By Chris Mooney
11 April 2017
(The Washington Post) – The largest glacier in Greenland is even more vulnerable to sustained ice losses than previously thought, scientists have reported.
Jakobshavn glacier, responsible for feeding flotillas of icebergs into the Ilulissat icefjord — and possibly for unleashing the iceberg that sank the Titanic — is an enormous outlet for the larger Greenland ice sheet, which itself contains enough ice to raise seas by more than 20 feet.
Of that ice, more than 6 percent flows toward the ocean through Jakobshavn, which has raced inland since the 1990s, pouring ever more of its mass into the seas — a change that scientists believe has been caused by warming ocean temperatures. If all of the ice that flows through this region were to melt, it would raise global sea levels by nearly two feet. Just from 2000 to 2011, the ice loss through Jakobshavn alone caused the global sea level to rise by a millimeter.
The U.S. east coast in particular is already a “hot spot” for sea level rise, with rates in many areas exceeding the current global average of a little over 3 millimeters per year.
But until now, researchers have not been sure how far Jakobshavn’s ice extends below sea level — or how much deeper it gets further inland. That’s crucial because Jakobshavn is undergoing a dangerous “marine ice sheet instability,” in which oceanfront glaciers that grow deeper further inland are prone to unstoppable retreat down what scientists call a “retrograde” slope.
That’s where the new science comes in: Researchers who flew over Jakobshavn in a helicopter toting a gravimeter, used to detect the gravitational pull of the ice and deduce its mass, say they’ve found the glacier extends even deeper below sea level than previously realized, a configuration that sets the stage for further retreat.
“The way the bed looks, sort of makes it more prone to continuous retreat for decades to come,” said one of the study’s authors, Eric Rignot, a researcher with NASA’s Jet Propulsion Laboratory and the University of California-Irvine. […]
“It’s a direct conduit to the heart of Greenland,” Rignot said. (For a NASA video showing the two vast canyons beneath Greenland’s ice, one connected to Jakobshavn and one to Petermann, see here.) [more]
ABSTRACT: Jakobshavn Isbræ, West Greenland, which holds a 0.6-m sea level volume equivalent, has been speeding up and retreating since the late 1990s. Interpretation of its retreat has been hindered by difficulties in measuring its ice thickness with airborne radar depth sounders. Here, we employ high-resolution, helicopter-borne gravity data from 2012 to reconstruct its bed elevation within 50 km of the ocean margin using a three-dimensional inversion constrained by fjord bathymetry data offshore and a mass conservation algorithm inland. We find the glacier trough to be asymmetric and several 100 m deeper than estimated previously in the lower part. From 1996-2016, the grounding line migrated at 0.6 km/yr from 700 m to 1,100 m depth. Upstream, the bed drops to 1,600 m over 10 km then slowly climbs to 1,200 m depth in 40 km. Jakobshavn Isbræ will continue to retreat along a retrograde slope for decades to come.