On a GRACE satellite map, the Earth looks like a warty ball, with red bumps and deep blue holes highlighting fluctuations in the planet's gravity. The red spots represent places where the Earth's gravity is unusually strong. The blue ones are where it's weak. Not that the force of gravity itself varies. Rather, it's an indication that the Earth's mass distribution isn't quite uniform. Mountain-building in South America and the Himalayas produces dense, red zones; elsewhere, tectonic movements produce thin, blue, ones. All of this is interesting enough because it gives geologists a new way to visualise global processes. But even more interesting is the fact that the map changes over time.
Some changes are geological. For example, much of Canada, centred around Hudson Bay, is undergoing "postglacial rebound" as the continental crust slowly rises after being depressed, thousands of years ago, by the weight of Ice Age glaciers. Other changes are related to redistributions of water. Melting ice sheets, heavy rains, changes in soil moisture: all of these shift around enough water to make discernable changes in the Earth's gravitational field. Some of these are signs of global warming. Others provide early warning of floods, crop failures, and aquifer depletion in remote corners of the globe. "Water has weight," says project scientist Michael Watkins of NASA's Jet Propulsion Laboratory, in Pasadena, California. "It has gravitational attraction, and GRACE can detect it."
GRACE is a satellite launched in 2002 by the U.S. space agency. Or, more precisely, it's a pair of satellites. The name itself - Gravity Recovery and Climate Experiment - indicates the major role water was expected to play in its findings. The concept is simple. The two satellites, each about three metres long, follow each other in identical orbits roughly 400 kilometres above the Earth and 210 kilometres apart. Microwave instruments measure the distance between them, precisely enough to detect variations smaller than one percent of the width of a human hair. "[It's as though] you have two automobile-sized things, one in Los Angeles and one in San Diego, and you're measuring the distance between them to the size of a red blood cell," says Watkins. As one satellite and then the other passes through wrinkles in the Earth's gravity field, they speed up or slow down slightly, shifting the distance between them. By measuring these tiny yo-yos, scientists can calculate the gravity field that produced them, mapping the entire Earth about once a month...
Already there have been some dramatic findings. In late 2005, GRACE was instrumental in determining that the Greenland ice sheet was melting much faster than previously anticipated [^] enough to be contributing 0.4 millimetres per year to global sea level rise. But long-term changes aren't the only ones GRACE can detect. For example, it's possible to spot the difference between the tropical monsoon season and the dry season. "It's a big signal," Watkins says. One advantage of using gravity to monitor changes in wetness is that GRACE measures not just changes in surface water, but also in soil moisture or groundwater. "It's been difficult to measure [that] without an army of grad students or people monitoring [a] well all the time," adds Watkins...
Because GRACE has only been in orbit for five years, the research is just beginning. But already, results are coming in. At a 2006 geophysics meeting, John Wahr, of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, ran through a list of "off-beat anomalies" that GRACE found in Central Asia.One of these, he speculated, might be from the impoundment of water in China's Three Gorges Reservoir, which began filling in June 2003 and had then reached a depth of 300 feet. A bigger anomaly was southeast of the Aral Sea, between Kazakhstan and Uzbekistan. The Aral Sea is a large lake, fed by snowmelt from mountains to the south and southeast. For years, cotton farmers have diverted water from the incoming rivers, producing an ecological crisis as the sea shrinks. But some of the diverted water is accumulating in the desert soil, creating a manmade aquifer. GRACE can't reveal how polluted that aquifer might be, but it can show the rate at which water is accumulating in it. All told, Wahr said, increasing gravity in the surrounding desert indicates that about 25 cubic kilometres of water is percolating into the ground each year...
Northern India, on the other hand, shows a declining gravitational field. Again, the cause is irrigation, but in this case, water is being pumped out of the ground. Overall, it is known that about 150 cubic kilometres per year are being drawn out of wells, Wahr said. But some of that water percolates back into the ground to recharge the aquifer. The question is, how much? In the Aral Sea region, only about 20 to 25 per cent of the irrigation water is making its way into the ground. In India, Wahr's calculations show that the fraction is higher, but that 35 to 40 cubic kilometres per year are still being lost. The same calculation, of course, could have been done by monitoring water levels in wells. But with GRACE, it could be done cheaply, by one man, thousands of miles away. "GRACE is a new technique for hydrology," Watkins said...
GRACE is also giving the first-ever baseline measurements of important climate variables. One is the flow of water into the Arctic Ocean. Several big rivers drain the Arctic, and changes in their flow are likely to occur as glaciers retreat and the rate of snowmelt changes. This is important because the flow of freshwater into the Arctic Ocean is believed to play a major role in overall global climate. Traditionally, it requires river gages to measure the rate of discharge from a watershed. "Before GRACE, if we wanted to estimate freshwater discharge from an ungauged basin it was simply not possible," Famiglietti says. But now, GRACE allows scientists to measure all such flows, confirming other estimates that the rate of freshwater discharge into the Arctic has been increasing. On a seasonal basis, hydrologists are also looking for ways to use GRACE to issue water reports for farmers. "Our hope is that water-resource managers can integrate this into their reservoir allocations," Famiglietti says...
It's even possible to use GRACE to track mountain snowfall. "If you know there's a lot of snow up there, you can tell that this year is going to be a good year for crops," says Matt Rodell, a hydrologist at NASA's Goddard Space Flight Centre in Greenbelt, Maryland. In flatlands, like America's Midwest, where heavy, late snowfall poses a risk spring floods, GRACE data can also help downstream communities prepare. "GRACE won't replace ground-based observations," Rodell says. "[It] gives another piece of the puzzle, so you get a better prediction." As the scientists continue to fine-tune their data, applications will become ever more sophisticated. Soon, Watkins says, scientists hope to be able to track changes in individual glaciers in Antarctica and Greenland. And river-drainage studies will focus not just on big rivers, but smaller and smaller ones. Also on the horizon are efforts to map fluctuations in water depth in the ocean. Such changes, which GRACE should be able to map at the single-centimetre level, will allow scientists to chart shifts in ocean currents via minute alterations in sea-surface level. "I think we're about to see a new wave of applications to oceanography," says Watkins.
