A host of observations from satellites, aircraft, and scientists on the ground suggests that Greenland’s ice sheet diminished this year at a rate more than twice that seen just a few years ago.
By surveying the fringes of Greenland from aircraft equipped with laser altimeters, scientists have for several years noted the thinning of many glaciers (SN: 7/22/00, p. 54: Available to subscribers at Greenland’s ice is thinner at the margins). New assessments suggest that the thinning is accelerating, says William B. Krabill, a glaciologist at NASA’s Wallops Island (Va.) Flight Facility. In western Greenland, the heights of some glaciers are now dropping by 15 meters each year, he said last week at the fall meeting of the American Geophysical Union in San Francisco.
Losses are even greater along Greenland’s southeastern coast. The surface of Helheim Glacier has dropped about 50 m in the past 2 years. In the same interval, the surface of Kangerdlugssuaq Glacier fell about 80 m, says Krabill.
Those major glaciers drain about 8 percent of Greenland’s ice sheet, says Leigh A. Stearns, a glaciologist at the University of Maine in Orono. And in addition to thinning dramatically, they’re flowing faster.
Last June and July, using Global Positioning System equipment, Stearns and her colleagues clocked Helheim Glacier at a flow speed exceeding 14 kilometers per year. That’s almost three times its speed in 2001, which was the same as in 1996 and 1988, she notes. Kangerdlugssuaq Glacier’s flow speed this year was 40 percent faster than in 1995 and 2001.
Furthermore, although the spot where each of these glaciers meets the sea had been stable for many years, it retreated inland about 5 km in the past year, Stearns notes.
Across the island, scientists watching the Jakobshavn Isbrae Glacier have seen it retreat and speed up as well. The glacier’s seaward edge was stable until the 1990s but has moved inland about 12 km in the past 4 years, says Helmut Mayer of Darmstadt Technical University in Germany.
In 2004, that glacier’s ice mass flowed at a rate of about 20 m per day, as it had for the previous century, says Reinhard Dietrich of Dresden Technical University in Germany. This year, he clocked the glacier at 35 m/day.
Gravitational measurements made by satellites suggest that Greenland’s overall ice sheet has lost on average 162 cubic kilometers of ice annually over the past 3 years, says Isabella Velicogna of the University of Colorado at Boulder. However, month-to-month data show that much of that loss occurred in the past year. The average annual rate of melting raises sea level worldwide about 0.4 millimeter, she notes.
Between 1996 and 2000, changes in glacier behavior were mostly limited to latitudes below 66°N, but scientists are now seeing effects as far north as 70°N, says glaciologist Eric Rignot of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. At the San Francisco meeting, he estimated that Greenland is now losing about 220 km3 of ice per year.
Although the estimates of the 2005 Greenland ice loss put forth by Rignot and Velicogna differ, both figures far outstrip estimates made in the previous 3 years, which suggested yearly melt volumes of 70 to 100 km3 since 2000.