8 December 2004
Glacier Speed-Up Causes Concern
by Kate Melville
In findings reported in Nature, Greenland's Jakobshavn Isbrae, the world's fastest moving glacier, doubled its speed between 1997 and 2003. Researchers believe 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.
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 is important because this one glacier has increased the rate of sea level rise by about .06 millimeters per year, or roughly 4 percent of the 20th century rate of sea level increase.
Study co-author, University of New Hampshire glaciologist Mark Fahnestock, found the glacier's sudden speed-up also coincides with very rapid thinning, indicating loss of ice of up to 15 meters in thickness per year after 1997. Along with increased rates of ice flow and thinning, the floating 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.
"This speedup is the most dramatic change observed in a large glacier to date", Fahnestock said. "Observing a doubling of the flow of one of the large rivers of ice draining the Greenland Ice Sheet in such a short time raises questions about the nature and stability of the ice sheet." The NASA-funded study relies on data from satellites and airborne lasers to derive ice movements. The study's lead author, Ian Joughin, conducted much of this research while working at NASA's Jet Propulsion Laboratory in Pasadena, California.
"In many climate models glaciers are treated as responding slowly to climate change," Joughin said. "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."
"This finding suggests the potential for more substantial thinning in other glaciers in Greenland," co-researcher Waleed Abdalati added. "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 where the glaciers are accelerating as warming causes conditions to change at the boundaries of these ice streams."
The increase in speed 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 1950s 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 break up likely reduced its restraining effects 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.