Controls on West Greenland Outlet Glacier Sensitivity to Climate Forcing

Thesis

Presented in Partial Fulfillment of the Requirements for the Degree of Master of Science in the Graduate School of The Ohio State University

Ellyn Mary McFadden, B.S.

Graduate Program in Geological Science

The Ohio State University

2010

Thesis Committee:

Dr. Ian M. Howat, Advisor

Dr. W. Berry Lyons

Dr. Lonnie Thompson

Abstract

Significant changes in the dynamics of Greenland’s marine-terminating outlet glaciers within the past few years indicate a rapid and complex response of these systems to recent climatic forcing. Widespread and substantial accelerations in flow-speed of outlet glaciers in southeast Greenland have been linked to destabilization and retreat of glacier fronts triggered by thinning to flotation induced by warmer ocean temperatures.

There is concern that ongoing coastal thinning in western Greenland will trigger a similar response, further threatening the stability of the ice sheet. Despite regional ice thinning and retreat, the glaciers of Greenland’s northwest coast have not yet undergone substantial acceleration. This suggests a lessened dynamic sensitivity of these glaciers to changes at the ice front than southeastern glaciers, likely due to differences in glacier geometry. To investigate the potential controls behind this contrasting behavior, we derive time series’ of front position, surface elevation, and surface thinning for 59 marine-terminating outlet glaciers in west Greenland from 2000-2009. Surface speeds are derived for several glaciers to determine sensitivity to large front retreats in this region.

Using these data, we look for patterns in the relationships between retreat, thinning, acceleration, and geometric variables, such as surface slope, to determine the first-order controls on glacier sensitivity.

ii Data are compared to regional changes in air and ocean temperatures to assess similarities in climate forcing conditions along Greenland’s west coast. We conclude there is no direct relationship between front retreat and measured geometric parameters applicable to the entire study region. The relative importance of surface slope as a control of glacier behavior is highly variable and must be coupled to bathymetric data in order to understand and accurately model outlet glacier response to climate forcing.

iii

Acknowledgements

I’d like to thank my advisor, Dr. Ian Howat, for his guidance and support with my Masters research as well as his extensive mentoring in Glaciology, of which I knew little prior to my research. I’d like to thank Dr. Berry Lyons and Dr.

Lonnie Thompson for providing helpful suggestions while serving on my thesis committee. I’d also like to thank Dr. Yushin Ahn (Byrd Polar Research Center) for providing velocity data, Dr. Wieslaw Maslowski (Naval Postgraduate School) for providing ocean temperature reanalysis data, and Dr. Ian Joughin (University of Washington) for providing complete RADARSAT mosaics for Greenland.

Without their help and support, this paper would not have been possible.

Vita

January 2008 ...... B.S. Environmental Science, Lehigh University September 2008 ...... University Fellowship, The Ohio State University December 2008 ...... “Controls on Greenland outlet glacier sensitivity to climate forcing: A comparative approach”, AGU Annual Meeting September 2009 ...... Rick Toracinta Graduate Scholarship, Byrd Polar Research Center December 2009 ...... “West Greenland outlet glacier sensitivity (2000-2009)”, AGU Annual Meeting

Fields of Study

Major Field: Geological Science

Table of Contents

Abstract ...... ii Acknowledgements ...... iv Vita ...... v List of Tables ...... viii List of Figures ...... ix 1. Introduction ...... 1 2. Methods ...... 9 2.1 Data and Sources ...... 9 2.2 Front Positions ...... 15 2.3 Surface Elevations and Surface Slopes ...... 16 2.4 Surface Speed ...... 17 2.5 Additional Observed Parameters ...... 18 3. Results ...... 20 3.1 Overview of Changes in Front Position ...... 20 3.2 Overview of Changes in Surface Elevation and Slope ...... 21 3.3 Case Studies of Glacier Change ...... 23 4. Discussion ...... 43

4.1 Climate Forcing Conditions ...... 43 4.2 Front Retreat, Thinning, and Surface Slope ...... 45

4.3 Comparison of Grounded and Floating Termini ...... 51

4.4 Focus on Smaller Spatial Scales ...... 53 5. Conclusions ...... 63 6. References ...... 66

vii

List of Tables

Table 1: Glacier Slopes ...... 32

viii

List of Figures

Figure 1: Location map ...... 19

Figure 2: Front position change (2000-2009) ...... 29

Figure 3: Surface elevation profiles ...... 30

Figure 4: Front positions (A) and surface elevation profiles (B) for Edvard Glacier (76°18'24.26''N, 61°58'28.31''W) ...... 34

Figure 5: Front positions (A) and surface elevation profiles (B) for Rink South Glacier (76°14'13.71''N, 60°57'46.79''W) ...... 35

Figure 6: Front positions (A), surface elevation profiles (B), and surface speeds (C) for Alison Glacier (74°37'21.31''N, 56°13'4.99''W) ...... 36

Figure 7: Front positions (A) and surface elevation profiles (B) for Upernavik North Glacier (73°0'2.33''N, 54°26'32.14''W) ...... 38

Figure 8: Front positions (A), surface elevation profiles (B), and surface speeds (C) for Umiamako Glacier (71°44'3.32''N, 52°24'53.34''W) ...... 39

Figure 9: Front positions (A), surface elevation profiles (B), and surface speeds (C) for Jakobshavn Isbræ (69°10'59.33''N, 49°37'27.44''W) ...... 41

Figure 10: Air Temperatures...... 57

Figure 11: SSTs for the northern (A), central (B), and southern (C) regions ...... 58

Figure 12: Heat flux data for the northern (A), central (B), and southern (C) gates ...... 60

Figure 13: Surface elevation profiles for glaciers with stable fronts from 2000-2009...... 62

1. Introduction

Recent studies have revealed retreat, thinning, and acceleration of Greenland’s marine-terminating outlet glaciers (e.g. Joughin et al. 2004, Moon and Joughin 2008,

Howat et al. 2005, 2007, 2008, Luckman et al. 2006, Rignot and Kanagaratnam 2006).

Although the frontal retreats of glaciers along the southeast coast of Greenland have been well documented (Howat 2005, 2007, 2008), the stabilization and slight advance of previously retreating fronts following 2005 (Howat et al. 2008, Moon and Joughin 2008) lead to questions regarding external controls on front positions and speed of Greenland’s outlet glaciers. Focus on Greenland glaciers has been primarily concentrated on large outlet glaciers in southeast Greenland, and Jakobshavn Isbræ in the west, because these regions together represent nearly half of the total ice discharge from Greenland (Rignot et al. 2004, Rignot and Kanagaratnam 2006). Despite the spatially comprehensive analysis of changes in ice front position of Greenland’s outlet glaciers by Moon and Joughin

(2008), little work has been done to create a spatially and temporally robust assessment of glacier change in west Greenland. Current analysis of Greenland outlet glaciers shows variable trends in retreat over time scales of several years (Moon and Joughin 2008).

Substantial changes in outlet glacier dynamics, however, have been shown to occur on annual timescales or less (Howat et al. 2007). Therefore, analysis of glacier change at higher temporal resolution must be examined in order to better understand processes controlling glacier response to climate forcing.

To resolve the primary physical controls on Greenland outlet glacier dynamics, and specifically the response to climate forcing, variability in many glacier parameters must be assessed spatially and temporally. Previous research on the controls of glacier

dynamics has investigated variations in basal lubrication, effective pressure and resistive

stress at the front, and ungrounding/flotation of the front as primary controls of glacier

behavior in response to climate forcing. The relative importance of these parameters must

be addressed prior to examining additional controls of outlet glacier response to climate

forcing.

Changes in ice flow speed have been linked to additions of meltwater to the bed

of the Greenland Ice Sheet margin (e.g. Zwally et al. 2002, Das et al. 2008, Price et al.

2008). From measurements over a 3-year period, Zwally et al. (2002) found that ice

speed at a point on the ice sheet interior, near the equilibrium line altitude (ELA),

correlated with Positive Degree Days (PDD). They noted that networks of moulins could

provide a drainage path from the surface to the base of the glacier. Das et al. (2008) was

able to infer a well-connected subglacial drainage network from the transient uplift and

subsidence of the glacier surface following the drainage of a large meltwater lake. An

increase in basal meltwater through moulins can cause increased basal sliding, but the

associated speed-up is only ~100 m a-1 throughout the ice sheet, making its impact

relatively unimportant for the fast-moving outlet glaciers (Joughin et al. 2008a).

Increased basal sliding near the terminus could propagate up-stream through longitudinal

stress gradients (Price et al. 2008) but recent modeling studies suggest the patterns of

high stretching rates observed on marine-terminating outlet glaciers are not consistent

with what one would expect from an increase in basal lubrication (Nick et al. 2009).

Ground-based studies have further confirmed that speed-ups due to increased basal lubrication are short-lived and variations in velocity over multiple years are more likely related to changes in ice thickness and surface slope (Van de Wal et al. 2008).

Thinning of a calving glacier with a grounded front causes a reduction of resistive stress near the glacier front from a loss of effective pressure (ice overburden minus water pressure) at the bed. It has been hypothesized that this loss of effective pressure may result in increased basal sliding, acceleration, thinning and retreat of polar and subpolar outlet glaciers (Pfeffer 2007). The stability of glaciers with grounded fronts therefore may be heavily dependent on the surface elevation of the glacier and bed geometry near the front. Glaciers are relatively stable if they are able to maintain a marine shoal at the calving face, effectively reducing the calving rate and maintaining effective pressure at the front (Vieli et al. 2001, Meier and Post 1987, and others). If an ice front retreats into deeper water behind a marine shoal, calving will accelerate and the glacier will undergo rapid irreversible retreat (Pfeffer 2007). Retreat is more rapid than advance due to the positive feedback between ice thinning at the front, loss of effective pressure, increased ice speed and stretching (Weertman 1974,Vieli et al. 2001, Schoof 2007). Retreat and thinning will continue until the glacier front retreats into the bottom of an over- deepening, where the feedback reverses and a new equilibrium state is reached (Howat et al. 2007, Joughin et al. 2008b, Nick et al. 2009). Once retreat is initiated the behavior of grounded tidewater glaciers becomes relatively insensitive to climate change and more

dependent on glacier geometry and interactions with the bed (Trabant et al. 2003, Vieli et

al. 2001, Meier and Post 1987).

In order to balance forces in a glacier, the downhill pull of gravity (i.e. the driving stress) must be equally opposed by resistance generated by shearing along the ice bed

(i.e. basal drag) and along the fjord walls (i.e. lateral drag). The flow of land-terminating

glaciers is primarily controlled by basal drag, so that thinning leads to slowing as the

driving stress and basal drag decrease by a proportional amount. In contrast, the flow of

fast-moving outlet glaciers, ice streams, and glaciers terminating at floating ice tongues is primarily resisted by lateral drag, so that a reduction in glacier thickness will therefore reduce resistive stress by shrinking the contact area between the glacier and fjord walls, resulting in acceleration as the rate of shearing must increase to compensate. Thinning near the front may bring the glacier close to the flotation level, causing the break-up and retreat of the floating tongue (Howat et al. 2005, 2007, 2008, Pfeffer 2007, Joughin et al.

2008b). Once flotation is reached resistive stress will be reduced further due to thinning of the ice tongue from basal melting from ice-ocean interaction. Penetration of ocean water to the grounding zone can cause additional retreat of the grounding zone and faster ice flow (Schoof 2007). Decreased resistive stress near the front also causes the glacier surface to steepen at the grounding line, increasing the gravitational driving stress and further increase in flow speed and thinning (Schoof 2007).

Following retreat, a glacier thins in response to increased stretching caused by steeper velocity gradients near the front. Inland propagation of dynamic thinning reduces driving stress near the front, causing a decline in speed and discharge until an equilibrium

state is reached (Howat et al. 2007, 2008). If dynamic thinning does not cause the front to reach flotation, slower velocities may create positive mass balance at the front, resulting in re-advance. Alaskan tidewater glaciers typically take an order of 1000 years to re- advance because effective pressure is maintained by the formation of a sediment platform

(marine shoal) at the front (Meier and Post 1987). The re-advance of Greenland outlet glaciers may be faster than Alaskan glaciers because basal drag can be regained by re- grounding floating ice tongues on the seaward side of bed over-deepenings (Howat et al.

2007, Joughin et al. 2008b).

Modern retreat of Greenland outlet glaciers is likely a result of thinning and ice front destabilization from increased air and ocean temperatures (Holland et al. 2008,

Howat et al. 2008). Destabilization occurs as the glacier approaches flotation due to thinning and increased crevassing from progressively steepening longitudinal stress gradients caused by acceleration. Therefore, glaciers with shallow surface slopes should be particularly sensitive to dynamic thinning because thinning will bring a greater inland extent of ice closer to flotation, potentially triggering grounding zone retreat and further acceleration. The subsequent dynamic response of the outlet glacier and interior ice sheet should then be controlled by the surface slope. Thinning of glaciers with steep upper slopes should be concentrated within the lower outlet, making them prone to rapid retreat of ice near flotation without extensive inland migration of the grounding zone. This occurs because diffusion decreases with steeper slopes, limiting the distance thinning can propagate from the front (Weertman et al. 1958). In general, propagation of thinning in ice is described by a kinematic wave of the form

∂h ∂c  ∂D  ∂h ∂ 2h 1 = b − 0 h − c − 0  1 + D 1 , (1) ∂t 1 ∂x 1  0 ∂x  ∂x 0 ∂x 2 where q is flux per unit width, h is ice thickness, b is the mass balance, x is the along-

∂q ∂q flow direction, c = is the speed of the kinematic wave, D = is diffusivity, α is ∂h ∂α

surface slope, c0 and D0 represent equilibrium state conditions, and perturbation states

are denoted by h1 and b1 (Nye 1960, Hooke 2005). Thus, longitudinal diffusion of thinning is determined by the magnitude of the perturbation, along-flow velocity gradient, speed of the kinematic wave, and diffusive dampening (Nye 1960, Price et al.

2001, Hooke 2005). The motion of the kinematic wave and the diffusion of the wave are inversely proportional to changes in the mass flux with respect to surface slope, or initial

diffusivity ( D0 ) in Equation 1, meaning thinning is concentrated near the front for glaciers with steeper slopes.

The width of an outlet glacier may control glacier retreat due to the inverse relationship between width and lateral drag at the glacier center line. Lateral stresses propagate from the shear margin to the center line for a distance of up to 30 times ice thickness if basal shear stress is low (Raymond et al. 1996), such as for Greenland outlet glaciers. As glaciers approach flotation and basal stress is reduced, lateral shearing, and thus flow velocity, will increase, raising the relative importance of lateral stress

(Echelmeyer et al. 1994).

Due to the potential for near-future climate warming and the associated risk of accelerated rates of sea level rise due to ice sheet deterioration partially caused by

changes in glacier dynamics, it is important to resolve the controls of glacier response to

similar climate forcing. As displayed by Moon and Joughin (2008), marine-terminating

outlet glaciers in Greenland have responded to climate change in a non-uniform manner

since the 1990’s, potentially due to differences in glacier geometry. Pfeffer (2007) and

Meier and Post (1987) propose that climate controls the long-term mass balance of a

glacier, thus influencing glacier geometry over long periods of time (centuries to

millennia). Trabant et al. (2003) hypothesized that tidewater glaciers only become

sensitive to climate change once expansion of the front causes the ratio of the

accumulation area to the total glacier surface area (termed the Accumulation Area Ratio,

or AAR) to decrease below 0.95, at which time the ice flux reaching the glacier front

becomes less than mass loss, resulting in thinning and retreat. Mass loss is attributed only

to ablation in this situation, assuming melting below the water line is negligible. Since the

AAR >>0.95 for Greenland outlet glaciers, they should be relatively insensitive to

climate change. Fjord topography and width act as pinning points, additionally

controlling the location of the grounding line during retreat and advance (Warren and

Glasser 1992). However, melting below the water line is substantial for Greenland outlet

glaciers and has been directly attributed to the retreat of Jakobshavn Isbræ (Holland et al.

2008) suggesting outlet glaciers are sensitive to climate change despite the AAR >>0.95.

Therefore, to better understand the controls of marine-terminating outlet glaciers experiencing similar climate forcing, data must be spatially and temporally robust.

Spatial coverage includes large (>2 km-wide) marine-terminating outlet glaciers identified along the west Greenland coast from ~76°37’N to ~61°35S. The approximate

locations of the glaciers included in the study are shown in Figure 1. Although the glaciers included in the study are not a comprehensive list of all west Greenland outlet glaciers, a total of 59 glaciers are included to ensure all possible variability in glacier parameters (i.e. width, orientation, floating/grounded front, etc) is encompassed by the data set. Data are collected using satellite images from 2000 through 2009 to analyze for seasonal, annual, and decadal variability. Satellite images are compiled to compare geometric parameters such as surface slope, glacier width, and orientation along with changes in front position and surface elevation throughout the time series. Analysis of glacier behavior throughout the record should provide insight into the controls of west

Greenland outlet glacier response to climate forcing.

2. Methods

Data sources and general methodology used in this study are discussed below.

Background information on data sources is briefly described to provide rationale for data usage.

2.1 Data and Sources

Data were acquired from the visible to near infrared (VNIR) wavelengths (bands

1-3) of the Advanced Spaceborne Thermal Emissivity and Reflection Radiometer

(ASTER) and Landsat 7 Enhanced Thematic Mapper Plus (ETM+) Scan Line Corrector- on (SLC-on) and Scan Line Corrector-off (SLC-off), ortho-images from the French Space

Centre’s SPOT 5 stereoscopic survey of Polar Ice: Reference Images and Topographics

(SPIRIT), radar images from the Canadian Space Agency’s Radar Satellite

(RADARSAT), and surface elevation profiles along central flow lines from laser- altimeter surveys with NASA’s Airborne Topographic Mapper (ATM). All sets of remotely sensed data were utilized to create a temporally complete assessment of glacier behavior as described below.

2.1.1 ASTER Data

ASTER data were acquired through the Land Processes Distributed Active

Archive Center (LP DAAC, https://lpdaac.usgs.gov/). ASTER data were selected based

on its high resolution (15 m) and summer acquisition dates. Front positions were mapped

using the visible to near infrared (VNIR) wavelengths (bands 1-3) of orthorectified image products. The orthorectified images provide near-vertical views registered to UTM coordinates. Surface elevations were measured using on demand digital elevation model and registered radiance at the sensor-orthorectified (ASTDMO) products from the LP

DAAC. DEMs produced by ASTDMO were created using nadir and backward-looking

VNIR image pairs taken 57 seconds apart. Starting in 2006, DEMs were produced with automated stereo-correlation, eliminating the use of ground control points. Final DEMs were geodectically referenced to the UTM coordinate system and claim accuracy typically better than 25-m RMS error. Random elevation errors are significantly reduced when averaged over scales much greater than the pixel size (15 m). Therefore, for the purpose of this study, ASTER DEMs provide accurate (±5 m) elevations over the several kilometer scales near the relatively flat glacial margins analyzed in this study.

2.1.2 Landsat 7 ETM+ Data

Landsat 7 ETM+ SLC-on and SLC-off data were acquired through the United

States Geological Survey (USGS) Global Visualization Viewer (GLOVIS, http://glovis.usgs.gov/). Landsat 7 ETM+ data were selected due to its high resolution (15

m), repeat image cycling (16 days), and temporal coverage (spring, summer). Front positions were mapped using SLC-on VNIR wavelengths from April 2000-May 2003 and

SLC-off VNIR wavelengths from July 2003-September 2009. A malfunction in the scan line corrector (SLC) occurred on May 31, 2003 creating data gaps in the final product available to the public. Despite the data gaps, the images are still useful for mapping front positions, particularly for the central portion of each image as data gaps become larger towards image margins. All Landsat images were made freely available to the public through the GLOVIS website in November 2008, further encouraging the use of Landsat imagery to fill data gaps in our ASTER time series.

2.1.3 SPIRIT Data

SPIRIT DTMs from the SPOT 5 satellite were obtained at no cost through the

French Space Centre during the International Polar Year (June 2007-June 2009). SPIRIT

imagery was primarily utilized due to the high resolution (~5 m) of its ortho-images and high vertical accuracy (10 m) of its DTMs. DTMs were created using stereoscopic pairs taken from telescopes oriented 20° toward the rear and 20° toward the front relative to nadir. The image repeat cycle was scheduled for ~26 days but may be as small as 1-2 days at high latitudes. Precision between images pairs was quoted as better than 30-m

RMS error.

Ortho-images and DTMs were pre-processed and converted to polar stereographic coordinates prior to purchase. SPIRIT DTMs were manually checked for horizontal

accuracy to ensure elevation profiles obtained from the both the SPIRIT DTMs and

ASTER DEMs were representative of the same approximate flow lines. The high resolution DTMs provide better spatial and temporal coverage from 2007-2009 than available through ASTER data alone.

2.1.4 RADARSAT Data

RADARSAT mosaics were provided by Dr. Ian Joughin, University of

Washington. RADARSAT images were acquired using a synthetic aperture radar (SAR) instrument in 2000/2001, 2005/2006, and 2006/2007. Mosaics were created using fine-

beam (20 m) resolution images and were coregistered to ensure errors of less than 20 m

between mosaics. Use of the SAR instrument enables imaging of surface conditions

through cloud cover, allowing images to be compiled for the entire ice sheet. Mosaics of

the ice sheet were used to map winter front positions for comparison with summer fronts.

If summer fronts could not be mapped for years with RADARSAT data, winter fronts

were examined to monitor potentially rapid changes in front position.

2.1.5 NASA ATM Data

NASA ATM surface elevation profiles were utilized when available because their

airborne conically scanning laser altimeter provides swaths of highly dense, accurate

surface elevation measurements. Data were ideally collected along central flow lines of

each glacier with deviations from the flow direction occurring when necessary. Data were 12

fit with 70-m planar surfaces described by center coordinates, surface slope, and roughness of the ice surface. Planar surfaces were used in conjunction with repeat surveying to ensure accurate measurement of elevations regardless of slight deviations in flight paths. Data were pre-processed prior to acquisition and provided as three-

dimension geographic coordinates. Elevations were initially referenced to the World

Geodetic System 1984 (WGS84) ellipsoid and were manually corrected to a standard

base level ocean elevation (0 m) for appropriate comparison to ASTER DEMs and

SPIRIT DTMs. Variations in tidal height (±2 m) were within DEMs/DTM error and were not considered.

2.1.6 Relative Errors

Relative errors between the different image types used in the study were briefly

compared to ensure accuracy of collected data. Relative errors between RADARSAT

mosaics are less than 20 m while absolute errors range from 20-60 m near ice fronts.

Offsets between ASTER, Landsat, RADARSAT, and SPIRIT images caused by

topographic distortion were examined using off-ice control points. Offsets of ±74 m, ±87

m, and ±25 m for Landsat, RADARSAT, and SPIRIT images respectively relative to

ASTER images were recorded. The reasonably small offset between images enabled

accurate mapping of front positions to well within ±100 m. Elevation profiles were

manually corrected to mean sea level (0 m) following data extraction. Seasonal

differences between NASA ATM data and other DEMs/DTMs are expected to cause only

minor discrepancies in interpretation of surface elevation profiles and surface slopes. 13

2.1.7 Climate Data

Meteorological data were obtained through Goddard Institute for Space Studies

(GISS). Monthly and annual average air temperatures from Nuuk/Godthab (64.2°N,

51.8°W) and Aasiaat/Egedesminde (68.7°N, 52.8°W) were analyzed from 2000-2009 for comparison with trends in observed glacier behavior. Extended records of air temperatures, excluding data from our study period, were analyzed for long-term

(decadal) trends. Temperature data from Thule (76.5°N, 68.8°W), Upernavik (72.8°N,

56.2°W), Jakobshavn (69.2°N, 51.1°W), and Sdr. Stromfjo (67.0°N, 50.8°W) were compared for the extent of their records, typically ending in the 1970’s or 1980’s.

Records excluding data from 2000-2009 are not presented in this paper because they were used to analyze long-term air temperature trends, and do not directly reflect climate forcing in this region during our observational record of glacier change.

Sea surface temperatures (SSTs) were acquired from the Physical Oceanography

Distributed Active Archive Center (PO.DAAC, http://podaac.jpl.nasa.gov/), providing monthly average SSTs from the Moderate Resolution Imaging Spectroradiometer

(MODIS) aboard NASA’s Terra and Aqua satellites. Both satellites image the entire

Earth surface every 1-2 days at relatively high spatial resolution (4.63 km x 9.26 km) creating a comprehensive record of global SSTs. Data were analyzed at several locations along the west coast to examine regional SST trends. Data were compared to ocean temperature reanalysis data provided by the Naval Postgraduate School from 2000-2004

to verify the validity of the reanalysis model. The reanalysis model was then used to examine changes in total heat flux integrated through the water column. Heat flux was measured across three arbitrarily determined “gates” through which both the direction and magnitude of heat flow were modeled. The three gates divide Greenland’s west coast into northern, central, and southern regions. SST data were also subset according to these regions to better resolve differences in climate forcing throughout the west coast.

Coupling of air temperatures, SSTs, and heat flux data allows a simple description of climate forcing and climate change along the west Greenland coast from 2000-2009.

Neither air temperatures nor SSTs used in the analysis are representative of conditions

near the surface of the outlet glaciers, as the locations of measurements range in distance

from outlet glacier fronts (Figure 1). However, for the purpose of this study, these data simplify the recent climate regime along the coast of west Greenland.

2.2 Front Positions

Front positions were mapped for 59 marine-terminating outlet glaciers in west

Greenland using orthorectified Landsat 7 ETM+ images and ASTER VNIR images from

2000 through 2009. Due to the limited spatial coverage of ASTER imagery in western

Greenland, front positions were also mapped using 5-m SPIRIT ortho-mosaics.

Ice front positions were mapped using a technique similar to that of Moon and

Joughin (2008). Since mapping ice front positions along centerlines will often provide arbitrary assessment of change, front positions are compared according to mean change.

The mean change in front position is determined by first delineating the sides of the glacier with a rectangle. The long axes of each box are oriented parallel to the direction of flow, and the box is terminated at an arbitrary distance up-glacier to allow for future mapping. Line vectors are then drawn along the front position for each available image and the mean position of each vector is calculated using the known width of the rectangle. Front positions were mapped for all available imagery to examine inter- and

intra-annual variability. Locations of glaciers included in the study are shown in Figure 1.

2.3 Surface Elevations and Surface Slopes

Surface elevations were extracted from ASTER DEMs and SPIRIT DTMs, either

along a central flow line visually identified from flow features in ASTER VNIR images,

or along the flight path of the NASA ATM data when applicable. Although flight paths

may not follow central flow lines, a brief comparison of elevation profiles along flow

lines and those obtained with NASA ATM flight paths indicates that the differences in

the data are minimal. Surface slopes were then calculated using the “best” or averaged

annual surface elevation profile. Surface slopes were obtained for four different distance

ranges to encompass all possible variability due to changes in front position. Changes in

slope near the front were monitored from the front to distances of 1-km and 5-km inland.

We refer to these slopes as the 1-km and 5-km slopes for simplicity. Slopes were

calculated for the retreated section of each glacier as well. The retreated distance used in

the calculation was determined by the largest consecutive retreat throughout the record

for each glacier. Slopes were calculated up to the summer snowline to monitor potential 16

changes in ablation zone geometry. These slopes, referred to as snowline slopes, were calculated from the grounding zone to the summer snowline to monitor slope changes of grounded ice only. The grounding zone was approximated for each glacier based on surface elevation profiles and surface slope. The location of the grounding zone was inferred to coincide with an abrupt increase in surface slope for glaciers terminating as floating ice tongues. The location of the grounding zone coincided with the front position for glaciers with grounded fronts.

2.4 Surface Speed

Surface velocities were determined using multiple-image/multiple-chip (MIMC) repeat image feature tracking (RIFT) software developed by Dr. Yushin Ahn, Byrd Polar

Research Center (Ahn and Howat 2009). MIMC feature tracking was performed using pairs of Landsat 7 ETM+ and ASTER VNIR imagery with acquisition date separation of

10-90 days. The RIFT algorithms use cross-correlation of spatial variations in image intensity, such as crevasses, to match images. Cross-correlation produces a pixel displacement field, which is further processed to produce a velocity field. Image pair offsets were minimized by co-registration of stationary control points within both images.

Here we present time series of average speeds calculated near the front of each glacier rather than velocity fields to simplify the analysis. Speeds were obtained from along the centerline or flight path used for surface elevation profiles. Surface speeds are presented for select glaciers with large front retreats during the study to demonstrate variability in response to large perturbations at the front. 17

2.5 Additional Observed Parameters

Additional parameters were examined for each glacier to aid in interpretation of ice surface elevation change and speed observations. The width of each glacier was measured from the ASTER and LANDSAT imagery to calculate the average distance of retreat and to examine the differences between glaciers with wide, broad fronts versus those with more confined fronts through the role of lateral resistive stress as described above. The inland distance to which flow is bounded by fjord walls was also noted.

Glaciers which are bounded by rock walls, as opposed to ice, may undergo more rapid thinning because thinning at the front would be concentrated over a smaller area whereas thinning of an unconfined glacier would likely diffuse over the surrounding ice. Glacier latitude was also recorded to assess a possible relationship between behavior and location.

Figure 1: Location map. Red circles denote 59 marine-terminating outlet glaciers in this study. Yellow triangles denote weather stations. Blue squares denote SST sites. Solid white lines define heat flux gates and dashed white lines differentiate boundaries of the heat flux gates. Numbers coincide with glaciers presented in detail in the text: (1) Edvard, (2) Rink South, (3) Alison, (4) Upernavik North, (5) Umiamako, (6) Jakobshavn. 19

3. Results

From 2000 through 2009, there is no evidence suggesting synchronous changes in

west Greenland outlet glaciers in response to climate forcing on annual time scales.

Changes in front positions indicate all glaciers either retreated or maintained stable front

positions on a decadal time scale (2000-2009), but the timing and magnitude of retreat varied significantly between consecutive summer measurements. Front positions were considered stable if they varied by no more than ±0.1 km a-1 because measurement uncertainty is of comparable magnitude (See Section 2.1.6). While most glaciers thinned, the timing and magnitude of thinning differed throughout the data set.

3.1 Overview of Changes in Front Position

Throughout the time series, of the 59 marine-terminating outlet glaciers in the study, 50 retreated and 9 maintained stable front positions. The magnitude of front

change was highly variable with the largest total retreat in the sample being 12.8 km

(Jakobshavn: 69°10'59.33''N, 49°37'27.44''W) and mean retreat of 1.4 km for all 59

glaciers. There were no latitudinal trends in front position change on annual or decadal

timescales. Figure 2 provides an overview of retreat for all glaciers included in the study

from 2000-2009.

3.2 Overview of Changes in Surface Elevation and Slope

Figure 3 is a composite of the surface elevation profiles for 58 of the 59 glaciers

included in the study. Dietrich glacier is excluded from the plot because the flight path

used to extract the elevation profiles does not reach the front. Profiles are arranged

according to latitude from the left to right and from top to bottom, to facilitate

comparison of neighboring glaciers. Changes in surface elevation varied from ~120 m

thinning to 10 m thickening between 2000 and 2009. All glaciers thinned during a portion

of the study period though the timing was asynchronous on annual time scales. Slight

thickening occurred for a few glaciers from 2000-2009, despite thinning during a portion

of the record. No obvious latitudinal trends in the magnitude of or rate of thinning were

evident. However, if one considers decadal changes in elevation, a greater number of

glaciers with small thinning rates are found at lower latitudes compared to those at higher

latitudes.

Several glaciers (Edvard, Sverdrup, Steenstrup, Upernavik North, and Umiamako)

experienced periods of relatively little surface elevation change followed by thinning of

up to ~100 m a-1 between consecutive summer seasons. These rapid thinning periods

were then followed by several years of relatively stable surface elevations, indicating the

relative distribution of resistive stresses was temporarily steady near the front. Records

indicate thinning periods occurred between 2003-2007 for Edvard, 2002-2005 for

Sverdrup, 2002-2005 for Steenstrup, 2005-2006 for Upernavik North, and 2007-2008 for

Umiamako. Rapid thinning periods were constrained to consecutive summers for

Upernavik North and Umiamako glaciers. Thinning rates could not be calculated for

consecutive summers for Edvard, Sverdrup, and Steenstrup due to data gaps within the time series for each glacier. These five glaciers have no distinct shared geometric characteristics (i.e. surface slope, width, orientation, etc.) differentiating them from glaciers exhibiting more gradual changes in surface elevation. Their front widths range from ~3-14 km and their pre-retreat snowline slopes range from 45-111%. The asynchronous timing of the thinning periods is a result of differences in the timing and rate of front retreat.

Slopes were highly variable overall, with snowline slopes ranging from 0.018 to

0.092, most likely reflecting changes in basal topography more so than differences in glacier thickness. To determine changes in surface slopes over time due to dynamic thinning near the front, slopes were calculated prior to and following front retreat.

Glaciers with a limited time series or multiple low quality profiles were excluded, leaving average slopes calculated using 48 of the 59 glaciers. Individual slopes are presented as a percentage relative to the calculated average for the data set unless otherwise specified.

The percent slopes for each glacier are presented in Table 1. The average 1-km

(5-km) slope was 0.044 (0.040) prior to retreat and 0.052 (0.041) following retreat. The average snowline slope was 0.036 prior to retreat, and 0.037 following retreat. The average slope for the retreated section was 0.053. Slopes were highly variable and had no direct relationship with retreat magnitude. Glaciers are examined individually in the next section to assess possible relationships between slope and front retreat on smaller spatial and temporal scales.

3.3 Case Studies of Glacier Change

Several glaciers are presented in detail below to obtain a better understanding of the large changes in front position, surface elevation, and surface slope within the data set. The following glaciers do not encompass the total range of variability, but rather, they represent some of the most significant changes recorded from 2000-2009.

3.3.1 Edvard Glacier

Edvard Glacier (76°18'24.26''N, 61°58'28.31''W) is southwest-facing and located in northwest Greenland in the Melville Bay region (Figure 1). The glacier terminates as a floating tongue into a short, ~5.5km-wide fjord. The presence of a floating tongue was inferred from the abrupt slope change from 0.0313 to ~0.001 near the front. The summer snowline was ~20 km from the 2000 front position. The break-up of the floating tongue reached its peak retreat rate of 2.7km a-1 from summer 2003-2004. The glacier retreated

4.9 km (Figure 4A) from 2000-2009. Surface elevation data are sparse throughout the time series, including during the period of most rapid retreat. Elevation data for 2004 include inland surface elevations similar to those measured in 2007 (Figure 4B), when the front was at its most retreated position. Retreat coincided with nearly 100 m of thinning at the grounding zone. Loss of the floating ice tongue and thinning near the grounding zone caused the 5-km slope to steepen by 75% and the snowline slope to steepen by 5%.

Surface speed increased from 2.2 m d-1 in 2002 to 3.3 m d-1 in 2009 at a location 2.4 km

inland from the grounding zone, concurrent with retreat and thinning.

3.3.2 Rink South Glacier

Rink South Glacier (76°14'13.71''N, 60°57'46.79''W) is southwest-facing and located ~30 km south of Edvard Glacier, in the Melville Bay region (Figure 1). The glacier front is ~2.8 km-wide at its most retreated position, located between two rock outcrops. Break-up of the glacier’s floating tongue occurred in 2005, following a period of relative stability of the front position from 2000-2004 (Figure 5A). The average surface elevation of the floating tongue decreased from ~30 m a.s.l. in 2000 to ~1 m a.s.l. in 2004 prior to its break-up. Surface lowering implies ~300 m of total thinning of the floating tongue indicating the tongue was reduced to an ice mélange prior to its final break-up (Figure 5B). Thinning was concentrated at the grounding zone, causing grounding zone retreat of >1 km from 2000-2004. Snowline slopes were 223% and 241% prior to and following retreat, respectively, due to ~100 m of thinning at the grounding zone. Surface speed increased by 39% from 2001-2009 at a location 2.5 km inland from the grounding zone. The average June speed throughout the study was 1.4 m d-1.

Considerable seasonal variability was evident despite limited data availability, with peak speeds typically occurring at the end of July.

3.3.3 Alison Glacier

Alison Glacier (74°37'21.31''N, 56°13'4.99''W) is west-facing and terminates at a

~4.8 km-wide floating ice tongue (Figure 1). The snowline is located ~10 km from the

grounding zone, where the slope steepens by 250%. Front position measurements indicate a gradual, steady break-up of the floating tongue from 2001-2009 (Figure 6A).

The total front retreat was 10.4 km, second only to Jakobshavn Isbræ. The large retreat coincided with an approximate doubling of surface speed (Joughin et al. in prep.), resulting ~70 m of dynamic thinning near the grounding zone. The retreat of the floating ice tongue evident in surface elevation profiles (Figure 6B) causes the 5-km slope to steepen from 3% prior to retreat to 97% following retreat. Snowline slopes prior to and following retreat were 130% and 141% of the respective average slopes. Changes in surface speed within 1 km of the grounding zone were directly related to summer front retreat rates (Figure 6C). From 2000-2005, surface speed increased by 0.7 m d-1 annually,

corresponding to an average retreat rate of 1.5 km a-1. Following 2005, surface speed stabilized at ~7.4 m d-1 and the average front retreat rate slowed to ~0.8 km a-1.

3.3.4 Upernavik North Glacier

Upernavik North Glacier (73°0'2.33''N, 54°26'32.14''W) is southwest-facing and

terminates at a small floating tongue (Figure 1). The glacier is confined to a 4.3 km-wide bedrock fjord for ~10 km upstream of the front. The front retreated 4.8 km from 2000-

2009. Front retreat was likely affected by interaction with the neighboring glacier,

Upernavik Northwest, which provided resistive stresses transverse to flow at the beginning half of the observation period. Following the separation of the fronts in 2005,

Upernavik North advanced slightly prior to retreat to its 2009 position (Figure 7A). The front retreated at a maximum rate of 2.8 km a-1 between 2007 and 2008. Thinning of >50 25

m occurred for the extent of the elevation profiles, with the largest amount of thinning

(~120 m) near the grounding zone, 8-12 km from the front (Figure 7B). The snowline

slope steepened by 13% and the 5-km slope to shallowed by 70% during the study period,

potentially bringing ice close to flotation at the current grounding zone. Observations of

surface speed are limited due to poor image quality but available data indicate peak

speeds in 2005 corresponding to a small front advance. The average speed within 1 km of

the grounding zone for 2000-2008 was ~8.5 m d-1 with peak speed of 9.8 m d-1 in 2005.

Rignot et al. (2008) recorded a 20% increase in ice speed near the front in 2006/2007,

within a gap in our temporal coverage, suggesting speed may have increase more than

indicated in our data set.

3.3.5 Umiamako Glacier

Umiamako Glacier (71°44'3.32''N, 52°24'53.34''W) is southwest-facing and confined within a fjord for over 40 km upstream from the front (Figure 1). The glacier is

~3-km wide throughout most of the length of the fjord, becoming slightly more narrow at the 2000 front location. From 2000-2005, the front retreat rate accelerated to 0.9 km a-1

then decelerated to ~0.4 km a-1 by 2007. The front then retreated behind the narrow

section of the fjord (Figure 8A), reaching its maximum retreat rate of 1.3 km a-1 between

2007 and 2008, followed by 0.4 km of retreat between 2008 and 2009. Front retreat totaled 3.8 km from 2000-2009. The surface elevation profile is distinct from other large- retreat glaciers because it terminates as a grounded front with a steep surface slope

(Figure 8B). Thinning near the front caused the 5-km slope to relax from 169% to 110% 26

and the snowline slope to steepen by 16% from 2000-2009. Apparent thickening in 2009 may be caused by differences in acquisition dates for the 2008 and 2009 elevation profiles. The ~20-m difference in elevations may be partly due to the early acquisition date (April) for the 2009 data, though seasonal melt is unlikely to account for such large differences in elevation because we expect an average accumulation rate of ~2 m water equivalent (we) per year in this region. Changes in surface speed can be directly related to retreat rates (Figure 8C). Speeds within ~2 km of the 2009 front remained relatively constant at ~1.9 m d-1 from 2000-2004 when front retreat was minimal. The glacier reached its maximum July speed of 4.7 m d-1 in 2009 and may continue to accelerate in

the future.

3.3.6 Jakobshavn Isbræ

Jakobshavn Isbræ (69°10'59.33''N, 49°37'27.44''W) is west-facing and located

within the Disko Bay region of west Greenland. Jakobshavn is the most well studied

glacier in west Greenland because it drains an ice equivalent of ~7% of the Greenland Ice

Sheet’s total accumulation. The front retreated from within a 9.4 km-wide fjord in 2004

as its northern and southern branches began to separate. From 2000-2009, Jakobshavn

retreated 12.8 km due to a significant break-up of its floating tongue (Figure 9A). Retreat rates varied from year-to-year, with the fastest retreat rates occurring from 2002-2003

(3.3 km a-1) and from 2003-2004 (2.5 km a-1). Surface thinning was minimal during years

of fastest retreat, but increased to >50 m from 2005-2007 (Figure 9B). From 2000-2009,

the 5-km slope decreased by 7% and the snowline slope increased by 6%. Changes in 27

surface slope were relatively small due to the nearly uniform surface lowering that occurred up to the snowline. On average acceleration was ~0.6 m d-1 annually, causing the speed to increase from 13.7 m d-1 in 2000 to 20.0 m d-1 in 2009 approximately 8 km

upstream from the 2009 front (Figure 9C).

Figure 2: Front position change (2000-2009) for all marine-terminating outlet glaciers included in the study. The circles represent the average front position change of each glacier. The colored circles denote varying amounts of front retreat. Warmer colors indicate larger retreat and cooler colors indicate stable fronts.

continued Figure 3: Surface elevation profiles for the 58 glaciers included in the study, plotted according to latitude, with the highest latitude glacier plotted in the upper left and the lowest latitude glacier plotted in the lower right. Dietrich Glacier is excluded because its elevation profiles do not intersect the ocean, causing elevations to be considerably higher.

Figure 3 continued

1km Pre- 1km Post- 5km Pre- 5km Post- Snowline Snowline Retreated Glacier Name Retreat Retreat Retreat Retreat Pre-Retreat Post-Retreat Area Harold Moltke 52 43 46 47 47 49 39 Pitugfik 127 84 63 60 70 59 127 De Dodes Central 62 53 84 57 93 63 53 Meteor 162 148 100 89 126 112 78 Yngvar Nielsen -2 7 18 8 102 94 4 Norujupaluk 71 74 116 103 140 146 59 Gade 225 160 88 90 81 85 380 Edvard 3 9 55 93 84 88 34 Docker Smith 23 75 50 64 82 79 26 Fisher 187 175 170 143 147 138 136 Rink North 17 194 244 234 245 258 69 Rink South 37 125 114 231 223 241 45 Balgoni 81 54 195 194 82 87 55 Igssuarssuit 123 119 41 30 41 38 99 Kong Oscar 8 20 44 56 70 73 27 Nordenskiold 108 59 121 134 112 163 94 Nansen 43 N/A 75 78 68 68 35 Sverdrup 15 61 50 37 45 38 35 Dietrich 19 101 132 149 127 121 5 Alison -11 37 3 97 130 141 4 Igdlulik 94 123 86 65 55 57 69 Comell 410 232 136 131 148 145 475 Sugar Loaf 238 107 73 60 117 118 390 Ussing 1 -6 67 69 88 86 -19 Continued Table 1: Glacier slopes. Slopes calculated for each glacier are expressed as a percent relative to the average of all glaciers. Glaciers with incomplete surface elevation data sets are excluded because they were not included in calculated averages.

Table 1 continued

1km Pre- 1km Post- 5km Pre- 5km Post- Snowline Snowline Retreated Glacier Name Retreat Retreat Retreat Retreat Pre-Retreat Post-Retreat Area Giesecke 267 169 99 97 93 93 223 Tasiussaq -64 -21 49 76 77 76 7 Tuvssaq 29 30 77 77 99 77 4 Upernavik Northwest 149 123 143 102 142 119 96 Upernavik North 34 103 85 58 111 124 46 Upernavik Central 58 50 98 81 104 84 64 Upernavik South 44 81 42 52 67 59 82 Ingia 49 79 44 41 82 83 35 Umiamako 261 343 170 111 71 82 216 Karrat 132 99 71 74 67 69 133 Perdlerfiup 168 95 247 284 217 224 148 Silardleq 107 70 83 65 89 92 90 Kangigdleq 95 104 181 175 119 117 43 Store 308 170 64 55 65 62 74 Avangnardleq 8 15 97 94 67 72 10 Kangilerngata 115 64 63 84 76 77 110 Kujatdleq 22 46 48 65 54 59 31 Eqip 40 36 133 119 80 78 341 Torssukatak 94 195 141 100 110 106 74 Jakobshavn -1 42 50 45 63 66 41 Akugdlerssup 188 203 156 138 106 108 83 Kangiata 77 53 155 161 116 121 84 Kvane 339 257 252 251 127 129 272 Sermilik 187 242 80 77 77 76 180 Average 0.044 0.055 0.040 0.041 0.036 0.037 0.053

Figure 4: Front positions (A) and surface elevation profiles (B) for Edvard Glacier (76°18'24.26''N, 61°58'28.31''W). Limited cloud-free images prevented analysis of front positions for 2005-2006 and DEM construction for 2000 and 2005-2006.

Figure 5: Front positions (A) and surface elevation profiles for Rink South Glacier (76°14'13.71''N, 60°57'46.79''W). The floating tongue was only an average of 1 m a.s.l. prior to its break-up in 2005.

continued Figure 6: Front positions (A), surface elevation profiles (B), and surface speeds (C) for Alison Glacier (74°37'21.31''N, 56°13'4.99''W). Limited cloud-free images prevented DEM construction for 2000 and 2006. 36

Figure 6 continued

Figure 7: Front positions (A) and surface elevation profiles (B) for Upernavik North Glacier (73°0'2.33''N, 54°26'32.14''W). The intersection of the front with Upernavik Northwest strongly influenced front positions until 2005 when the fronts separated.

continued Figure 8: Front positions (A), surface elevation profiles (B), and surface speeds (C) for Umiamako Glacier (71°44'3.32''N, 52°24'53.34''W). Plotted surface speeds for 2009 are from early summer and may not reflect maximum summer speeds. 39

Figure 8 continued

continued Figure 9: Front positions (A), surface elevation profiles (B), and surface speeds (C) for Jakobshavn Isbræ (69°10'59.33''N, 49°37'27.44''W). The break-up of the floating tongue occurred in pulses, with the largest retreats from 2003-2004, 2005-2006, and 2007-2008. 41

Figure 9 continued

4. Discussion

Changes in front position and surface elevation for the 59 marine-terminating outlet glaciers in this study indicate the majority of marine-terminating outlet glaciers in west Greenland retreated and thinned from 2000-2009. Although there was no distinct relationship between the magnitude of front retreat, surface elevation change, and surface

geometry applicable to the entire data set, the geometric parameters of the several

glaciers may explain their distinct behavior. The discussion presented below addresses

climate forcing conditions from 2000-2009 in relation to observed front retreat. We also

address relationships between front retreat, thinning, and surface slopes, differences

between grounded and floating termini, and contrasting glacier behavior on small spatial

scales in order to resolve controls of outlet glacier behavior within our observational

record.

4.1 Climate Forcing Conditions

We first examined the data for similarities in the magnitude, timing, and rate of

front retreat to better resolve any relationship between front retreat and climate forcing.

Total front retreat throughout our study area had a mean of 1.4 km, median of 0.7 km,

and mode of 0.2 km. Approximately 2/3 of the glaciers retreated less than 1.0 km, many

of which had stable front positions (fluctuated by less than ±0.1 km) from 2000-2009. In

contrast, several glaciers reached maximum retreat rates exceeding 1.0 km a-1 during the

record. Rapid retreat rates were only sustained for several consecutive summers for

Alison Glacier and Jakobshavn Isbræ, suggesting the response to a perturbation at the

front was short-lived for most glaciers included in the study. Large front retreats were

asynchronous, occurring between all consecutive summers other than 2000-2001.

To assess the potential influence of regional climate forcing on observed front

retreats, available air and sea surface temperature (SST) data were compared to the

observational record. Average monthly and annual temperatures from two weather

stations, Aasiaat/Egedesminde (68.7°N, 52.8°W) and Nuuk/Godthab (64.2°N, 51.8°W),

were examined to determine seasonal and annual changes in air temperatures (Figure 10)

potentially effecting front retreat. These stations recorded an average annual temperature

increase of 0.136°C a-1 (R2=0.337) and 0.110°C a-1 (R2=0.404), respectively, from 2000-

2007. Average annual temperatures then decreased at both stations from 2007-2009.

Large SST anomalies were present in the northern, central, and southern regions of the

west coast of Greenland (Figure 1 for locations, Figures 11A, 11B, 11C). Timing and

magnitude of SST anomalies varied within each region and throughout the entire data set.

Positive SST anomalies had no direct relationship with timing or magnitude of front

retreat or thinning of floating ice tongues. Although high SSTs may contribute to front

destabilization, our data suggest they are not the direct cause of front retreat of outlet

glaciers in west Greenland.

Modeled heat flux integrated through the water column was examined as a

potential trigger for front retreat. Heat flux data were divided into 3 regions (northern,

central, and southern) to assess changes in the magnitude, direction, and timing of heat flux from 2000-2004 relative to heat flux from 1990-1999 (Figure 1 for locations, Figures

12A, 12B, 12C). Positive numbers indicate heat flux toward the coast, whereas negative numbers indicate heat flux away from the coast. These data indicate lessening heat flux into the northern region throughout the 1990’s, with net annual heat flux directed away from the coast during our observational period. Data from the central and southern regions indicate increasing heat flux directed toward the coast beginning in the late

1990’s. Although Holland et al. (2008) correlated the initial thinning and retreat of

Jakobshavn Isbræ with anomalously warm subsurface ocean temperatures in the late-

1990’s, further supported by our data, our observational data do not record a similar response to increasing heat flux for other glaciers located throughout the central and southern regions. Additionally, both Edvard Glacier and Alison Glacier located in the northern region thinned >50 m and retreated >2.0 km from 2000-2004, concurrent with an increase of ~3 TW in the average annual heat flux directed away from the coast.

Therefore, our data support the initial thinning of Jakobshavn Isbræ by subsurface ocean warming (Holland et al. 2008) but we cannot directly relate changes in glacier behavior throughout the data set to heat/temperature change at the front.

4.2 Front Retreat, Thinning, and Surface Slope

In the absence of a direct relationship between climate forcing and front retreat, observational data were examined in further detail to resolve a relationship between retreat, thinning, and surface slope. Outlet glaciers were divided into 3 categories based 45

on magnitude of front retreat for comparison with thinning data. The 3 categories include glaciers with stable fronts (< ±0.1 km front change), small retreats (<1.0 km retreat), and large retreats (>1.0 km retreat).

We first address glaciers with stable fronts because they are an example of end-

member behavior within our observational record. Of the nine glaciers with stable fronts,

eight have fairly complete records of elevation change and were used in the slope

analysis. These eight glaciers were divided into two separate categories based on geometry. The first category includes glaciers with above average 5-km and snowline slopes. Glaciers in this category are Meteor, Kvane, and Akugdlerssup. The second category includes glaciers with shallow 5-km and snowline slopes. Glaciers in this category include Upernavik South, Ussing, Tuvssaq, Avangnardleq, and Sermilik. Slopes are directly related to driving stress near the front assuming relatively smooth, flat basal topography. The relationship between slope and driving stress is described by

 dh τ = ρgH  , (2) d  dx 

where τ b is basal shear traction, ρ is ice density, g is gravitational acceleration, H is ice

dh thickness, and is surface slope. Using the relationship in Equation 2, we found the dx

first category of stable glaciers has high driving stress throughout the record by

maintaining steep slopes and the second category creates higher driving stress

periodically by thinning concentrated at the front (Figure 13). Estimated driving stress

was higher for glaciers in the first category, ranging from ~170-250 kPa, compared to the

second category, ranging from ~70-140 kPa. Variability in driving stress for individual glaciers was less than ±2% for glaciers in the first category and 4-10% in the second category, reflecting relative changes in surface slope and ice thickness. The periods of thinning and thickening exhibited by glaciers in the second category are potentially due to large changes in speed near the front on semi-annual time scales. Joughin et al. (in prep) has noted large changes in speed from nearly stagnant to >500 m a-1 for

Avangnardleq Glacier, providing support for our hypothesis.

Glaciers with small (< 1.0 km) retreats had variable surface elevations and slopes,

including snowline slopes ranging from 38% to 217% relative to the average. The

average pre-retreat and post-retreat snowline slopes are 0.037, approximately equal to the average snowline slopes for the entire data set. When compared to the average pre-retreat

and post-retreat slopes for the large retreat category, 0.037 and 0.038 respectively, our

data suggest that surface slope cannot explain differences in the magnitude of retreat.

These glaciers are not discussed in further detail because their small front retreats should

not cause a substantial, sustained perturbation to the glacier stress regime, and therefore

should not induce significant changes in glacier velocity or discharge (Joughin et al.

2008a, Rignot and Kanagaratnam 2004).

Glaciers with large retreats were examined in detail due to the potential effects of

front retreat on outlet glacier force budgets. The magnitude and timing of thinning varied

widely even though all glaciers thinned from 2000-2009. Total thinning ranged from ~40

m (Kong Oscar) to ~120 m (Upernavik North). Slopes were highly variable as well. The

5-km slope ranged from 3% (Alison, pre-retreat) to 244% (Rink North, pre-retreat)

relative to the average. Snowline slopes were slightly less variable, with a range from

38% (Sverdrup, post-retreat) to 257% (Rink North, post-retreat) relative to the average.

Snowline slopes steepened for glaciers with floating tongues likely due to reduced buttressing at the front, subsequent acceleration, and stretching/thinning of ice near the grounding zone. The time between retreat and stabilization of the snowline slope and grounding zone location was monitored to determine variability in response time for glaciers with large front retreats. Here we define response time as the time required for a glacier to reach a steady distribution of force budget components for several consecutive summers following front perturbation. The response time following loss of floating ice was relatively short for glaciers with steep snowline slopes because thinning was concentrated at the front, causing slopes to steepen, increasing driving stress at the grounding zone. In contrast, response times were longer for glaciers with shallow slopes because thinning/stretching brought ice closer to flotation, causing continued redistribution of basal and lateral drag near the front. We present several examples below to clarify variability in front perturbation response due to differences in surface slope.

We first present Rink North Glacier (76°14'14.07''N, 61°3'34.85''W) and Rink

South Glacier to provide examples of perturbation response for glaciers with steep slopes.

Surface slopes for Rink North and South are similar throughout the record (See Section

3.3.2 and Table 1). Thinning was concentrated at grounding zones because inland diffusion of thinning was slope-limited, causing additional slope steepening and grounding zone retreat of ~1 km. Following loss of the floating ice tongues in 2004, fronts and surface slopes stabilized for both glaciers, indicating the distribution of

resistive stress in the force budget had temporarily stabilized following front perturbation.

The short response time is additionally attributed to their minimally confined fronts that enable replacement of mass loss at the front by increased ice flux from the inland ice sheet. Therefore, the short response time (~5 years) of these glaciers was a result of both limited diffusion of thinning and increased ice flux from the inland ice sheet.

The effect of large front retreat on the force budget of shallow-sloped glaciers is

best demonstrated by Upernavik North Glacier and Upernavik Northwest Glacier

(73°0'36.52''N, 54°31'44.94''W). The Upernavik glaciers were connected by shallow-

sloped, floating ice tongues similar to those of the Rink glaciers in the first half of the

study period. However, retreat rates for the Upernavik glaciers increased following front

separation, due to propagation of dynamic thinning along shallow slopes for >20 km-

inland from the margin. The extensive inland propagation of thinning decreased effective

pressure at the bed and increased basal sliding, resulting in further thinning/stretching

near the front. Continued retreat and thinning may potentially be influenced by basal

over-deepenings. If the Upernavik fronts were located on the seaward side of basal over-

deepenings prior to the large retreat from 2007-2008, the retreats could be caused by

buoyancy-driven feedbacks as well as propagation of thinning from subsurface melt. In

either case, as inland ice is brought closer to flotation, resistive stress is transferred from

the bed to the lateral margins. Additionally, relatively warm, saline ocean water may

penetrate inland beneath the ice, causing further grounding zone retreat and acceleration.

Therefore dynamic thinning and grounding zone retreat drastically affect perturbation

response of an outlet glacier by causing continued redistribution of resistive stress from basal to lateral drag, contributing to further acceleration and shearing near the front.

Alison Glacier and Jakobshavn Isbræ provide an additional example of the control

of surface slope on perturbation response. These glaciers serve as end-member examples

of rapid dynamical changes triggered by the break-up of floating ice tongues and do not

reflect behavior applicable to the entire data set. The glaciers have similar orientations

(west-facing) and length of confinement to straight fjords (~10 km) but differ in fjord

width and snowline slope. Fjord widths are 4.9 km and 9.4 km for Alison and Jakobshavn

respectively. Retreat rates varied for both glaciers, potentially due to seasonal changes in

force budgets. Jakobshavn’s peak retreat rate of 3.3 km a-1 occurred from 2002-2003 and

Alison’s peak retreat rate of 2.7 km a-1 occurred from 2004-2005. Surface speeds for both

glaciers increased during front retreat as a result of continued reduction of lateral resistive

stress (See Sections 3.3.5 and 3.3.6). Although both glaciers accelerated considerably

during the study, Alison’s retreat rate and acceleration slowed towards the end of the

record as the front approached the estimated grounding zone. In contrast, Jakobshavn’s

retreat rate slowed at the end of the record but speed continued to increase at a steady rate

of ~0.8 m d-1 annually. Differences in surface slope likely influence front retreat and

acceleration in the later portion of the observational record by influencing the location of

the grounding zone. Alison’s snowline slope steepened by 10% during break-up of its

floating tongue, causing driving stress and surface speed to increase, stabilizing the

location of the grounding zone. Jakobshavn’s snowline slope steepened by 6% during

break-up of its floating tongue but extensive thinning of shallow-sloped ice near the front

brought more ice closer to flotation, resulting in continued grounding zone retreat and redistribution of resistive stress from the bed to the lateral margins of the ice stream.

These glaciers demonstrate how surface slopes and ice thickness affect the force budget for outlet glaciers by influencing the volume of inland ice reaching flotation and grounding zone retreat. Though the initiation of the break-up of each glaciers floating ice tongue was likely triggered by an increase in subsurface ocean temperatures in the late

1990’s (Holland et al. 2008), these glaciers continued to respond to front perturbation for throughout our observational period, indicating greater sensitivity to climate forcing than initially predicted by previous observations of outlet glacier behavior (Trabant et al.

2003, Meier and Post 1987).

4.3 Comparison of Grounded and Floating Termini

Although the three examples of large front retreat mentioned above include glaciers terminating as floating ice tongues, the presence or absence of a floating ice tongue had no direct relationship with front retreat applicable to the entire data set.

Several glaciers with large retreats terminated as floating ice tongues (i.e. Alison,

Jakobshavn, Edvard), but many glaciers with floating tongues retreated less than 1.0 km

(i.e. Yngvar Nielsen, Docker Smith, Tasiussaq). Glaciers with grounded fronts such as

Umiamako and Sverdrup also experienced large front retreats, suggesting the presence of

floating ice tongues is not a necessary criterion for large retreat. These data indicate that

glaciers with resistive stress from lateral drag alone are not necessarily more susceptible

to thinning and retreat near the front. Glaciers with grounded fronts may therefore respond similarly to climate forcing as those terminating as floating tongues.

To further investigate the behavior of glaciers with grounded fronts in comparison

to those terminating as floating ice tongues, we analyzed our data for relationships

between the magnitude, timing, rate, and type (grounded vs. floating ice) of front retreat

and changes in surface speed. Joughin et al. (in prep) found that front retreat was

independent from changes in surface speed for many outlet glaciers in west Greenland,

with most glaciers experiencing only minor changes in speed from 2000/2001 to

2005/2006. Surface speeds increased for all glaciers with large retreats in our dataset.

Availability of image pairs for speed measurement varied depending on the extent of

overlapping flight paths near the front and cloudiness. In general, prevalence of useable

image pairs decreased with glacier latitude. Speeds from Edvard and Rink South indicate

front retreat, thinning, and increased surface speed were concurrent but records are too

sparse to determine exact timing of acceleration. Upernavik North data indicate

acceleration prior to retreat, causing a small front surge and stretching/thinning at the

grounding zone. The more complete time series of speed for Alison Glacier (Figure 6C)

and Umiamako Glacier (Figure 8C) reveal a direct relationship between retreat rate and

acceleration. The retreat rate of Alison glacier was most rapid from 2000-2005

corresponding to a 65% increase in speed. The front continued to retreat from 2006-2009

while the speed stabilized and slowed slightly (Figure 6C). Response to perturbation at

the front had two phases for Alison Glacier, abrupt thinning and acceleration followed by

a slow stabilization of front position, speed, and surface elevation. Umiamako Glacier

displays a similar relationship between retreat rate, acceleration, and surface elevation.

Following retreat, Umiamako accelerated by 192% with no sign of stabilization at the end of the record (Figure 8C). The continued acceleration of Umiamako in contrast to

Alison’s speed stabilization may be due to differences in the loss of grounded versus floating ice. If grounded ice is lost at the front, the glacier experiences a large force budget perturbation because basal and lateral drag are rapidly reduced, causing acceleration and stretching/thinning until a new stable geometry is reached near the front.

The loss of floating ice results in a smaller force budget perturbation because resistive stress generated by lateral drag is small relative to basal drag (Bamber et al. 2007).

Therefore glaciers with grounded fronts should be more sensitive to retreat than those terminating as floating ice tongues, as suggested by our data.

4.4 Focus on Smaller Spatial Scales

Due to the high spatial variability in observed relationships between front retreat, surface slopes, and surface speed as discussed above, we analyzed data on regional spatial scales to determine smaller-scale trends. This analysis was performed on glaciers within the same fjord system that are presumably experiencing similar climate and ocean

forcing conditions. By dividing the data into smaller subsets, we hoped to constrain more

parameters effecting glacier behavior to resolve distinct controls of glacial response to

regional climate forcing.

Upernavik Northwest, North, Central (72°56'20.31''N, 54°22'22.82''W), and South

(72°51'9.60''N, 54°21'31.91''W) share a single fjord system, but exhibited large variability in front retreat. Basal topography data are only available for Upernavik South, limiting interpretation of ocean circulation and locations of basal over-deepenings near modern fronts. Data for Upernavik South indicate a relatively smooth, horizontal bed located

~100 m below sea level. We therefore assume similar climate/ocean forcing and interpret differences in front retreat based on surface geometry. From 2000-2009 the glaciers’ fronts retreated 2.4 km, 4.8 km, 0.2 km, and 0.1 km for Upernavik Northwest, North,

Central, and South respectively. In this sampling of glaciers, those with more shallow snowline slopes experienced smaller front retreats. Snowline slopes were 111%

(Northwest), 143% (North), 97% (Central), and 62% (South) relative to the average snowline slope for the entire data set. The inverse relationship between slope and retreat is partially explained by the break-up of floating ice tongues at the Northwest and North glaciers during the study as discussed above. The Central and South glaciers may terminate as small floating tongues, but the grounding zone is likely located within a few hundred meters of the front, stabilizing the front at its current location. The relative insensitivity to climate forcing for Upernavik Central and South may also be related to basal topography if fronts are currently grounded on bathymetric highs. In the absence of basal topography data, we cannot fully resolve relative controls of response to climate forcing for these glaciers and must look to other glaciers for additional comparison.

Silardleq (70°48'14.54'' N, 50°48'39.55'' W) and Kangigdleq (70°43'42.35'' N,

50°38'24.35'' W), located in the Uummannaq District, provide an additional comparison

of glaciers with different responses to similar climate forcing. Orientation, front width, and distance from the grounding zone to the summer snowline were comparable for the glaciers constraining comparison for the controls of glacier behavior. From 2000-2009,

Silardleq retreated 1.5 km and Kangigdleq retreated 0.3 km. The retreat of Silardleq’s front was periodic, with several years of stable front position punctuated by years of retreat. Thinning of up to ~50 m concurrent with front retreat was confined to within 7 km of the front. Kangigdleq had periods of summer-to-summer thinning and thickening resulting in minimal thinning from 2000-2009. The average snowline slope was 94% and

120% relative to average for Silardleq and Kangigdleq, respectively. Although

Kangigdleq’s steep snowline slope contributed to its stability throughout the record, its front position is likely controlled by the presence of the large outcrop at its front, visible in satellite imagery. The presence of the outcrop has a similar affect as a submarine bathymetric high because changes in the force budget caused by thinning/thickening near the front are compensated by changes in backstress generated by the interaction of the ice front and the rock outcrop. If basal topography is similar in the Silardleq and Kangigdleq fjords with the exception of the outcrop, the observed behavior for the glaciers should be comparable under similar climate forcing conditions.

The observed changes in outlet glacier dynamics in response to climate forcing are highly variable for our given data set. Although several glaciers are presented in detail in this paper, no single glacier can be used as a regional archetype. Our data indicate no uniform, clear relationship between front retreat and climate forcing, surface slope, or speed. However, our analyses of surface slope and speed are limited due to poor

image quality and availability. Future airborne laser altimetry missions through NASA’s

ICE BRIDGE program will hopefully improve surface elevation profiles, allowing more accurate comparisons of changes in surface slope. Current slope data for many glaciers in our data set suggest steep slopes limit the volume of ice near flotation and cause high driving stresses at the grounding zone. If slopes are sufficiently steep, such as for Rink

North and Rink South, steepening due to concentrated thinning at the front causes glaciers to re-equilibrate to perturbations at the front within a few years. Shallow slopes

enable propagation of dynamic thinning well inland from the front, often bringing a large

volume of ice closer to flotation, promoting further retreat. Initial thinning causes a

positive feedback of lower backstress, acceleration, thinning, lower driving stress,

acceleration, retreat, etc. such as the case for Upernavik North. In order to better resolve

the controls of glacier dynamics in the future, we must expand and improve our

observational record and improve our current models of outlet glacier behavior.

Figure 10: Air temperatures. Temperatures are from the Aasiaat/Egedesminde and Nuuk/Godthab weather stations. Temperatures are plotted as monthly averages to determine changes in seasonality.

continued Figure 11: SSTs for the northern (A), central (B), and southern (C) regions. Locations for each point measurement are shown in Figure 1.

Figure 11 continued

continued Figure 12: Heat flux data for the northern (A), central (B), and southern (C) gates. See Figure 1 for the locations of the heat flux gates. Positive numbers indicate heat flux toward the coast and negative numbers indicate heat flux away from the coast. 60

Figure 12 continued

Figure 13: Surface elevation profiles for glaciers with stable fronts from 2000-2009. Meteor, Kvane, and Akugdlerssup glaciers maintain above average slopes throughout the study. Upernavik South, Ussing, Tuvssaq, Avangnardleq, and Sermilik thin and thicken periodically, changing near-front driving stress to maintain a stable front position.

5. Conclusions

The response of west Greenland marine-terminating outlet glaciers reveals complex, variable relationships between glacier behavior and changing climate under similar climate forcing conditions. The synchronous retreat of outlet glaciers in southeast

Greenland in 2003, coincident with anomalous warming of the ocean and atmosphere in the region (Howat et al. 2008) demonstrated a high sensitivity of outlet glacier dynamics to external forcing which we examine in west Greenland. Warming of subsurface ocean waters beginning in the late 1990’s as mapped by Holland et al. (2008) provided further support for the potentially synchronous retreat of west Greenland glaciers. Front positions and surface elevation profiles measured for 59 marine-terminating outlet glaciers reveal asynchronous retreat and surface thinning under similar climate forcing conditions.

We hypothesized that geometric parameters such as front width, surface slope, and the formation of floating ice tongues are the primary factors controlling variability in front retreat under similar climate forcing. Width near the front should affect lateral propagation of lateral drag because it can only propagate from the shear margin to the center line for a distance of up to 30 times ice thickness. In this study there was no direct relationship between terminus width and magnitude of front retreat applicable to the entire data set. However, our data indicate the interaction of two floating ice tongues

generated enough stress transverse to ice flow to stabilize Rink North/South and

Upernavik Northwest/North floating ice tongues in the beginning part of the study.

The retreat of many glaciers in west Greenland was limited by surface slope near the grounding zone for two distinct reasons; (1) Steep slopes concentrated thinning near the grounding zone, limiting the inland extent of ice brought closer to flotation and (2) steep slopes indicate high driving stress and therefore high basal drag near the grounding zone, limiting the importance of a reduction in lateral drag due to thinning. The importance of surface slope is limited for most glaciers in the data set but can be the primary control of front position for glaciers such as Rink North and Rink South as previously demonstrated. The presence of a floating ice tongue was also a primary control of glacier retreat for several glaciers but a relationship between the extent of a floating ice tongue and front retreat does not apply to the entire data set.

Our results do not provide a technique to predict glacier retreat based on glacier geometry, and in particular, surface slope as was the original objective. However, more work can be done to investigate a relationship between surface slope and glacier behavior in this region. Interpretation of the relationship between slope and front retreat was limited by data gaps in the time series, limiting spatial and temporal coverage for many glaciers. Errors in the data set were also caused by cloud coverage within surface elevation profiles derived from ASTER imagery. Incorrect elevations were examined on

ASTER VNIR images and removed manually, creating data gaps in many elevation profiles. Slopes calculated along profiles with data gaps may not represent ground truth because fewer values were used to calculate the average slope. Future work will continue

to utilize NASA ATM elevation profiles when they are available due to their high resolution and minimum data errors.

Bathymetric data must also be obtained in order to incorporate the effects of buoyancy driven feedbacks accelerating front retreat into our current observational record. The absence of bathymetric data prevents complex quantitative analysis of driving and resistive stress at the front. Furthermore, if bathymetric data cannot be obtained, then predictions of glacier behavior must be performed using assumed bathymetry in numerical models that reproduce the observational record of outlet glacier behavior. Accurate numerical models encompassing the full range of outlet glacier variability are necessary for future predictions of glacier dynamics under continued climate forcing. Thus, the implications of glacier variability are far-reaching because the observational record must be used to predict how changes in glacier dynamics will continue to affect the total mass balance of the Greenland Ice Sheet and future projections of sea level rise from ice sheet deterioration.

6. References Ahn, Y. and I.M. Howat 2009. Automated glacier surface velocity using multi- image/multi-chip (MIMC) feature tracking. Proceedings from the American Geophysical Union Annual Meeting, Session C23C (0510). Bamber, J.L. R. B. Alley, and I. Joughin 2007. Rapid response of modern day ice sheets to external forcing. Earth and Planetary Science Letters 257: 1-13. Das, S.B., I. Joughin, M.D. Behn, I.M. Howat, M.A. King, D. Lizzarralde, and M.P. Bhatia 2008. Fracture propagation to the base of the Greenland Ice Sheet during supraglacial lake drainage. Science 320: 778-781, doi:10.1126/science.1153360. Echelmeyer, K.A., W.D. Harrison, C. Larsen, and J.E. Mitchell 1994. The role of the margins in the dynamics of an active ice stream. Journal of Glaciology 40 (136): 527-538. Holland, D.M., R.H. Thomas, B. De Young, M.H. Ribergaard, and B. Lyberth 2008. Acceleration of Jakobshavn Isbræ triggered by warm subsurface ocean waters. Nature Geoscience 1: 659-664. Hooke, R.L. 2005. Principles of Glacier Mechanics. Second Edition. United Kingdom: Cambridge University Press, 368-373. Howat, I.M., I Joughin, S. Tulaczyk, and S. Gogineni 2005. Rapid retreat and acceleration of Helheim Glacier, east Greenland. Geophysical Research Letters 32 (L22502): doi:10.1029/2005GL024737. Howat, I.M., I. Joughin, and T. Scambos 2007. Rapid changes in ice discharge from Greenland outlet glaciers. Science 315: 1559-1561. Howat, I.M., I. Joughin, M. Fahnestock, B.E. Smith, and T. Scambos 2008. Synchronous retreat and acceleration of southeast Greenland outlet glaciers 2000-06: ice dynamics and coupling to climate. Journal of Glaciology 54 (187): 646-660. Joughin, I., W. Abdalati, and M. Fahnestock 2004. Large fluctuations in speed on Greenland’s Jakobshavn Isbræ glacier. Nature 432: 608-610.

Joughin, I., S.B. Das, M.A. King, B.E. Smith, I.M. Howat, and T. Moon 2008a. Seasonal speedup along the western flank of the Greenland Ice Sheet. Science 320: 781- 783, 10.1126/science.1153288. Joughin, I., I.M. Howat, R.B. Alley, G. Ekstrom, M. Fahnestock, T. Moon, M. Nettles, M. Truffer, and V.C. Tsai 2008b. Ice-front variation and tidewater behavior of Helheim and Kangerdlugssuaq Glaciers, Greenland. Journal of Geophysical Research 113 (F01004): doi:10.1029/2007JF000837. Joughin, I. (in prep). Greenland flow variability from ice sheet wide velocity mapping. Luckman, A., T. Murray, R. de Lange, and E. Hanna 2006. Rapid and synchronous ice- dynamic changes in East Greenland. Geophysical Research Letters 33 (L03503): doi:10.1029/2005GL025428. Meier, M.F. and A. Post 1987. Fast tidewater glaciers. Journal of Geophysical Research 92 (B9): 9051-9058. Moon, T. and I. Joughin 2008. Changes in ice front position on Greenland’s outlet glaciers from 1992 to 2007. Journal of Geophysical Research, 113 (F02022): doi:10.1029/2007JF000927. Nick, F.M., A. Vieli, I.M. Howat, and I. Joughin 2009. Large-scale changes in Greenland outlet glacier dynamics triggered at the terminus. Nature Geoscience 2: 110-114, doi:10.1038/NGEO394. Nye, J.F. 1960. The response of glaciers and ice-sheets to seasonal and climatic changes. Proceedings of the Royal Society of London, Series A, 256 (1287): 559-584. Pfeffer, W.T. 2007. A simple mechanism for irreversible tidewater glacier retreat. Journal of Geophysical Research 112 (F03S25): doi:10.1029/2006JF000590. Price, S.F., R.A. Bindschadler, C.L. Hulbe, and I.R. Joughin 2001. Post-stagnation behavior in the upstream regions of Ice Stream C, West Antarctica. Journal of Glaciology 47 (157): 283-294. Price, S.F., H. Conway, E.D. Waddington, and R. A. Bindschadler 2008. Model investigations of inland migration of fast-flowing outlet glaciers and ice streams. Journal of Glaciology 54 (184): 49-60. Trabant, D.C., R.M. Krimmel, K.E. Echelmeyer, S.L. Zirnheld, and D.H. Elsberg 2003. The slow advance of a calving glacier: Hubbard Glacier, Alaska, U.S.A.. Annals of Glaciology 36: 45-50. Raymond, C, 1996. Shear margins in glaciers and ice sheets. Journal of Glaciology 42 (140): 90-102.

Rignot, E., D. Braaten, S.P. Gogineni, W.B. Krabill, and J.R. McConnell 2004. Rapid ice discharge from southeast Greenland glaciers. Geophysical Research Letters 31 (L10401): doi:10.1029/2004GL019474. Rignot, E. and P. Kanagaratnam 2006. Changes in the velocity structure of the Greenland Ice Sheet. Science 311: 986-990, doi:10.1126/science.1121381. Rignot, E., J.E. Box, E. Burgess, and E. Hanna 2008. Mass balance of the Greenland ice sheet from 1958 to 2007. Geophysical Research Letters 35 (20): 5. Schoof, C. 2007. Ice sheet grounding line dynamics: Steady states, stability, and hysteresis. Journal of Geophysical Research 112 (F03S28): doi:10.1029/2006JF000664. Van de Wal, R.S.W., W. Boot, M.R. van den Broeke, C.J.P.P. Smeets, C.H. Reijmer, J.J.A. Donker, and J. Oerlemans 2008. Large and rapid melt-induced velocity changes in the ablation zone of the Greenland Ice Sheet. Science 321: 111-113, doi: 10.1126/science.1158540. Vieli, A., M. Funk, and H. Blatter 2001. Flow dynamics of tidewater glaciers: a numerical modeling approach. Journal of Glaciology 47 (159): 595-606. Warren, C.R. and N.F. Glasser 1992. Contrasting response of South Greenland glaciers to recent climate change. Arctic and Alpine Research 24 (2): 124-132. Weertman, J. 1958. Travelling waves on glaciers. IAHS Publications 47: 162-168. Weertman, J. 1974. Stability of the junction of an ice sheet and an ice shelf. Journal of Glaciology 3: 3-11. Zwally, H.J., W. Abdalati, T. Herring, K. Larson, J. Saba, and K. Steffen 2002. Surface melt-induced acceleration of Greenland Ice-Sheet flow. Science 297: 218-222.