Scientists from the University of California, Irvine and NASA's Jet Propulsion Laboratory have for the first time quantified how warming coastal waters affect individual glaciers in the Greenland fjords. Her work is the subject of a study recently published in Advances in science.
Working under the auspices of the Oceans Melting Greenland mission for the past five years, the researchers used ships and planes to study 226 glaciers in all sectors of one of the largest islands on earth. They found that 74 glaciers in deep, steep-walled valleys accounted for almost half of the total ice loss in Greenland between 1992 and 2017.
It was found that such fjord-bound glaciers are undercut the most. Warm, salty water at the bottom of the gorges melts the ice from below and the masses break apart faster than usual. In contrast, the team found that 51 glaciers in shallower gorges were less undercut and only contributed about 15 percent to total ice loss.
"I was surprised at how crooked it was. The largest and deepest glaciers are undercut much faster than the smaller glaciers in shallow fjords," said senior author Michael Wood, a postdoctoral fellow at NASA's Jet Propulsion Laboratory in Southern California, who began this research as a PhD student at the UCI. "In other words, the largest glaciers are most sensitive to the warming water, and it is these that are really driving Greenland's ice loss."
The study highlighted the dynamics by which deeper fjords allow for warmer seawater to penetrate than shallow ones, accelerating the process of undercut with some of Greenland's largest glaciers.
Greenland has one of the only two ice sheets on earth, the largest of which is Antarctica. The ice in Greenland is more than three kilometers thick in places. At the edges of the landmass, the giant glaciers that extend from the ice sheet slowly migrate through valleys like icy conveyor belts that move into the fjords and then melt or break off as icebergs. The ice is filled by snowfall, which over time is pressed into the ice bag.
If the ice sheet were in equilibrium, the amount of snow that accumulates on top would roughly equal the amount of ice lost through melting, evaporation, and calving – chunks that detach from anchored masses and float into the ocean.
But the ice sheet has been out of whack since the 1990s. The melt has accelerated and calving has increased, causing glaciers that extend into the sea to retreat back towards land. Together these result in a shrinkage of the ice sheet.
According to the research team, the build-up of warm salt water at the bottom of the fjords has been accelerated by rising temperatures in the summer months, which warm the surfaces of the glaciers and form meltwater pools. This liquid leaks through cracks in the ice and forms underground freshwater rivers that flow into the sea, where they interact with salt water under fjords.
Glacier melt water is salt-free, therefore lighter than sea water and rises to the surface as a cloud, draws warm water and brings it into contact with the bottom of the glacier. Fjord depth is a fairly constant factor, but other factors such as sea water temperature and the amount of melt water from glacier surfaces are heavily influenced by global warming. All three factors combine to accelerate the deterioration of the Greenland ice sheet, the researchers said.
Since the water temperature on the Greenland coast is expected to continue to rise in the future, these results suggest that some climate models may underestimate glacial ice loss by at least a factor of two if they do not take into account the undercut by a warm ocean.
The study also sheds light on why many of Greenland's glaciers never recovered from sudden ocean warming between 1998 and 2007 that caused sea temperatures to rise by almost 2 degrees Celsius. Although ocean warming paused between 2008 and 2017, glaciers had undergone such extreme undercutting in the past decade that they continued to retreat at an accelerated rate.
"We have known for well over a decade that the warmer ocean plays an important role in the development of Greenland's glaciers," said Eric Rignot, OMG deputy principal investigator, also from JPL and UCI. "For the first time, we were able to quantify the undercutting effect and demonstrate its dominant effects on glacier retreat over the past 20 years."