Jane J. Lee
A Greenland glacier named Jakobshavn Isbrae, which many believe spawned the iceberg that sank the Titanic, has hit record speeds in its race to the ocean. Some may be tempted to call it the king of the glacier world, but this speedy river of ice is nothing to crow about.
A new study published February 3 in the journal Cryosphere finds that Jakobshavn’s averaged annual speed in 2012 and 2013 was nearly three times its rate in the 1990s. Its flow rate during the summer months was even faster.
“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, glaciers in Greenland,” Ian Joughin, a researcher at the University of Washington in Seattle, told the BBC.
In summer 2012, Jakobshavn reached speeds of about 150 feet (46 meters) per day.
Other glaciers may periodically flow faster than Jakobshavn, but Greenland’s most well known glacier is the bellwether of climate change in the region and likely contributes more to sea-level rise than any other glacier in the Northern Hemisphere—as much as 4 percent of the global total, Joughin and his colleagues found in an earlier study. (Read about glacial meltdown in National Geographic magazine.)
In Too Deep?
When glaciers flow into the ocean, their floating edge, or terminus, slows the river of ice behind it. Where the terminus is grounded on the seafloor, it can act like a doorstop, slowing the glacier’s flow even further.
As warming temperatures in Greenland cause the Jakobshavn glacier to retreat up a long fjord, however—shedding icebergs in the process—the depth of the seafloor directly beneath the terminus varies.
In 2012 and 2013, the study authors say, the glacier’s terminus retreated over a trough in the fjord that is 4,265 feet (1,300 meters) deep. As ice flowed into this deeper terminus, they think, the glacier behind the terminus accelerated and thinned—much as taffy thins in the middle when it’s pulled from one end. The thinning in turn causes the glacier to melt faster. (See pictures of icebergs from James Balog’s Extreme Ice Survey project.)
The Jakobshavn terminus now seems to be retreating up a hump in the seafloor, and that may slow the ice loss a bit in the coming years. But beyond that hump lies 30 miles (48 kilometers) of deep fjord.
The researchers suggest that in the coming decades Jakobshavn’s speed could hit ten times that seen in the 1990s, slowing down only when the glacier has retreated to the head of the fjord.
The good news, says Penn State glaciologist Richard Alley, who was not involved in the new study, is that “the scariest possibilities are not happening.” The Greenland ice sheet does not seem to be collapsing wholesale into the ocean over a timescale of years or decades.
Nevertheless, Alley says, the changes detected by Joughin and his colleagues “are not good news for the ice sheet or for people living near sea level.”
“As the retreat of Jakobshavn ‘unzips’ this part of Greenland to let the warm waters in along the fjord,” Alley says, it will allow more ice from the sides and the head of the glacier to fall into the sea. If other fjords around Greenland follow suit, he explains, the country’s glaciers could accelerate sea-level rise.