How we look kilometers below the Antarctic ice sheet | Dustin Schroeder

33,537 views ・ 2018-03-22

TED


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I'm a radio glaciologist.
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That means that I use radar to study glaciers and ice sheets.
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And like most glaciologists right now,
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I'm working on the problem of estimating
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how much the ice is going to contribute to sea level rise in the future.
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So today, I want to talk to you about
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why it's so hard to put good numbers on sea level rise,
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and why I believe that by changing the way we think about radar technology
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and earth-science education,
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we can get much better at it.
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When most scientists talk about sea level rise,
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they show a plot like this.
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This is produced using ice sheet and climate models.
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On the right, you can see the range of sea level
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predicted by these models over the next 100 years.
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For context, this is current sea level,
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and this is the sea level
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above which more than 4 million people could be vulnerable to displacement.
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So in terms of planning,
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the uncertainty in this plot is already large.
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However, beyond that, this plot comes with the asterisk and the caveat,
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"... unless the West Antarctic Ice Sheet collapses."
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And in that case, we would be talking about dramatically higher numbers.
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They'd literally be off the chart.
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And the reason we should take that possibility seriously
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is that we know from the geologic history of the Earth
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that there were periods in its history
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when sea level rose much more quickly than today.
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And right now, we cannot rule out
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the possibility of that happening in the future.
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So why can't we say with confidence
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whether or not a significant portion of a continent-scale ice sheet
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will or will not collapse?
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Well, in order to do that, we need models
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that we know include all of the processes, conditions and physics
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that would be involved in a collapse like that.
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And that's hard to know,
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because those processes and conditions are taking place
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beneath kilometers of ice,
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and satellites, like the one that produced this image,
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are blind to observe them.
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In fact, we have much more comprehensive observations of the surface of Mars
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than we do of what's beneath the Antarctic ice sheet.
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And this is even more challenging in that we need these observations
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at a gigantic scale in both space and time.
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In terms of space, this is a continent.
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And in the same way that in North America,
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the Rocky Mountains, Everglades and Great Lakes regions are very distinct,
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so are the subsurface regions of Antarctica.
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And in terms of time, we now know
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that ice sheets not only evolve over the timescale of millennia and centuries,
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but they're also changing over the scale of years and days.
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So what we want is observations beneath kilometers of ice
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at the scale of a continent,
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and we want them all the time.
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So how do we do this?
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Well, we're not totally blind to the subsurface.
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I said in the beginning that I was a radio glaciologist,
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and the reason that that's a thing
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is that airborne ice-penetrating radar is the main tool we have
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to see inside of ice sheets.
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So most of the data used by my group is collected by airplanes
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like this World War II-era DC-3,
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that actually fought in the Battle of the Bulge.
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You can see the antennas underneath the wing.
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These are used to transmit radar signals down into the ice.
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And the echos that come back contain information
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about what's happening inside and beneath the ice sheet.
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While this is happening,
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scientists and engineers are on the airplane
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for eight hours at a stretch,
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making sure that the radar's working.
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And I think this is actually a misconception
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about this type of fieldwork,
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where people imagine scientists peering out the window,
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contemplating the landscape, its geologic context
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and the fate of the ice sheets.
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We actually had a guy from the BBC's "Frozen Planet" on one of these flights.
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And he spent, like, hours videotaping us turn knobs.
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(Laughter)
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And I was actually watching the series years later with my wife,
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and a scene like this came up, and I commented on how beautiful it was.
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And she said, "Weren't you on that flight?"
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(Laughter)
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I said, "Yeah, but I was looking at a computer screen."
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(Laughter)
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So when you think about this type of fieldwork,
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don't think about images like this.
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Think about images like this.
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(Laughter)
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This is a radargram, which is a vertical profile through the ice sheet,
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kind of like a slice of cake.
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The bright layer on the top is the surface of the ice sheet,
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the bright layer on the bottom is the bedrock of the continent itself,
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and the layers in between are kind of like tree rings,
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in that they contain information about the history of the ice sheet.
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And it's amazing that this works this well.
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The ground-penetrating radars that are used
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to investigate infrastructures of roads or detect land mines
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struggle to get through a few meters of earth.
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And here we're peering through three kilometers of ice.
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And there are sophisticated, interesting, electromagnetic reasons for that,
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but let's say for now that ice is basically the perfect target for radar,
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and radar is basically the perfect tool to study ice sheets.
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These are the flight lines
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of most of the modern airborne radar-sounding profiles
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collected over Antarctica.
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This is the result of heroic efforts over decades
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by teams from a variety of countries and international collaborations.
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And when you put those together, you get an image like this,
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which is what the continent of Antarctica would look like
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without all the ice on top.
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And you can really see the diversity of the continent in an image like this.
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The red features are volcanoes or mountains;
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the areas that are blue would be open ocean
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if the ice sheet was removed.
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This is that giant spatial scale.
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However, all of this that took decades to produce
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is just one snapshot of the subsurface.
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It does not give us any indication of how the ice sheet is changing in time.
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Now, we're working on that, because it turns out
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that the very first radar observations of Antarctica were collected
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using 35 millimeter optical film.
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And there were thousands of reels of this film
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in the archives of the museum of the Scott Polar Research Institute
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at the University of Cambridge.
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So last summer, I took a state-of-the-art film scanner
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that was developed for digitizing Hollywood films and remastering them,
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and two art historians,
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and we went over to England, put on some gloves
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and archived and digitized all of that film.
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So that produced two million high-resolution images
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that my group is now working on analyzing and processing
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for comparing with contemporary conditions in the ice sheet.
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And, actually, that scanner -- I found out about it
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from an archivist at the Academy of Motion Picture Arts and Sciences.
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So I'd like to thank the Academy --
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(Laughter)
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for making this possible.
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(Laughter)
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And as amazing as it is
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that we can look at what was happening under the ice sheet 50 years ago,
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this is still just one more snapshot.
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It doesn't give us observations
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of the variation at the annual or seasonal scale,
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that we know matters.
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There's some progress here, too.
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There are these recent ground-based radar systems that stay in one spot.
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So you take these radars and put them on the ice sheet
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and you bury a cache of car batteries.
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And you leave them out there for months or years at a time,
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and they send a pulse down into the ice sheet
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every so many minutes or hours.
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So this gives you continuous observation in time --
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but at one spot.
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So if you compare that imaging to the 2-D pictures provided by the airplane,
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this is just one vertical line.
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And this is pretty much where we are as a field right now.
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We can choose between good spatial coverage
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with airborne radar sounding
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and good temporal coverage in one spot with ground-based sounding.
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But neither gives us what we really want:
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both at the same time.
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And if we're going to do that,
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we're going to need totally new ways of observing the ice sheet.
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And ideally, those should be extremely low-cost
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so that we can take lots of measurements from lots of sensors.
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Well, for existing radar systems,
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the biggest driver of cost is the power required
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to transmit the radar signal itself.
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So it’d be great if we were able to use existing radio systems
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or radio signals that are in the environment.
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And fortunately, the entire field of radio astronomy
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is built on the fact that there are bright radio signals in the sky.
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And a really bright one is our sun.
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So, actually, one of the most exciting things my group is doing right now
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is trying to use the radio emissions from the sun as a type of radar signal.
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This is one of our field tests at Big Sur.
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That PVC pipe ziggurat is an antenna stand some undergrads in my lab built.
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And the idea here is that we stay out at Big Sur,
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and we watch the sunset in radio frequencies,
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and we try and detect the reflection of the sun off the surface of the ocean.
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Now, I know you're thinking, "There are no glaciers at Big Sur."
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(Laughter)
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And that's true.
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(Laughter)
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But it turns out that detecting the reflection of the sun
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off the surface of the ocean
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and detecting the reflection off the bottom of an ice sheet
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are extremely geophysically similar.
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And if this works,
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we should be able to apply the same measurement principle in Antarctica.
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And this is not as far-fetched as it seems.
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The seismic industry has gone through a similar technique-development exercise,
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where they were able to move from detonating dynamite as a source,
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to using ambient seismic noise in the environment.
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And defense radars use TV signals and radio signals all the time,
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so they don't have to transmit a signal of radar
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and give away their position.
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So what I'm saying is, this might really work.
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And if it does, we're going to need extremely low-cost sensors
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so we can deploy networks of hundreds or thousands of these on an ice sheet
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to do imaging.
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And that's where the technological stars have really aligned to help us.
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Those earlier radar systems I talked about
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were developed by experienced engineers over the course of years
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at national facilities
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with expensive specialized equipment.
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But the recent developments in software-defined radio,
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rapid fabrication and the maker movement,
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make it so that it's possible for a team of teenagers
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working in my lab over the course of a handful of months
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to build a prototype radar.
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OK, they're not any teenagers, they’re Stanford undergrads,
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but the point holds --
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(Laughter)
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that these enabling technologies are letting us break down the barrier
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between engineers who build instruments and scientists that use them.
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And by teaching engineering students to think like earth scientists
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and earth-science students who can think like engineers,
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my lab is building an environment in which we can build custom radar sensors
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for each problem at hand,
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that are optimized for low cost and high performance
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for that problem.
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And that's going to totally change the way we observe ice sheets.
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Look, the sea level problem and the role of the cryosphere in sea level rise
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is extremely important
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and will affect the entire world.
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But that is not why I work on it.
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I work on it for the opportunity to teach and mentor
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extremely brilliant students,
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because I deeply believe that teams of hypertalented,
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hyperdriven, hyperpassionate young people
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can solve most of the challenges facing the world,
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and that providing the observations required to estimate sea level rise
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is just one of the many such problems they can and will solve.
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Thank you.
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(Applause)
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