Weighing Greenland

Spotlight: Scott Luthcke, NASA's Goddard Space Flight Center -- by Seth Shulman

Scott Luthcke

According to the latest scientific evidence, Greenland's vast ice sheet is melting at a dramatic rate. Over the past six years, Greenland has lost an average of 183 gigatons (or 200 cubic kilometers) of ice annually. That's a third of the volume of water in Lake Erie every year. Greenland's shrinking ice sheet offers powerful evidence that global warming is underway. But how do we know the exact amount of ice that is melting and in many cases breaking off in large chunks into the ocean? That's where Scott Luthcke, a scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, comes in. His job is to regularly weigh Greenland, looking at the data every ten days.

Luthcke specializes in space geodesy, a branch of earth sciences that monitors Earth from space by measuring changes in its shape, orientation, and gravitational field. Much of Luthcke's work over the past several years has focused on measuring Greenland by processing and interpreting data from one of the most sophisticated gravitational "scales" ever built: the U.S.-German satellite mission called GRACE—the Gravity Recovery and Climate Experiment.

The GRACE mission consists of two satellites orbiting Earth in tandem at a relatively low altitude of 450 to 500 kilometers. Working together, the satellites operate in much the same way a scale uses a spring to gauge weight. As Luthcke explains, "If you use a spring scale and attach a bucket full of tennis balls to it, the spring expands. When you take some of the balls out of the bucket, the spring correspondingly contracts and you can measure that variation."

The GRACE satellites, Luthcke explains, measure the distance between themselves with remarkable accuracy. Even though the satellites travel 220 kilometers (137 miles) apart from each other (roughly the distance from Philadelphia to Washington, D.C.), their sophisticated ranging system, developed by NASA's Jet Propulsion Laboratory, can detect the tiniest variations in that distance, down to a micron—one-hundredth the width of a human hair.

The ranging system of the GRACE satellites functions as a vast weighing machine by measuring variations in Earth's gravitational pull of different land masses. Big land masses such as mountains exert a slightly stronger gravitational pull upon the satellites, causing minute fluctuations in their speed as they fly over them.

For example, several times a day the two GRACE satellites fly over Greenland. As the first satellite approaches, Greenland's mass causes it to accelerate and thereby move slightly away from its trailing companion. By carefully measuring the fluctuations in the distance between the two satellites over time as they fly over Greenland, the GRACE mission can accurately measure its mass.

By examining the data GRACE gathers every time the satellites fly over Greenland, Luthcke can monitor subtle changes in the gravitational pull that the land mass exerts on them to get a reliable measure of Greenland's shrinking mass. The system is accurate enough, Luthcke says, to "detect the loss of just a centimeter of ice over an area the size of Delaware."

Luthcke, whose father ran a small contractor business in New Jersey, says he became hooked on his field when, as a physics major in college, he got a summer job at NASA. Since then, he jokes, he has worked his way "up from the mailroom," contributing to many NASA missions in various ways. "Space geodesy is a fantastic field," Luthcke says. "I love my work because not only do I get to help develop and refine new space-borne sensors, but I get to be involved in using them to gather useful information. It is the best of both worlds."

The challenge, Luthcke says, is being sure to "carefully analyze how well we know all the steps involved to turn the satellite's raw observations into usable data." He uses sophisticated techniques to try to correct for any "errors" in the data caused by localized variations in mass as well as other factors such as solar radiation pressure acting on the satellite to get a usable regional calculation of Greenland's ice sheet.

The good news, Luthcke says, is that a team using an entirely different method [note: led by Jay Zwally using ICESat-1 data] has come up with measurements of Greenland's melting ice that he says are "equivalent within the error bars" to the GRACE data. The bad news, of course, is that the measurements make it all the more certain that Greenland's ice is melting faster than anyone expected. Because so many important decisions are likely to rest on his data, the key for Luthcke is accuracy.

"My goal is to do everything I can to minimize the uncertainties involved in these measurements," he says. "It's a good job for me because I'm a cautious person by nature who always wants to know how things will stand up under scrutiny."

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