Article: GRACE Watches Earth's Water

Earth’s water is always in motion, shifting among different forms and traveling to different locations. AMNH

Earth’s water is always in motion, shifting among different forms and traveling to different locations.


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.

The GRACE mission uses a pair of satellites to measure Earth’s gravitational field. AMNH

The GRACE mission uses a pair of satellites to measure Earth’s gravitational field.


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 show the areas that are wetter than usual (blue) and drier than usual (red). NASA/DLR

GRACE’s data show the areas that are wetter than usual (blue) and drier than usual (red).


Watching Freshwater

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.