The Greenland Ice Sheet has gone through an "unprecedented" period of ice loss within the last two decades, according to scientists studying years and years worth of satellite data.

The study, published in Proceedings of the National Academy of Sciences, says that satellites revealed a four-fold increase in mass being lost from Greenland's ice sheet from 2003-2013.  It's causing worry to scientists who've spent decades studying areas of Greenland and how ice melt could contribute to sea-level rise in future.

Usually, the researchers watch Greenland's southeast and northwest regions, where glaciers continually force large chunks of ice into the Atlantic Ocean.  But the satellite data showed that the largest sustained acceleration in ice loss occurred in southwest Greenland, which is largely devoid of these large glaciers.

"Whatever this was, it couldn't be explained by glaciers, because there aren't many there," said Professor Michael Bevis of Ohio State University and the study's lead author.  "It had to be the surface mass - the ice was melting inland from the coastline.

"We knew we had one big problem with increasing rates of ice discharge by some large outlet glaciers," Bevis added, "But now we recognize a second serious problem: Increasingly, large amounts of ice mass are going to leave as meltwater, as rivers that flow into the sea."