What's hidden under the Greenland ice sheet? | Kristin Poinar

2,631,257 views ・ 2017-11-06

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When was I was 21 years old,
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I had all this physics homework.
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Physics homework requires taking breaks,
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and Wikipedia was relatively new, so I took a lot of breaks there.
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I kept going back to the same articles,
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reading them again and again,
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on glaciers, Antarctica and Greenland.
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How cool would it be to visit these places
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and what would it take to do so?
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Well, here we are
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on a repurposed Air Force cargo plane
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operated by NASA
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flying over the Greenland ice sheet.
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There's a lot to see here,
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but there's more that is hidden,
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waiting to be uncovered.
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What the Wikipedia articles didn't tell me
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is that there's liquid water hidden inside the ice sheet,
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because we didn't know that yet.
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I did learn on Wikipedia that the Greenland ice sheet is huge,
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the size of Mexico,
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and its ice from top to bottom is two miles thick.
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But it's not just static.
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The ice flows like a river downhill towards the ocean.
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As it flows around bends,
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it deforms and cracks.
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I get to study these amazing ice dynamics,
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which are located in one of the most remote physical environments
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remaining on earth.
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To work in glaciology right now is like getting in on the ground floor
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at Facebook in the 2000s.
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(Laughter)
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Our capability to fly airplanes and satellites over the ice sheets
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is revolutionizing glaciology.
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It's just starting to do for science
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what the smartphone has done for social media.
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The satellites are reporting a wealth of observations
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that are revealing new hidden facts about the ice sheets continuously.
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For instance, we have observations of the size of the Greenland ice sheet
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every month going back to 2002.
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You can look towards the bottom of the screen here
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to see the month and the year go forward.
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You can see that some areas of the ice sheet melt
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or lose ice in the summer.
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Other areas experience snowfall
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or gain ice back in the winter.
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This seasonal cycle, though, is eclipsed by an overall rate of mass loss
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that would have stunned a glaciologist 50 years ago.
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We never thought that an ice sheet could lose mass into the ocean this quickly.
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Since these measurements began in 2002,
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the ice sheet has lost so much ice
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that if that water were piled up on our smallest continent,
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it would drown Australia knee-deep.
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How is this possible?
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Well, under the ice lies the bedrock.
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We used radar to image the hills, valleys, mountains and depressions
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that the ice flows over.
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Hidden under the ice sheet are channels the size of the Grand Canyon
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that funnel ice and water off of Greenland and into the ocean.
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The reason that radar can reveal the bedrock
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is that ice is entirely transparent to radar.
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You can do an experiment.
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Go home and put an ice cube in the microwave.
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It won't melt,
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because microwaves, or radar,
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pass straight through the ice without interacting.
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If you want to melt your ice cube, you have to get it wet,
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because water heats up easily in the microwave.
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That's the whole principle the microwave oven is designed around.
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Radar can see water.
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And radar has revealed a vast pool of liquid water
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hidden under my colleague Olivia,
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seven stories beneath her feet.
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Here, she's used a pump
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to bring some of that water back to the ice sheet's surface.
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Just six years ago, we had no idea this glacier aquifer existed.
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The aquifer formed
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when snow melts in the summer sun
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and trickles downward.
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It puddles up in huge pools.
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From there, the snow acts as an igloo,
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insulating this water from the cold and the wind above.
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So the water can stay hidden in the ice sheet
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in liquid form year after year.
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The question is, what happens next?
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Does the water stay there forever?
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It could.
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Or does it find a way out to reach the global ocean?
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One possible way for the water to reach the bedrock
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and from there the ocean
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is a crevasse, or a crack in the ice.
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When cracks fill with water,
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the weight of the water forces them deeper and deeper.
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This is how fracking works
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to extract natural gas from deep within the earth.
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Pressurized fluids fracture rocks.
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All it takes is a crack to get started.
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Well, we recently discovered
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that there are cracks available in the Greenland ice sheet
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near this glacier aquifer.
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You can fly over most of the Greenland ice sheet
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and see nothing,
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no cracks, no features on the surface,
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but as this helicopter flies towards the coast,
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the path that water would take on its quest to flow downhill,
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one crack appears,
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then another and another.
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Are these cracks filled with liquid water?
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And if so, how deep do they take that water?
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Can they take it to the bedrock
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and the ocean?
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To answer these questions,
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we need something beyond remote sensing data.
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We need numeric models.
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I write numeric models that run on supercomputers.
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A numeric model is simply a set of equations
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that works together to describe something.
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It can be as simple as the next number in a sequence --
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one, three, five, seven --
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or it can be a more complex set of equations
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that predict the future
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based on known conditions in the present.
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In our case, what are the equations for how ice cracks?
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Well, engineers already have a very good understanding
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of how aluminum, steel and plastics fracture under stress.
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It's an important problem in our society.
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And it turns out that the engineering equations
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for how materials fracture
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are not that different from my physics homework.
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So I borrowed them, adapted them for ice,
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and then I had a numeric model for how a crevasse can fracture
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when filled with water from the aquifer.
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This is the power of math.
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It can help us understand real processes in our world.
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I'll show you now the results of my numeric model,
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but first I should point out
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that the crevasse is about a thousand times narrower than it is deep,
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so in the main panel here,
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we've zoomed in to better see the details.
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You can look to the smaller panel on the right
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to see the true scale for how tall and skinny the crevasse is.
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As the aquifer water flows into the crevasse,
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some of it refreezes in the negative 15 degree Celsius ice.
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That's about as cold as your kitchen freezer.
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But this loss can be overcome
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if the flow rate in from the glacier aquifer is high enough.
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In our case, it is,
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and the aquifer water drives the crevasse all the way to the base of the ice sheet
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a thousand meters below.
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From there, it has a clear path to reach the ocean.
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So the aquifer water is a part
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of the three millimeters per year of sea level rise
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that we experience as a global society.
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But there's more:
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the aquifer water might be punching above its weight.
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The ice flows in complex ways.
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In some places, the ice flows very fast.
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There tends to be water at the base of the ice sheet here.
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In other places, not so fast.
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Usually, there's not water present at the base there.
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Now that we know the aquifer water is getting to the base of the ice sheet,
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the next question is:
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Is it making the ice itself flow faster into the ocean?
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We're trying to uncover these mysteries hidden inside the Greenland ice sheet
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so that we can better plan for the sea level rise it holds.
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The amount of ice that Greenland has lost since 2002
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is just a small fraction of what that ice sheet holds.
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Ice sheets are immense, powerful machines that operate on long timescales.
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In the next 80 years, global sea levels will rise at least 20 centimeters,
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perhaps as much as one meter,
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and maybe more.
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Our understanding of future sea level rise is good,
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but our projections have a wide range.
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It's our role as glaciologists and scientists
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to narrow these uncertainties.
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How much sea level rise is coming,
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and how fast will it get here?
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We need to know how much and how fast,
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so the world and its communities can plan for the sea level rise that's coming.
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Thank you.
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(Applause)
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