Outlet glacier and margin elevation changes: Near‐coastal thinning of the Greenland ice sheet

Abstract

Abstract. Repeat surveys by aircraft laser altimeter in 1993/1994 and 1998/1999 have revealed significant thinning along many parts of the Greenland ice sheet at elevations below about 2000 m. In this paper we examine elevation changes from 29 repeat aircraft surveys over the lower portions of some of the larger outlet glaciers and parts of the ice sheet margin. Here thinning rates in excess of 1 m/yr are common in the lower sections of the flight lines, but in some cases, this rate is measured at elevations as high as 1500 m. Warmer summers along parts of the coast may have caused a few tens of cm/yr additional melting, but the magnitudes and character of the elevation changes suggest that in many cases they are more likely a result of glacier dynamics and creep thinning. The most extreme thinning was observed near the terminus of the Kangerdlugssuaq Glacier in southeastern Greenland where rates as high as 10 m/yr were measured. There are a few areas of significant thickening (over 1 m/yr), which is probably related to higher than normal accumulation rates during the observation period; but one location L, Bistrup Brae, had local regions of thickening of 8 to 9 m/yr. Three glaciers in the northeast show patterns of thickness change that may suggest surging behavior, and one has been independently documented as a surging glacier. Overall, the lowest reaches of the outlet glaciers and ice sheet edges appear to be changing significantly, with thinning observed more frequently than thickening. IntroductionOutlet glaciers are particularly important to the mass balance of the Greenland ice sheet because they are the means by which the ice is discharged into the surrounding seas. Such discharge comprises nearly half of the ice sheet mass loss [Weidick, 1985]. An understanding of the balance of these glaciers and the mechanisms that affect this balance is essential to understanding the present and future states of the ice sheet. The factors affecting outlet glacier mass balance are complex, because this balance is not simply a result of accumulation and surface ablation, but rather it is largely influenced by flow characteristics, which are often nonlinear.Until recently, large-scale assessment of glacier and ice sheet mass balance was not possible by conventional means. While in situ measurements can provide detailed information about specific locations, large-scale coverage is often not feasible because of the costs, difficulties, and dangers associated with work on these remote glaciers. Satellite radar altimetry facilitated study of the higher portions of the Greenland ice sheet through elevation-change measurements from which mass balance of the surveyed areas can be estimated. This technique, however, is limited to shallow-sloped and relatively smooth areas and provides no useful information about steeper areas

Testing hypotheses of the cause of peripheral thinning of the Greenland Ice Sheet: is land-terminating ice thinning at anomalously high rates?

“…2.4. Although it is possible that data from these coastal stations are not representative of prevailing conditions on the ice sheet, coastal meteorological stations have been used in many previous PDD analyses (Abdalati et al, 2001;Hanna et al, 2005Hanna et al, , 2006Hanna and Valdes, 2001;Thomas et al, 2003). Hanna et al (2008:332), suggest that coastal "DMI data reflect changing meteorological conditions on the adjacent low-lying marginal ablation zone of the ice sheet, where much of the seasonal melt and subsequent runoff occurs".…”

“…Recent airborne altimetry observations show that the greatest thinning (and mass loss) is along fast flowing marine-terminating outlet glaciers where rates often exceed 10 m yr −1 (Abdalati et al, 2001;Krabill et al, 2004). Between 1996 and 2005 the 21 largest outlet glaciers accelerated on average by 57% (Rignot and Kanagaratnam, 2006) while mass loss from the ice sheet increased from 91±31 km 3 ice/year to 224±41 km 3 Published by Copernicus Publications on behalf of the European Geosciences Union.…”

“…Ice dynamics are currently thought to account for about half of GrIS mass loss and contribution to sea level rise (Rignot and Kanagaratnam, 2006). For three of the GrIS's largest marine-terminating outlet glaciers (Helheim Gletscher, HG; Kangerdlugssuaq Gletscher, KG; and Jakobshavn Isbrae, JI) these changes in ice dynamics are thought to have been caused by perturbations at the calving front due to climate induced thinning of their termini (Abdalati et al, 2001;Howat et al, 2007). However, it is not yet clear to what extent calving is responsible for the thinning and acceleration of other GrIS marine-terminating outlet glaciers .…”

“…The Greenland Ice Sheet (GrIS) is losing mass from its periphery through increased surface melting (Abdalati et al, 2001) and increased ice discharge into the surrounding ocean (Rignot and Kanagaratnam, 2006). Recent airborne altimetry observations show that the greatest thinning (and mass loss) is along fast flowing marine-terminating outlet glaciers where rates often exceed 10 m yr −1 (Abdalati et al, 2001;Krabill et al, 2004).…”

“…The rapid thinning of some GrIS marine-terminating outlet glaciers has been attributed to changes in ice dynamics as well as rising temperatures since thinning rates exceed those expected from changes in surface mass balance alone (Abdalati et al, 2001). Dynamic thinning is defined as D=(U ∂H /∂x)−H E Z where D is dynamic thinning, U is the depth averaged horizontal velocity, ∂H /∂x is the icethickness gradient taken along the direction of ice flow, H is the ice thickness and E Z is the depth-averaged vertical creep rate (Thomas et al, 2003).…”