information that may help us decide
00:05how physical parameters control the motion of this ice stream.
00:10And once we have a clear picture of what this material, the till is, if the till is controlling
00:16the motion and what its properties are, we can then address the question, how could it
00:22respond to changes in the physical situation of the ice stream, if that change, say, due
00:28to climatic change?
00:30We haven't got to that point yet, but what you can, among the various thoughts you can
00:34have about this is one that I think is fairly striking is this.
00:40What controls the strength of that till and the rate of its deformation?
00:45Well it seems clear that the biggest control on it is the water pressure.
00:50If the water pressure is increased, it's going to become weaker, it's going to flow faster
00:54and vice versa.
00:56Well, what controls the water pressure?
00:58This is much like the story in surging.
01:00We ask the same questions because we have a similar situation.
01:03In this case, what controls the water pressure?
01:06Well, the only evident thing is the basal melting, which we were talking about.
01:10Remember, the heat coming up and the heat generated at the bottom, melting ice off the
01:13bottom of the ice stream and producing water.
01:16That water has to move through this conduit system and through that process of water being
01:21generated, transported.
01:23a distribution of pressure has to be achieved to make that flow possible.
01:30Now, think.
01:32If you had the thing moving along in a certain situation in equilibrium and you pushed it
01:40faster, you caused it to go a little faster somehow, just a little perturbation, you would
01:44increase the melting rate.
01:46Because the melting rate is directly proportional to the rate of sliding because the generation
01:51of heat, which is produced by the sliding of the mass at this shear stress, is directly
01:57proportional to the rate of sliding.
02:00So you would increase the melting rate.
02:03You would therefore, I think, increase the basal water pressure because you're going to
02:07have to have a higher pressure there to get that extra water generated out of the system
02:12and prevent it from accumulating.
02:15If the pressure went up, then the till would get weaker.
02:19It would slide faster.
02:21Then more heat would be generated and so on.
02:23So you see, you have there an instability, potentially.
02:27And now, what is really going on?
02:30What determines the level at which it actually moves, the 1.2 meter a day at upstream B and
02:36so on?
02:37Is it really stable?
02:39Well, not too sure about that because ice stream C is stopped.
02:42That's making a manifestation there is kind of instability, a kind of start and stop instability.
02:48What kind of speed could it actually reach?
02:51Well, you think about surging.
02:55One meter a day is pretty puny for a surge velocity.
02:59We readily had in our glacier in Alaska 10 meters a day and in some parts we had up to
03:0360, 65 meters
03:05a day and surging speeds of 100 a day or no.
03:08Are those possible in the Antarctic ice sheet?
03:12It reminds me of one day on Variegated Glacier in Alaska.
03:15I was in my sleeping bag trying to go to sleep and suddenly there was a great shaking.
03:20The glacier was surging underneath.
03:22There was a great shaking.
03:23I thought, is this it?
03:24Is this thing really taking off?
03:25Is this what it feels like to be in a real surge and it's starting to shake?
03:31I rushed out of the tent and I heard rocks rolling down the mountainside.
03:34I realized it was an earthquake.
03:36It wasn't the glacier taking off like a galloping.
03:40Well, it was galloping, but it didn't gallop that hard.
03:42So, we don't know, but this is the kind of question we would ask.
03:46How do we figure out what kind of speed is ultimately attainable?
03:50And this bears on what kind of, you know, what sort of disintegration could really possibly take place.
03:57If you could have an instability like that, where the thing would go faster and faster,
04:02then suppose you could have a speed of 100 meters a day.
04:05You can move the ice out of the West Antarctic ice sheet remarkably fast that way.
04:10You could get rid of that ice in the order of decades if you could have speeds that fast.
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