WEBVTT FILE

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The lakes forming on the surface of Greenland are storing a large amount

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of meltwater throughout the season.

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Sometimes you see really shallow, kind of lagoon, almost snow swamp sort of thing,

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all the way up to lakes that are almost 30 feet in depth.

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As a glaciologist, I am looking at this and

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I see bright ice, some darker ice,

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that happens to have more dust in it. I see really bright snow

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I see the lakes. You can see kind of a network structure

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where they are draining together.

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So intuitively, you see this beautiful blue color, and you know the darker the blue, the deeper the lake.

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Which is nice, but we want to be able to quantify that.

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So imagine that you're a little beam of light. You get emitted by the sun,

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you hit the surface of the lake. As the light goes through the lake, some of it is absorbed by the water

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and a little bit of it gets scattered back.

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Then the light hits the bottom of the lake and a bunch gets return reflected,

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goes back through the column of water in the lake,

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and that is what Landsat can see.

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So we need to be able to understand how much energy is being absorbed by the water

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what the reflectance of the bottom of the lake is,

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and what proportion of light is being scattered back by the water.

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Once we have those three pieces of information

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and what the brightness that particular pixel is,

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then we can say how deep the water is in the lake there.

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These lakes are darker than the ice around them so they are going to start absorbing

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more energy from the sun and melt more.

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A little bit of melt concentrates in one place,

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and then melts more, which melts more, and it's a feedback mechanism.

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And when the lakes get big enough they can force open fractures

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that then drill all the way down to the bed of the glacier,

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transporting this water to the base where it can temporarily speed up the flow of the ice.

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And so we're interested in the lakes

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because they might be important for speeding up the ice sheet.

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