\t\t\t\tby Seth Shulman <\/p>\n
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Scott Luthcke weighs Greenland—every 10 days. And the island has been losing weight, an average of 183
\ngigatons (or 200 cubic kilometers)—in ice—annually during the past six
\nyears. That’s one third the volume of water in Lake Erie
\nevery year. Greenland’s
\nshrinking ice sheet offers some of the most powerful evidence of global
\nwarming.<\/p>\n
<\/p>\n
Luthcke is a scientist at NASA’s Goddard Space
\nFlight Center
\nin Greenbelt, Maryland. He specializes in space geodesy, a
\nbranch of earth sciences that monitors Earth from space by measuring changes in
\nthe planet’s shape, orientation, and gravitational field.<\/p>\n
<\/p>\n
Lutchke “weighs” Greenland by processing and
\ninterpreting data from one of the most sophisticated gravitational “scales”
\never built: the U.S.-German satellite mission called GRACE—the Gravity Recovery
\nand Climate Experiment.<\/p>\n
<\/p>\n
GRACE consists of two satellites which orbit
\nEarth in tandem at a relatively low altitude (450 to 500 kilometers). The pair operate in much the same way as a
\nscale that uses a spring to gauge weight.
\n“If you use a spring scale and attach a bucket full of tennis balls to
\nit, the spring expands,” explains Luthcke. “When you take some of the balls out
\nof the bucket, the spring correspondingly contracts and you can measure that
\nvariation.”<\/p>\n
<\/p>\n
The two GRACE satellites can measure the
\ndistance between them with remarkable accuracy. Even though the satellites
\ntravel 220 kilometers (137 miles) apart from each other (roughly the distance
\nfrom Philadelphia to Washington, D.C.), their sophisticated ranging systems,
\ndeveloped by NASA’s Jet Propulsion Laboratory, can detect variations in that
\ndistance down to a micron-or one-hundredth the width of a strand of human hair.<\/p>\n
<\/p>\n
The GRACE ranging system functions as a giant
\nscale by measuring variations in the gravitational pull of different land
\nmasses. Big land masses such as mountains exert a slightly stronger
\ngravitational pull upon the satellites, causing minute fluctuations in their
\nspeed as they fly over.<\/p>\n
<\/p>\n
For example, the two GRACE satellites fly
\nover Greenland several times each day. As the
\nfirst satellite approaches, the island’s mass causes the satellite to
\naccelerate and thereby move slightly away from its trailing companion. Over
\ntime, the Grace mission carefully records fluctuations in the distance between
\nthe two satellites each time they pass over
\nGreenland. <\/p>\n
<\/p>\n
By examining the GRACE data, Luthcke can
\nmonitor subtle changes in the gravitational pull that the land mass exerts on the
\nsatellites to get a reliable measure of Greenland’s
\nshrinking mass. The system, says Luthcke, is accurate enough to “detect the
\nloss of just a centimeter of ice over an area the size of Delaware.”<\/p>\n
<\/p>\n
Luthcke says he got
\nhooked on space geodesy when, as a physics major in college, he landed a summer
\njob at NASA. He never left, working his way “up from the mailroom” and contributing
\nover the years to many NASA missions. “Space geodesy is a fantastic field,” Luthcke says. “I love my work because not only do I
\nget to help develop and refine new space-borne sensors, but I get to be
\ninvolved in using them to gather useful information. It is the best of both
\nworlds.”<\/p>\n
<\/p>\n
Because so many important decisions are
\nlikely to rest on his data, the key for Luthcke is accuracy. The challenge, he
\nsays, is being sure to “carefully analyze how well we know all the steps
\ninvolved to turn the satellite’s raw observations into usable data.”<\/p>\n
<\/p>\n
Luthcke strives to come up with a usable,
\nregional calculation of the size of Greenland’s
\nice sheet. To do so, he uses sophisticated techniques to correct for “errors” that
\ncan creep into the data due to localized variations in mass or to other factors
\nsuch as solar radiation pressure on the satellites themselves. “My goal is to do everything I can to minimize
\nthe uncertainties involved in these measurements,” he says. “It’s a good job
\nfor me because I’m a cautious person by nature who always wants to know how
\nthings will stand up under scrutiny.”<\/p>\n
<\/p>\n
The good news for Luthcke is that a separate
\nteam using an entirely different method has come up with measurements of
\nGreenland’s melting ice that, he says, are almost identical to his GRACE data.
\nThe bad news, of course, is that both sets of measurements make it all the more
\ncertain that Greenland’s ice is melting faster
\nthan anyone expected.<\/p>\n
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<\/p>\n
This is the second installment of America’s Click here<\/a> to read all the climate scientist profiles.<\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n Related Links:<\/strong><\/p>\n Battle of the Carbon Titans<\/a><\/p>\n American PRIDE – alternative to the Lieberman-Kerry Disaster<\/a><\/p>\n Finding evidence of climate change in the caves of the American Southwest<\/a><\/p>\n
\nClimate Scientists: A series from the Union
\nof Concerned Scientists<\/a>. <\/p>\n
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