The melting of the ice caps has been charted since 2002 using the measurements produced by the two GRACE satellites. From space they detect small changes in the Earth's gravitational field and these changes are related to the exact distribution of mass on Earth, including ice and water. When ice melts and lands in the sea, this therefore has an effect on the gravitational field.
Based on this principle, previous estimates for the Greenland ice cap calculated that the ice was melting at a rate of 230 giga-tons per year (i.e. 230,000 billion kg), which would result in an average rise in global sea levels of around 0.75 mm a year. For West Antarctica, the estimate was 132 giga-tons per year.
However, it now turns out that these results were not properly corrected for glacial isostatic adjustment, the phenomenon that the Earth’s crust rebounds as a result of the melting of the massive ice caps from the last major Ice Age around 20,000 years ago. These movements of the Earth’s crust have to be incorporated in the calculations, since these vertical movements change the Earth’s mass distribution and therefore also have an influence on the gravitational field.
Researchers from the Jet Propulsion Laboratory in Pasadena (US), TU Delft and SRON Netherlands Institute for Space Research have now succeeded in carrying out that correction far more accurately. They did so using combined data from the GRACE mission, GPS measurements on land and sea floor pressure measurements. These reveal that the sea floor under Greenland is falling more rapidly than was first thought. One of the researchers, Dr Bert Vermeersen of TU Delft, explains: 'The corrections for deformations of the Earth’s crust have a considerable effect on the amount of ice that is estimated to be melting each year. We have concluded that the Greenland and West Antarctica ice caps are melting at approximately half the speed originally predicted.' The average rise in sea levels as a result of the melting ice caps is also lower.
"The innovative aspect of our method is that we simultaneously matched the current changes in the ice mass and glacial isostatic adjustment to the observations, instead of assuming that a particular glacial isostatic adjustment model is correct," says Dr Vermeersen. "For Greenland in particular, we have found a glacial isostatic adjustment model that deviates rather sharply from general assumptions. But at present there are too few data available to verify this independently. A more extensive network of GPS readings in combination with geological indicators for the local and regional changes in sea level changes around Greenland over the last 10,000 years, will possibly be able to provide conclusive evidence on this matter in the years to come."
Xiaoping Wu, Michael B. Heflin, Hugo Schotman, Bert L. A. Vermeersen, Danan Dong, Richard S. Gross, Erik R. Ivins, Angelyn W. Moore&Susan E. Owen, 'Simultaneous estimation of global present-day water transport and glacial isostatic adjustment', Nature Geoscience 3, 642 - 646 (2010) Published online: 15 August 2010 | doi:10.1038/ngeo938
David H. Bromwich, Julien P. Nicolas, 'Sea-level rise: Ice-sheet uncertainty', Nature Geoscience 3, 596 - 597 (2010) Published online: 15 August 2010 | doi:10.1038/ngeo946