Greenland Melt Accelerating
Pastor Dale Morgan
Greenland Melt Accelerating*
An iceberg calved in the the Jakobshavn fjord in 2005. The Jakobshavn
Glacier has sped up two-fold in the last decade as the result of melt
water lubricating the glacier bed. Credit: Konrad Steffen, University of
Colorado at Boulder
by Staff Writers
Boulder CO (SPX) Dec 12, 2007
The 2007 melt extent on the Greenland ice sheet broke the 2005 summer
melt record by 10 percent, making it the largest ever recorded there
since satellite measurements began in 1979, according to a University of
Colorado at Boulder climate scientist.
The melting increased by about 30 percent for the western part of
Greenland from 1979 to 2006, with record melt years in 1987, 1991, 1998,
2002, 2005 and 2007, said CU-Boulder Professor Konrad Steffen, director
of the Cooperative Institute for Research in Environmental Sciences. Air
temperatures on the Greenland ice sheet have increased by about 7
degrees Fahrenheit since 1991, primarily a result of the build-up of
greenhouse gases in Earth's atmosphere, according to scientists.
Steffen gave a presentation on his research at the fall meeting of the
American Geophysical Union held in San Francisco from Dec. 10 to Dec.
14. His team used data from the Defense Meteorology Satellite Program's
Special Sensor Microwave Imager aboard several military and weather
satellites to chart the area of melt, including rapid thinning and
acceleration of ice into the ocean at Greenland's margins.
Steffen maintains an extensive climate-monitoring network of 22 stations
on the Greenland ice sheet known as the Greenland Climate Network,
transmitting hourly data via satellites to CU-Boulder to study ice-sheet
processes.
Although Greenland has been thickening at higher elevations due to
increases in snowfall, the gain is more than offset by an accelerating
mass loss, primarily from rapidly thinning and accelerating outlet
glaciers, Steffen said. "The amount of ice lost by Greenland over the
last year is the equivalent of two times all the ice in the Alps, or a
layer of water more than one-half mile deep covering Washington, D.C."
The Jacobshavn Glacier on the west coast of the ice sheet, a major
Greenland outlet glacier draining roughly 8 percent of the ice sheet,
has sped up nearly twofold in the last decade, he said. Nearby glaciers
showed an increase in flow velocities of up to 50 percent during the
summer melt period as a result of melt water draining to the ice-sheet
bed, he said.
"The more lubrication there is under the ice, the faster that ice moves
to the coast," said Steffen. "Those glaciers with floating ice 'tongues'
also will increase in iceberg production."
Greenland is about one-fourth the size of the United States, and about
80 percent of its surface area is covered by the massive ice sheet.
Greenland hosts about one-twentieth of the world's ice -- the equivalent
of about 21 feet of global sea rise. The current contribution of
Greenland ice melt to global sea levels is about 0.5 millimeters annually.
The most sensitive regions for future, rapid change in Greenland's ice
volume are dynamic outlet glaciers like Jacobshavn, which has a deep
channel reaching far inland, he said. "Inclusion of the dynamic
processes of these glaciers in models will likely demonstrate that the
2007 Intergovernmental Panel on Climate Change assessment underestimated
sea-level projections for the end of the 21st century," Steffen said.
Helicopter surveys indicate there has been an increase in cylindrical,
vertical shafts in Greenland's ice known as moulins, which drain melt
water from surface ponds down to bedrock, he said. Moulins, which
resemble huge tunnels in the ice and may run vertically for several
hundred feet, switch back and forth from vertical to horizontal as they
descend toward the bottom of the ice sheet, he said.
"These melt-water drains seem to allow the ice sheet to respond more
rapidly than expected to temperature spikes at the beginning of the
annual warm season," Steffen said. "In recent years the melting has
begun earlier than normal."
Steffen and his team have been using a rotating laser and a
sophisticated digital camera and high-definition camera system provided
by NASA's Jet Propulsion Laboratory to map the volume and geometry of
moulins on the Greenland ice sheet to a depth of more than 1,500 feet.
"We know the number of moulins is increasing," said Steffen. "The bigger
question is how much water is reaching the bed of the ice sheet, and how
quickly it gets there."
Steffen said the ice loss trend in Greenland is somewhat similar to the
trend of Arctic sea ice in recent decades. In October, CU-Boulder's
National Snow and Ice Data Center reported the 2007 Arctic sea-ice
extent had plummeted to the lowest levels since satellite measurements
began in 1979 and was 39 percent below the long-term average tracked
from 1979 to 2007.