The findings were reported in the journal Nature on December 2, 2004. Co-authoring the study was University of New Hampshire glaciologist Mark Fahnestock of the Institute for the Study of Earth, Oceans, and Space (EOS).
The researchers found the glacier's sudden speed-up also coincides with very rapid thinning, indicating loss of ice of up to 15 meters (16.4 yards) 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.
The authors began looking at the flow of this glacier using satellite pictures after pronounced thinning was measured by a NASA aircraft.
The satellite imagery revealed the dramatic acceleration. Fahnestock found this signal to be quite striking.
"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, Pasadena, Calif. Joughin is currently a glaciologist at the Applied Physics Laboratory at the University of Washington, Seattle. Co-authors include Waleed Abdalati, a senior scientist at NASA's Goddard Space Flight Center, Greenbelt, Md., and Fahnestock.
"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," 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 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 (4.2miles) per year in 1985 to 5700 meters (3.5 miles) per year in 1992. This latter speed remained somewhat constant until 1997. By 2000, the glacier had sped up to 9400 meters (5.8 miles) per year, topping out with the last measurement in spring 2003 at 12,600 meters (7.7miles) per year.
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 (16.4 yards) a year just as its velocity began to increase between 1997 and 2003.
The authors used synthetic aperture radar (SAR) data from Canada's RADARSAT and the European Space Agency's European Remote Sensing Satellites to measure the glacier's velocity in 1992, 1994, 1995 and 2000. They also tracked distinct features in NASA Landsat image pairs to determine velocities in 2001, 2002 and 2003. Speeds from 1997 were determined using airborne laser survey data. Researchers at the University of Maine and Ohio State University used airborne photographs to calculate historical 1985 velocities.
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 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.
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.
Mark Fahnestock can be reached at 603-862-5065 or firstname.lastname@example.org.