A new study reveals one of the largest glaciers in Greenland is shrinking and speeding to the sea faster than scientists expected. If it continues, Greenland itself could become much smaller during this century and global seas could rise as much as 3 feet.
"The rates of change that we're observing are much higher than expected," said Ian M. Howat of the University of California, Santa Cruz. "If these rates of response continue, it is not unlikely that Greenland could shrink by several tens of percent this century."
Howat cautions, however, that it's not known how quickly this coastal response of the Greenland ice sheet melting will affect the vast inland ice.
"The ice sheet becomes less sensitive to climate as it retreats," Howat told LiveScience. "It sort of flies off the handle at first and then re-adjusts, so it is extremely unlikely that all of the ice sheet will disappear."
Greenland is the world's largest island, covering an area more than three times the size of Texas. Some 81 percent of it is permanently capped by ice, and there are many glaciers.
Glaciers are like slow-moving rivers of ice. Where a glacier meets the sea, its weight keeps it firmly resting on the bottom. A glacier's front is the point where the water is deep enough that the glacier floats. It becomes brittle and crumbles into icebergs, which ultimately melt.
The new study, to be detailed in the journal Geophysical Research Letters, used satellite images of the Helheim glacier.
Since the 1970s, the front of Helheim stayed in the same place. Then it began retreating rapidly, moving back 4.5 miles from 2001 through this past summer. It has also grown thinner, from top to bottom, by more than 130 feet since 2001. And over these past four years, its trek to the sea has sped up from about 70 feet per day to nearly 110.
"This is a very fast glacier, and it's likely to get faster," Howat said.
As the glacier's front retreats, its like a dam has been removed, and the inland portion can move more swiftly. The process has been seen in Antarctica by other researchers. A similar runaway effect has struck Greenland's Jakobshavn Isbrae glacier.
If the Helheim glacier thins beyond a critical point, it would simply float and rapidly disintegrate. In fact, the changes seen since 2001 were probably underway long before then but just not noticed. "Glaciers may have been thinning for over a decade," Howat said. "But it's only in the last few years that thinning reached a critical point and began drastically changing the glacier's dynamics."
The melting is driven by a warmer climate. Temperatures in Greenland have risen more than five degrees Fahrenheit (three degrees Celsius) in the last decade.
Helheim's speedup has been noted 12.5 miles up the glacier. The center of the Greenland ice sheet is 150 miles inland. The ultimate outcome depends on how far inland the speed-up occurs.
"Current models treat the ice sheet like it's just an ice cube sitting up there melting, and we're finding it's not that simple," Howat said.
Since most of Greenland's ice is on land, seas will rise as the ice melts. Predicting the extent of the rise is tricky, however. If Greenland's entire ice sheet melted, oceans would be 15-20 feet higher. Nobody expects that to happen anytime soon.
Relatively conservative estimates from climate models suggest Greenland alone could contribute about 4 inches of sea-level rise in 21st Century, said Howat's colleague Slawek Tulaczyk. Other recent models put the figure at up to 3 feet.
"Much of the near-future glaciologic work in Greenland will be focused on refining this estimate," Tulaczyk said.