Changes in the Marine-Terminating Glaciers of Central East Greenland, 2000-2010, and Potential Connections to Ocean Circulation

Thesis

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

Kaitlin Mary Walsh, B.S.

Graduate Program in Geological Sciences

The Ohio State University

2011

Thesis Committee:

Dr. Ian M. Howat, Advisor

Dr. Bryan Mark

Dr. Carolyn Merry

Kaitlin Mary Walsh

2011

Abstract

Outlet glaciers and ice caps on the periphery of the Greenland Ice Sheet have been

observed to be extremely sensitive to climate. The limited studies conducted on the

marine-terminating glaciers of eastern Greenland’s Geikie Plateau and Blosseville

Coast suggest exceptionally rapid rates of mass loss and short-term variability in ice dynamics. This study is targeted at a region of central east Greenland for which

GRACE mass-anomaly observations show substantial recent mass-loss since its launch in March 2002. Additionally, glaciers in this region terminate into Denmark

Straight, which is a thermodynamic transition zone between the Arctic and North

Atlantic oceans. Extensive glacial change has been more pronounced through the

Denmark Straight and south of the straight, which supports the hypothesis that ocean dynamics, specifically the Irminger Current and East Greenland Current, are supporting increased melt at the ice-ocean interface. It is possible that an appreciable amount of melt and ice loss south of Kangerdlugssuaq is occurring as a result of warmer subpolar water flowing into glacial fjords. We present changes to 38 marine-

terminating glaciers as observed using Landsat-7 ETM+ imagery to develop a time

series of changing front positions and flow speeds of these glaciers from 2000 to

2010. ASTER DEMs were used to quantify elevation change and thinning.

Additionally, we examine sea surface temperatures at five sites along the east

Greenland coast to identify possible correlations between warming of the sea surface and increased melt at the glacier termini.

iii

Acknowledgements

I’d like to thank my advisor, Dr. Ian Howat, for his extensive guidance and mentoring throughout my Masters research, for providing me with assistance on how to become proficient with new computer software and hardware, how to effectively work with satellite imagery and data, and for helping me further develop my understanding of glaciology. I’d also like to thank Dr. Carolyn Merry, Dr. Bryan

Mark, and Dr. Jason Box for their encouragement and suggestions while serving as mentors on my thesis committee. Additionally, I’d like to thank Dr. Yushin Ahn for providing the tools necessary to compute glacier velocity. Finally, I’d like to thank

Bryan Blair (NASA GSFC), Michelle Hofton (NASA GSFC), and John Sonntag

(NASA WFF) for including me on the 2010 Antarctic campaign for NASA’s

Operation IceBridge and showing me how airborne remote sensing is actually conducted in the field.

Vita May 2009……………………………………………………B.S. Geography, Pennsylvania State University September 2009……………………………………………..Graduate Research Assistantship, The Ohio State University August 2010………………………………………………. “Changes in the marine- terminating glaciers of the Geikie Plateau, East Greenland, 2000-2009”, IGS International Symposium on Earth’s Disappearing Ice December 2010……………………………………………. “Changes in the marine- terminating glaciers of the Geikie Plateau and Blosseville Coast regions, East Greenland, 2000- 2010”, AGU Annual Meeting

Fields of Study

Major Field: Geological Science, Physical and Environmental Geography

Table of Contents

Abstract...... ii Acknowledgements...... iv Vita...... v Table of Contents...... vi List of Tables...... viii List of Figures...... ix 1. Introduction...... 1 2. Data Sources and Methods...... 8 2.1 Data Products and Sources...... 8 2.1.1 Landsat-7 ETM+ Imagery...... 9 2.1.2 ASTER Imagery and Digital Elevation Models...... 9 2.1.3 MODIS Ocean Temperature Data...... 10 2.2 Front Positions...... 11 2.3 Surface Elevations...... 11 2.4 Surface Velocities...... 12 3. Results...... 14 3.1 Overview of Front Position Changes...... 14 3.2 Overview of Surface Elevation Changes...... 15 3.3 Overview of Surface Velocity Changes...... 16 3.4 Spatial Patterns of Glacier Change...... 17 3.4.1 Sermilik Fjord to Kangerdlugssuaq Fjord...... 17 3.4.2 Blosseville Coast (Kangerdlugssuaq Fjord to Scoresby Sound) ...... 19 3.4.3 Scoresby Sound and North of the Geikie Plateau...... 21

4. Discussion...... 43 4.1 Surging Glaciers in East Greenland...... 44 4.2 Mechanisms for Glacier Thinning...... 45 4.3 Oceanographic Forcing of Glacier Melt...... 46 4.4 Implications for Future Ice Sheet Change...... 49 5. Conclusions...... 53 References...... 56 Appendices...... 60 Appendix A: Data Acquisition and Processing Procedures...... 61 Appendix B: Digitizing Front Positions using MATLAB...... 62 Appendix C: Creating Glacier Elevation Profiles using MATLAB...... 63

vii

List of Tables

Table 1: Front Retreat and Latitude...... 27 Table 2: Summer Surface Velocities and Latitude...... 35

viii

List of Figures

Figure 1: Map of Greenland and Ocean Currents...... 5 Figure 2: Glacier and SST Point Locations...... 6 Figure 3: Local Ocean Currents, Central East Greenland...... 7 Figure 4: Midgard Glacier (66°26’N, 36°45’W) Front Position Methods Comparison...... 13 Figure 5: Elevation Change and Front Retreat Bar Graph...... 23 Figure 6: Front Position Retreat Regional Histogram...... 24 Figure 7: Front Position Retreat Map...... 25 Figure 8: Glacier Width and Front Position Retreat...... 26 Figure 9: Elevation Change Regional Histogram...... 28 Figure 10: Glacier Thinning Map...... 29 Figure 11: Glacier Surface Elevation Profiles...... 30 Figure 12: Average Surface Velocities Map...... 34 Figure 13: Front Position Retreat and Surface Elevation Profiles for Midgard Glacier (66°26’N, 36°45’W) ...... 36 Figure 14: Front Position Retreat and Surface Elevation Profiles for Kangerdlugssuaq Glacier (68°36’N, 32°57’W); (A) Surface Velocity Time Series for Kangerdlugssuaq Glacier...... 37 Figure 15: Front Position Retreat and Surface Elevation Profiles for Frederiksborg Glacier (68°17’N, 31°30’W) ...... 39 Figure 16: Front Position Retreat and Surface Elevation Profiles for Sortebrae (68°43’N, 26°59’) ...... 40 Figure 17: Front Position Retreat and Surface Elevation Profiles for Sydbrae (70°10’N, 26°19’W) ...... 41

Figure 18: Front Position Retreat and Surface Elevation Profiles for Daugaard Jensen Glacier (71°54’N, 28°36’W) ...... 42 Figure 19: Front Retreat and Surface Velocity Time Series, Kangerdlugssuaq Glacier (68°36’N, 32°57’W) ...... 51 Figure 20: Sea Surface Temperatures, East Greenland...... 52

1. Introduction

Rapid, unpredicted changes in the dynamics of ice sheets have prompted efforts to observe and understand ice loss and understand mechanisms of change using satellite and airborne measurements. Of importance is understanding how outlet glaciers draining the periphery of ice sheets react dynamically to atmospheric and oceanographic changes, and how these glaciers may contribute to sea level rise.

With regard to the Greenland Ice Sheet (GrIS), air- and spaceborne altimetry have revealed that the higher elevations of the ice sheet interior are either stable or slightly thickening, while the margins, particularly at marine outlets, are thinning rapidly, resulting in increased rates of mass loss for the entire ice sheet (e.g., Rignot and

Kanagaratnam, 2006; Tapley et al., 2004; Krabill et al., 1999; Krabill et al., 2004;

Luthcke et al., 2006; Thomas et al., 2006; Pritchard et al., 2009). Multiple studies using a range of methods show that mass loss in Greenland is dominated by ice draining from low-elevation coastal regions, especially in southeast Greenland and the Baffin Bay region of west Greenland (Luthcke et al., 2006; Velicogna and Wahr,

2006; Velicogna, 2009).

Several studies have shown that Greenland’s outlet glaciers increased their ice discharge rates to the North Atlantic Ocean between 1999 and 2005 (Howat et al.,

2007; Joughin et al., 2008; Luthcke et al., 2006; Rignot and Kanagaratnam, 2006;

Thomas et al., 2006; Velicogna and Wahr, 2006). Results from the NASA

Geoscience Laser Altimetry System (GLAS) altimeter onboard the Ice, Cloud and

Land Elevation Satellite (ICESat) have shown that glaciers flowing faster than 100 m yr-1 thinned at an average rate of nearly 1 m yr-1 from 2003 to 2007 (Pritchard et al.,

2009). The concentration of thinning above fast-flowing outlets indicates that

increasing glacier flow speeds are driving thinning, as opposed to loss at the surface

due to reduced precipitation or increased melt (i.e., the surface mass balance).

Dynamic thinning of the margins appears to be migrating into the interior of the ice

sheet, and thinning and retreat of the margin is expected to continue under climate

warming and ocean-driven melt (Pritchard et al., 2009).

The ability to measure the ice sheet’s contribution to sea level rise has

improved dramatically over the past two decades. Spaceborne remote sensing

revealed remarkable increases in ice-flow speed of most glaciers south of 72˚N

latitude, doubling the ice sheet’s contribution to sea level rise to roughly 0.28 ± 0.04

mm yr-1 by 2006 (Rignot and Kanagaratnam, 2006; Luthcke et al., 2006).

Satellite altimetry measurements show that the large outlet glaciers of

southeast Greenland (specifically Helheim and Kangerdlugssuaq glaciers) account for

approximately 28% of the mass loss originating from southeast Greenland (Howat et

al., 2008b). This indicates that most of the mass loss from this region is the sum of

individual losses from numerous, smaller outlet glaciers. Accordingly, many marine-

terminating glaciers draining the GrIS show evidence of dynamic thinning (e.g.,

accelerated ice flux and subsequent thinning) (Krabill et al., 2004; Howat et al., 2005;

Pritchard et al., 2009). Measuring glacier changes associated with increasing ice loss

will require observations over large spatial scales at sub-annual resolution. Such

observations may only be obtained through synthesis of multiple remotely sensed datasets.

Previous studies have linked recent increases in mass loss to changes in ocean circulation. There is a correlation between the warming of the North Atlantic and accelerations of Greenland’s large tidewater glaciers during the late 1990s and early

2000s (Hanna et al., 2009). The observed speed-up of outlet glaciers in southeast

Greenland in 2004 coincided with the onset of a warming trend in the subpolar North

Atlantic Ocean (Straneo et al., 2010; Myers et al., 2007; Bersch et al., 2007; Thierry and Mercier, 2008). Additionally, abrupt warming of subsurface ocean temperatures in 1997 along Greenland’s west coast suggests that significant changes at the front of

Jakobshavn Isbrae were initiated by the influx of warmer water that most likely originated in the Irminger Sea, which is located off the southeast coast of Greenland

(Hanna et al., 2009; Holland et al., 2008).

It is possible that warming of the ocean surrounding the GrIS is increasing melt and retreat of the ice sheet’s outlet glaciers, either independently or in addition to atmospheric warming (Box, 2009). Although the mechanisms driving the circulation of warmer North Atlantic waters are not well understood (e.g., Straneo et al., 2010), one hypothesis is that increased glacier runoff promotes convection of warm fjord water deep into the fjord, increasing melt rates at the calving front

(Murray et al., 2010). Increased melt has been observed through limited in situ measurements at the ice-ocean interface throughout the last decade, with the temperature and renewal rates of ocean water suggesting that this water is causing increased submarine melt at the margin of the ice sheet (Straneo et al., 2010; Thomas,

2004; Nick et al., 2009). Recent oceanographic studies, however, have not

demonstrated that subtropical ocean waters reach glacier fjords in southeast

Greenland. Alternatively, warming of North Atlantic subsurface water itself could

increase melt and calving rates (Straneo et al., 2010).

Roughly 38 marine-terminating outlet glaciers wider than 2 km drain the

central-eastern part of the Greenland coast from Sermilik Fjord at roughly 66°N north

to Scoresby Sound at 71°N latitude, including the Geikie Plateau and Blosseville

Coast regions. This region spans the thermodynamic transition zone from the North

Atlantic Ocean into the Arctic Ocean through the Denmark Strait, so contrasting patterns of glacial change north and south of this strait would provide further evidence that changes in the circulation of the North Atlantic Ocean are causing accelerated melt of the marine-terminating outlet glaciers south of the Denmark Strait

(Figure 1). Observations of the glaciers in this region, however, are currently too

limited to test this hypothesis.

This study uses satellite measurements to develop a time series analysis of

change on 38 marine-terminating glaciers in central-eastern Greenland between

~65˚34’N to ~71˚53’ N over the past decade (Figure 2). From these data, we aim to

identify differences in behavior between glaciers north and south of the Denmark

Strait, and determine if the circulation of the North Atlantic Ocean triggered retreat of

these glaciers (Figure 3). Additionally, we assess the importance of the other

mechanisms of glacier change in this region, such as surging and variability in

dynamic thinning.

Figure 1: Map of Greenland and surrounding ocean showing oceanographic areas of interest (specifically the Arctic Ocean, Denmark Strait, and North Atlantic Ocean), and ocean currents around Greenland through the Denmark Straight on the east and into the Labrador Sea towards Baffin Bay on the west. The Irminger Current is the eastern branch of the North Atlantic Current, which transports subtropical water; the East and West Greenland currents transport polar water from the Arctic Ocean (modified from Straneo et al., 2010; image courtesy NASA Earth Observatory).

Figure 2: Radar mosaic showing specific glaciers and sea surface temperature points region of study. Red circles indicate the 38 marine-terminating glaciers included in this study; yellow circles indicate glaciers that will be discussed in depth in this paper (1=Midgard, 2=Kangerdlugssuaq, 3=Frederiksborg, 4=Sortebrae, 5=Sydbrae, 6=Gasegletscher, 7=Daugaard Jensen); blue circles indicate location of four of the five sea surface temperature measurement points.

Figure 3: RADARSAT mosaic of central east Greenland showing specific geographic locations and features within the area of study; Sermilik Fjord is the southern-most area where marine-terminating glaciers were examined. “IC” and the orange lines depict the Irminger Current; “EGC” and the blue lines depict the East Greenland Current. Dotted lines represent variations in each ocean current.

2. Data Sources and Methods

We constructed an improved spatial and temporal resolution record of front position, patterns in thinning, and glacier surface velocities for glaciers in east

Greenland with data from multiple sensors to enable a greater understanding of regional glacier change. Sea surface temperatures were used as a proxy for variations in subsurface ocean temperature to identify possible correlations between ocean warming and increased submarine melt at the glacier termini.

2.1 Data Products and Sources

Data acquired from 2000 to 2010 over the Geikie Plateau and Blosseville Coast regions include visible and near infrared (VNIR) bands of the Advanced Spaceborne

Thermal Emission Reflector and Radiometer (ASTER) and the panchromatic band of the Landsat-7 Enhanced Thematic Mapper Plus (Landsat-7 ETM+) sensor for creating time series of front position change, flow speed change, and elevation change

(from ASTER digital elevation models). Sea surface temperatures were derived from data collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument.

2.1.1 Landsat-7 ETM+ Imagery

Imagery from the Landsat-7 ETM+ satellite were obtained from the United

States Geological Survey (USGS) Global Visualization Viewer (GLOVIS, http://glovis.usgs.gov) public archive. The mostly cloud-free images were selected to create image mosaics of the area for each year in the study period. Composite mosaics were created for each year in the study period using ENVI, an image processing and analysis software tool. The data have a 16-day repeat pass cycle and a

pixel resolution of 15 m.

Permanent failure of Landsat-7 ETM+’s Scan Line Corrector (SLC) in 2003

resulted in null-data striping and loss of roughly 20% of the image for all data

acquired since this instrument failure. The stripes widen from 0 pixels at 11 km from

the image center to 200 pixels at the image edges, so the impact of these stripes on

glacier mapping depends on the position of the glacier in the image. The images are

of the same radiometric and geometric quality as pre-SLC failure data.

2.1.2 ASTER Imagery and Digital Elevation Models

Orthorectified images and digital elevation models (DEMs) from the Advanced

Spaceborne Thermal Emission Reflector and Radiometer (ASTER) were collected

from the USGS Land Processes Data Active Archive Center (LP DAAC,

https://lpdaac.usgs.gov), including images that were cloud-free or partially clouded in

order to quantify thinning rates. ASTER’s host satellite, Terra, has a 16-day repeat pass cycle but images are acquired only on-demand, so that only a few images are available for a given glacier each season (Joughin et al., 2008). DEMs are created

using nadir and backward-looking VNIR image pairs acquired 57 seconds apart.

Relative DEMs are produced without ground control and were later registered using

offsets over off-ice terrain (i.e., stationary bedrock). Following this correction, DEM

vertical accuracy is better than 10 m over glacier ice.

2.1.3 MODIS Ocean Temperature Data

Sea surface temperatures were obtained from the Physical Oceanography

Distributed Active Archive Center (PO DAAC, http://podaac.jpl.nasa.gov) which provided data from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor to determine sea surface temperatures. Identical MODIS instrument are onboard two NASA Earth Observing System (EOS) satellites, Terra and Aqua, making MODIS capable of covering the entire surface of the Earth twice daily.

Despite the poor spatial resolution (250 m), this orbital pattern provides unparalleled temporal resolution for monitoring surface processes. MODIS captures data in 36 spectral bands ranging in wavelength from 0.4 to 14.4 micrometers (near infrared to mid-infrared), and six of the bands are dedicated to measuring surface temperature.

MODIS utilizes a rotating scan mirror that measures radiation of varying wavelength amplitude reflected from the ocean surface. Since the wavelength distribution of thermal radiation emitted by an object is proportional to its temperature, these variations in amplitude are used to determine sea surface temperature.

2.2 Front Positions

Front positions for each glacier were manually digitized from Landsat-7 ETM+ and ASTER images using two methods of measurement: first, a polygon-vector method (also known as the “box” method, e.g. Moon and Joughin, 2008) was used to measure changes in the near-terminus area, and second, a centerline method which measures the intersection of the ice front with the central flow line of the glacier. The polygon-vector method accounts for asymmetric variations in front shape of each glacier, as front position vector tracings give information on net area change of each glacier, but this method is time-consuming. The centerline method, in contrast, is a less time-consuming process and measurements can be made even when the front is partially obscured by clouds, but this method only captures variability at a single arbitrary point on the front. In order to test the sensitivity of our results to which methods is chosen, we mapped the fronts of 3 glaciers using both methods for comparison (Midgard (Figure 4), Kangerdlugssuaq, Sortebrae). These two methods yield very similar results. We therefore used the more efficient centerline method to generate our dataset.

2.3 Surface Elevations

Transects were drawn along the central flow line from sea level to the accumulation zone on the first Landsat-7 ETM+ image for each glacier (usually from

1999), and this transect information was transferred to the ASTER DEM subsets.

Elevation profiles along these transects are generated for each glacier to quantify thinning. Individual elevation profiles were manually edited for elevation errors

resulting from clouds and failure of the DEM generation software (and subsequent

generation of spurious elevation data). The data were then vertically registered by

subtracting offsets over ice-free terrain. Multiple elevations for a given year were

averaged together to give a single elevation profile for each year in the time series.

2.4 Surface Velocities

We extracted glacier surface velocities from ASTER and Landsat image pairs

using multiple image/multiple chip (MIMC) repeat image feature tracking software

developed by Dr. Yushin Ahn, Byrd Polar Research Center (Ahn and Howat, in

press). This method of feature tracking utilizes pairs of Landsat-7 ETM+ and ASTER

VNIR images collected between 10-90 days apart. The MIMC algorithm fits a

surface to the two-dimensional cross-correlation of various spatial features (such as crevasses) between chronological image subsets (chips) and measures the offset between these image subsets (Howat et al., 2010). The glacier velocity map is calculated from the pixel displacement field produced by the image cross-correlation algorithm. This analysis includes a time series of average surface velocities taken from a point along the glacier transect centerline near the front of each glacier. Error in this method varies with pixel resolution and time between image pairs, and are less than 1 m d-1 for the data used in this study (Ahn and Howat, in press).

Figure 4: Front position retreat of Midgard Glacier (66°26’N, 36°45’W) with each marker representing glacier front position relative to the glacier’s initial position in 2000; retreat was measured using the centerline method (blue) and the polygon-vector method (red) demonstrating the similarity in results between the two methods.

3. Results

We find a wide range in the behavior of marine-terminating glaciers in our study area over the past decade. A greater magnitude of change, especially in terms of extent of front retreat, occurred on the southern- and eastern-terminating glaciers of the Blosseville Coast and coastline running from Sermilik Fjord to

Kangerdlugssuaq Fjord, supporting the hypothesis that the warming of the North

Atlantic Ocean is the primary trigger for change. Our results are presented in detail below. Figure 5 shows front position change and thinning for each glacier with respect to latitude.

3.1 Overview of Front Position Changes

All 38 glaciers retreated between 2000 and 2010, but to widely varying degrees of magnitude, ranging from < 0.1 km to 9 km. Only one glacier retreated less

than 0.1 km (Gasegletscher, 70°5’N, 28°36’W), which is within the measurement

error. The mean retreat for all 38 glaciers over the time period was 1.63 km, with the

largest change observed to be 9.193 km from 2000 to 2010 (Midgard, 66°26’N,

36°45’W) (Figure 6). Greater front retreat is found on the southern- and eastern-

terminating glaciers of the Blosseville Coast and coastline running from Sermilik

Fjord to Kangerdlugssuaq Fjord, with 14 of the 25 glaciers in this area retreating

more than 1 km over the time period. The mean total retreat of glaciers in this region

of the study area over the time period was 2.13 km, while the mean total retreat of

glaciers terminating into Scoresby Sound and further north was less than 0.5 km. The

large retreats of a few glaciers, however, have a large impact on the mean front

retreat; the median retreat for all 38 glaciers in the study area is only 0.94 km. This

indicates that although the glaciers that retreated more than 5 km are significantly

impacting the statistics of this dataset, half of the glaciers in this study area still

retreated roughly a kilometer or more during the time period. Figure 7 shows total

front change for each glacier as the difference in front position from 2000 to 2010.

Figure 8 shows the relationship between total front retreat and glacier width near the

terminus. Table 1 shows front position retreat for each glacier in the dataset based on

latitude.

3.2 Overview of Surface Elevation Changes

Thinning rates for the 38 glaciers in this study were much more variable and

not as spatially consistent as changes in front position. We observe generally no

change on the glaciers terminating into Scoresby Sound and its channels north of the

eastern-most point of the Geikie Plateau peninsula (70°9’N, 22°3’W) (Figure 9). For glaciers south of 70°9’N, rates of thinning 15-km above the front varied from > 10 m

of thinning to roughly 155 m of thinning between 2000 and 2010. For glaciers north

of 70°9’N, thinning rates varied from approximately 10 m of thickening to 35 m of thinning between 2000 and 2010. Only two glaciers in the region had thickened over the decade (on the order of 5-7 m of thickening at 15 km upglacier from the termini), and both were north of 70°9’N. Figure 10 shows thinning evident 15 km from the

terminus for each glacier as the difference in surface elevation from 2000 to 2010.

Figure 11 shows elevation profiles that were constructed for each glacier. Elevation

profiles show annual average elevation from the terminus upglacier to the

accumulation zone for each glacier.

3.3 Overview of Surface Velocity Changes

Surface speeds varied both spatially and temporally, with the highest speeds

measured on Kangerdlugssuaq (68°36’N, 32°57'W) and Helheim (66°20'N,

38°9’W) during periods of acceleration between 2005 and 2006 (e.g., Howat et al.,

2008a; Joughin et al., 2008; Luckman et al., 2006). Many of the glaciers in this study exhibit seasonal variability in surface speed, oscillating on the order of ± 7 m per year, especially in the Scoresby Sound region north of the Geikie Plateau. For example, Daugaard Jensen (71°54’N, 28°36’W) had speeds varying annually between

7.5 and 11 m d-1, while Rolige Brae (70°34'N, 28°16' W) had surface speeds varying seasonally between 6 m d-1 in the summer and near zero m d-1 in the winter. Surface

speeds at Rolige Brae varied seasonally roughly 100% around its mean surface speed

of 2.6 m d-1, and surface speeds at Daugaard Jensen seemed to vary 25% around its

mean annual surface speed of 9.5 m d-1. Kangerdlugssuaq experienced the largest

range in its surface speed as a result of its sustained acceleration between 2004 and

2006, accelerating from 10 m d-1 to 27 m d-1. Maximum surface speeds occurred in

the summer for each glacier in this study area, with a mean maximum summer

surface speed of 7.5 m d-1. Only six glaciers had maximum summer surface

velocities greater than 10 m d-1, but eliminating these measurements from the mean

surface velocity of glaciers in this region does not change the mean quantity

substantially (mean is reduced to 5.6 m d-1). Besides all maximum surface speeds

occurring in the summer months, maximum surface speeds of all glaciers in the study

area varied throughout the time series as opposed to occurring synchronously. Figure

12 shows average surface velocities for all glaciers in the study area. Table 2 shows all maximum surface velocities and coincident errors measured for each glacier according to latitude.

3.4 Spatial Patterns of Glacier Change

The study area is presented below to attain a greater understanding of changes

in front position, surface elevation, and surface velocity occurring throughout the data

set. We discuss the three major regions of the study area and the observed spatial

patterns of glacier change, with several glaciers discussed in detail to highlight the

regionally representative behavior of these glaciers relative to the latitude of the

Denmark Straight. The regions are presented from south to north.

3.4.1 Sermilik Fjord to Kangerdlugssuaq Fjord

This area experienced the greatest magnitude of change (i.e., front retreat, thinning, surface velocity), especially in terms of the degree of front retreat. Peak rates of change occurred between 2003 and 2005, suggesting a regional forcing that was at a maximum during this time. The mean front retreat of glaciers in this subset was 2.9 km, and the median front retreat was 1.6 km. The average thinning observed

15 km from the glacier terminus was 28 m, and the elevation profiles for roughly half

of these glaciers show evidence of extensive thinning (mean thinning at 15 km from the glacier terminus is greater than 30 m). The mean maximum surface velocity was

8.8 m d-1, with a median maximum surface velocity of 7.7 m d-1. All maximum

surface velocities occurred in the summer months.

Midgard Glacier (66°26’N, 36°45’W) is a glacier with particularly notable

changes observed during the time period. Midgard is southwesterly facing and

terminates into the long northeast channel of Sermilik Fjord (Figure 13). This glacier

underwent the largest magnitude of change (in terms of front position, thinning, and

surface velocity) of the glaciers sampled. Sustained, interannual retreat of Midgard

increased in 2003, when a clearly pronounced pattern of seasonal advance-and-retreat

began and lasted until late 2007. Between early 2008 and late 2009, the glacier

retreated 4 km before a brief period of re-advance in late 2009/early 2010 and an

additional retreat of 1.5 km. The most significant thinning occurred below 1000 m

elevation and within 40 km of the terminus; thinning is on the order of 100 m from

2000 to 2010 at 10 km from the terminus and is roughly 60 m at 15 km from the

terminus. This rate of thinning has been consistent with the pattern of front retreat

throughout the time series, with an overall acceleration in both front retreat and

thinning occurring in late 2007 into 2008. Surface speeds increased from roughly 4

m d-1 in 2000 to 9 m d-1 in 2009 at a center point roughly 5 km upglacier from the

glacier’s most retreated position.

3.4.2 Blosseville Coast (Kangerdlugssuaq Fjord to Scoresby Sound)

The glaciers along the Blosseville Coast experienced the most variable

changes in front position, thinning, and surface velocity observed throughout the

dataset. While glacier change was observed to be substantial in most cases, of

particular interest is the observation that certain glaciers may or may not reflect

behavior of other glaciers within a close spatial proximity. The mean front retreat of

glaciers in this subset was 1.6 km, with a median retreat of 0.94 km. The glaciers in

this subset of the data thinned variably, with dynamic thinning evident on two of the

14 glaciers in this area. The mean thinning apparent at 15 km from the glacier terminus was 34 m, which is primarily driven by the extreme thinning observed on

Kangerdlugssuaq (68°36'N, 32°57'W) of 156 m. The mean maximum surface velocity was 7.4 m d-1, with a median maximum surface velocity of 5.9 m d-1. All maximum

surface velocities occurred in June or July.

Kangerdlugssuaq and Frederiksborg (68°17’N, 31°30’W) glaciers represent

two glaciers within close proximity to one another yet display contrasting behavior in

glacier change. Kangerdlugssuaq (68°36'N, 32°57'W) is a large southeasterly facing

glacier, roughly 9 km wide, which terminates into the 40-km long Kangerdlugssuaq

Fjord (Figure 14). In this study, we observed the glacier to have a very strong

seasonal pattern of advance-and-retreat, with a cyclic progression and retreat of

almost 2 km from 2000 to 2004. Between late 2004 and early 2006, Kangerdlugssuaq

experienced extensive retreat, in which the terminus retreated roughly 5 km. As a

result of this retreat, the glacier experienced a rapid acceleration of its surface speed,

accelerating from roughly 12 m d-1 in 2000 to 27 m d-1 in 2006 (Figure 14a) and the

glacier thinned more than 150 m near the front during this time. This thinning

propagated upglacier following this acceleration event, and stable elevation on the

glacier is not evident roughly 40 km from the glacier’s most retreated front position.

Kangerdlugssuaq slowed following the 2005-2006 acceleration, but the glacier has

not been able to substantially readvance or thicken and the front of the glacier

remains approximately 7 km from its initial position in 2000.

Frederiksborg is a 55-km long glacier and terminates at two calving fronts in

Kangerdlugssuaq Fjord (Figure 15). Frederiksborg retreated roughly 1.5 km from

2000 to 2004, and has maintained a steady pattern of summer retreat and winter advance since 2005, advancing and retreating roughly 0.7 km every year. This glacier had fairly negligible elevation change from 2000 to 2009, however, in 2010, the glacier appeared to have thickened to greater than its 2006 elevation, specifically more than 10 km away from the front where elevation data are available (no elevation data available below 200 m elevation for 2010). Surface speeds for this glacier were relatively stable from 2000 to 2006, varying between 3 and 5 m d-1, and no seasonal

variation was observed. Beginning in 2007, the glacier began to show a large

seasonal cycle in surface velocities, from near zero m d-1 to 13 m d-1; a period of acceleration that coincided with the large seasonal variation in front position.

Some glaciers in this region have been previously identified as surging glaciers, with two examples evident in this study (Sortebrae and Johan Petersen

Bugt). Glacial surges occur as a result of changing water pressure in a subglacial lake, which causes the lake to drain rapidly, leading to a large speed-up of the overlying glacier over a short time period. Sortebrae (68°44’N, 26°59’W) is a surge-

type glacier (e.g., Jiskoot et al., 2001) that has a large central trunk with smaller

subsidiary glaciers occupying several channels on the south facing side of the Geikie

Plateau (Figure 16). Sortebrae retreated steadily at roughly 500 m per year from 2000

to 2010. Consistent with the steady retreat, the glacier thinned 60 m within 15 km of

its front with no resolvable thinning above. Since this glacier is in a quiescent period

of surge behavior (Murray et al., 2002; Jiskoot et al., 2001), surface velocities were

relatively low, ranging from near 0 m d-1 to 3 m d-1, and with no seasonal variation in surface speed observed. This glacier provides an excellent example of surging glaciers in this region. Further observation of potential surging glaciers on the

Blosseville Coast will contribute to the knowledge of surge potential of all glaciers in this region by understanding surge time scales and potential contributions to regional mass loss.

3.4.3 Scoresby Sound and North of the Geikie Plateau

The glaciers north of the Blosseville Coast terminating into Scoresby Sound and north of the sound exhibit minimal changes in front position, thinning, and surface speed. The mean front retreat of glaciers in this subset was 0.51 km, with a median retreat of 0.32 km. The glaciers in this region showed the most evidence of seasonal front position migration. The mean thinning apparent at 15 km from the glacier terminus was 9 m, with two of the 11 glaciers in this subset thickening over time. The mean maximum surface velocity was 6.6 m d-1, with a median maximum

surface velocity of 5.3 m d-1. The following two examples of glaciers show

characteristics of minimal glacier change that are representative of glaciers in this region.

Sydbrae (70°10’N, 26°19’W) is a wide (5 km) glacier terminating into

Scoresby Sound on the north-facing side of the Geikie Plateau (Figure 17). The glacier has a seasonal oscillation of its front position between 200 and 300 m. The surface speeds reflect a strong seasonal pattern as well, with velocities ranging from 1 m d-1 in the winter to 5 m d-1 in the summer.

Daugaard Jensen Glacier (71°54'N, 28°36'W) is 5 km wide and is the northern-most glacier in the study area, terminating into the northern-most channe l of

Scoresby Sound (Figure 18). This glacier experiences large amounts of variation in its front position and surface velocity, which set it apart from glaciers experiencing negligible change in the Scoresby Sound region. The glacier undergoes a seasonal advance-and-retreat of 1 km annually, which is the largest annual variation in front position observed in the study area. Its interannual front position, however, has not changed since 2000. The glacier thinned approximately 30 m below 400 m elevation, with negligible thinning above this elevation. Daugaard Jensen is the fastest moving glacier observed in the Scoresby Sound region, with a maximum velocity of 11 m d-1 in June 2006 and a pattern of acceleration and deceleration generally following the cyclic pattern of front advance and retreat.

Figure 5: Elevation change (blue bars) and front retreat (red bars) for all 38 glaciers in the study area from lower latitude (roughly 68˚N) to higher latitude (roughly 71.5˚N). It is clear that the glaciers with the most substantial front retreat and elevation change are south of 70°9’N, which is indicated by the dashed line.

Figure 6: Histogram showing distribution of glaciers based on front retreat and spatial distribution for all 38 marine-terminating glaciers in the dataset. There was a wider distribution of retreat values (and a greater magnitude of retreat) south of 70.1°N, with minimal front retreat and little decadal variability observed north of 70.1°N.

Figure 7: Change in front position (2000-2010) for all marine-terminating outlet glaciers in this analysis. Circles indicate the location of each glacier front and colors indicate magnitude of retreat.

Figure 8: Front position movement from a baseline position in 2000 as a function of glacier width; no strong trend exists between glacier width and front retreat.

Front Retreat Latitude (deg) Glacier Name (km) 66.3 Helheim 7.76 66.38 Fenris 1.64 66.45 Midgard 9.19 66.48 Midgard Nord 0.94 66.57 Steenstrup 0.38 66.78 Tasilaq 1.61 67.25 Kruuse 3.08 67.45 Nordre 2.79 67.88 Polaric 2 1.31 67.97 Polaric 1 0.31 68.28 Frederiksborg 1.59 68.33 Sorgenfri 0.27 68.4 Christian IV 0.19 68.41 Nordfjord 0.15 68.5 Rosenborg 1.98 68.53 Courtauld 0.42 68.57 Kronborg 1.47 68.63 Borggraven 1.36 68.63 Kangerdlugssuaq 7.43 68.73 Sortebrae 6.29 68.78 Johan Petersen 0.79 68.88 Storbrae 1.74 68.98 De Reste Bugt 1.04 69.25 Barclay Bugt 0.94 69.32 Dendrit 0.47 69.63 Barthinolisbrae 0.94 69.87 Steno Brae 0.33 69.95 West Krista Dan 0.67 69.96 Krista Dan 0.33 69.97 Magga-Dan 0.14 70.1 Gasegletscher 0.04 70.17 Sydbrae 0.3 70.27 Bredegletscher 1.09 70.37 Vestifjord 0.42 70.47 Dodebrae 0.11 70.6 Rolige Brae 0.81 71.17 Eilson 0.18 71.9 Daugaard Jensen 1.51

Table 1: Front retreat measured using the centerline method for all 38 marine- terminating glaciers in data set. Front retreat occurred at a greater magnitude south of 70°9’N, suggesting an oceanographic forcing of melt south of the Denmark Straight.

Figure 9: Histogram showing distribution of glaciers based on elevation change at 15 km from the glacier front and spatial distribution for all 38 marine-terminating glaciers in the dataset. There was a wider distribution of thinning (and an overall greater magnitude of thinning) south of 70.1°N, with minimal thinning observed north of 70.1°N.

Figure 10: Change in surface elevation (all values negative) (2000-2010) 15 km from each glacier’s terminus for all marine-terminating glaciers in this analysis. Circles indicate the location of each glacier front; warm colors indicate glaciers with substantial thinning; cool colors indicate glaciers with minimal thinning.

Figure 11: Elevation profiles for all 38 marine-terminating glaciers in dataset. Latitude increases from top to bottom; starting with Helheim Glacier (66°20'N, 38°9’W) to Daugaard Jensen Glacier (71°54’N, 28°36’W). Continued on next page.

Figure 11 (cont.): Elevation profiles for all 38 marine-terminating glaciers in the dataset.

(continued)

Figure 11 (cont.): Elevation profiles for all 38 marine-terminating glaciers in the dataset.

(continued)

Figure 11 (cont.): Elevation profiles for all 38 marine-terminating glaciers in the dataset.

Figure 12: Average surface velocities (in m d-1) for all marine-terminating glacier in the data set. Circles indicate the location of each glacier front and colors indicate magnitude of glacier speed.

Velocity Latitude Glacier Name (m d-1) Date Error 66.3 Helheim 20.99 Jul-07 1.09 66.38 Fenris 7.68 Jun-04 1.09 66.45 Midgard 14.21 Jul-10 1.25 66.48 Midgard Nord 4.74 Jul-00 0.38 66.57 Steenstrup 7.32 May-05 0.97 66.78 Tasilaq 7.77 Jul-09 0.97 67.25 Kruuse 7.81 Jun-09 0.70 67.45 Nordre 67.88 Polaric 2 3.85 Jun-04 0.97 67.97 Polaric 1 4.91 Aug-07 1.25 68.28 Frederiksborg 13.39 Jun-07 1.25 68.33 Sorgenfri 3.67 Jun-01 0.97 68.4 Christian IV 6.88 Jul-07 0.67 68.41 Nordfjord 6.93 Jun-05 0.51 68.5 Rosenborg 3.21 Jul-03 0.97 68.53 Courtauld 68.57 Kronborg 5.74 Jun-01 1.25 68.63 Borggraven 8.52 Jun-04 0.38 68.63 Kangerdlugssuaq 27.49 Jun-06 1.59 68.73 Sortebrae 1.92 Jul-10 1.09 68.78 Johan Petersen 6.03 Jun-10 0.70 68.88 Storbrae 5.29 Jun-04 0.32 68.98 De Reste Bugt 8.88 Mar-07 0.55 69.25 Barclay Bugt 3.97 Jun-01 1.25 69.32 Dendrit 3.36 Mar-02 1.25 69.63 Barthinolisbrae 7.05 Jun-00 0.76 69.87 Steno Brae 5.64 Jul-10 0.76 69.95 West Krista Dan 4.65 Aug-01 1.25 69.96 Krista Dan 3.72 Apr-10 0.45 69.97 Magga-Dan 22.78 Jun-07 1.09 70.1 Gasegletscher 5.28 Jul-05 0.55 70.17 Sydbrae 4.81 Jul-07 1.25 70.27 Bredegletscher 7.98 Jun-04 1.09 70.37 Vestifjord 5.31 Mar-02 0.83 70.47 Dodebrae 6.54 May-00 0.41 70.6 Rolige Brae 6.23 Apr-02 0.83 71.17 Eilson 2.91 Jun-04 0.97 71.9 Daugaard Jensen 11.93 Jun-06 1.59

Table 2: Maximum surface velocities for 36 of the 38 marine-terminating glaciers in the data set; poor image registration led to no velocity data for Nordre and Courtauld glaciers. All maximum velocities occurred during the late spring or summer months.

Figure 13: (top) Elevation profile time series at Midgard Glacier (66°26’N, 36°45’W) showing substantial thinning through the analysis period. (bottom) Landsat-7 ETM+ image from 2000 showing progression of retreat in 2005 and 2010; Midgard has retreated roughly 9 km since 2000.

Figure 14: (top) Elevation profile time series at Kangerdlugssuaq Glacier (68°36'N, 32°57'W) showing substantial thinning following its massive retreat in 2005, and relative stability in elevation for the remainder of the time period. (bottom) Landsat-7 ETM+ image from 2000 showing progression of retreat in 2005 and 2010; Kangerdlugssuaq has retreated roughly 7 km since 2000. (Continued on following page)

Figure 14 (cont.): Ice surface speed at Kangerdlugssuaq measured from feature tracking using Landsat-7 ETM+ imagery; acceleration is visible starting in 2004 and slowdown is visible starting in 2007.

Figure 15: (top) Elevation profile time series for Frederiksborg Glacier (68°17’N, 31°30’W) showing relatively marginal thinning until 2009. (bottom) Landsat-7 ETM+ image from 2000 showing progression of retreat in 2005 and 2010; relatively little change in front position with more change notable on a seasonal scale rather than an annual scale.

Figure 16: (top) Elevation profile time series for Sortebrae (68°43’N, 26°59’); thinning is localized within 25 km of the front as the glacier. (bottom) Landsat-7 ETM+ image from 2000 showing progression of retreat in 2005 and 2010; the glacier retreated roughly 6 km as part of the quiescent phase of the glacial surging process. 40

Figure 17: (top) Elevation profile time series for Sydbrae (70°10’N, 26°19’W) showing fairly negligible change in terms of surface elevation throughout the time period. (bottom) Landsat-7 ETM+ image from 2000 showing progression of retreat in 2005 and 2010; the glacier has shown negligible change at its front throughout the time series. 41

Figure 18: (top) Elevation profile time series for Daugaard Jensen Glacier (71°54'N, 28°36'W) showing moderate thinning below 400 m elevation, but relatively negligible elevation change above that elevation. (bottom) Landsat-7 ETM+ image from 2000 showing progression of retreat in 2005 and 2010; showing slight retreat to 2005 and then advance ahead of 2000 position in 2010; glacier experiences substantial seasonal changes in its front position with the terminus migrating ± 1 km annually.

4. Discussion

Changes in the front positions, surface elevations, and surface speeds of 38

marine-terminating glaciers in central east Greenland indicate that glaciers south of

the Denmark Strait thinned and retreated more extensively than glaciers north of the

Denmark Strait, suggesting a relationship between the warming of the North Atlantic

and enhanced melt of glaciers in southeast Greenland. The ten outlet glaciers on the coast from Sermilik Fjord to Kangerdlugssuaq fjords all underwent accelerated retreat following 2003, a year of anomalously high air and sea surface temperatures along the southeast Greenland coast (Howat et al., 2008a). While there is a clear latitudinal

distinction between the magnitudes of change, the relative magnitudes of thinning and

retreat varied substantially from glacier to glacier. The discussion presented below

focuses on the contribution of surging to regional glacier change and the effects of

dynamic thinning on glaciers in this region, particularly south of Kangerdlugssuaq

Fjord. Additionally, the discussion will outline the relationship between sea surface

temperatures and magnitude of glacier change, and how sea surface temperatures are

used as a proxy for ocean circulation patterns.

4.1 Surging Glaciers in East Greenland

Several tidewater glaciers in east Greenland have exhibited surge-like flow

speed behavior and morphological attributes (Jiskoot et al., 2003; Murray et al.,

2002). Glacial surges are short-term events (days to weeks) where a glacier advances

at rates of hundreds of meters per day due to a sudden release of trapped subglacial water, which lubricates the bed and causes the flow speed to accelerate by an order of magnitude. A glacial surge follows a quiescent period of slow retreat, which can vary from years to decades. Several means exist to identify “surging” glaciers, including warped moraine shapes, sheared-off ice tributaries, varying crevasse patterns, and rougher-than-usual glacial surfaces (Jiskoot et al., 2003). Surging has been documented on Sortebrae (68°44’N, 26°59’W), which surged for roughly 19-35 months from 1992 to 1995 following a quiescent phase of between 39-49 years. Ice flow during the surge event increased by up to 60-1500 times over the glacier’s quiescent-phase speeds and was sustained at rates of up to 30 m d-1 for more than 12

months (Murray et al., 2002).

Sortebrae exhibits characteristics typical of a quiescent phase; retreating

steadily without a seasonal oscillation of front position, unlike the other glaciers in

this region. The thickness of the glacier changed little over the decade, and it is one of

the slowest moving glaciers in this analysis, with a maximum surface speed measured

to be less than 2 m d-1 (occurring in July 2010). While Sortebrae is the only glacier in

central east Greenland where active surging has been observed by aerial imagery,

other studies (e.g., Jiskoot et al., 2003; Weidick, 1988) have identified morphological

signs on other glaciers in this region that indicate that surging is a regional

mechanism for glacier change (including Dendrit and Borggraven glaciers, both

which are on the south-facing coast of the Geikie Plateau along the Blosseville

Coast). There is disagreement in the literature as to what the predominant factor

dictating whether or not a glacier will exhibit surge-like qualities is. It is suggested

that underlying bedrock younger than Precambrian in age (roughly 542 Ma), a

common bedrock type underlying the glacier, relatively low glacier slopes, and high

equilibrium line altitudes may lead to surge-like behavior (Weidick, 1988; Hamilton,

1992; Jiskoot et al., 2003). However, Jiskoot and others (2003) found that the

underlying geology of known surging glaciers in east Greenland is variable both in

age and lithology, and that glacial valley shape, which has a significant impact on

glacier geometry, could also determine whether or not a glacier will be surge-like.

4.2 Mechanisms for Glacier Thinning

In addition to remotely sensed observations, experiments using ice flow

models suggest that marine terminating glaciers are sensitive to changes at the ice front and hydraulic basal conditions, leading to dynamic instabilities and thinning

(Nick et al., 2009). Dynamic thinning occurs when perturbations in glacier stresses caused by calving front retreat propagate rapidly upglacier, causing acceleration and thinning to migrate inland (Joughin et al., 2008; Nick et al., 2009). Surface melting is

only thought to be contributing to about half of the observed increase in ice loss,

suggesting that ice dynamics may be causing accelerated ice flux (and subsequent

thinning) through outlet glacier fjords. Dynamic thinning is thought to be the primary

cause of thinning on the northwest and southeast margins of the Greenland Ice Sheet

(Krabill et al., 2004; Howat et al., 2005; Pritchard et al., 2009). Thus, with front retreat occurring at a greater magnitude in glaciers south of Kangerdlugssuaq Fjord, it is likely that ice dynamics will continue to drive thinning upglacier and contribute to the already-notable glacier mass loss in this region.

Dynamic thinning has been observed and measured on several glaciers in southeast Greenland, including Kangerdlugssuaq (68˚36’N, 32˚57’W), where dynamic thinning is detectable from the glacier’s terminus to approximately 100 km inland (Pritchard et al., 2009) (Figure 19). Our observations suggest that roughly half of the glaciers between Sermilik and Kangerdlugssuaq fjords are experiencing rapid dynamic thinning as well, with the most dramatic example being Midgard Glacier

(see Figure 11). From the glacier’s elevation profile, it is apparent that thinning began near the front of the glacier at the beginning of the time series and has propagated at least 45 km upglacier through the time period of the study. Dynamic thinning is only potentially detectable on two glaciers north of Kangerdlugssuaq, which suggests that it is mostly confined to the area between Helheim and

Kangerdlugssuaq glaciers and that it is likely these less-studied glaciers are contributing a considerable amount of the mass loss measured from southeast

Greenland.

4.3 Oceanographic Forcing of Glacier Melt

As mentioned above, several studies have attributed changes in Greenland’s outlet glaciers over the past decade to changes at the ice-ocean interface. The annual discharge from the GrIS doubled during the early 2000s, and this was primarily a

result of glacier acceleration and consequent thinning of the southeast portion of the

ice sheet (Murray et al., 2010). Acceleration and the resulting thinning that took

place in southeast Greenland in the middle part of the 2000s suggest that a regional

forcing was causing a synchronous speed-up of glaciers in the region, and a possible

mechanism for regional glacier change on a synchronous scale is increased ocean temperatures flowing into glacier fjords (Luckman et al., 2006; Howat et al., 2008a;

Murray et al., 2010). The substantial rates of change observed on glaciers south of the

Denmark Straight give credence to the hypothesis that warmer ocean water may be

coming into contact with glaciers in southeast Greenland through the convection of

warm fjord water deep in these glacial fjords, and that this warm ocean water is

increasing melt and calving rates of southeast Greenland glaciers.

Limited studies of the subsurface ocean conditions of the North Atlantic

surrounding southeast Greenland revealed a spike in temperatures of the subpolar

North Atlantic Ocean from 2003 to 2004, which coincided with this regional

acceleration of glaciers in southeast Greenland (Straneo et al., 2010; Myers et al.,

2007; Bersch et al., 2007; Thierry et al., 2008). Surveys conducted in the fjords and

nearby ocean area near three of Greenland’s largest glaciers (Kangerdlugssuaq and

Helheim on the east coast; Jakobshavn Isbrae on the west coast) indicate that the

ocean is capable of triggering major changes at the termini of Greenland’s marine

terminating glaciers (e.g., Holland et al., 2008; Straneo et al., 2010). Holland and

others (2008) documented a sudden increase in ocean temperatures along the west

coast of Greenland in 1997 that corresponded with rapid thinning at Jakobshavn

Isbrae, a glacier that had been slowly thickening and decreasing its velocity

throughout the 1990s. Warmer, more saline waters originating from the Irminger Sea

off the southeast coast of Greenland likely migrated along the West Greenland

Current and infiltrated glacial fjords along the west Greenland coast. Before 1997,

Jakobshavn Isbrae terminated into a 15-km long floating ice tongue. A sudden

increase in the temperature (+1.1ºC) of the fjord water could explain the roughly 80

m yr-1 of thinning that occurred between 1997 and 2001, and the disintegration of the

glacier’s floating tongue soon after (Motyka et al., 2011; Holland et al., 2008). The

loss of its floating tongue has caused Jakobshavn Isbrae to continue to accelerate and

its grounding line has continued to retreat.

Widespread measurements of ocean conditions at depth are not readily

available and sea surface temperatures (SSTs) may not be the best proxy for

subsurface ocean temperatures. However, the data derived from MODIS used in this

study show a clear increase in SSTs in late 2003 at the southern-most study site (SST-

A), which is roughly 80 km south of the mouth of Sermilik Fjord and 140 km south of the terminus of Helheim Glacier. Temperatures at this location increased from 6°C in late 2002 to 9.5°C in late 2003. Two other SST-observation sites, SST-B and SST-C, also show a slight increase in temperature from 2002 to 2003. Additionally, the temperature at SST-A appears to have increased in late 2010 (though not as dramatically as in 2003), so continued close monitoring of the glaciers in southeast

Greenland is necessary to see if another regional glacier change event is initiated.

The two northern-most points chosen to observe SSTs do not show the spike in temperature in 2003 or in 2010, and these data indicate the interesting variability in ocean temperatures through five sites that are spread over a length less than 1000 km

(Figure 20). This spike in SSTs at the southern-most measuring point in 2003 (in

addition to the anomalously warm air temperatures observed in southeast Greenland

at this time) may have been the impetus for the peak in glacier change from Sermilik

Fjord to Kangerdlugssuaq Fjord from 2003 to 2005. Definitive conclusions cannot be

drawn from this increase in sea surface temperatures and subsequent acceleration and

thinning of southeast Greenland’s marine-terminating glaciers, although the

connection between ice and ocean dynamics is evident in this study and in previously

published results (e.g., Holland et al., 2008; Howat et al., 2008a; Straneo et al., 2010;

Murray et al., 2010).

4.4 Implications for Future Ice Sheet Change

As the subpolar North Atlantic experiences potential changes in circulation

and temperature as a result of natural and anthropogenic causes, the potential exists

for the marine-terminating outlet glaciers of the GrIS to respond regionally and synchronously to this warming by accelerating and thinning rapidly in response to melting at the terminus. The GrIS contains roughly 10% of the world’s freshwater, and has the potential to add approximately 7 m to sea level if it were to melt entirely

(Hanna et al., 2009; Hanna and Braithwaite, 2003; Dowdeswell, 2006). Mass loss measurements have shown that GrIS mass loss annually contributes approximately

10% of the total annual sea level rise (3 mm yr-1) measured by satellite altimetry

(Rignot and Kanagaratnam, 2006; Luthcke et al., 2006; Cazeneve and Nerem, 2004).

Changes over the past decade have demonstrated that marine-terminating glaciers are

highly sensitive to changes in ocean conditions. Yet, the degree to which the margins

of the ice sheet will react to future warming, as well as the rate at which the interior of the ice sheet will respond to changes at the margin, remains unknown.

Figure 19: Front retreat (blue) and surface velocities (red) of Kangerdlugssuaq Glacier (68˚36’N, 32˚57’W) from 2000 to 2010. Substantial front retreat (~ 5 km) took place in 2005, followed by an extensive acceleration from 10 m d-1 to 27 m d-1 in 2006. This suggests that the glacier thinned dynamically as a result of this rapid retreat and acceleration.

Figure 20: Sea surface temperatures (SSTs) derived from MODIS data from 2000 to 2010 from five sites off of the central east Greenland coast; site SST-A is the furthest south and site SST-E is the furthest north. There is no overall trend of significant interannual temperature increase or decrease; however, of note is the clear spike in SSTs in 2003, which corresponds to a period of anomalously warm atmospheric temperatures as well (Howat et al., 2008a).

5. Conclusions

Analysis of changes in 38 marine-terminating glaciers in the Geikie Plateau and Blosseville Coast regions of Greenland’s central east coast over the past decade indicates that front retreat and thinning have become more prevalent across central east Greenland over the past decade. A potential major driver for this change, especially along the Blosseville Coast and from the Sermilik Fjord to

Kangerdlugssuaq Fjord, is the interaction between ice and a warming North Atlantic

Ocean. Glacier change was more pronounced south of the Denmark Straight, suggesting that accelerated melt and calving in southeast Greenland is linked to changes in the circulation of the North Atlantic Ocean. In addition, glaciers south of the Denmark Straight appeared to be more synchronous in their behavior than glaciers north of the Denmark Straight, which also indicates that a regional forcing may be behind glacier acceleration and melt in this region.

Our initial hypothesis stated that there would be a more pronounced change in front position, elevation, and ice velocity on glaciers south of Kangerdlugssuaq Fjord and south-facing glaciers of the Blosseville Coast compared to north-facing glaciers of the Geikie Plateau as a result of exposure to the warming of the North Atlantic and its associated coastal currents. While this analysis suggests that oceanographic forcing may be affecting glacier melt south of the Denmark Straight, observations of

glaciers in this region and of the surrounding ocean are still too limited to state

definitively that changing ocean dynamics are the sole regional forcing affecting

glacier change in this region. Future studies are required that incorporate ocean

monitoring stations near the outlet glaciers lining the Blosseville Coast and along the

coast from Sermilik Fjord to Kangerdlugssuaq Fjord in order to obtain more

information about the subsurface oceanographic conditions along the continental shelf and closer to these outlet glaciers.

The methods used in this study to quantify glacier change in central east

Greenland can be applied to other areas where outlet glaciers are being assessed for

ongoing changes in front position, surface elevation, and ice velocity. However, the

methods and imagery used in this analysis have limitations. The east Greenland coast

is often obscured with clouds for much of the year, making it difficult to have a

complete time series of Landsat-7 ETM+ or ASTER imagery over a particular

location. Future studies should incorporate all-weather and all-year synthetic aperture

radar data and high-resolution commercial satellite imagery to provide a much more

complete picture of change. Additionally, elevation data collected by airborne laser

altimetry acquired as part of NASA’s Operation IceBridge initiative will provide a

sufficient alternative to using moderate resolution ASTER digital elevation models

for measuring changes in glacier surface elevation.

Using a wide and growing array of airborne and satellite remote sensing

platforms, observations and measurements of the continued mass loss and glacier

change on the Greenland Ice Sheet are becoming more abundant and accessible for

studying the effects of a warming climate on the ice sheet. Mass loss is occurring at

accelerated rates on marine-terminating glaciers in the southeast drainage areas of the ice sheet, as shown by gravity anomalies, altimetry data, and imagery showing loss at glacier termini. The implications of melting glaciers in Greenland are global in scope; ongoing changes in ice cover are expected to continue contributing to changes in sea level, which is one of the greatest challenges facing coastal communities and heavily populated coastal urban centers. These areas of extreme glacier change discussed in this analysis indicate that continuous monitoring of ice sheet changes is necessary to develop a better understanding of how the GrIS is adjusting to atmospheric and oceanographic changes.

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Appendices

7.1 APPENDIX A: Data Acquisition and Processing Procedures

7.2 APPENDIX B: Digitizing Front Positions using MATLAB

7.3 APPENDIX C: Creating Glacier Elevation Profiles using MATLAB