Friday, June 02, 2006

Antartica ( GRACE Project )

The two GRACE satellites flying in formation and communicating by a microwave beam, which is shown as a blue line; (image courtesy of NASA)

To precisely measure the separation between the two GRACE satellites, each one sends out a microwave signal, which the other detects, as shown in the drawing. On-board instrumentation measures the time of transmission and relative phase of the generated and received microwave signals. In addition, the global positioning system accurately specifies the satellites’ positions in three dimensions to within a few centimeters.

Since the measured change in gravity is so small, there are many corrections. The atmosphere beneath the satellites has mass and therefore exerts a small gravitational force. This effect can be modeled and accounted for using global air pressure data. Also, they are subject to non-gravitational forces, such as atmospheric drag, the pressure of sunlight, and even the pressure of reflected sunlight from Earth. Accelerometers on each satellite detect the effects of these forces, and a star-tracker determines the attitude of the spacecraft so that atmospheric drag can be determined.
When these data are combined, the separation of the pair of satellites can be specified to within a micron, about 1/50 the diameter of a human hair. Since the satellites fly in and out of sunlight, thermal expansion must be minimized, so their frames are constructed of plastic reinforced with carbon fiber.

Iceberg formed when a part of the ice shelf broke off. The shelves are believed to play a role in preventing the movement of the rest of the ice sheet towards the ocean. (photo by Robert Reeves © Commonwealth of Australia)

A big complication in the GRACE measurement of the Antarctic ice sheet thickness is the fact that the land beneath is lifting, part of the “post-glacial rebound” after the last Ice Age, which ended only about 10,000 years ago (melting in Antarctica continued up until only about 4,000 years ago). The crust—the uppermost layer of Earth—floats on the rock beneath, which is plastic and responds to changes in its “load” (see last link). The melting of the huge ice age glaciers removed a large mass from Antarctica, and the continent is slowly lifting to reestablish equilibrium. The GRACE scientific team used geological estimates of ice thickness changes and a model of rock flow beneath Earth’s surface to estimate the uplifting of the land, which is a significant effect.
After taking into account all the above effects, the GRACE scientists find that from 2002 through 2005, the volume of the Antarctic ice sheet decreased substantially, corresponding to .4 mm plus or minus .2 mm of sea-level increase per year. This result was a surprise, both because of the extremely low temperatures in Antarctica, as mentioned above, and because forecasts of global warming had predicted increased snowfall in Antarctica.

In an earlier study, GRACE determined that the Greenland ice sheet is melting more rapidly than previously thought—in fact, the melting in Greenland and in Antarctica each produce about the same rate of sea level rise. So these ice sheets together add about .8 mm/yr. The overall rate of sea level rise over the last ten years, as obtained from statistical studies of radar measurements, is about 3 mm/year, and roughly half of this increase is due to thermal expansion of the oceans.
As the GRACE project continues, more data will accumulate and delineate the trend in ice sheet thickness over a longer time interval. Also, any change in the rate of melting would be unaffected by the correction for post-glacial rebound, which is presumed to be constant over long periods of time.
A meltstream in high summer on the Greenland ice sheet, (photo by Roger J. Braithwaite, The University of Manchester)


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