Movement of Greenland ice during past 9,000 years mapped

Scientists have created the first map that shows how the Greenland Ice Sheet has moved over time, and found that ice in the interior is moving more slowly towards the edges than it has during the past 9,000 years.
The findings do not change the fact that the ice sheet is losing mass overall and contributing to sea level rise, researchers said.
Along Greenland's periphery, many glaciers are rapidly thinning. However, the vast interior of Greenland is slowly thickening, a process the new study clarifies, they said.
"Scientists are very interested in understanding how ice sheets flow and how that flow may have been different in the past. Our paleo-velocity map for Greenland allows us to assess the flow of the ice sheet right now in the context of the last several thousand years," said lead author Joe MacGregor of The University of Texas at Austin's Institute for Geophysics (UTIG).

The study builds on earlier research that developed a database of the many layers within Greenland's ice sheet.
Using this database, the scientists determined the flow pattern for the past 9,000 years - in effect creating a "paleo-velocity" map.

Researchers identified three causes for this deceleration.
First is that snowfall rates were generally higher during the past 9,000 years, second is the slow stiffening of the ice sheet over time, and third is the collapse of an "ice bridge" that used to connect Greenland's ice to that on nearby Ellesmere Island.
"Like many others, I had in mind the ongoing dramatic retreat and speedup along the edges of the ice sheet, so I'd assumed that the interior was faster now too. But it wasn't," said MacGregor.

"In comparing the paleo-velocity map with modern flow rates, researchers found that the ice sheet's interior is moving more slowly now than during most of the Holocene, a geological period that began about 11,700 years ago and runs to the present.
"The ice that formed from snow that fell in Greenland during the last ice age is about three times softer than the ice being formed today," according to William Colgan of York University's Lassonde School of Engineering, a co-author of the study.
Because of this difference, the ice sheet is slowly becoming stiffer. As a consequence, the ice sheet is flowing more slowly and getting thicker over time.

This effect is most important in southern Greenland, where higher snowfall rates have led to rapid replacement of ice from the last glacial period with more modern Holocene ice.
The study was published in the journal Science.