East Antarctic ice sheet reveals rivers and lakes
By Robyn Williams on The Science Show
Tas van Ommen's study group flew a DC3 from Casey Station in East Antarctica, mapping parts of the ice sheet which had not been studied before. The plane flew 1,000km radials from a hub. One valley went to 3km below sea level. Some of the ice went to a thickness of 4km. The total area was similar to that of NSW. Radar was used to map the land below the ice. The original thought was that the bedrock was well above sea level. But actually, the underlying rock is many kilometres below sea level. River and even lakes of water have been discovered below the ice. Knowing the composition of the ice sheet will allow more accurate predictions for its future in a warming world.
Tas van Ommen
Principal Research Scientist: ice cores and climate
Australian Antarctic Division
Tas van Ommen: It's fascinating, going back some 34 million years the ice sheet first formed and then went through a succession of expansions and retreats, carving out the landscape as it did. And the information we learn from looking at the pattern in the landscape...there are buried fjords, for example, that you can see with different directions...tell you about how large the ice sheet at what sea level was like at the time and you can put that information together with other geological information to learn a little bit more about the interaction between climate, ice and sea level on these very long timescales.
Robyn Williams: Tas van Ommen at the University of Tasmania in Hobart, a paper on the immense study of the southern ice cap was published in Nature this week. Both the scale of their survey and the implications of what they found are really impressive.
Tas van Ommen: What we've been doing is working with the group from Texas and also Edinburgh, and flying over the last three seasons an old World War II DC3 that has been virtually completely rebuilt, had radars fixed under the wings, and flying a pattern out of Casey Station in East Antarctica mapping parts of the ice sheet that nobody had ever looked through before.
Robyn Williams: That must really be something of a challenge because that's not exactly an old plane but it doesn't go as fast as some of the planes we travel around in, it's not like a jet, so is it risky?
Tas van Ommen: I don't know about risky, although they have lost one or two of these planes, not our group but generally this has happened in Antarctica. No, they do a fantastic job, very professional, and you feel pretty darn safe when you're on the plane, even if it does feel like an old-world experience. So we started out at Casey and we flew...if you can imagine a bicycle hub with spokes extending out 1,000 kilometres, it's a bit like the hub at Sydney, and we flew radials that would extend as far as Brisbane in one direction, and Melbourne in the other, western New South Wales, and map this very large chunk of East Antarctica that we had only really a handful of lines dating back from the '70s and '80s. So we found these really deep valleys, a basin that goes down to over three kilometres below sea level called the Aurora Sub Glacial Basin. We sort of knew it was there from the limited information, but we've mapped it out in much greater detail than we ever had possible before.
Robyn Williams: So are you looking at the slab of ice and what's underneath it, or hills and valleys in the ice itself?
Tas van Ommen: No, it's a slab of ice sitting on top of hills and valleys. And so the ice might be four kilometres thick and the deepest parts of the valleys extend down to a couple of kilometres below sea level. In fact of that area we looked at, the whole survey area was probably equivalent in size to New South Wales. We found that probably something like the Northern Territory area was actually below 500 metres below sea level, and if you look at the deepest bits, something like the size of Tasmania was more than one kilometre deep.
Robyn Williams: Isn't that incredible, because people forget how huge Antarctica really is, one and a half times the size of Australia itself.
Tas van Ommen: It's an amazing massive continent. We'd go out for a seven-hour flight, three and a half hours out, three and a half hours back. It might sound a little bit boring but it was hard to tear your eyes off the window looking outside to actually monitor the equipment as you went, watching the radar trace come through, and you'd see the peaks and valleys go past underneath, beneath the ice, and these amazing deep features when you are over some of the fast-flowing deep glaciers.
Robyn Williams: And of course when people are talking about whether the ice is changing, melting or whatever, when you've got it that thick and that cold...so what is actually happening to it?
Tas van Ommen: Well, that's the thing, if people had thought that East Antarctica, which is the bit we're working in, the bit south of Australia, because that's so high, so thick, so cold, it really wasn't going to change very much. And we also thought that most of it was actually sitting on bedrock hills that were well above sea level, so that even if you get a little bit of melt around the edges where it touches the ocean, the ice would just shrink back onto its hills, end of story, you're not going to get much of a contribution to sea level from that.
But in actual fact because we are finding that so much of this area is grounded below sea level, it means that if it melts a little at the edge, it can't actually retreat from the ocean, the ocean can follow it in and the process continues, it melts a little bit more. And so what this does is it starts to actually make the edge of the ice flow faster and it thins and flows into the ocean, so you get an enhanced contribution to sea level over what we might have expected before we knew this.
Robyn Williams: Yes, that's also what is happening in Greenland, they reported that three or four years ago, didn't they.
Tas van Ommen: Yes, in fact the Jakobshavn Glacier, for example, has been speeding up quite considerably, and around the other side of Antarctica in the west, that's the bit to the south of South America, a lot of that was known to be resting on rock below sea level. So scientists have been concerned for a long time that West Antarctica has the potential to accelerate what's called this marine instability were the ocean can follow the ice in. We now know that at least significant chunks of East Antarctica are vulnerable to the same type of process, and we are watching very closely.
Robyn Williams: And what's going to happen to all this information? What are you going to do when you put it all together?
Tas van Ommen: One of the big challenges that came out of the last IPCC round of assessments was that when we tried to estimate what sea level was going to do by the end of the century, they could say, well, we can estimate what's going to happen in terms of just melting from the surface because the air gets a bit warmer, but we don't know enough about what happens at the bottom of the ice sheet to really make a good assessment of how much it's going to speed up in response to ocean warming, particularly at the edge.
So the big challenge for glaciologists is to actually get really good computer models that tell us what Antarctica is going to do as it warms up, and that means knowing about where there is water at the bottom of the ice sheet because many people are surprised to hear that the base of Antarctica has got lots of melting going on. The geothermal heat comes from underneath, melts the ice away, and in places there is quite a lot of liquid water. There are even lakes under Antarctica. You can have either hard rock or you can have soft sediments, and that will change the way the ice flows. So our radar maps are providing all the information that is needed to get much better models of the way the ice sheets are going to flow.
Robyn Williams: And what's going to happen with that plan? Will you continue those expeditions indefinitely?
Tas van Ommen: Certainly for the next few years. There are two strands to what we're doing. First of all there are still areas we haven't mapped out well, there are still big gaps between the existing radar lines. And the other thing we're doing is flying missions where we repeat-fly certain areas of the ice sheet that are changing rapidly, and we can actually see year upon year the surface changing if the ice is melting quickly. For example, there is one glacier that is an outlet for a large chunk of East Antarctica, it's called the Totten Glacier and its surface is going down between one and two metres a year near the coast where it actually is an outlet. We knew that from satellites but there aren't enough satellite observations at the moment because we are between missions of the particular satellite of concern. So our aircraft is actually overflying that area and doing repeats to keep track of what is happening year by year.
Robyn Williams: How are you getting on with the Scots and the Texans?
Tas van Ommen: It's great fun actually, I still think it's kind of amusing to go to a fridge in the science building in Australia's Casey Station and plastered on the door was 'Don't mess with Texas' stickers.
Robyn Williams: I can imagine!
An intrepid team, still having fun. But imagine four kilometres of ice with lakes and rivers underneath and huge stretches way, way below sea level. A paper published in the journal Nature this week. Tas van Ommen from the Australian Antarctic Division in Hobart.