Rapid Volume Loss From Two E. Greenland Glaciers Quantified Using Repeat Stereo Sat. Imaging - AGU

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Rapid volume loss from two East Greenland outlet glaciers quantified using repeat stereo satellite imagery
Leigh A. Stearns and Gordon S. Hamilton
Climate Change Institute, University of Maine,
Orono, Maine, USA

Abstract

<1> The coastal portions of Kangerdlugssuaq and Helheim glaciers in southeast Greenland lost at least 51 ± 8 km3 yr−1 of ice between 2001–2006 due to thinning and retreat, according to an analysis of sequential digital elevation models (DEMs) derived from stereo ASTER satellite imagery. The dominant contribution to this ice loss was dynamic thinning caused by the acceleration in flow of both glaciers. Peak rates of change, including thinning rates of ~90 m yr−1, coincided with the rapid increases in flow speed. Extrapolation of the measured data to the ice divides yields an estimated combined catchment volume loss of ~122 ± 30 km3 yr−1, which accounts for half the total mass loss from the ice sheet reported in recent studies. These catchment-wide volume losses contributed ~0.31 ± 0.07 mm yr−1 to global sea level rise over the 5-year observation period with the coastal regions alone contributing at least 0.1 ± 0.02 mm yr−1.

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<3> The largest changes in ice dynamics are observed in southeast Greenland. Kangerdlugssuaq Glacier (68.5°N, 33°W) accelerated from 5 km yr−1 to 14 km yr−1 in the past 4 years , with most of the acceleration taking place between 2004–2005 . Helheim Glacier (66.5°N, 38°W) accelerated from 6 km yr−1 to 11 km yr−1 between 2002 and 2005 . Similar changes are apparently widespread on Greenland outlet glaciers south of ~70° , but less evident farther north .

<4> Flow acceleration leads to dynamic thinning by increasing longitudinal stretching rates. Several Greenland outlet glaciers have been thinning at rates of 1–10 m yr−1 since the mid-1990s , perhaps as a result of this process. We observed unweathered ice stranded on fjord walls approximately 100 m above the present surfaces of Kangerdlugssuaq and Helheim glaciers during field visits in July 2005, indicating a relatively recent drawdown and a possible increase in the rates of glacier thinning. Here we quantify rates and timings of surface elevation and ice volume change on both glaciers for the period 2001–2006 using repeat stereo satellite images.

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Figure 1. Kangerdlugssuaq Glacier (68.5°N, 33°W). (a) ASTER image acquired June 21, 2005. (b) Surface topography derived from the image in Figure 1a. (c) Surface elevation change on the along-flow profile labeled in Figure 1b (0 km is the 2005 terminus position). (d) Surface elevation change on the across-flow elevation profile labeled in Figure 1b. Note the consistent elevation profiles on the steep fjord walls on either side of the glacier, indicating the good quality of the sequential DEMs. Enhanced TIF <45.1 MB>


Figure 2. Helheim Glacier (66.5°N, 38°W). (a) ASTER image acquired August 29, 2005. (b) Surface topography derived from the image in Figure 2a. (c) Surface elevation change on the along-flow elevation profile labeled in Figure 2b (0 km is the terminus). (d) Surface elevation change on the across-flow elevation profile labeled in Figure 2b. Enhanced TIF <34.2 MB>


Figure 3. (a) Map of surface elevation change on Kangerdlugssuaq Glacier, over a 1,750 km2 area between July 2001–July 2006. (b) Map of surface elevation change on Helheim Glacier over a 1,040 km2 area between June 2002–August 2005. (c) Fourth-order polynomial fitted to measured surface elevation changes on Kangerdlugssuaq Glacier and extrapolated to the ice divide along the black line in Figure 3a. (d) Fourth-order polynomial fitted to measured surface elevation changes on Helheim Glacier and extrapolated to the ice divide along the black line in Figure 3b. Enhanced EPS <62.9 MB>

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<17> We evaluate equation (1) for the lower sections of each glacier using the values in Table 1. The calculations yield a total 4-year (2001–2005) thinning due to ice dynamics of ~212 m for Kangerdlugssuaq Glacier, and ~130 m for Helheim Glacier. These estimates are consistent with thinning amounts derived from the repeat DEM analysis (Figures 1d and 2d). Small differences between the calculated and measured values are to be expected because of assumptions about ice thickness and omission of the surface balance terms, and because of likely transient speed variations not captured in the velocity data. Rignot and Kanagaratnam <2006> calculate a similar amount of dynamic thinning for Kangerdlugssuaq Glacier (~250 m), but a slightly smaller amount for Helheim Glacier (~75 m) perhaps because of differences in assumed ice thickness. The overall good agreement between the observations and the amounts of calculated thinning indicates that the recent accelerations of Kangerdlugssuaq and Helheim glaciers are driving the rapid volume losses.

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1. *Abstract* Freely Available Here

http://www.agu.org/pubs/crossref/2007/2006GL028982.shtml