The Greenland and West Antarctic ice caps are melting at half the expected rate, according to a joint US-Dutch team.
The melting has been charted since 2002 by the two GRACE satellites, which detect small changes in the Earth's gravitational field -changes related to the exact distribution of mass on Earth, including ice and water.
Previous estimates for the Greenland ice cap reckoned it was melting at a rate of 230,000 billion kg per year, resulting in an average rise in global sea levels of around 0.75mm a year.
For West Antarctica, the estimate was 132 gigatonnes a year.
However, it now turns out that the figures may have been wrong all along. They weren't properly corrected for glacial isostatic adjustment, the rebounding of the Earth’s crust as a result of the melting of the massive ice caps from the last major Ice Age around 20,000 years ago.
Researchers from the Jet Propulsion Laboratory, Delft University of Technology and SRON Netherlands Institute for Space Research have now redone the math. Using combined data from the GRACE mission, GPS measurements on land and sea floor pressure measurements, they now believe that the sea floor under Greenland is falling more rapidly than was first thought.
"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," says Dr Bert Vermeersen of TU Delft.
"We have concluded that the Greenland and West Antarctica ice caps are melting at approximately half the speed originally predicted."
As a result, they say, projected sea level rises could also be inaccurate.
The team now wants a more extensive network of GPS readings which could be interpreted in combination with geological indicators for changes in sea level around Greenland over the last 10,000 years. This, they say, "will possibly be able to provide conclusive evidence on this matter in the years to come".