US: NASA and European researchers have conducted a novel study to simultaneously measure 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 centre of mass.
To calculate the changes, scientists at NASA’s Jet Propulsion Laboratory; 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.
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. 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.
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 centre of mass by 0.88 millimetres (.035 inches) a year toward the North Pole.
Wu said previous GRACE-based estimates of the movement of mass at Earth’s surface have been calculated by correcting the data using a post-glacial rebound model, while estimates of post-glacial rebound itself have been estimated using a hydrological model. These models are not as precise as the geodetic data, however, and contain unknown and potentially large errors that will throw off estimates of the other process.
GRACE project scientist Michael Watkins of JPL said that although some of the new results, such as those for Greenland, are surprising, they are not due to a reanalysis of GRACE or GPS data alone. Rather, they are a result of the simultaneous use of GRACE, GPS and other geodetic measurements to help objectively sort out the relative sizes of post-glacial rebound and present-day ice mass loss. “Both the GPS and gravity measurements are accurate on their own, but untangling the relative contributions of the two processes as observed by satellites is difficult. This technique provides a first global attempt at doing that,” Watkins said.