Greenland’s permanent ice-cap is melting at half the speed previously predicted, according to new research by the Delft Technical University in the Netherlands and the US Jet Propulsion Laboratory.

The researchers say that previous conventional estimates have suggested that the Greenland ice-cap is melting at the rate of some 230 gigatonnes each year, which would result in an average rise in global sea levels of some 0.75 mm per year.

The researchers claim, however, that previous estimates have been carried out without the proper corrections for adjustment of the earth’s crust. The  GRACE satellite that measures magnetic changes on earth.

“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,” the group says.

“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,” Dr Bert Vermeersen of TU Delft says on the project website, adding that the average rise in sea levels as a result of melting ice-caps is also lower.

Vermeersen says that for Greenland in particular the group found a glacial isostatic adjustment model that deviated sharply from general assumptions, although he adds that not enough data is currently available to verify the findings 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,' Vermeersen says.

Apart from Greenland, the researchers have also said that current estimates for West Antarctica are also in question. Estimates have been at 130 gigatonnes per year.

The group says the melting of the ice caps has been charted since 2002 using two Gravity Recovery and Climate Experiment (GRACE) satellites which detect small changes in the earth’s gravitational field caused by the distribution of mass on earth, including ice and water.

The group’s findings are published in Nature Geoscience, which recently also published a report from Ohio University saying that measurements of ice-mass loss in Greenland and Antarctica are complicated by glacial isotatic adjustment. Although signals do confirm substantial losses of ice-sheet mass, they do not confirm the magnitude of the loss, the report says.