June 16, 2014
Refreezing Meltwater Could Be Speeding Up Greenland Ice’s Journey To Sea
redOrbit Staff & Wire Reports - Your Universe Online
Newly discovered massive blocks of ice located underneath the ice of Greenland could help scientists learn more about the behavior of ice sheets and how they will respond to global warming, according to research appearing in the latest edition of Nature Geoscience.
Lead author Robin Bell, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory, and her colleagues used ice-penetrating radar to discover the ragged blocks of frozen water, which they said are as tall as a skyscraper and as wide as the entire island of Manhattan.
The blocks were found at the very bottom of the ice sheet, and the researchers believe that they were formed after meltwater trapped beneath the surface ice refroze, modifying the temperature of the ice surrounding it and warping it upwards. In turn, this phenomenon influences ice flow and grounding line melting, the authors wrote.
“We see more of these features where the ice sheet starts to go fast,” Bell explained in a Columbia University statement on Sunday. “We think the refreezing process uplifts, distorts and warms the ice above, making it softer and easier to flow.”
According to Bell and her colleagues, the newly-discovered structures cover an estimated 10 percent of northern Greenland, and become both larger and more common as the ice sheet narrows into glaciers bound for the sea. As the meltwater at the bottom refreezes over a period of hundreds or thousands of years, they believe that it radiates heats to the surrounding ice, causing it to move more quickly as it becomes softer.
As recently as 1998, scientists flying over the region mistakenly believed that radar images of these structures were hills, Bell’s team explained. However, thanks to new technology available as part of NASA’s Operation IceBridge campaign revealed that these so-called hills were actually made of ice and not rock.
Bell was able to recognize these features, as she had previously mapped similar structures at the base of the East Antarctic ice sheet. During research at the ice-covered Gamburtsev Mountains in 2008 and 2009, she and her fellow investigators located extensive regions of melting and refreezing along ridges and valley walls.
While scientists have long known that the bottom of ice sheets can be melted due to friction and pressure, they were unaware until now that refreezing water could alter the layers of ice located closer to the surface. Three years ago, Bell and her co-authors suggested that ice sheets could grow from the bottom up, and now the new study builds on those findings by associating the bottom features to faster ice sheet flow.
“The researchers looked at Petermann Glacier in the north of Greenland, which made headlines in 2010 when a 100-square mile chunk of ice slipped into the sea,” the university said. “They discovered that Petermann Glacier is sweeping a dozen large features with it toward the coast as it funnels off the ice sheet; one feature sits where satellite data has shown part of the glacier racing twice as fast as nearby ice.”
Their findings suggest that the refreezing process is somehow affecting the advance of the Petermann Glacier, even though it is located hundreds of miles from where it actually floats onto the sea. They added that Greenland’s glaciers are apparently moving more quickly towards the sea as the climate grows warmer, but they are not yet certain how this trend will be impacted by this refreezing process.
“They expected to find bottom features in the ice sheet's interior, as they did in Antarctica, but did not expect to see features at the edges, where lakes form and rivers flow over the surface,” the university explained. “Water from those lakes and rivers appears to fall through crevasses and other holes in the ice to reach the base of the ice sheet.”
Once it reaches that location, some of it appears to refreeze, the study authors said. Based on this discovery, Bell and her colleagues believe that the refreezing and deformation phenomenon could be far more widespread than they had previously believed, and that it could exist in other parts of Antarctica and Greenland as well.
Image 2 (below): Melting and refreezing at the bottom of ice sheets warps the layer-cake structure above, as seen in this radar image from Greenland. Credit: Mike Wolovick