VO: Of all the freshwater on Earth, 99 percent is stored
in ice sheets, the large frozen masses that form over land.
As climate changes, melting ice sheets can contribute to rising
sea levels, which can place vulnerable cities around the world in
jeopardy. For decades
NASA has studied the polar regions, and a new mission, the Ice, Cloud
and Land Elevation Satellite, or ICESat-2, will elevate our
understanding of these complex ice sheets. Brunt: ICESat-2 is a NASA mission
whose goals include precisely measuring changes in our ice sheets
and how that's actually contributing to mean sea level rise.
Ice sheets form in our polar regions. We have two major ice sheets.
We have Greenland, and we have Antarctica. Antarctica is the larger one,
it's approximately the size of the continental United States, and it's really
thick. In places it gets to be about 4500 meters,
or just under 15,000 feet thick. So it's really thick.
Ice sheets are actually really dynamic and they flow under their own weight from
the center of the ice sheet out to the perimeter of the continent.
In the really cold regions and way high up on our ice sheets,
we get a lot of snow accumulation and over time that accumulation
can build up. If it stays cold enough and that snow persists,
and then you get another year of snow and another year of snow, you can imagine the
weight of the snow on top of itself forces some of the lower layers to compact. We call that the
firn densification of the top layer of the ice sheet.
When we talk about the health of our ice sheets, we talk about
the mass balance of the ice sheet. Basically that means snow coming
in is in balance with all the terms of water or ice
going out. VO: The health of the ice sheets depends on a balance
of these terms of input and output, but the interaction of the atmosphere, ocean
currents and temperatures can force the ice sheets out of this balance.
Brunt: At a big scale, the winds in Antarctica are kind of
spinning in a big clockwise direction around the continent.
But you can imagine a big dome of ice has very little obstruction,
like trees or mountains kind of steering the winds. Consequently,
winds that are sort of gravity-driven come down the continent can build up speed
really quickly, and again, uninterrupted by any sort of
disturbance. And we call those katabatic winds. And they have a major
influence on what happens at the edge of the continent. Around Antarctica
there's a massive current that we call the Antarctic Circumpolar
Current, and it flows clockwise around the continent.
Close to the continent, we also have the Antarctic Coastal Current, stays really
close to the coastline and flows counterclockwise around the
continent. In addition to these continent-scale currents, we
also have regional scale currents, such as gyres. Gyres are these
parts of the oceans that are sort of isolated because of topography,
or ocean bottom topography. They're usually closed currents that
often circulate. The gyres have a big role in
sea ice formation and also in the currents that actually
flow underneath our ice shelves. You can imagine that
around the edge of the continent, near those ice shelves,
warm water from the ocean can intrude into that cavity and contribute to
basal melting, the melting from warm ocean waters
of the bottoms of our ice shelves. Calving in Antarctica
is a little bit sporadic and it's hard to actually model, but some of the
contributing factors associated with calving include those strong katabatic
winds pushing on the on the edge of the ice sheet, pushing on
the edge of the ice shelf and calving large icebergs.
So we're measuring surface elevation, and we can take that
vertical measurement and kind of integrate it over a whole ice sheet and get a volume
change. And then the real science of ICESat-2 is taking that volume change
and turning it into a mass change. And from that we can determine how much
ice is actually turning into water in our oceans and raising sea levels.
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