GRACE Teaching Case

GRACE: Tracking Water From Space

Video transcript

The video is 7 minutes and 10 seconds long.

Produced by the American Museum of Natural History, July 2009.

Video begins here.

Visual: Students conducting a science experiment in a classroom.

Speaker: Michael Watkins

Gravity is one of the fundamental forces of the Universe.

Visual: A scuba diver enters the water from a boat.

And it's connected with mass.

Visual: Geologists examine a large rock sample.

Anything that has mass has gravity.

Visual: A section of glacier crumbles into the ocean.

We tend to think of the weight of the Earth as being entirely rocks and magma deep in the Earth.

Visual: Montage of mountains, lava flows, clouds, the sun in the sky, the ocean, snow fields, and glaciers.

But actually, on the surface of the Earth, as you look around you see the atmosphere, and you see the ocean, and you see snowfields, and polar ice caps.

Visual: Snow and ice melt into a creek.

All of those things have mass, too. But those things change much more frequently.

Visual: Michael Watkins, Jet Propulsion Laboratories, NASA

So what we realized is, if we could design a mission that was accurate enough to observe those small changes, we could actually watch the polar ice caps melt.

Visual: Snow melts, forming pools of water on the ground.

We could measure polar ice mass loss. We could even measure how much water is in the ground after it rains. And that led us to come up with the GRACE mission.

Visual: Title: GRACE: Tracking Water from Space

Visual: Michael Watkins, Jet Propulsion Laboratories, NASA

Deep in the Earth, or even on the surface of the Earth, the way mass is distributed with mountains and trenches and all kinds of things is slightly different, slightly nonuniform.

Visual: Michael Watkins walks the grounds of the NASA Jet Propulsion Unit.

So as you're to walk around the Earth, you'd actually sense more or less gravity. You'd actually weigh a little bit more, a little bit less on different parts of the Earth.

Visual: Michael Watkins holds an apple.

Isaac Newton taught us that the amount of mass that I'm standing on top of affects the gravity of where I'm standing.

Visual: Michael Watkins tosses the apple straight up into the air.

So the rate at which this apple comes down is a function of how much mass is below me on the Earth.

Visual: Michael Watkins walks through a wooded area.

So no matter where you are, what's below you will be different.

Visual: A three-way split screen showing people walking over different terrain; New York City, geyser pools, a Peruvian village, a rocky path.

So gravity will be different.

Visual: Michael Watkins continues to toss the apple up into the air.

And so the apple will drop at a different rate.

Visual: An animation of Earth in space.

The orbit of a satellite is highly dependent on the gravity field of the planet that it's orbiting. So what we wanted to do was to put up a satellite whose orbit we could measure incredibly accurately.

Visual: An animation of two satellites chasing each other over the Pacific Ocean, headed towards the California coast.

The best way to do that was to observe one satellite with another satellite. So the satellites are sort of chasing each other pole to pole, flying around the Earth, and as the first one comes up, for example, on a mountain, it feels that mountain first. So it starts to get pulled toward the mountain.

Visual: The first satellite is pulled toward the mountain, creating a distance from the satellite behind it.

The second one isn't quite there yet, so the satellites tend to drift apart. Then, of course, as they're leaving the mountain, the second one is still feeling the effect of the mountain.

Visual: The second satellite passes the mountain, and catches up with the first satellite.

So they come together again.

Visual: Michael Watkins

And that sort of dance that the two satellites do as they go around the Earth is what tells us what the gravity field underneath them was.

Visual: The satellites over the ice of Greenland.

Now, in addition to that, the gravity field is also changing every day, every week, every month, because water is moving all around the Earth, and it's raining here, or the polar ice cap is melting.

Visual: The different position of the satellites over several months, over the same spot over Greenland.

So water is the primary thing that's changing. But what scientists really want to understand is not just what happens across one year. They want to really see that play out over many years, over decades, to try to understand what's really happening in the climate system.

Visual: Heavy traffic on the highway, industrial smokestacks, an airliner landing, the Sun's reflection in a river.

Visual: James Famiglietti, University of California, Irvine

Speaker: James Famiglietti, University of California, Irvine

We know that the world's distribution of water is being affected by climate change. One of the key things that GRACE data does is help us to monitor where the water is now, and how it's changing over time.

Visual: James Famiglietti, with other researchers carrying equipment, head into a wooded field.

Before GRACE, it was nearly impossible to get that measurement. You would have to set up just an incredible array of equipment and an army of dedicated students and volunteers, working several hours a day, a few times a week, year-round. And that wouldn't even be enough.

Visual: James Famiglietti

GRACE is allowing us to see, for the first time, how water, how freshwater, is being redistributed across the continents.

Visual: James Famiglietti at a computer displaying the GRACE data.

So, here are the GRACE data.

Visual: The GRACE data laid over a map of Earth, with differently-colored areas over the land masses.

So what GRACE really does is make a map like this once a month, and the blue areas are areas that are wetter than usual, and the red areas are areas that are drier than usual. If the whole map were green, that would imply no storage change, so that would imply sort of a steady state.

Visual: James Famiglietti at the computer.

And, you know, for a long time, that's what people thought.

Visual: Civic water management structures; drains, pipes, canals, etc.

And a lot of engineering and a lot of our infrastructurestorm drains, bridgeswere built based on the idea that there were no long-term trends.

Visual: A flow of water in an urban drainage system.

What GRACE is showing us is that there are these trends, that there are in fact storage changes, and that we have to deal with the changes.

Visual: James Famiglietti points out certain data on the computer monitor.

Let me point some of them out to you. Some of the biggest signals are the ice sheetsGreenland, Antarctic ice sheets that are melting away at a steady pace because of climate change.

Visual: Glacial melt tumbles into the sea, streams of snowmelt forms waterfalls.

Visual: James Famiglietti points to Tibet on the computer monitor. A line graph of the data.

Some of the things we're really just starting to learn about are the decline of snow-water storage in the Tibetan Plateau. That's a region that feeds many of the world's major rivers. We have the annual oscillation.

Visual: A trending line is drawn through points of data.

We also have a trend, and that is an area where the trend is a linear decline.

Visual: The GRACE data for Australia.

This is the Murray-Darling Basin in Australia.

Visual: Montage of various areas of the Murray-Darling Basin; cracked dry earth, a river, a pond.

This is a basin that supplies a huge amount of water to people of Australia. That's where most of their water resources are developed.

Visual: A graph of the GRACE data for the Murray-Darling Basin.

And for as long as we've been looking at the GRACE data, that basin has been losing water.

Visual: A downward-sloping trend line is drawn through the graphed GRACE data.

And it is not a good situation for the people of Australia.

Visual: Animated GRACE data laid over a map of Earth, from 2003 to 2009.

A surprise that we get from the GRACE data is that we're able to see groundwater mining, where groundwater is being removed from subsurface aquifers at a rate that's greater than it's being naturally replenished.

Visual: A montage of groundwater sources. The GRACE data laid out on a map of Earth.

One place where there is active groundwater mining is in the High Plains Aquifer.

Visual: The GRACE data on a map of the High Plains Aquifer. The High Plains.

The water levels have been in steady decline there for about 50 years.

Visual: James Famiglietti

There's something like a billion people in the world today that don't have access to clean water.

Visual: Dry, cracked, earth.

We need to be able to manage it.

Visual: An animation of the GRACE satellites in orbit over South America.

We need to be able to allocate it in ways that are consistent with climate change, and so GRACE can really help us track the distribution of water, to better help us predict how climate and water availability is going to change for the future.

Visual: Michael Watkins

Speaker: Michael Watkins

With GRACE, we have a new way to measure these things that scientists have been looking for for so long.

Visual: Ocean waves break.

How much water is in the ocean?

Visual: An arctic glacier.

How much does a polar cap weigh?

Visual: A lake.

How much did it rain in the Amazon this year?

Visual: A waterfall.

You know, things that you think of as very gut-feel, physical measurements that turn out to just be hard to make any other way.

Video ends here.

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