GRACE: Tracking Water from Space
GRACE: Tracking Water From Space
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.
© American Museum of Natural History
NASA’s Gravity and Climate Experiment (GRACE) is an audacious mission to track the impact of climate change on the planet’s vast tracts of freshwater, saltwater, and ice. GRACE’s pair of satellites responds to the gravitational pull of these massive stores, effectively “weighing” Earth’s shifting water resources month by month. The satellites can detect where water is accumulating and drying up on a grand scale—data that were unavailable before. GRACE’s unprecedented view of our water planet could prove critical in the effort to anticipate and manage the consequences of climate change for people worldwide.
Classroom discussion activity for use with the video.
Read this related article.
GRACE Watches Earth's Water
Earth's water is in constant motion. It cycles through the planet's atmosphere, its surface, and its depths. Lakes, rivers, and oceans lose water to the air through evaporation. Plants draw water from the soil and release it to the air. Water falls to Earth as rain or snow. In the polar regions, it freezes into ice caps, which melt at their edges. Water even finds its way deep underground, hidden among particles of soil and in crevices and pores of rock.
This water cycle is fundamental to Earth's climate. Changes in the water cycle cause changes in climate, and vice versa. This link explains why global warming is dramatically affecting the movement of water on Earth. In some areas, higher temperatures are evaporating water reserves faster than ever. In other areas, rainstorms are becoming more intense and more frequent. The polar ice sheets are melting rapidly, causing global sea levels to rise. But how fast, and by how much? Measuring the changes in Earth's water stores is critical in tracking and predicting the consequences of climate change on human societies and the natural world.
Now, a satellite experiment called GRACE is giving scientists their widest perspective yet on our blue planet. By harnessing the laws of gravity, GRACE is obtaining the first global measurements of Earth's shifting water from space.
Mapping Earth's Gravitational Field
GRACE, which stands for Gravity Recovery and Climate Experiment, had its beginnings in the mid-1990's. That's when a NASA team proposed "weighing" Earth's water by measuring its gravitational force. "Newton's laws tell us that anything that has mass will have a gravitational attraction," explains GRACE project scientist Michael Watkins. "The bigger the mass, the more gravity there is." That means a mountain exerts more gravitational pull than a hill, an ocean more than a stream. Earth's gravity is not the same wherever you go. This uneven distribution is called Earth's gravitational field.
"If you actually look at the gravitational field from one month to the next, what's changed the most is Earth's thin fluid layerthe oceans and rivers, the polar ice caps, the groundwater," says Watkins. "So we realized, after decades of looking at satellite orbits, that if we could design a mission accurate enough to observe those small changes, we could actually watch the polar ice caps melt." By regularly measuring changes in the gravitational field with satellites, Watkins' team proposed, they could indirectly track the motions of large masses of water as they cycle around Earth.
Because satellites use Earth's gravitational field to stay in orbit, they're sensitive to its changes. For example, when a satellite flies over a high-mass areasay a mountain range or an ice capthe increase in gravitational force causes it to speed up slightly. After the satellite passes the area, it slows back down. These velocity changes are an indirect measure of the mass of that mountain range or ice cap.
Yet tracking a satellite's changing velocity would require a dense network of ground stationsan impractical solution. So the GRACE team decided to track a satellite with another satellite following right behind it. When the first satellite speeds up while passing a high-mass feature on Earth, the second satellite is left further behind. Then it, too, speeds up as it passes the same feature, closing the gap between them. The two satellites constantly gauge the changing distance between them by beaming microwave signals back and forth. These readings produce a gravitational field map for the strip of Earth beneathand thus, its concentrations of mass.
Spying on Ice
The GRACE satellites began orbiting in March 2002. They circle Earth once every 90 minutes, taking 30 days to map Earth's entire gravitational field. Since water's movements can be detected on this timescale, GRACE's ever-growing dataset is revealing long-term changes in Earth's water and its relationship to the changing climate.
One of the first researchers to analyze the data was geophysicist Isabella Velicogna, who has appointments at both the University of California–Irvine and NASA's Jet Propulsion Laboratory. Velicogna was excited about GRACE's early measurements of the ice sheets in Greenland and Antarctica. "It was the first time that you could directly measure polar ice mass loss," she says. Without GRACE, she'd have to use indirect methods to estimate ice loss, such as measuring the changing altitude of the ice surface. Now, says Velicogna, "we basically weigh the ice sheets with GRACE every month" to accurately measure long-term change.
In Greenland, GRACE's data have revealed that the ice sheet has lost 240 cubic kilometers of ice since 2002. That's 240 times the annual water consumption of metropolitan Los Angeles. In Antarctica, GRACE data show a loss of approximately 150 cubic kilometers of ice per year. These rates are much higher than the estimates from the Intergovernmental Panel on Climate Change, the main organizing body of climate change research.
If the ice sheets continue to melt into the ocean this quickly, they'd contribute to a sea level rise of one meter by the year 2100high enough to submerge large areas of coastline around the world. "If you have one meter of sea level rise, that affects not only the coast, but 10 kilometers inland," says Velicogna. "The surface is going to change, the vegetation is going to change, the animals that live there are going to change."
GRACE's data are also revolutionizing scientists' understanding of the little-studied store of freshwater beneath the ground. "One of the original justifications for the GRACE mission was that it could be used to track both water storage on land and groundwater in the world's major aquifers for the first time," says Jay Famiglietti, a hydrologist and colleague of Velicogna's at UC­–Irvine.
Before GRACE, the only way to measure water storage underground was to take a huge number of samples of an area at various locations and depths. "It would take an army," says Famiglietti, to achieve the scale of data that GRACE offers. Furthermore, hydrological field studies are rarely conducted in nations at war or with political strife, even though those regions are often the ones where the supply of clean water is most compromised. Because GRACE takes its measurements from space, it can safely "see" the weight of water in subsurface aquifers everywhere in the world.
GRACE's globetrotting is revealing a wealth of freshwater information. It has alerted hydrologists to areas, such as Australia, where water storage is decreasing faster than it is replenishing from precipitation. As climate change continues, some water stores will dry up while others will overflow, forcing human populations to adapt. "There are some very important things that we can do with a long time-series of GRACE data," says Famiglietti. "We can integrate it into our computer models that help us understand the climate and predict its changes. If we can improve our climate prediction, we can better allocate our water resources."
Scientists are just beginning to analyze the seven years of water data that GRACE has gathered so far. But climate change unfolds over decades, centuries, millennia. "We need three, four, five decades of this kind of information to truly understand the behavior of Earth's water," says Famiglietti. While GRACE's satellites likely won't last that long, climate scientists hope the mission's successors will keep the water data flowing. This information will become ever more critical to the world's growing population as climate change continues.