The largest glacier in Greenland is moving ice from land into the ocean at a speed that appears to be the fastest ever recorded.
“We are now seeing summer speeds more than four times what they were in the 1990s, on a glacier which at that time was believed to be one of the fastest, if not the fastest, glacier in Greenland,” says lead author Ian Joughin, a glaciologist at the University of Washington’s Polar Science Center.
Joughin and colleagues measured the speed of the Jakobshavn Glacier in 2012 and 2013. The results were published in the journal The Cryosphere.
The new observations show that in the summer of 2012 the glacier reached a record speed of more than 10.5 miles (17 kilometers) per year, or more than 150 feet (46 meters) per day. These appear to be the fastest flow rates recorded for any glacier or ice stream in Greenland or Antarctica.
The summer speeds are temporary, with the glacier flowing more slowly over the winter months. But scientists point out that even the glacier’s average annual speed over the past couple of years is nearly three times its average annual speed in the 1990s.
The speedup of Jakobshavn means that the glacier is adding more and more ice to the ocean, contributing to sea-level rise, Joughin says.
“We know that from 2000 to 2010 this glacier alone increased sea level by about 4/100 of an inch (1 millimeter). With the additional speed it likely will contribute a bit more than this over the next decade.”
Sank the Titanic
Jakobshavn Glacier, which is widely believed to be the glacier that produced the large iceberg that sank the Titanic in 1912, drains the Greenland ice sheet into a deep-ocean fjord on the west coast of the island.
At its calving front, where the glacier effectively ends as it breaks off into icebergs, some of the ice melts while the rest is pushed out, floating into the ocean. Both of these processes contribute about the same amount to sea-level rise from Greenland.
As the Arctic region warms, Greenland’s glaciers have been thinning and calving icebergs farther and farther inland. This means that even though the glacier is flowing toward the coast and carrying more ice into the ocean, its calving front is actually retreating. In 2012 and 2013, Jakobshavn’s front retreated around 0.6 miles (1 km) each year compared to its position the previous summer.
In Jakobshavn’s case, the thinning and retreat coincide with the increase in speed. The calving front of the glacier is now located in a deeper area of the fjord, where the underlying rock bed is about 0.8 miles (1.3 km) below sea level, which the scientists say explains the record speeds.
“As the glacier’s calving front retreats into deeper regions, it loses ice—the ice in front that is holding back the flow—causing it to speed up,” Joughin says.
The team used satellite data to measure the glacier. “We used computers to compare pairs of images acquired by the German Space Agency’s satellites. As the glacier moves we can track changes between images to produce maps of the ice flow velocity,” Joughin says.
The researchers believe Jakobshavn is unstable, meaning it will continue to retreat further inland. By the end of this century its calving front could retreat as far back as the head of the fjord through which the glacier flows, about 31 miles (50 km) upstream from where it is today.
“The thing that’s remarkable about the Jakobshavn Glacier is that even after all the mass that it has already lost, it is able to keep doing it, year after year,” says glaciologist Benjamin Smith, a co-author on the study.
“A smaller glacier would settle down after losing that much mass. Jakobshavn’s ability to drain ice from the ice sheet is really exceptional among all of the glaciers in Greenland.”
NASA and the National Science Foundation funded the work.
Source: University of Washington