When people talk about something moving at a glacial pace, they are referring to speeds that make a tortoise look like a hare. While it is all relative, glaciers actually flow at speeds that require time lapses to recognize. Still, researchers who study Earth's ice and the flow of glaciers have been surprised to find the world's fastest glacier in Greenland doubled its speed between 1997 and 2003.
The finding is important for many reasons. For starters, as more ice moves from glaciers on land into the ocean, it raises sea levels. Jakobshavn Isbrae is Greenland's largest outlet glacier, draining 6.5 percent of Greenland's ice sheet area. The ice stream's speed-up and near-doubling of ice flow from land into the ocean has increased the rate of sea level rise by about .06 millimeters (about .002 inches) per year, or roughly 4 percent of the 20th century rate of sea level increase.
Also, the rapid movement of ice from land into the sea provides key evidence of newly discovered relationships between ice sheets, sea level rise and climate warming.
The researchers found the glacier's sudden speed-up also coincides with very rapid thinning, indicating loss of ice of up to 15 meters (49 feet) in thickness per year after 1997. Along with increased rates of ice flow and thinning, the thick ice that extends from the mouth of the glacier into the ocean, called the ice tongue, began retreating in 2000, breaking up almost completely by May 2003.
The NASA-funded study relies on data from satellites and airborne lasers to derive ice movements. The paper appears in this week's issue of the journal Nature.
"In many climate models glaciers are treated as responding slowly to climate change," said Ian Joughin, the study's lead author. "In this study we are seeing a doubling of output beyond what most models would predict. The ice sheets can respond rather dramatically and quickly to climate changes." Joughin conducted much of this research while working at NASA's Jet Propulsion Laboratory, Pasadena, Calif. Joughin is currently a glaciologist at the Applied Physics Laboratory at the University of Washington, Seattle.
The researchers used satellite and other data to observe large changes in both speeds and thickness between 1985 and 2003. The data showed that the glacier slowed down from a velocity of 6700 meters (4.16 miles) per year in 1985 to 5700 meters (3.54 miles) per year in 1992. This latter speed remained somewhat constant until 1997. By 2000, the glacier had sped up to 9400 meters (5.84 miles) per year, topping out with the last measurement in spring 2003 at 12,600 meters (7.83 miles) per year.
"This finding suggests the potential for more substantial thinning in other glaciers in Greenland," added Waleed Abdalati, a coauthor and a senior scientist at NASA's Goddard Space Flight Center, Greenbelt, Md. "Other glaciers have thinned by over a meter a year, which we believe is too much to be attributed to melting alone. We think there is a dynamic effect in which the glaciers are accelerating due to warming."
Airborne laser altimetry measurements of Jakobshavn's surface elevation, made previously by researchers at NASA's Wallops Flight Facility, showed a thickening, or building up of the glacier from 1991 to 1997, coinciding closely with the glacier's slow-down. Similarly, the glacier began thinning by as much as 15 meters (49 feet) a year just as its velocity began to increase between 1997 and 2003.
The acceleration comes at a time when the floating ice near the glacier's calving front has shown some unusual behavior. Despite its relative stability from the 1950's through the 1990s, the glacier's ice tongue began to break apart in 2000, leading to almost complete disintegration in 2003. The tongue's thinning and breaking up likely reduced any restraining effects it had on the ice behind it, as several speed increases coincided with losses of sections of the ice-tongue as it broke up. Recent NASA-funded research in the Antarctic Peninsula showed similar increases in glacier flow following the Larson B ice shelf break-up.
Mark Fahnestock, a researcher at the University of New Hampshire, Durham, N.H., was also a co-author of this study.
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