Changes in the Marine-Terminating Glaciers of Central East Greenland, 2000–2010
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The Cryosphere, 6, 211–220, 2012 www.the-cryosphere.net/6/211/2012/ The Cryosphere doi:10.5194/tc-6-211-2012 © Author(s) 2012. CC Attribution 3.0 License. Changes in the marine-terminating glaciers of central east Greenland, 2000–2010 K. M. Walsh1,2,*, I. M. Howat1,2, Y. Ahn2,**, and E. M. Enderlin1,2 1School of Earth Sciences, The Ohio State University, Columbus, Ohio, USA 2Byrd Polar Research Center, The Ohio State University, Columbus, Ohio, USA *now at: NASA GSFC, Greenbelt, Maryland, USA **now at: the School of Technology, Michigan Technical University, Houghton, Michigan, USA Correspondence to: K. M. Walsh ([email protected]) Received: 1 August 2011 – Published in The Cryosphere Discuss.: 21 October 2011 Revised: 27 January 2012 – Accepted: 6 February 2012 – Published: 17 February 2012 Abstract. Marine-terminating outlet glaciers of the Green- 1 Introduction land Ice Sheet have undergone substantial changes over the past decade. The synchronicity of these changes suggest a Multiple studies using a range of methods show that the regional external forcing, such as changes in coastal ocean recent increase in the Greenland Ice Sheet’s rate of mass heat transport and/or increased surface melt and subglacial loss has been due to both increased surface melting and dis- runoff. A distinct contrast in rates of ice front retreat has charge from fast-flowing, marine-terminating outlet glaciers, ◦ been observed between glaciers north and south of 69 N lat- predominately along the southeast and northwest coasts itude on along the East Greenland coast. This latitude corre- (e.g. Krabill et al., 1999, 2004; Tapley et al., 2004; Luthcke sponds with the northward limit of subtropical waters carried et al., 2006; Rignot and Kanagaratnam, 2006; Thomas et by the Irminger Current, suggesting variability in ocean heat al., 2006; Pritchard et al., 2009; Velicogna and Wahr, 2006; transport as the dominant forcing. Glacier surging, however, Velicogna, 2009; van den Broeke, 2009). Previous studies is yet another mechanism of change in this region. In order have attributed retreat and acceleration of these glaciers to to provide further spatial and temporal constraint on glacier changes in ocean (e.g. Holland et al., 2008; Hanna et al., change across this important oceanographic transition zone, 2009; Straneo et al., 2010) and atmospheric circulation (Box we construct time series of thinning, retreat and flow speed of et al., 2006; Zwally et al., 2011). The observed speedup of 37 marine-terminating glaciers along the central east Green- outlet glaciers in southeast Greenland in the early 2000’s co- land coast from 2000 to 2010. We assess this dataset for spa- incided with increasing trend in sea surface temperatures in tial and temporal patterns that may elucidate the mechanisms the subpolar North Atlantic Ocean (Bersch et al., 2007; My- of glacier change. We confirm that glacial retreat, dynami- ers et al., 2007; Thierry et al., 2008; Straneo et al., 2010). cal thinning, and acceleration have been more pronounced Additionally, thinning and retreat of Jakobshavn Isbræ co- ◦ south of 69 N, with a high degree of variability along the incided with a sharp increase in subsurface ocean tempera- Blosseville Coast and little inter-annual change in Scoresby tures along Greenland’s west coast due to increased influx of Sound. Our results support the conclusion that variability warmer water originating in the Irminger Sea from the south- in coastal ocean heat transport is the primary driver of re- east after 1996 (Holland et al., 2008; Hanna et al., 2009). gional glacier change, but that local factors, such as surging Similarly, Seale et al., 2011, suggests that increased subtrop- and/or individual glacier morphology, are overprinted on this ical water transport onto the East Greenland continental shelf regional signal. and into glacier fjords may explain retreat, and subsequent stabilization, of glaciers south of 69◦ N, as well as the rela- tive stability of glaciers to the north of that latitude. Instead of, or in addition to, to changes in the surround- ing ocean, increased melting of glacier calving faces may Published by Copernicus Publications on behalf of the European Geosciences Union. 212 K. M. Walsh et al.: Central east Greenland marine-terminating glaciers be linked to an increase in melt water runoff due to re- and thermodynamic transition zone between the Irminger cent atmospheric warming (Box et al., 2009; Christoffer- and Greenland Seas (Fig. 1), providing an ideal study area son et al., 2011). An increase in melt water discharge at for this comparative approach. In the above-mentioned study, the grounding zone would invigorate buoyancy-driven cir- Seale et al. (2011) examined patterns of front retreat for 32 culation along the front, entraining warmer ocean water and glaciers in East Greenland between 2000 and 2009 and found increasing melt rates and, potentially, calving rates through a distinct contrast in retreat between those in south, which undercutting (Motyka et al., 2003; Rignot et al., 2010). Such underwent widespread synchronous retreat followed by sta- an increase in runoff is supported by an observed, ice sheet- bilization, and those in the north, which remained largely un- wide increase in melt season (June–August) surface temper- changed. The transition between these two regimes, centered ature, averaging 0.8 ◦C between 1997 and 2007 (Box et al., at approximately 69◦ N, corresponds with the northern extent 2009). There are, however, no direct observations of in- of warm subtropical waters transported within the Irminger creased runoff leading to increase submarine melting. Ad- Current. The spatial correlation with this boundary, as well ditionally, many glaciers underwent large-scale retreat in the as temporal correlation between glacier variability and vari- winter months (e.g. Howat et al., 2008, 2011; Seale et al., ability in heat transport, suggests that retreat was primarily 2011), and no simple causality between runoff and front re- due to a circulatory change associated with this current. treat is apparent from the existing data (Luckman et al., 2006; In this study, we add further constraints on the spatial Seale et al., 2011). and temporal patterns of glacier change across this impor- Finally, substantial inter-annual changes in these glaciers tant transition zone. From a range of remote sensing data, may be the result of glacier surging. Most of the marine- we construct time series’ of front position, surface elevation terminating glaciers on Greenland’s east coast between and flow speeds for all 38 marine-terminating East Greenland Kangerdlugssuaq Glacier (∼68◦ N) and northern Scoresby glaciers wider than 2 km between ∼65◦340 N to ∼71◦530 N Sound (∼72◦ N) have either shown surging behavior or are over the period 2000 and 2010 (Fig. 1). We examine tem- expected to be surging glaciers based on their surface mor- poral and spatial variability in these records for patterns that phology (Jiskoot et al., 2003). Both Alaskan-type surging may elucidate the mechanisms of change. (i.e. surging induced by basal hydrology) and Svalbard-type surging (i.e. surging induced by ice thermodynamics) have 2 Data sources and methods been observed in this region (Jiskoot and Juhlin, 2009), with the latter thought to be more prevalent. Alaskan-type surge Our data sources include (1) visible and near-infrared events are typified by acceleration lasting several months (VNIR) bands of the Advanced Spaceborne Thermal Emis- to a year or more, followed by a sudden deceleration and sion Reflector and Radiometer (ASTER) and (2) panchro- quiescent phase of several decades. In contrast, Svalbard- matic band imagery from the Landsat-7 Enhanced Thematic type events typically have a shorter phase of acceleration and Mapper Plus (Landsat-7 ETM+), both spanning the period longer phases of deceleration and periods of quiescence last- 2000 to 2010. Sequences of imagery from both sensors were ing centuries (Jiskoot et al., 2001). Unlike climate and ocean- used to delineate front position and flow speed, and ASTER induced change, however, there is no clear reason why the digital elevation models (DEM) yielded elevation change. timing of surges should be synchronous between multiple More detail regarding these measurements is provided below. glaciers. Also, whereas dynamic thinning following retreat Imagery from Landsat-7 were obtained from the United is due to increased along-flow ice stretching, thinning fol- States Geological Survey (USGS) Global Visualization lowing a glacier surge occurs due to melting. This results Viewer (GLOVIS, http://glovis.usgs.gov) public archive. in a distinctly different pattern of thinning; dynamic thin- ASTER orthoerectified images and DEMs (ASTDMO prod- ning propagates inland from the front with acceleration while uct) were obtained from the USGS Land Processes Data thinning following a surge remains concentrated at low ele- Active Archive Center (LP DAAC, https://lpdaac.usgs.gov). vations, where mass balance rates are most negative, and is Both sensors have a pixel resolution of 15 m and nominal concurrent with deceleration. Patterns of thinning thus far repeat interval of 16 days, although higher temporal resolu- observed for rapidly changing glaciers in the northwest and tion is obtained due to overlap of the Landsat paths and by southeast over the past decade have been associated with re- using both ascending and descending passes. Unlike Land- treat and acceleration, and shown a clear inland propagation sat, ASTER images are only acquired on-demand, so that with accelerations, indicating dynamic thinning (e.g. Howat few images are available for a given glacier each melt sea- et al., 2008; McFadden et al., 2011). son (Joughin et al., 2008). ASTER DEMs are created us- One approach for determining which of the possible mech- ing nadir and backward-looking band-3 and 3N image pairs anisms are responsible for recent glacier change in Green- acquired 57 seconds apart.