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Oral History Transcript — Dr. Charles Bentley

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Interview with Dr. Charles Bentley
By Will Thomas
At the University of Wisconsin at Madison
August 7, 2008

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Charles Bentley; August 7, 2008

ABSTRACT: This interview was conducted as part of a series documenting the history of scientific work on the West Antarctic Ice Sheet (WAIS). Charles Bentley has been a geophysicist at the University of Wisconsin since 1959. This interview discusses his entry to geophysics and graduate work at Columbia University under Maurice Ewing, and his inclusion by Frank Press in Antarctic traverses associated with the International Geophysical Year. He discusses his discovery with Ned Ostenso of the marine nature of WAIS during this field work, and then the building of the geophysics program at Wisconsin. There is detailed information about the organization and work of the Ross Ice Shelf Geophysical and Glaciological Survey, and the Siple Coast Project, and his group's subsequent field work. He also discusses his interest in the subject of the prospective disintegration of WAIS, and his shift in interest from geophysics to glaciology on account of this problem.

Transcript

Session I | Session II

Thomas:

This is Will Thomas. Itís now August 7. Iím still talking with Charlie Bentley at the University of Wisconsin, picking up from yesterdayís interviews. We just thought that today we might discuss a little bit of the research that has taken place in the Antarctic from the 1980s up through the Ď90s, and perhaps a bit into the present decade. So, when Siple Coast first started, what was the main concern? What were the main things that you and the people that you were working with wanted to find out, and how did that evolve over time?

Bentley:

Well, we wanted to find out what ice streams were in greater detail than we knew. We had only a general picture of what ice streams were in terms of the speed of movement, and particularly how they move. Oh, and what happened at the crevasse boundary zones, what the force balance — I mean, glaciers are driven by gravity, and then there has to be some restraining forces, otherwise theyíd accelerate to the speed of light, as John Nye pointed out in a paper once years ago. So, we didnít know how much of the resistance to that flow came from the sides, and how much from the bed, and how much internally within the ice just due to shear. So, we were interested in learning more about how and why ice streams behave, in terms of their mechanics. Itís really mechanics not dynamics. You used the term ďglacier mechanics,Ē which is correct, because itís not a dynamic, because thereís no acceleration term in the equations that we use to study glaciers. So, itís not really dynamics, but itís commonly called ďglacier dynamics,Ē where it should properly be called ďglacier mechanics,Ē which you probably got from either MacAyeal or Weertman, who would use the right term. So, we wanted to learn that, and then we were particularly interested also in — well, there were two particularly interesting ice streams: B, which was way out of balance, it was going so fast, and C, which had stagnated. I mean, we knew that the crevasses were buried, and that it was no longer active, even though we didnít have any direct velocity measurements on it yet. And they were right next to each other. So, we spent several seasons studying B. We worked first on the central part of the ice stream, and then worked down near the grounding line, which was clearly an important part [of] how the ice stream interacts with the ice shelf. Because itís a very different situation than the simple model that Weertman had in his original paper on the boundary zone. And the instability that Weertman found was, to some extent, just caused by the discontinuity in the physics that he was using to represent the ice shelf and the ice stream — or the inland ice. He didnít have an ice stream in his model. So, we were interested in what was really going on at the grounding line of an ice stream to see whether the instability mechanism would apply there or not, which is still somewhat of an open question. Incidentally, on that matter, I was influenced in my turn towards the negative thought about instability — In other words, when I was supporting the stable side, [I was] influenced quite a bit by Richard Hindmarshís theoretical work. Heís with the British Antarctic Survey. And, he found that [in] the more detailed analysis that he did, that the instability went away, and it was artificial just due to a model mismatch at the grounding line between the two systems. And then they had some supporting — he and Philippe Huybrechts from Belgium, but at AWI, the German Institute, Alfred Wegener Institute, had some measurements over on the Filchner Ice Shelf side, suggesting the same thing, that the ice shelf was not acting to restrain the flow from the interior. I mean, thatís the point. Thatís the cork-in-the-bottle argument.

Thomas:

Right. The buttressing versus not.

Bentley:

Of course, those studies were before Larsen B broke up and it turned out that the glaciers did indeed speed up quite dramatically, as I mentioned yesterday. But at any rate, so we were interested in the grounding line, so we spent a season or two — I think it was just one season — at the downstream end of Ice Stream B, and then we moved to Ice Stream C. And while we were at Ice Stream C, we did [an airborne] radar survey of Ice Streams B and C. And, at the same time we started, we were doing the seismic reflections. We did special things with radar, like one interesting experiment was to record in detail — on a two-dimensional grid at the surface — record the radar signal in detail, radar profiles. I mentioned the Picket Fence Effect yesterday. So, the rough bed acts like a reflecting diffraction grating, so you get interference patterns, and you get maxima and minima in the amplitude just due to this interference pattern. That interference pattern is the first approximation. If youíre not changing the ice thickness particularly, then itís fixed relative to the bed. Right? So, if you measure the diffraction pattern at the surface, and the surface is moving relative to the bed, the diffraction pattern, if you can identify it, will appear to move upstream at the same speed that the ice is actually moving downstream. You see what I mean?

Thomas:

I think so.

Bentley:

Because, the diffraction comes from reference to the bed. The ice itself is just transparent to the radio wave. So the ice is just acting as if weíre in a slow-moving airplane flying over this rough surface and looking at the diffraction pattern from it. Well, the point of that is, then, if you can watch this diffraction pattern moving upstream, youíve got a direct measurement of how fast the ice is moving relative to the bed.

Thomas:

Mhmm. I see.

Bentley:

And, thatís important when weíre trying to study the ice stream dynamics, because one of the first early things we found was that the bed itself was deforming. And then it becomes of great interest to know how much of the motion of the ice is due to deforming of the bed, and how much is due to the motion at the ice-bed interface. Now, if all the motion is by deformation within the bed, the roughness of the interface being fixed (through short periods of time at any rate), that roughness, if itís associated with the base of the ice, then it will move (and all the motion of the ice is caused by deformation within the bed below that level) then that diffraction pattern will move right along with the surface, and youíll get no relative motion between the surface and the bed in this diffraction experiment. On the other hand, if it had a rigid bed, or the bed was deforming the base of the ice — the ice was moving over a bed whose shape did not change with time — then the diffraction pattern would be tied to the bed rather than the bottom of the ice and you would have a relative motion. So, the point of all that is that we can get information about the part of the movement thatís caused by deformation within the ice as distinct from deformation within the bed. Kind of a neat experiment, taking advantage — I mean, the way we do it is [we] just compare sequential measurements on the same surface grid, and, since the signal amplitude changes in fractions of a meter, you have to drive the radar system — towed by a Sno-Cat — you have to drive it very carefully as close as possible to the same spot each time next to a line of flags. We put flags out every meter over a square kilometer. We drove back and forth, and back and forth, and back and forth several times in the course of a season. And there wasnít any detectable motion. I mean, the experiment was a success in the sense that we were able to show that there wasnít any detectable motion, or it was below some level indicating that there was very little deformation within the ice. But that was a little nonstandard. I mean, we think of radar as just measuring ice thickness, but there are other things you can do with it like that. Another useful thing to do with the radar system is look at crystal orientations within the ice. The ice crystals in the glacier, in most cases theyíre not randomly distributed. Ice is a hexagonal crystal and, mechanically, it acts as if it were a little cylinder. So it has one behavior parallel to the C axis, which is the axis of the equivalent cylinder. And another behavior in directions normal to that. Well, one of the things that differs is the dielectric constant, which determines the speed at which the radio waves travel. So, if you imagine the ice is a single crystal, and waves that are polarized parallel to the axis of the crystals will travel at different speed — Iíve forgotten now, whether itís faster or slower than those that are polarized perpendicular to that C axis. And so you have birefringence. You get different echo times from the bed depending on whether youíre on — If you assume that thereís a preferred orientation of the ice crystals, you get a birefringence that you can measure. You can measure by looking at the reflections — as the antennas are rotated in one spot through 180 degrees you can measure the birefringence and thereby get a handle on what the orientation of the ice crystals in the ice is. And that orientation comes about because of deformation that the ice has undergone. So, we get sort of a paleo strain. I mean, we get a map of the strain thatís occurred within the ice thatís produced the crystal orientation. So, thatís another trick to do with radar other than just measuring ice thickness. And we do similar things with the seismic work in measuring — the seismic wave velocity also varies strongly depending on whether itís parallel or perpendicular to the C axis. And so we do essentially the same sort of thing with seismic and radar in determining the internal structure of the ice. But they donít behave in precisely the same way as the two different — I mean, oneís an elastic wave and the otherís an electromagnetic wave. And so we get complimentary information from the seismic and the other radar systems. So, thatís the kind of — oh, and then we studied internal structure of the ice using electrical resistivity.

Thomas:

Yeah. I think you spoke a little bit about that yesterday.

Bentley:

Yeah. Now the resistivity is a function primarily of the temperature [Ö] Well, oh, yeah no, it depends, it depends on the density, of course, and the temperature. And then, again, on the crystal structure. So, we get still more information about the internal characteristics of the ice from electrical resistivity, as well as getting information on what the mechanism of electrical conduction through ice is, through a glacier. You know, there are — well, no, I wonít go into that in any more detail. And then when we got to Ice Stream C then we started doing this small-scaleÖ

Thomas:

When was it found that Ice Stream C was inactive?

Bentley:

Well, it was actually found by the NSF-Scott Polar Research Institute-Technical University of Denmark radar flying, NSF-SPRI-TUD.

Thomas:

That was before RIGGS?

Bentley:

That was in the late Ď60s.

Thomas:

That was in the late Ď60s? Okay.

Bentley:

There were no velocity measurements, but the crevasses were buried.

Thomas:

Right. Okay. So, that was how they couldÖ

Bentley:

So that was the first clue that there was a dead ice stream there.

Thomas:

Okay. So, you moved on to Ice Stream C?

Bentley:

So then we moved, yeah, to Ice Stream C. And now we were in a place where there were no crevasses at the surface. It made it a lot easier to run around with our vehicles. We didnít have to worry about whether we were going to fall in a crevasse or not.

Thomas:

Ever had any close calls with those?

Bentley:

No, not really. Back in IGY we put a Sno-Cat pontoon into a crevasse, but it was a small one and we were kind of expecting we might see one because we were driving up the side of a mountain, which was not what we were supposed to be doing [laughs]. That was the end of that. I mean, all we had to do was turn around and go back. Fortunately we didnít have to go through the crevasse field. I had what in a way seemed like a close call. When we were working at Crary Ice Rise — you know where Crary Ice Rise is?

Thomas:

Yeah. Yeah. Itís kind of where the Ross Ice Shelf is snagged on the floor, so thereís an ice rise above as well.

Bentley:

Yeah. Right. And, of course thatís all heavily crevassed, even on the upstream side. And, we had a camp on the upstream end of Crary Ice Rise, and we had a flagged route to get from the camp off to the side of the Crary Ice Rise, onto the uncrevassed part of the ice shelf. But, we had an aerial photo or a satellite image — I guess it was a satellite image — they were looking at late in the season, the satellite image of the very area we were working on, and you could actually see the Sno-Cat tracks, the vehicle tracks on the surface. And, they could also see the crevasses, which are revealed by subtle changes in the lighting, because there are gentle sags in the surface. And some of these were really big. And I was shocked to see that one of guys, when heíd gone out with a Sno-Cat, had decided to take a shortcut, and, instead of following this L-shaped track that we had, he cut across, and the satellite image revealed that heíd gone right over a huge crevasse that was big enough to swallow the Sno-Cat and the sled, [Laugh] and everything. And, fortunately he didnít go in, because the big crevasses often have very thick bridges and can support a lot of weight. But it was a rather dramatic picture, and it was a close call in the sense that the bridge could very well not have been strong enough, and it would have been a total loss of humans and vehicles. So, needless to say, I showed him, he saw this image too, and I told him not to take that shortcut anymore.

Thomas:

It seems like a very sobering thing to look at.

Bentley:

Yeah. Yeah.

Thomas:

So, sorry, Iíve distracted you a bit from the work on Ice Stream C that you moved to after B.

Bentley:

Yeah. So, when we got to Ice Stream C, then we started running a radar across the crevasse boundaries to see how deeply buried they were. And we could do the same thing — there were lots of buried crevasses within the body of the ice stream too. And, we started trying to find out if we could get a history of the stagnation. I mean, one of the mysteries was how the ice stream stagnates. Nobody knew at the time what caused it to stagnate. One idea was that the lubricating water thatís required at the bed was diverted by the hydrologic potential, which is driven largely by a combination of the surface slope and the bed slope. I mean, which way water will flow at the base of the glacier. But, the surface slope is ten times more important than the bed slope. So, the water, even though itís at the bed, largely flows down the surface slope. And so, if youíve got a good map of the surface topography (it does help to have the bed topography too because it is ten percent important), then you see where the water flows. And it looked like maybe the water was flowing away from the head of Ice Stream C, and therefore maybe it stagnated because the bed lost its lubricating water. But another way that it could happen is that the efficient water drainage channels developed first down near the other end of the ice stream, down near the ice shelf. And when you get effective drainage channels, then they pull all the surplus water off and they could eliminate the lubricating water at the downstream end by that mechanism, in which case the ice stream should stagnate. The wave of stagnation would move upstream. In the first model the wave of stagnation should move downstream. And so we were trying to see if we could determine that by measuring the age of the crevasse burial as a function of distance up along the ice stream. And the first one was somewhat ambiguous. I mean, we didnít have enough information, and thatís why I did the second one that I mentioned yesterday. Itís the last field program of mine even though I wasnít there. I mean, the last one done by my group, that Ben Smith did looking at a more-or-less continuous profile up along the margin of the ice stream. But itís still a puzzling result.

Thomas:

Who were some of the people that you were working with inÖ

Bentley:

In that work?

Thomas:

Yeah. In that work.

Bentley:

I think I mentioned most of them yesterday. At the same field camp Whillans, Barclay Kamb, Keith Echelmeyer, Will Harrison. Bob Bindschadler, we never worked at the same field camp with him. He liked to get around more. He had a very lightweight mobile party. He didnít operate out of a fixed field camp. We operated out of a fixed field camp up — well there was one at Crary Ice Rise, and then there was one at Upstream B, and then there was one at Downstream B, and then there was one at Upstream C. Oh, and then there was one on a little — itís called Out B. It was in between the two branches of Ice Stream B. And so we had Jamesways [shelters] and a cookÖ so we stayed in one place for a whole season. Bob liked to travel around. He worked more down at the lower end of the ice streams in the days that we were working farther in the interior. And then he did some surveys on Ice Stream D, now MacAyeal Ice Stream, but at that time we were doing something different. So we never actually were at the same place. But when we had our planning meetings for the WAIS project, we all planned together. But, the mobile way he liked to work, I think, was the primary reason we didnít actually work together in the field. Oh, and then Bob Jacobel I mentioned yesterday, he also worked out of the base camp.

Thomas:

I wanted to ask you about a couple of names that you havenít mentioned, they were coauthors of some of the papers in the 1980s. Iím not sure if I have the pronunciation right. Shabtaie?

Bentley:

Oh yeah. He was my own graduate student.

Thomas:

Okay.

Bentley:

Yeah. I forgot — I havenít mentioned any of my own people. Yeah, so [Sion] Shabtaie was one. Richard Alley was my graduate student at that time and he did work on the theoretical side. He didnít actually do geophysical fieldwork. He did go down and collect an ice core on the ridge between Ice Stream B and Ice Stream C. In fact, that was the first thing he did when he joined our group, and unfortunately it melted on the way home, so he had to go down and collect another core the next season. But, then there was Don Blankenship. There was Sean Rooney.

Thomas:

Schulz?

Bentley:

Don Schulz was somebody who worked in the early stages of B. He worked with our first digital radar system. About the time we started at Up B, we also were developing a new digital recording system instead of a — before that our radar recording was just on film, or a dry recording type of film. So about in the early Ď80s we developed — it was about the same time we developed the digital seismic system, we also were developing a digital radar system.

Thomas:

Okay. He was part of your group, or he was somebody else?

Bentley:

Who Schulz?

Thomas:

Yes.

Bentley:

Yeah, he was, he was part of the group but he finished up. Well, he didnít finish up, actually. He left and took a job without getting his degree after that first season at Up B. Bentley, Blankenship, Rooney, and AlleyÖ Yeah. Those four: Bentley, Blankenship, Rooney, and Alley were the ones who were on a whole series of papers. Yeah, Rooney went off, heís now a big shot at one of the oil companies. Heís head of their whole South American operation. Iíve forgotten which oil company it is.

Thomas:

Sounds lucrative.

Bentley:

Yeah. And then there were people that I worked with, Jamie Robertson, Ken Jezek. They were on the Ross Ice Shelf Project.

Thomas:

Oh, OK. But he was a student?

Bentley:

These are all students now that Iím talking about. Yeah. But, letís see, who else am I — what other names have you run across? [Shuffling papers]

Thomas:

I donít know. I mean, I have your list of publications right here so maybe youíll Ö

Bentley:

Yeah, okay. Oh, and Sridhar Anandakrishnan. Yeah, heís an important one because heís still at it.

Thomas:

Iíve been told to talk to him.

Bentley:

And, heís the only one of my students who stayed in Antarctic glaciology and who does the same sort of thing that I used to do. So, heís a favorite. [Laughter] Yeah, you see the same names over again.

Thomas:

Yeah.

Bentley:

[Ralph] Von Frese is a geomagnetics guy at Ohio State that I published with. I never worked in the field with him. But, this is a different kind ofÖ I told you that I had a split interest between the glaciology and the solid earth geophysics, and this is part of the solid earth geophysics.

Thomas:

Oh, okay. Still in Ď86 then?

Bentley:

Bill Hinze is a magnetics guy who graduated from Wisconsin back before I even got here. Olivier is a French magnetics person.

Thomas:

Okay. I see.

Bentley:

Perepezko had published with Alley. He is a professor in materials science. Iíve forgotten what the exact name of the — in mechanical engineering. Bogorodsky was my colleague in the Soviet Union. I visited Leningrad several times, and we wrote a book together. Gudmandsen was a Dane that was a coauthor on that book. [Radioglaciology]

Thomas:

I noticed you had a couple of papers on Lake Vostok.

Bentley:

Yeah. I havenít been to Vostok. I think those papers werenít research papers. They were commentary on the significance of Lake Vostok. But, I think maybe there were two of them.

Thomas:

Okay.

Bentley:

When you said ďa couple,Ē is that meaning between one and three, or does that mean two?

Thomas:

I think it means two, but I wouldnít say that.

Bentley:

I may very well — I have the idea that I did do that twice.

Thomas:

They were Nature papers. So, they were probablyÖ

Bentley:

Yeah. I think I did a little commentary.

Thomas:

Commentaries, yes.

Bentley:

I have to get off this [page of the publications list]. This is all the same guys. We did a lot of publishing. Thereís Whillans. I did a lot with Sion Shabtaie. He was a graduate student for many, many years, and in the end he still hadnít gotten his degree when I retired. So he went off without it, and isnít even in geophysics let alone glaciology anymore, which is too bad. That was a terrible waste. He was a really good research guy. Don Albert here, Kirchner, these are guys — Don Albert is now at CRREL. He did his work here with me, although his degree actually came from one of the University of Texas campuses. But he was physically here in Madison, and he was working on Antarctic work, but we agreed that he could use it to finish his degree program in Texas.

Thomas:

Yeah. Iíve known people to escape their original degree program, but still get their degree from the place.

Bentley:

Yeah. Jamie Robertson has been very successful in ARCO.

Thomas:

What is that?

Bentley:

With Atlantic Richfield Oil Company. He worked, for the first couple of seasons, on the RIGGS project, but then he went to earn an honest living, and heís done very well at it. Rory Retzlaff got a masterís degree. He was the first one to do the radar sounding measurement of buried crevasses across the margin of Ice Stream C, and then he also was the one who did the field work and write-up of the airborne radar sounding of Ice Streams B and C. But he only got a masterís degree, because he had to get out and earn some money. I heard from him not too long ago, that heís interested in getting back and working on a Ph.D. again. But, by the time he felt he was in a position to do that, I didnít have a program anymore. Ted Clark was here for several seasons. He was an excellent field man. He did primarily seismic work, and as part of the Siple Coast Project at Ice Stream C. He did a big refraction profile in the interior or West Antarctica, deep seismic refraction profile, a couple hundred kilometers, 250 kilometers long. And then he did seismic work on that little island between the two parts of Ice Stream B. So, he was part of that same group. Chen Liu was a little bit later, but he also worked atĖ oh, that radar depolarization experiment, thatís what I was explaining about, measuring ice crystal orientations. He did that. Drew Novick was only here for a short time. He did some work, though, particularly at Downstream B with the radar. Shashank Atre was another one who got a masters with me and then moved to, more standard geophysics — I mean non-glaciological geophysics. But, he was studying the characteristics of the bed from the phase of the seismic reflections. The phase of the reflection, whether itís reversed in phase or not, depends on the acoustic impedance contrast. Acoustic impedance is a product of the density and the seismic wave speed. And, if itís a wet bed, a soft bed, the phase will be one way, and if thereís a rigid bed, itíll be the other way. And, this is the time we were trying to map where thereís a really soft bed. So, weíre using the phase of the seismic reflection to determine whether the bed was soft or hard.

Thomas:

At what point are we at here now? We must be in the Ď90s?

Bentley:

This is still the Siple Coast Project.

Thomas:

Oh, okay.

Bentley:

We did this both on Ice Stream B and Ice Stream C. But, I mentioned it because it wasÖ

Thomas:

Because you saw the name?

Bentley:

Drew Novick. No, no. Shashank Atre. Then, [Donghui] Yi and [Mark] Stenoien were part of my satellite radar altimetry research, and they were not related to the West Antarctic Ice Sheet field program. Oh, Neal Lord, youíll see Nealís not a glaciologist. Heís actually an electronics technician whoís still here in the department. Heís a very valuable person, because he kept all of the equipment going. And, nobody was more important than that. I mean without a good electronics technician you just simply donít get anything done. My ability to do electronics terminated when they went from vacuum tubes to solid-state devices.

Thomas:

Right. [Laugh] How persistent are maintenance issues in the field?

Bentley:

Oh, theyíre always there. I mean, you can always expect to have them. In the course of a field season youíre going to have maintenance issues, but not day in and day out. Most of the time the equipment worked pretty well. And Neal was always there, and he could make sure that the equipment kept going, and he could catch things that werenít quite right before they actually caused any problems. And he did a lot of the actual fieldwork. I mean, when the equipment was working okay, then he was doing the work. And he came to know a lot about the glaciology, actually, just from working with the glaciological graduate students for so many years. But he doesnít do glaciology anymore because he is one of the electronics engineers for the department. And the department doesnít do glaciology anymore, so Neal doesnít do glaciology anymore. So, letís see. I think, I have a feeling that there are a few others maybe back — I mentioned Ken Jezek. Heís a professor at Ohio State now, and he was one of our primary people on the Ross Ice Shelf Project. Sion Shabtaie worked on the Ross Ice Shelf, on RIGGS as well as on the WAIS project. John Clough was one of the primary guys on RIGGS. I think Iím back in RIGGS.

Thomas:

I think so. Yeah.

Bentley:

Maybe earlier.

Thomas:

Now, youíre even before it.

Bentley:

Yeah. Okay. Anybody else you want to ask me about?

Thomas:

No. Thatís actually a pretty good biographical overview. Itís nice to know, you know, who these people are and itís useful to know where people go, too, if they havenít stuck around with the glaciological thing.

Bentley:

Yeah. The ones who stayed in glaciology, letís see, Richard Alley, Sridhar Anandakrishnan, Ken Jezek. Oh, Iím sure there are more than that. My mind is drawing a blank. Iím trying to think where — Don Albert. Oh, well, Iíd have to see aÖ

Thomas:

All right.

Bentley:

I donít know why Iím — I know there are others.

Thomas:

Well, we donít need a comprehensive list.

Bentley:

I know there are others.

Thomas:

Mhmm. So, after the Ice Stream C measurements then, I mean, what season was the Ice Stream C work that you were talking about before we got on the biographical information?

Bentley:

It was about 1990.

Thomas:

Oh, okay. Oh, so this is actually over a period of about seven or eight years that you went fromÖ

Bentley:

The whole thing was over a period of more like fifteen years.

Thomas:

Oh, okay.

Bentley:

I mean, the time that I spent from the early Ď80s to the middle Ď90s, all that work was on (Thomas: On those two ice streams?) on those two ice streams. We never got farther north except in that last airborne program, which was the one that was done by a survey group, and we werenít allowed to have any say in where the flights went. I was complaining about it yesterday. That was on Ice Stream D (or MacAyeal Ice Stream). No, wait a minute. D is Bindschadler Ice Stream. I have trouble. I remember the old names better than the — yeah. D is Bindschadler Ice Stream. E, old E, is MacAyeal Ice Stream. Yeah, D is Bindschadler Ice Stream, and thatís appropriate because he did a lot of work on that ice stream. At any rate, so that airborne survey over the trunk of Bindschadler Ice Stream was the only time we got north of Ice Stream C. The rest of those years were all spent working in the Ice Stream B and Ice Stream C areas. Plus, the beginning, just on Crary Ice Rise, which you can think of really as a downstream extension of Ice Stream B anyway.

Thomas:

Did you have anything to do with Bindschadlerís workshops? I know, we were talking about SeaRISE and how you didnít like the presupposition of ďsea riseĒ.

Bentley:

I missed the SeaRISE. Iíve forgotten why. I missed it. Yeah, those workshops — now, there was a period there when I didnít go to — I think I missed a couple of SeaRISE workshops. Iíve forgotten now just why that was.

Thomas:

Was that 1990 and 1991, I think, before they changed the name?

Bentley:

No, I donít remember. But, I did miss it. I mean, I used to go to the WAIS workshops, and I started going to them later, but for a few years there I didnít go, and I donít remember why.

Thomas:

Okay. I donít think itís so important that we nail down the reason.

Bentley:

No. I donít remember. Itís not going to come to me. Itís not just right there waiting to seep through.

Thomas:

Itís just nowhere.

Bentley:

Itís deeper in my memory, or maybe it totally has fled from my memory.

Thomas:

But, you went later on in the Ď90s?

Bentley:

Yeah. Well, I didnít go to the WAIS workshop — Oh, you mean the WAIS — maybe weíre not — the SeaRISE workshops were one thing and then I guess they developed into the WAIS workshops?

Thomas:

Yeah. Yeah. I think thatís how it worked.

Bentley:

Yeah. Thatís right. And, of course, when I stopped doing any fieldwork then I stopped going to the WAIS workshops because I wasnít doing that anymore.

Thomas:

Okay.

Bentley:

All my work then was in satellite radar and laser altimetry. So then I stopped going to the WAIS workshops, and I started going again only in recent years when they went back to West Antarctica to do ice core drilling, which is my current business. I donít study ice cores, but Iím now PI on an engineering group called Ice Coring and Drilling Services, and have been since the year 2000. And we provide ice cores on NSF contract for anybody that wants to use our services.

Thomas:

Oh. Tell me a little bit about that.

Bentley:

Well, itís an outgrowth of the Polar Ice Coring Office that was an outgrowth of the Ross Ice Shelf Project. It all started with the preparations to drill that hole through the Ross Ice Shelf. And then that led to the Polar Ice Coring Office, which for ten years or so was at University of Nebraska. And they provided drills and drillers for projects for PIs that wanted ice cores, but didnít have their own drills, which are most people. It moved then, for just one five-year stretch, to the University of Alaska. It was at Alaska when they supported the drilling to the bed in Greenland at the Summit, GISP II Project. This is all paleo climate stuff — most of the most important information about paleo climate back for the last 800,000 years comes from ice cores. And one of the most important ice cores for that was the GISP II core drilled at Summit, Greenland, ďSummitĒ meaning the highest point on the ice sheet. And there were two drilling projects there. One, the U.S., was GISP II, and one by the Europeans called GRIP. And, everything you read about rapid climate change — the evidence for the climate can change over the course of just a few decades — that all comes from ice core information. And past changes in temperatures and carbon dioxide levels in the atmosphere, that all comes from ice cores, particularly at Vostok, [Ö] and Dome C, and the Japanese Dome Fuji now. All of that is deep ice core stuff. Well, thereís a lot of work done in shallow ice cores. A lot of thatís paleo climate too, but looking at a much shorter time record. At any rate somebody has to collect these cores, and geochemists normally donít have the equipment to do it, and arenít particularly interested in maintaining that equipment. So, NSF — oh, well then it went back to Nebraska for one contract period, and then the year 2000 we bid on bringing it here because that was the time that the deep drilling, at the AMANDA Project and the IceCube Project, were getting started at South Pole. Thatís another — do you know about those?

Thomas:

I donít think so. No.

Bentley:

Oh, well, thatís another big field. Thatís neutrino detectors. The ice is very transparent, particularly down below 2,000 meters or so, because all the air bubbles go into solution. And, itís more transparent than the clearest ocean water anywhere.

Thomas:

I didnít know that.

Bentley:

Now, thereís a project to bury neutrino detectors, which are really muon detectors. Well theyíre really light detectors. Neutrinos mostly pass through the earth without interacting with anything. But occasionally, just by random chance, one will run into a particle in the material that itís going through, and then it produces a muon, and the muon produces Cerenkov radiation, and the radiation then is detected by these detectors that are buried in the ice at the South Pole. And that project started in the Ď90s. The PIs were here at the University of Wisconsin and then the PICO organization was doing the — the holes were drilled by hot water, not taking a core, just so they could emplace these detectors quick before it froze up again. But, in the course of that drilling program, a lot of the work was transferred here to Wisconsin at a physical sciences laboratory, which is an outgrowth of the physics department here. And so the guys who were doing this decided, as long as they were doing that part of the drilling, they might as well bid on the whole drilling contract. And so, after one unsuccessful try, we were successful in some sense. And then they roped me in because they wanted a glaciologist, because I had retired and didnít have any post-retirement job yet. So, since the year 2000, Iíve been the PI with a full-time engineering staff that does all the work doing the contract ice core drilling.

Thomas:

Okay. So, what do you do?

Bentley:

We collect ice cores. We have shallow drills that collect cores down to about a maximum [of] 300 meters. And then we just started our first season of deep ice core drilling in central West Antarctica. And itís the connection with that deep ice core drilling in central West Antarctica that got me back into going to the WAIS meetings. Because now Iím part of WAIS again through this drilling program, which is part of WAISCORES, which is an offshoot of the WAIS program. And thatís the connection back to Bob Bindschadler, because itís one of the things that they talked about was the coring program, WAISCORES, in developing the WAIS programs. We also have shot-hole drills, so now I support Sridhar providing him with a rapid series of shot holes that are much better than anything we had when he was working with me. And then we do some hot water access hole drilling. But at any rate, so now Iím an ice-drilling engineer. Except not really.

Thomas:

Does that bring you down there, or is it just managing?

Bentley:

Yeah. Thatís why I went down last January.

Thomas:

Oh, okay. I didnít know you were down there.

Bentley:

Yeah, I was down last January to go out to the central West Antarctic site, because it was the first season of our drill in operation, and I wanted to see it in operation. We did have a chance to test it in Greenland two years ago now. Thatís the summer of 2006, and I was scheduled to go up there and see it in operation for the test, but then all the C-130 Hercules were all grounded because they had some kind of insulation that was breaking loose and clogging their fuel filters, which is [Laugh] not (Thomas: You donít want that) — you donít want to fly under those conditions. So, they had to ground the whole fleet and go through and fix this problem. And, that meant that they had only very limited flight resources to get to the same Summit site where the drill was being tested. So, anybody who wasnít totally essential didnít get to go, and I wasnít totally essential. Anyway, so I got into all that, thatís what I do now, and itís taken me back into West Antarctica. And, actually, one of the reasons I enjoy going back to this site in central West Antarctica was that itís not very far from where I ran a traverse over fifty years ago. Well, actually, I was back on site, in a sense, at my fiftieth anniversary of my first IGY traverse out at Byrd Station. So, all of that came from Ė you asked me if I went to the WAIS workshops?

Thomas:

Yes.

Bentley:

So, now I go again. Because now I can report on — although Iím not doing any of the science, I still have an interest in the science aspects, and other people are interested in the deep coring project.

Thomas:

Bindschadler has told me that he thinks that the workshops have really lent real focus to the research thatís going on. I guess, a bit more collaboration, especially with oceanographers and people who were outside the ordinary community. I was just wondering if you had a perspective on the workshops?

Bentley:

Oh, itís absolutely true. And itís a very important part of the glaciological calendar. I mean, itís an indication of the health of glaciological science. These WAIS workshops are far bigger than any glaciological meeting. Theyíre bigger than the meetings back — I was talking about the early days of the Committee on Glaciology when there werenít very many glaciologists around? Well, WAIS workshop is bigger than, in those days, a meeting of all the glaciologists in the United States. These are just the ones who are interested in the WAIS Project. But yeah, another important interaction, I think, is with the Europeans, particularly the British, who have what was called the Filchner-Ronne Ice Shelf Project, originally FRISP. FRISP, it started not long after the beginning of RISP, they started doing a similar — well it was some years after the RIGGS and RISP program. But at any rate, they started doing a similar work on, on the Filchner-Ronne Ice Shelf. So, they had a Filchner-Ronne Ice Shelf Project, although they never did an aerial survey like we did with the RIGGS Program. But, theyíve done a lot of drilling through the ice shelf and a lot of other measurements. At any rate, so FRISP now has a broader remit, and FRISP no longer stands for Filchner-Ronne Ice Shelf. Theyíve made up a name. Letís see, Forum on Research inÖ

Thomas:

[Laugh] They kept the acronym!

Bentley:

Forum on Research — itís sort of an odd name, and thatís because they had to pick the words to fit the preexisting acronym. Forum on Research in Ice Shelf — Ice Sheet Processes. I donít think itís even ďIce ShelfĒ anymore. Forum on Research in Ice Sheet Processes.

Thomas:

Sounds about right.

Bentley:

Thatís close anyway. [Ed.: Forum for Research into Ice Shelf Processes, founded in 1984] And so they have joint meetings now, and actually Iíll be going to one in September, a joint WAIS — Oh and then the FRISP, although itís sort of an analog of RISP, thatís not what the U.S. is doing anymore. But, of course, anybody interested in West Antarctic Ice Sheet behavior is very much interested in ice shelves. And, so thatís sort of the converse, or the inverse, of the FRISP Project, the Filchner-Ronne Ice Shelf Project getting interested more broadly in Antarctic glaciology. So, at any rate, thereíll be a meeting in September that Iím going to go to in England. The one last fall was a joint meeting in Virginia. And glaciologists, particularly from the British Antarctic Survey, have been attending the WAIS meetings for some time. So, thatís another fruitful aspect is the international aspect of WAIS. But there are really two primary venues for glaciological presentations and discussions. One is FRISP and one is the annual fall American Geophysical Union meeting. I mean, not FRISP, the WAIS workshop. Iím sorry. And, the other is the annual fall American Geophysical Union meeting in San Francisco, where the glaciological activity is so great that they canít even schedule them all in the course of a week without running parallel sessions. And the same thing has happened with the International Glaciological Society. I mean glaciology has just grown by leaps and bounds.

Thomas:

About, ball park, how many people are involved now?

Bentley:

In glaciology in the United States?

Thomas:

Sure.

Bentley:

A couple hundred.

Thomas:

Okay.

Bentley:

I mean that includes students. And, you know, itís gotten high visibility. It used to be really an obscure science. And the renaissance — well no, itís not really a renaissance because the growth of glaciology really started with John Nye around 1950, and John Glen, and a few others who started treating glaciology as a branch of physics rather than a branch of geography.

Thomas:

Right. Iíve asked John Nye to send me along some materials on how he originally started out on that, and he did. So, that was useful to get kind of that origin point.

Bentley:

Yeah he was studying material deformation. Well, no, he actually worked with [Ö] oh I canít remember now. Somebody from Rutherfordís lab, I think, it seems to me.

Thomas:

Yeah. Yeah. He started out Cavendish, and I forget, there were a couple of people. I know Perutz communicated all of his papers to the Royal Society, but — ah I forget the guyís name.

Bentley:

Max Perutz?

Thomas:

Yeah. But, well, he was involved, but there was somebody else who got him involved in ice. But, I have the information, and itís in my computer.

Bentley:

Orowan, maybe? [Ed. Yes, Egon Orowan] Well, at any rate, then, my comment about the speed of light was from a paper, a footnote actually, of one of John Nyeís papers, a commentary on — oh, what the hell? Extrusion flow. Before proper physics was applied to glacier motion, there was the idea, particularly championed by an American glaciologist named Max Demorest, that the ice flowed by extrusion, under the high pressure, and that the lower part of the glacier was actually moving faster than the upper part of the glacier. And John Nye showed that that was a system of unbalanced forces. There was no restraining force for the upper part of the ice. So if there really were extrusion flow, the upper part of the ice would have to be riding along on the top of this, and since there was no restraining force it would have acceleration without limit, and, eventually, by the time it reached the coast of Greenland, it would be traveling at the speed of light. He was making sport, showing how ridiculous the extrusion flow was from the standpoint of just basic physics, Newtonís first law. But anyway, so that was really the start of the modern — Modern glaciology, you can say, is fifty-eight years old, because it starts with John Nyeís papers in 1950.

Thomas:

Iíve dug those out. And yeah, I kind of felt that when I did find those that, ďOh, here we go. I found the beginning.Ē Because, there wasnít really much that he was going on except for applying the principles from somewhere else to ice.

Bentley:

Well, my first trip to Europe, when I was still in college in 1948, we took one of the standard tourist trips up the Jungfrau, to the Jungfraujoch, where thereís an observation platform looking down on a glacier. Iíve forgotten what the name of the glacier was. And, when I was up there we looked down and we saw these guys working around a pipe and didnít know really — it was curious, it was interesting to see people actually seeming to be doing some kind of work down on the glacier down below. But, it wasnít until many years later that I was reading Max Perutzís work. He drilled a hole in the glacier and measured the deformation of a rod down the hole, and he showed that it was not extrusion flow, and in fact the top moves the fastest, and just through the shear process you get a profile of velocity that decreases with depth. It may go to zero, if thereís no sliding at the bed. And, what I realized — and this is the classic experiment. It was the first one to show this deformation profile in the ice. And, what I realized was that I was actually watching Perutzís group and I knew nothing about glaciology. I had no interest in glaciology at the time. I didnít even have any interest in geology. I was in physics. But I realized many years later that I had actually watched this famous experiment in process just by pure chance. So, maybe I should say itís sixty years old. Well, Iím not sure what the date of Perutzís publication was. I think maybe it was Ď49. So, at any rate, yeah, the British physicists were the ones who turned glaciology into a good solid physical science. But even, for years, there werenít very many glaciologists. I used to tell people that one of the reasons I liked to be in glaciology was because, in order to have a professional meeting, it had to be an international meeting, because there werenít enough glaciologists in any one country to have a meeting. So every time you went to a glaciological meeting you got to go to some interesting place abroad instead of having to go to Detroit, or Cleveland, or someplace.

Thomas:

Whatís kind of the inflection point where it really kind of started to grow?

Bentley:

IGY.

Thomas:

Okay, IGY?

Bentley:

Yeah. I think so. Yeah, because there were glaciology programs in a dozen different countries. And I think the visibility of the IGY Program in the Antarctic was the beginning of recognition that glaciers and ice sheets really are an important part of the world system. And that has just grown as people have learned more and more about how ice sheets and glaciers have changed through time, and how much they interact with the environment, with the climate, with the ocean levels. But, yeah, I think you can say IGY was a turning point as far as increasing the activity and the visibility and popularity of glaciology.

Thomas:

Sort of on a similar theme, but a different topic, going back to the WAIS issue: of course, youíve written a number explanatory articles for, well, I donít know, I suppose Science or Nature. Well, also the one you wrote for Scientific American. At what point did you see that this issue had kind of entered a place where its profile had become more public than its original kind of fringe status?

Bentley:

Really with the beginning of the discussions about stability of the ice sheet. I mean, when the question arose with Terry Hughes and — well not so much with John Mercer — nobody followed up on that in his early papers, until Terry followed up in his way. But that then started the debate on whether there really was an instability that could be triggered by a climate change.

Thomas:

But, I mean you hadnít even really heard about it until you had those papers with Thomas in Ď78. And so, Iím kind of wondering — I mean, then you have the workshops in the 1980s, and so it enters in with the whole climate change, anthropogenic climate change discussion. And so, Iím wondering when — I mean, in my early memories, you know, climate change always had to do with changes in sea level and that sort of thing. So, there was a point where it became also sort of a popular notion as well. And so, Iím just wondering if you might have a sense of when these changes took place.

Bentley:

Well, what Iím saying is I think the changeÖ

Thomas:

The hot-button issue, I guess I should say.

Bentley:

I think that what initiated the ice sheet stability as a hot-button issue was the same thing that initiated it for glaciologists themselves. Because there were some very dramatic predictions and warnings that came out at that time. I mean, people were talking about West Antarctic disappearance and sea level rise of five meters in fifty years. The time scales were that short. In fact, it seems to me that there was a fifty-year version of Thomas, Sanderson, and Rose, one of their models. I may be wrong about that. And I remember there was a well-known glacial geologist in England who claimed that the ice sheet could disappear so fast that the sea level would rise so fast in the Thames that small animals would not be able to run fast enough to get away from sea level rise, and only the large animals could escape. I donít know whether he was basing this on some fossil evidence or paleontological evidence or what. Well, I mean, thatís absurd.

Thomas:

Itís only a short step from there to Kevin Costnerís Waterworld, I suppose?

Bentley:

Yeah. Yeah. But, the very early attempts to put a time scale on it came up with really extremely rapid time scales. And so, I think that it started to catch public attention right away at the same time it was catching the attention of the glaciologists, because these first predictions were so dramatic in their consequence.

Thomas:

I see. Okay. I didnít realize that it happened so quickly?

Bentley:

Yeah, so itís all around 1980, the late Ď70s.

Thomas:

Mhmm. So, I mean, itís interesting then that when, you know, I speak to Michael Oppenheimer, or anybody else whoís been involved in the IPCC process, that they view it as sort of a fringe topic even around 1990. So, thereís sort of a public aspect to it, then, but then in the appraisal process itís still viewed as sort of fringe. Is that a correct perception of it, or is itÖ?

Bentley:

Well, I think, as we talked about this a little yesterday, I think it was not so much that there was a fringe as they didnít know how to handle it. I mean there was no valid prediction. There wasnít any way that they could — Iím talking about the IPCC process — there wasnít any way that they could factor it into their quantitative predictions.

Thomas:

I think thatís a good distinction to make, you know. Something being ďfringeĒ versus something being unhandleable. Because thatís something that — I donít know, itís just good to be precise in our use of language here so that I know exactly what the situation was around that time. Whether it was seen as something that was truly fringe or as just something that couldnít be handled. So, thatís useful.

Bentley:

Yeah, I think there was general recognition fairly early that ice sheets could change rapidly enough to cause significant sea level change on short time scales, speaking from a geological perspective. Where the short geological time scale is quite different from a short human. Well, I mean, it turns out that in some cases thatís not true. I mean, these short time scales are on a human — I mean, this rapid climate change in Greenland that I mentioned that showed up in the ice cores shows that climate can change on human time scales. And melt-water pulses — that the most rapid rises of sea level at the end of the last ice age would have happened within the human lifetime. I mean, significant sea level rise within human lifetimes. But there was a lot of skepticism about how fast it could happen, but I think it was recognized as something that really could and did happen. Itís all a matter of how fast.

Thomas:

Right. So, the question is which changes take place on which time scales and trying to sort that out? (Bentley: Yeah.) Right. Well, is there anything that weíve skipped over that, you know, in your professional activities that we ought to have covered? OrÖ

Bentley:

Craig Lingle is another one. We havenít mentioned him. He was a glacier modeler. He got his Ph.D. here. He never went to the field. He tried, but he couldnít pass the physical. And heís been at the University of Alaska. He stayed in glaciology. Heís been doing work at the University of Alaska ever since and heís just retiring now. Makes me feel funny when my students start to retire.

Thomas:

Have you dealt much with modeling? We havenít really discussed it too much.

Bentley:

No. Only when Craig was here.

Thomas:

Okay.

Bentley:

Well, I had a postdoc who did some modeling, but it wasnít successful. And, his tenure here was — I mean he was fine, but he and his family didnít like the school system here. He had a couple small children. He was from France and they decided to go back to France. And we tried for a while to run the modeling program with him there, but it didnít work. So, Craig is another one who has stayed in glaciology. [Ö] Hugh Bennett stayed in more or less for a while. Heís somebody I havenít mentioned. Thatís going all the way back to IGY. He was one of the people who was on the Ross Ice Shelf traverse in the first IGY year. He finished up his Ph.D. here. In fact, he was one of my very first students, and he went to Michigan State, and he did do some more work in glaciology, but then he drifted out. (Iím just worried that Iím slighting somebody because of myÖ) Oh, Mario Giovinetto has stayed in glaciology. He was one of the few who wintered over both IGY years. He was at Byrd Station the first winter and then he went to South Pole for a winter, and he works with Jay Zwally as a contractor for NASA. He was many years at the University of Calgary, in the geography department doing glaciology, and then his post-retirement job is with NASA Goddard. Letís see. Oh, Blankenship. Blankenship has stayed

Thomas:

You discussed him a little bit.

Bentley:

Yeah, but I didnít mention that heís one of the ones who stayed in glaciology. Thatís right. Yeah. I knew I was forgetting somebody, at least one person. Now I hope Iím not forgetting anymore.

Thomas:

Well, we can always come back later and paste them in if necessary.

Bentley:

Well, I mentioned Ben Smith. Heís still in glaciology. Heís still working at the University of Washington. Then Donghui Yi, he works at NASA Goddard also. He stayed in glaciology, in the satellite altimetry business. He works with the GLAS [Geoscience Laser Altimeter System]/ICEsat team.

Thomas:

Was there anything additionally that we wanted to talk about as far as laser altimetry is concerned? You mentioned your dealings with Bob Thomas on that a little bit, but I donít know if there was anything you really wanted to go into?

Bentley:

I didnít do any research myself in laser altimetry. The one who did was Ben Smith. [Ö] Icesat was a long time in coming, and I was on the science team for a decade or more, it seems. Iíve forgotten how long it was. And in the early years we used to joke that every time that the launch date for Icesat was always the same number of years ahead of the meeting that we were at at the time. We used to meet about three times a year, and it seems like the launch date was always getting set back. Maybe not in four months intervals, but at least every year it was delayed by another year. So, we worked on and on in the team activities, but we never got any closer to launch. But, of course, it did finally get launched, because itís been in orbit for several years now. I got interested in satellite altimetry because itís such a powerful way of studying the ice sheet elevations. I mentioned a little bit about my complaints, but on the other hand it certainly gave a lot of useful information. But the early radar altimeter satellites were not designed for the Antarctic. So they didnít go very far south. The U.S. ones only covered the margins of East Antarctica. And then the European satellites came along, ERS-1 and ERS-2, and they did substantially better, but they went only to 82.5 South, so they still only covered less than half of West Antarctica. They did better for East Antarctica, which goes a lot farther north. But, because it was such an exciting way to study the ice sheet, I proposed and was accepted as a member of the laser altimeter team. Because the laser has much higher accuracy both in height measurement and in precision of the point on the surface. Because, the laser focuses on a spot, whereas the radar just sends out a broad spherical wave front. And so you donít know within kilometers often where the reflection point on the surface —You get a return from the surface, but you donít know where it came from, particularly on the sloping parts of the ice sheet. And the laser, you know exactly where it comes from because they have precise pointing and a spot thatís very small. Actually, it was enlarged to 75 meters — thatís the spot size on the surface of the laser altimeter — in order to have a good statistical sampling of the sastrugi, the small-scale roughness of the surface, wind carved roughness of the surface. It could have been focused down even more than that. But those were interesting times. I mean I enjoyed the work and I never regretted the fact that it was so delayed that by the time the data were coming in I was retired. Well, thatís not quite true because I guess we started getting — well no, I think that is true. I donít think we started getting — I think Ben didnít start getting any data to work with until he was already at the University of Washington. Because I wanted him to continue on this laser altimeter work. Because it was the only way by proxy that I was going to get any result out of all these years of work on the development team. So, we worked out a deal with Washington. He would be a student there, but I would be supporting him on the GLAS contract. So, I supported him there. He worked on the laser altimeter data over West Antarctica, but he was being paid by the University of Wisconsin. Well, he was being paid by my contract with NASA. I think we found a way to pay University of Washington directly. But at any rate, it was my project that was supporting him. So, he got some good work out of that, and has continued to do good work, well, partly with laser altimeter and partly with other glaciology. So, that was a happy ending to my glaciological career, and a happy ending to my time with the laser altimeter.

Thomas:

All right. I mean, thatís a good note if we donít have anything else?

Bentley:

A good note to end on?

Thomas:

I think so. Well, yesterday we ended on a bit more of a dour note, as I recall. You were concerned about things not being done in your lifetime. So, the happy end to the Ö

Bentley:

Oh yeah, well. How are we ever going to be able to have a valid prediction?

Thomas:

Yeah. So, this seems good to me. Yeah, any final thoughts or anything besides that?

Bentley:

No.

Thomas:

All right, then. Thank you very much. Itís been a fantastic interview, I think, very detailed, very informative for me certainly.

Bentley:

Good. I hope so.

Thomas:

I hope you enjoyed it.

Bentley:

Yeah. Itís nice of you to say itís detailed, because to me it seems to me I forget all the details.

Thomas:

Well, you know.

Bentley:

Different perspective.

Thomas:

I guess so. I mean, compared to what we sometimes get it was very detailed. So, thank you very much.

Session I | Session II