“It’s one more side of Greenland that’s starting to lose mass … It’s like a boat that is taking on water from all sides.” Jeremie Mouginot of the University of California (UC), Irvine
Beneath the calm and ice covered waters of northeast Greenland is a massive flow of ice known as the Northeast Greenland Ice Stream that is flowing rapidly into the ocean.
Greenland’s icy reaches are far more vulnerable to warm ocean waters from climate change than had been thought, according to new research by UC Irvine and NASA glaciologists. The work, published today in Nature Geoscience, shows previously uncharted deep valleys stretching for dozens of miles under the Greenland Ice Sheet.
The bedrock canyons sit well below sea level, meaning that as subtropical Atlantic waters hit the fronts of hundreds of glaciers, those edges will erode much further than had been assumed and release far greater amounts of water.
These warm ocean waters that circulate so deeply to the base of the bedrock appear to be the major cause of the rapid retreat of Greenland’s Zachariæ Isstrøm, one of three massive northern glaciers that form the Northeast Greenland Ice Stream. The glacier, along with another large glacier, Nioghalvfjerdsfjorden, helps to drain 12 percent of the Greenland Ice Sheet and has the potential to raise sea level significantly if it were to melt. A eye popping 95 percent of the ice shelf that used to stabilize it and keep it grounded has decayed, crumbled, and is now lost to the Atlantic. The glacier front is a 230 foot (approximately the size of a 22 story building) high cliff of ice extending itself above the water. The glacier front extends deep below the sea for hundreds of more feet. The retreat has tripled since 2012 speeding up about a mile for every year until 2015.
See image of Zachariæ Isstrøm in 1999 with ice shelf.
See image of Zachariæ Isstrøm in 2015 with ice shelf remnant and mélange (mixture of sea ice types, icebergs, and snow from shearing and fracture at the ice front).
Scientists share what concerns them the most on the study findings which was published in an article from Science.
What is going on under the ice, however, raises the most concern. Using satellite inter-ferometry, the team mapped how the ice bobbed with the tides to infer the point at which the ice floats free of the bedrock. Before 2012, they found, that “grounding line” re-mained stable, because the glacier was firmly anchored to the sea floor by an underwater rise, or sill. In that year, however, warm ocean waters under-mined the glacier far enough to detach it from the sill, they say. “It took a while to push it free of this anchor,” Rignot says, which allowed it to flow—and retreat—faster.
The glacier is likely to continue to retreat at its current pace for another 20 to 30 years, he adds, based on the shape of the sea floor. On the inland side of the sill, the sea floor drops and forms a deep basin, into which relatively warm water can easily intrude. Even after the glacier retreats for about another 30 kilometers, reaching the inland end of that basin, a deep channel in the fjord will still let sea-water gnaw at the glacier from below.
Major ice drainages in Greenland overlain on a map of measured ice surface velocities. The northeast Greenland ice stream (NEGIS), Jakobshavn Isbræ (JI), Helheim Glacier (HG) and Kangerdlugssuaq (KG) catchments are shown. Credit: Combined image by Kristian Kjellerup Kjeldsen, Natural History Museum of Denmark; courtesy of The Ohio State University.NASA’s Jet Propulsion Laboratory highlights the study named “Fast retreat of Zachariæ Isstrøm, northeast Greenland” that was published in the journal Science.
The scientists determined the bottom of Zachariae Isstrom is being rapidly eroded by warmer ocean water mixed with growing amounts of meltwater from the ice sheet surface. "Ocean warming has likely played a major role in triggering [the glacier's] retreat," Mouginot said, "but we need more oceanographic observations in this critical sector of Greenland to determine its future."
"Zachariae Isstrom is being hit from above and below," said the study's senior author Eric Rignot, Chancellor's Professor of Earth system science at UCI, and Joint Faculty Appointee at JPL. "The top of the glacier is melting away as a result of decades of steadily increasing air temperatures, while its underside is compromised by currents carrying warmer ocean water, and the glacier is now breaking away into bits and pieces and retreating into deeper ground."
Adjacent to Zachariae Isstrom is another large glacier, Nioghalvfjerdsfjorden, which is also melting rapidly but is receding at a slower rate because it's protected by an inland hill. The two glaciers make up 12 percent of the Greenland ice sheet and would boost global sea levels by more than 39 inches (99 centimeters) if they fully collapsed.
The sector where these two glaciers reside is one of three major marine-based basins in Greenland, along with Jakobshavn Isbrae in central west Greenland and the Petermann-Humboldt sector in central north Greenland. The latter two sectors hold enough water to raise global sea level by 2 feet (0.6 meters) each, and both are also undergoing significant changes at present. The authors conclude it is likely that Nioghalvfjerdsfjorden and Petermann-Humboldt glaciers will lose their ice shelves in coming years, further increasing Greenland's future contributions to global sea level rise.
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