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The Greenland ice sheet, the planets second largest ice mass, has most recently been in a state of negative mass balance, contributing to about 15% of global sea level rise between 1991 and 2000 (Box et al., 2004). Results from NASA's Airborne Topographic Mapper (ATM) show an increase in the rate of Greenland's ice loss from 50 to 90 km3/yr in the past 11 years (Krabill et al., 2004) corresponding to a global sea level rise of .25 mm/year since 1997. Debate exists as to whether or not this loss is caused by recent increases in temperature, or by dynamic processes.
Kangerlussuaq Glacier, in SE Greenland, is the largest outlet glacier on the east coast of Greenland, draining approximately 3% of the Greenland Ice Sheet (GrIS). In 2004/05 this glacier underwent a dramatic retreat, as well as acceleration and mass loss, indicating a significant change in ice dynamics. During this time, the ice velocity increased from 6-8 km/yr to 14 km/yr, resulting in a peak mass loss of 40 Gt/yr by 2005, approximately 20% of the mass loss of the whole SE GrIS. Other SE Greenland outlet glaciers exhibited synchronous acceleration, retreat and thinning, and thus in 2004/05 the mass loss from SE Greenland dominated the overall mass balance of the GrIS. My study investigated the possible causes of increased outlet glacier mass loss in this sector by reconstructing the surface history and using the force budget technique to quantify the forces that control the flow of Kangerlussuaq Glacier before and after its major acceleration event. I used multiple sets of remotely sensed data, including repeat stereo imagery from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument on the Terra satellite and from the Satellite Pour l`Observation de la Terre (SPOT) satellite, as well as a bedrock DEM from radar observations provided by Center for Remote Sensing of Ice Sheets (CReSIS) to reconstruct the ice sheet surface topography and velocity in 2003 and 2006. These input data were then used to generate 2D force balance models. Previous studies have suggested that speed-up and thinning of Kangerlussuaq Glacier was caused by a collapse of the calving front in 2004/05 resulting in a loss of back-stress. However, my surface reconstruction revealed that thinning began in the summer of 2002; at least two years before the start of the rapid thinning and retreat of the calving front. This discovery was made possible by using the Surface Elevation Reconstruction And Change Detection (SERAC) method to combine the laser altimetry data and the stereo-image DEMs to improve the accuracy of the DEMs and to generate a high-resolution, accurate elevation change record. The force balance analysis showed only small changes in driving and resisting stresses between 2003 and 2006 despite the significant retreat. Therefore, I reject the hypothesis that the speed-up was the result of a collapsed calving front. My results suggest the speed-up was in part due to a change in the subglacial hydrology that caused a change in effective basal pressure. The 2003-2004 period showed below-average meltwater runoff that may have reduced water entering the subglacial drainage system. If subglacial drainage is through a network of tunnels, a reduction in the subglacial water flux would lower the effective basal pressure. Because the sliding velocity is inversely proportional to the effective pressure, this would increase the sliding speed. The increase in surface melt and runoff starting in 2005 would have increased the subglacial water flux again, and the resulting increase in effective pressure would have caused the glacier to slow down.
Repeat surveys by aircraft laser altimeter in 1993/4 and 1998/9 reveal significant thinning along 70% of the coastal parts of the Greenland ice sheet at elevations below about 2000 m. Thinning rates of more than 1 m/yr are common along many outlet glaciers, at all latitudes and, in some cases, at elevations up to 1500 m. Warmer summers along parts of the coast may have caused a few tens of cm/yr additional melting, but most of the observed thinning probably results from increased glacier velocities and associated creep rates. Three glaciers in the northeast all show patterns of thickness change indicative of surging behavior, and one has been independently documented as a surging glacier. There are a few areas of significant thickening (over 1 m/yr), and these are probably related to higher than normal accumulation rates during the observation period.
The Greenland Ice Sheet has been losing mass at an accelerating rate since 2003, in part due to changes in ice sheet dynamics. As ocean-terminating outlet glaciers retreat, they initiate thinning that diffuses inland, causing dynamic mass loss from the ice sheet interior. Although outlet glaciers have undergone widespread retreat during the last two decades, the inland extent of thinning and, thus, the mass loss is heterogeneous between glacier catchments. There remains a lack of a unifying explanation of the cause of this heterogeneity and accurately projecting the sea-level rise contribution from the ice sheet requires improvement in our understanding of what controls the upstream diffusion of thinning, initiated by terminus retreat. In this dissertation, I use observations and modeling to identify limits to the upstream diffusion of dynamic thinning for ocean-terminating glaciers draining the Greenland Ice Sheet. I start by using diffusive-kinematic wave theory to describe the evolution of thinning and I calibrate a metric that identifies how far upstream a thinning perturbation can diffuse from glacier termini. This metric is calculable from the observed glacier bed and surface topography and I use it to predict inland thinning limits for the majority of Greenland's outlet glaciers. I find that inland thinning limits often coincide with subglacial knickpoints in bed topography. These are steep reaches of the bed that are located at the transition between the portion of the bed that is below sea level and the upstream portion that is above sea level. I use the predicted thinning limits to help identify individual glaciers that have the largest potential to contribute to sea-level rise in the coming century. Finally, I use higher-order numerical modeling to validate the predicted thinning limits from the first-order kinematic wave model, and to investigate the timing and magnitude of glacier mass loss over the coming century. I find that glaciers that have small ice fluxes but are susceptible to thin far into the interior of the ice sheet have the potential to contribute as much to sea-level rise as their higher-flux counterparts. These lower-flux glaciers are often not discussed in literature but will be significant contributors to sea-level rise by 2100.
Current estimations of the contribution of ice sheets to future sea level rise are solely based on changes in Surface Mass Balance (SMB) of Antarctic and Greenland Ice Sheets. However, the reported SMB changes over the Greenland Ice Sheet explain only about 50% of the observed total mass loss of the Greenland Ice Sheet (GrIS). The other 50% is caused by ice dynamic processes, which have not been included in most sea level rise predictions. The goal of this study was to investigate surface elevation changes of the entire GrIS in 2003-2009. In addition to the total elevation changes, elevation changes due to ice dynamics were also estimated by computing the difference between surface elevation changes measured by laser altimetry and those caused by SMB processes. I applied the Surface Elevation And Change Detection (SERAC) approach to derive surface elevation changes from laser altimetry observations. By fusing satellite laser altimetry (Ice, Cloud, and land Elevation Satellite (ICESat)) and airborne laser altimetry (Airborne Topographic Mapper (ATM) and Land, Vegetation, and Ice Sensor (LVIS)) data, I have reconstructed the elevation and volume change history of the GrIS at more than 55,000 locations. To estimate elevation changes due to SMB, SMB anomalies from RACMO2/GR were converted into height changes using a simple firn-densification model. To facilitate the visualization of elevation changes and the computation of volume changes I interpolated the irregularly distributed observations of ice sheet elevation changes into regular grids. Finally, I partitioned the ice sheet elevation and volume changes into SMB-related and ice dynamics-related changes and computed the contributions of major drainage basins. I have shown that the southeast GrIS was the main contributor of ice loss in Greenland inx2003-2009. The Kangerlussuaq Glacier drainage basin exhibited the largest ice-dynamics related volume loss from the twelve major drainage basins of southeast Greenland. The regions below 2000 m elevation, despite constituting only about 28% of the southeast GrIS, contribute to more than 92% to its ice-dynamics related volume loss. Ice sheet elevation changes, as well as annual volume changes of the twelve major southeast Greenland drainage basins, show a complex spatial and temporal pattern. Finally, the effect of ocean and air temperature changes as external forcing mechanisms on the observed volume change patterns is also discussed. I have shown that the trend of ocean temperatures anomalies along the southeast coast of the GrIS shows a close similarity to the estimated ice-dynamics related volume change pattern.
Outlet glacier ice dynamics, including ice-flow speed, play a key role in determining Greenland Ice Sheet mass loss, which is a significant contributor to global sea-level rise. Mass loss from the Greenland Ice Sheet increased significantly over the last several decades and current mass losses of 260-380 Gt ice/yr contribute 0.7-1.1 mm/yr to global sea-level rise (~10%). Understanding the potentially complex interactions among glacier, ocean, and climate, however, remains a challenge and limits certainty in modeling and predicting future ice sheet behavior and associated risks to society. This thesis focuses on understanding the seasonal to interannual scale changes in outlet glacier velocity across the Greenland Ice Sheet and how velocity fluctuations are connected to other elements of the ice sheet-ocean-atmosphere system. 1) Interannual velocity patterns Earlier observations on several of Greenland's outlet glaciers, starting near the turn of the 21st century, indicated rapid (annual-scale) and large (>100%) increases in glacier velocity. Combining data from several satellites, we produce a decade-long (2000 to 2010) record documenting the ongoing velocity evolution of nearly all (200+) of Greenland's major outlet glaciers, revealing complex spatial and temporal patterns. Changes on fast-flow marine-terminating glaciers contrast with steady velocities on ice-shelf-terminating glaciers and slow speeds on land-terminating glaciers. Regionally, glaciers in the northwest accelerated steadily, with more variability in the southeast and relatively steady flow elsewhere. Intraregional variability shows a complex response to regional and local forcing. Observed acceleration indicates that sea level rise from Greenland may fall well below earlier proposed upper bounds. 2) Seasonal velocity patterns. Greenland mass loss includes runoff of surface melt and ice discharge via marine-terminating outlet glaciers, the latter now making up a third to a half of total ice loss. The magnitude of ice discharge depends in part on ice-flow speed, which has broadly increased since 2000 but varies locally, regionally, and from year-to-year. Research on a few Greenland glaciers also shows that speed varies seasonally. However, for many regions of the ice sheet, including wide swaths of the west, northwest, and southeast coasts where ice loss is increasing most rapidly, there are few or no records of seasonal velocity variation. We present 5-year records of seasonal velocity measurements for 55 glaciers distributed around the ice sheet margin. We find 3 distinct seasonal velocity patterns. The different patterns indicate varying glacier sensitivity to ice-front (terminus) position and likely regional differences in basal hydrology in which some subglacial systems do transition seasonally from inefficient, distributed hydrologic networks to efficient, channelized drainage, while others do not. Our findings highlight the need for modeling and observation of diverse glacier systems in order to understand the full spectrum of ice-sheet dynamics. 3) Seasonal to interannual glacier and sea ice behavior and interaction Focusing on 16 northwestern Greenland glaciers during 2009-2012, we examine terminus position, sea ice and ice m??lange conditions, seasonal velocity changes, topography, and climate, with extended 1999-2012 records for 4 glaciers. There is a strong correlation between near-terminus sea ice/mélange conditions and terminus position. In several cases, late-forming and inconsistent sea ice/mélange may induce sustained retreat. For all of the 13-year records and most of the 4-year records, sustained, multi-year retreat is accompanied by velocity increase. Seasonal speedup, which is observed across the region, may, however, be more heavily influenced by melt interacting with the subglacial hydrologic system than seasonal terminus variation. Projections of continued warming and longer ice-free periods around Greenland suggest that notable retreat over wide areas may continue. Sustained retreat is likely to be associated with multi-year speedup, though both processes are modulated by local topography. The timing of seasonal ice dynamics patterns may also shift.
The Arctic is thawing. In summer, cruise ships sail through the once ice-clogged Northwest Passage, lakes form on top of the Greenland Ice Sheet, and polar bears swim farther and farther in search of waning ice floes. At the opposite end of the world, floating Antarctic ice shelves are shrinking. Mountain glaciers are in retreat worldwide, unleashing flash floods and avalanches. We are on thin ice—and with melting permafrost’s potential to let loose still more greenhouse gases, these changes may be just the beginning. Vanishing Ice is a powerful depiction of the dramatic transformation of the cryosphere—the world of ice and snow—and its consequences for the human world. Delving into the major components of the cryosphere, including ice sheets, valley glaciers, permafrost, and floating ice, Vivien Gornitz gives an up-to-date explanation of key current trends in the decline of ice mass. Drawing on a long-term perspective gained by examining changes in the cryosphere and corresponding variations in sea level over millions of years, she demonstrates the link between thawing ice and sea-level rise to point to the social and economic challenges on the horizon. Gornitz highlights the widespread repercussions of ice loss, which will affect countless people far removed from frozen regions, to explain why the big meltdown matters to us all. Written for all readers and students interested in the science of our changing climate, Vanishing Ice is an accessible and lucid warning of the coming thaw.
This updated and expanded version of the second edition explains the physical principles underlying the behaviour of glaciers and ice sheets. The text has been revised in order to keep pace with the extensive developments which have occurred since 1981. A new chapter, of major interest, concentrates on the deformation of subglacial till. The book concludes with a chapter on information regarding past climate and atmospheric composition obtainable from ice cores.
The Greenland Ice Sheet's (GIS) magnitude of change has been of the utmost importance in understanding cryospheric contributions to the earth's climate change. Of particular interest is the long-term record of Jakobshavn Isbræ, one of the largest outlet glaciers draining an estimated 6.5% of the GIS. Its recent rapid thinning, associated with nearly doubled velocities, indicates that Greenland's outlet glaciers are likely to make faster contributions to sea-level rise than previously believed. To evaluate whether ongoing observed changes are climatically significant, changes must be determined over longer time frames. Although the 35 km retreat of its calving front since the LIA (1850) is well documented, it cannot be used to accurately reconstruct the glacier's history; in particular, since much of its recent retreat, the terminus was likely floating and thus susceptible to small and short-lived climate perturbations.^Here, we combine a chronology of the LIA readvance and subsequent retreat determined from ice sheet threshold lake sediments, along with a 3D reconstruction of ice marginal retreat, measured from stereo imagery to investigate the evolution of the floating ice tongue and land-based margins in the Jakobshavn drainage basin. For this study, we constrain Jakobshavn Isbræ's longer-term context with proglacial threshold lake sediments. Four AMS radiocarbon dates from macrofossils immediately below the LIA sediments from three lake basins to the north of the fjord reveal that Jakobshavn Isbrae reached its LIA maximum extent between 530"10 and 370"60 cal yr BP (1400-1640 AD). Two AMS radiocarbon dates from a lake south of the fjord state that Jakobshavn reached its LIA maximum between 2250"70 and 2420"60 cal yr BP.^Furthermore, the continuous nature of the LIA-sediment units in all sites indicates that Jakobshavn remained at or near its LIA maximum position between 1400-1640 AD and into the 20th century. Using stereoscopic pairs of aerial images taken in 1985, and SPOT satellite images acquired in 2007, vegetation trimlines marking the LIA ice extent and 1985 and 2007 ice sheet margins were mapped in 3D by using a soft-copy workstation. Maximum retreat and thinning rates were measured at Jakobshavn Isbræ, where the calving front retreated at an average rate of 0.178 km yr-1 between the LIA and 1985. Retreat rates increased to 0.545 km yr-1 between 1985 and 2007. Land based margins in the Jakobshavn area record average retreat rates from the LIA to 1985 at .007 km yr-1 and increasing from 1985 to 2007 at 0.030 km yr-1.^However, an outlet glacier just 30 km south of Jakobshavn Isbræ, Alanngorliup Sermia, is at or just above its LIA margin, and has only retreated at a rate of 0.017 km yr-1 since 1985. Thinning rates had a similar trend of increasing at the calving front at a rate of -2.15 m yr-1 from the LIA to 1985 and increased to -4.48 m yr-1 between 1985 and 2007. Land based margins in the Jakobshavn Isbræ area averaged -0.67 m yr-1 from the LIA to 1985 and increased to -1.34 m yr-1 thinning rate between 1985 and 2007. Alanngorliup Sermia has no thinning rate recorded between the LIA and 1985 because of its position at the LIA trimline and has thinned at a rate of -0.31 m yr-1 between 1985 and 2007. These results suggest the greatest retreat and thinning occurred between 1985 and 2007. They also suggest different termini environments respond differently to the same climatic changes.^Varied patterns of retreat and elevation indicate dynamic controls of the Jakobshavn study area. These data suggest that although climate may be the greater driving force of the Jakobshavn margin, ice dynamics play a key role in the marginal evolution since the LIA. The net loss of ice from the GIS plays an important role in global sea-level rise, and therefore more detailed investigations of the causes for marked changes of margins are needed to assess ongoing future changes.
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.