Over many years, glaciers helped form Greenland’s fjords, those narrow and deep inlets in the sea that are often surrounded by steep cliffs and serve as exit routes for the vast ice sheet’s sea-terminating outlet glaciers.
The shape and depth of fjords have big implications for the ice sheet, which has been melting both from the top and the bottom and contains 20 feet of potential sea level rise in total. Warm air erodes ice above the water, but warmer waters — which reside at deep levels in some parts of the polar regions — undercut glaciers and melt ice from below. “As they melt faster, they can slide out to sea,” said Eric Rignot, leader researcher and a glaciologist at the University of California at Irvine.
Deeper fjords means there are “a lot more places where the warm water, subsurface water, can reach the glaciers,” Rignot said. Shallow fjords don’t pose as much of a threat.
On average, the fjords in this region are about 200 to 300 meters deeper than previously thought in some areas, he added. Glaciers undercut by warm water can melt twice as fast as those in colder waters, all other things being equal.
Estimates on how fast sea levels are rising “will have to change because these processes are not accounted for in existing models,” Rignot said. “It’s part of an ongoing story, that the projections of sea level rise are underestimated.”
Looking underwater at the shape of the seafloor is “the new frontier” in ice sheet research, Rignot said. It’s been understudied, in part, because it is so difficult, costly and time-intensive.
Researchers in this expedition collected data in treacherous and ice-laden conditions over the span of three years, spending three to four weeks at a time on a vessel. They measured the seafloor by lowering instruments that use sound signals. They also created images of glacier faces and their points of contact with the ocean. “Most of the ice is not visible to the naked eye,” Rignot said.
The “side-looking, multibeam echo sounding observations” reveal that glaciers’ frontal cliffs “are grounded deeper below sea level than previously measured,” the researchers write. What’s more, glaciers’ “ice faces are neither vertical nor smooth but often undercut by the ocean and rough,” they write.
That means more cavities and crevices that can be exploited by warmer waters, making the ice break apart faster.
“A lot of the changes we’ve seen in Greenland … are related to the action of the ocean on ice,” Rignot said. “It’s not something we looked at a lot in the past.”
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