NASA Uses New Method to Estimate Earth Mass Movements
NASA and European researchers have conducted a novel study to simultaneously measure, for the first time, trends in how water is transported across Earth's surface and how the solid Earth responds to the retreat of glaciers following the last major Ice Age, including the shifting of Earth's center of mass. To calculate the changes, scientists at NASA's Jet Propulsion Laboratory, Pasadena, Calif.; Delft University of Technology, Delft, Netherlands; and the Netherlands Institute for Space Research, Utrecht, Netherlands, combined gravity data from the NASA/German Aerospace Center Gravity Recovery and Climate Experiment satellites with direct measurements of global surface movements from GPS and other sources and a JPL-developed model that estimates the mass of Earth's ocean above any point on the ocean floor. Results are reported in the September issue of Nature Geoscience.

Using the new methodology, the researchers, led by Xiaoping Wu of JPL, calculated new estimates of ice loss in Greenland and Antarctica that are significantly smaller than previous estimates. According to the team's estimates, mass losses between 2002 and 2008 measured 104 gigatonnes a year in Greenland, 101 gigatonnes a year in Alaska/Yukon, and 64 gigatonnes a year in West Antarctica. A gigatonne is one billion metric tons, or more than 2.2 trillion pounds. The smaller but significant ice loss estimates reflect the revised role that post-glacial rebound was found to play in relation to current ice mass loss in Greenland and Antarctica. Post-glacial rebound is the response of the solid Earth to the retreat of glaciers following the last Ice Age. After the weight of ice from the land surface was removed, the land under the ice rose and continues to slowly rise.

In addition, the team found that the shift of water mass around the globe, combined with the post glacial rebound of Earth's surface, is shifting Earth's surface relative to its center of mass by 0.88 millimeters a year toward the North Pole. The estimate of the shift due to rebound-0.72 millimeters per year is believed to be the first estimate based on actual data, rather than a model prediction. Wu said the shift of Earth's surface is due primarily to the melted Laurentide ice sheet, which blanketed most of Canada and a part of the northern United States around 21,000 years ago. "The new estimate of shift is much larger than previous model estimates of 0.48 millimeters per year," said Wu. "This suggests that either Earth's lower mantle must be much more viscous than previously believed, or that the history of Earth's deglaciation needs to be significantly revised."

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