The study said there was evidence that the rise in flows would soon spread to glaciers farther north in Greenland, which is covered with an ancient ice sheet nearly two miles thick in places, and which holds enough water to raise global sea levels 20 feet or more should it all flow into the ocean.
The study compared various satellite measurements of the creeping ice in 1996, 2000 and 2005, and was done by researchers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the University of Kansas...
Sometimes the rate of movement in a particular glacier can change abruptly, but the speedup in Greenland has been detected simultaneously in many glaciers, said Eric J. Rignot, the study's author, who has extensively studied glacier flows at both ends of the earth.
"When you have this widespread behavior of the glaciers, where they all speed up, it's clearly a climate signal," he said in an interview. "The fact that this has been going on now over 10 years in southern Greenland suggests this is not a short-lived phenomenon."
Richard B. Alley, an expert on Greenland's ice at Pennsylvania State University who did not participate in the study, agreed that the speedup of glaciers in various places supported the idea that this was an important new trend and not some fluke.
For educational purposes only, let's go to the original paper:
Science 17 February 2006:
Vol. 311. no. 5763, pp. 986 - 990
DOI: 10.1126/science.1121381
Changes in the Velocity Structure of the Greenland Ice Sheet
Eric Rignot1* and Pannir Kanagaratnam2*
Using satellite radar interferometry observations of Greenland, we detected widespread glacier acceleration below 66° north between 1996 and 2000, which rapidly expanded to 70° north in 2005. Accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year. As more glaciers accelerate farther north, the contribution of Greenland to sea-level rise will continue to increase.
...Repeat-pass airborne laser altimetry measurements (1) showed that the ice sheet is nearly in balance in the interior but its periphery is thinning, with deterioration concentrated along the channels occupied by outlet glaciers (2). The most recent surveys revealed that the mass loss from the periphery is increasing with time, with approximately half of the increase caused by enhanced runoff and half by enhanced glacier flow (3).
Although these airborne surveys crisscrossed a large fraction of Greenland, they left major gaps in glacier coverage, particularly in the southeast and northwest. The mass loss from nonsurveyed glaciers was estimated using an ice melt model, thereby assuming no temporal changes in ice flow. If glacier dynamics is an important factor, the contribution to sea level from Greenland is underestimated using this approach. To address this issue and understand the exact partitioning between surface mass balance and ice dynamics, it is essential to estimate glacier discharge and its variability over time.
Here, we measure glacier velocities using satellite radar interferometry data collected by Radarsat-1 in fall 2000 (4, 5) along the entire coast of Greenland except the southwest (Fig. 1) and repeatedly in spring and summer 2005 along selected tracks covering major glaciers. We also use European Remote Sensing satellites ERS-1 and ERS-2 data from winter 1996 in the north, east, northwest, and central west, and Envisat Advanced Synthetic Aperture Radar (ASAR) data from summer 2004 in the southwest. Ice velocity is measured with a precision of 10 to 30 m/year depending on satellite, data quality, and processing and is combined with ice thickness to calculate ice discharge.
Fig. 1. Ice-velocity mosaic of the Greenland Ice Sheet assembled from year 2000 Radarsat-1 radar data, color coded on a logarithmic scale from 1 m/year (brown) to 3 km/year (purple), overlaid on a map of radar brightness from ERS-1/Radarsat-1/Envisat. Drainage boundaries for flux gates in Table 1 are in red. Drainage boundaries with no flux estimates but discussed in the text are in blue. Numbers refer to drainage basins in Table 1
(Table 1 is not shown due to size. Losses in areas and changes in velocity over the last ten years are compared for 31 different glaciers as summarized in Fig.1 above. Likewise, I omit a technical discussion of the changes in the glaciers, broken down by regional differences, and their significance for the change in the ice mass.)
...Greenland's mass loss therefore doubled in the last decade, well beyond error bounds. Its contribution to sea-level rise increased from 0.23 ± 0.08 mm/year in 1996 to 0.57 ± 0.1 mm/year in 2005. Two-thirds of the loss is caused by ice dynamics; the rest is due to enhanced runoff minus accumulation. Ice dynamics therefore dominates the contribution to sea-level rise from the Greenland Ice Sheet.
Glacier acceleration in the east probably resulted from climate warming. Temperature records at Angmassalik (65.6°N, 37.6°E) show a +3°C increase in yearly air temperature from 1981–1983 to 2003–2005. The processes that control the timing and magnitude of glacier changes are, however, not completely characterized and understood at present. Glacier accelerations have been related to enhanced surface meltwater production penetrating to the bed to lubricate its motion (20), and ice-shelf removal (13), ice-front retreat, and glacier ungrounding (21, 22) that reduce resistance to flow. The magnitude of the glacier response to changes in air temperature (surface melting) and ocean temperature (submarine melting at calving faces) also depends on the glacier-bed properties, geometry, and depth below sea level and the characteristics of the subglacial and englacial water-storage systems (3, 20). Current models used to project the contribution to sea level from the Greenland Ice Sheet in a changing climate do not include such physical processes and hence do not account for the effect of glacier dynamics. As such, they only provide lower limits to the potential contribution of Greenland to sea-level rise. If more glaciers accelerate farther north, especially along the west coast, the mass loss from Greenland will continue to increase well above predictions.
References and Notes
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* 2. W. Abdalati et al., J. Geophys. Res. 106, 33279 (2001).
* 3. W. Krabill et al., Geophys. Res. Lett. 31, L24402 (2004).
* 4. The methodology used to map ice velocity has been developed in the 1990s with ERS-1/2 interferometric phase in north Greenland [e.g., (5, 23)], augmented with speckle tracking data from Radarsat-1 in the 2000s (24) during the background mission of the second Antarctic mapping (25), which we also applied to 35-day repeat ERS-1 data.
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* 7. Snow accumulation averaged for the period 1960 to 1990 is from (12). Surface melt is from a degree day model parameterized with 1960s temperatures (23), which should represent average conditions in 1960 to 1990. These models yield 265 ± 26 km3 ice/year runoff and 573 ± 50 km 3 ice/year accumulation for the 1.7-million-km2 ice sheet, consistent with published estimates.
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* 18. M. Carbonnell, A. Bauer, "Exploitation des couvertures photographiques aériennes répétées du front des glaciers vêlant dans Disko Bugt et Umanak Fjord, Juin-Juillet 1964" (Meddelelser om Grønland, Rep. 173, no. 5, 1968).
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