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VO: Of all the freshwater on Earth, 99 percent is stored

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in ice sheets, the large frozen masses that form over land.

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As climate changes, melting ice sheets can contribute to rising

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sea levels, which can place vulnerable cities around the world in

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jeopardy. For decades

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NASA has studied the polar regions, and a new mission, the Ice, Cloud

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and Land Elevation Satellite, or ICESat-2, will elevate our

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understanding of these complex ice sheets.
Brunt: ICESat-2 is a NASA mission

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whose goals include precisely measuring changes in our ice sheets

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and how that's actually contributing to mean sea level rise.

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Ice sheets form in our polar regions. We have two major ice sheets.

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We have Greenland, and we have Antarctica. Antarctica is the larger one,

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it's approximately the size of the continental United States, and it's really

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thick. In places it gets to be about 4500 meters,

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or just under 15,000 feet thick. So it's really thick.

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Ice sheets are actually really dynamic and they flow under their own weight from

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the center of the ice sheet out to the perimeter of the continent.

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In the really cold regions and way high up on our ice sheets,

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we get a lot of snow accumulation and over time that accumulation

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can build up. If it stays cold enough and that snow persists,

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and then you get another year of snow and another year of snow, you can imagine the

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weight of the snow on top of itself forces some of the lower layers to compact. We call that the

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firn densification of the top layer of the ice sheet.

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When we talk about the health of our ice sheets, we talk about

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the mass balance of the ice sheet. Basically that means snow coming

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in is in balance with all the terms of water or ice

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going out.
VO: The health of the ice sheets depends on a balance

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of these terms of input and output, but the interaction of the atmosphere, ocean

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currents and temperatures can force the ice sheets out of this balance.

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Brunt: At a big scale, the winds in Antarctica are kind of

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spinning in a big clockwise direction around the continent.

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But you can imagine a big dome of ice has very little obstruction,

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like trees or mountains kind of steering the winds. Consequently,

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winds that are sort of gravity-driven come down the continent can build up speed

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really quickly, and again, uninterrupted by any sort of

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disturbance. And we call those katabatic winds. And they have a major

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influence on what happens at the edge of the continent. Around Antarctica

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there's a massive current that we call the Antarctic Circumpolar

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Current, and it flows clockwise around the continent.

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Close to the continent, we also have the Antarctic Coastal Current, stays really

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close to the coastline and flows counterclockwise around the

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continent. In addition to these continent-scale currents, we

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also have regional scale currents, such as gyres. Gyres are these

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parts of the oceans that are sort of isolated because of topography,

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or ocean bottom topography. They're usually closed currents that

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often circulate. The gyres have a big role in

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sea ice formation and also in the currents that actually

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flow underneath our ice shelves. You can imagine that

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around the edge of the continent, near those ice shelves,

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warm water from the ocean can intrude into that cavity and contribute to

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basal melting, the melting from warm ocean waters

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of the bottoms of our ice shelves. Calving in Antarctica

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is a little bit sporadic and it's hard to actually model, but some of the

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contributing factors associated with calving include those strong katabatic

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winds pushing on the on the edge of the ice sheet, pushing on

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the edge of the ice shelf and calving large icebergs.

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So we're measuring surface elevation, and we can take that

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vertical measurement and kind of integrate it over a whole ice sheet and get a volume

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change. And then the real science of ICESat-2 is taking that volume change

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and turning it into a mass change. And from that we can determine how much

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ice is actually turning into water in our oceans and raising sea levels.

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