Cryosphere [C]

C41A MCC:2002 Thursday

The Dynamics of Glacier System Response: Tidewater Glaciers and the Ice Streams and Outlet Glaciers of Greenland and Antarctica I

Presiding: M Truffer, University of Alaska Fairbanks; M Fahnestock, University of New Hampshire

C41A-01

Recent Near-Coastal Elevation Changes on the Greenland Ice Sheet From Aircraft Laser Altimetry

* Krabill, W B (william.b.krabill@nasa.gov) , NASA/GSFC/Wallops Flight Facility, Building N159, Room E201, Wallops Island, VA 23337 United States
Thomas, R H (thomas@osb.wff.nasa.gov) , EG&Amp;G Services, Inc., Building N159, Wallops Island, VA 23337 United States
Swift, R N (swift@osb.wff.nasa.gov) , EG&Amp;G Services, Inc., Building N159, Wallops Island, VA 23337 United States
Fredrick, E B (earlb@osb.wff.nasa.gov) , EG&Amp;G Services, Inc., Building N159, Wallops Island, VA 23337 United States
Manizade, S (manizade@osb.wff.nasa.gov) , EG&Amp;G Services, Inc., Building N159, Wallops Island, VA 23337 United States
Yungel, J K (yungel@osb.wff.nasa.gov) , EG&Amp;G Services, Inc., Building N159, Wallops Island, VA 23337 United States
Martin, C F (martin@osb.wff.nasa.gov) , EG&Amp;G Services, Inc., Building N159, Wallops Island, VA 23337 United States
Sonntag, J G (sonntag@osb.wff.nasa.gov) , EG&Amp;G Services, Inc., Building N159, Wallops Island, VA 23337 United States

The Arctic Ice Mapping group (Project AIM) at the NASA Goddard Space Flight Center Wallops Flight Facility has been conducting systematic topographic surveys of the Greenland Ice Sheet (GIS) since 1993, using scanning airborne laser altimeters combined with Global Positioning System (GPS) technology. Earlier surveys showed the ice sheet above 2000-m elevation to be in balance, but with localized regions of thickening or thinning. Thinning predominates at lower elevations and thinning rates have recently increased, resulting in a negative mass balance for the entire ice sheet. In May 2005 critical segments of near-coastal flight lines were re-surveyed. Results from the new data will be presented, showing substantial accelerated thinning in several SE Greenland glaciers.

C41A-02

Leakage of the Greenland Ice Sheet through accelerated ice flow

* Rignot, E (eric.rignot@jpl.nasa.gov) , Jet Propulsion Laboratory/Caltech, MS 300-319 4800 Oak Grove Drive, Pasadena, ca 91109-8099 United States

A map of coastal velocities of the Greenland ice sheet was produced from Radarsat-1 acquired during the background mission of 2000 and combined with radio echo sounding data to estimate the ice discharge from the ice sheet. On individual glaciers, ice discharge was compared with snow input from the interior and melt above the flux gate to determine the glacier mass balance. Time series of velocities on several glaciers at different latitudes reveal seasonal fluctuations of only 7-8 percent so that winter velocities are only 2 percent less than the yearly mean. The results show the northern Greenland glaciers to be close to balance yet losing mass. No change in ice flow is detected on Petermann, 79north and Zachariae Isstrom in 2000-2004. East Greenland glaciers are in balance and flowing steadily north of Kangerdlussuaq, but Kangerdlussuaq, Helheim and all the southeastern glaciers are thinning dramatically. All these glaciers accelerated, Kangerdlussuaq in 2000, Helheim prior to 2004, and southeast Greenland glaciers accelerated 10 to 50 percent in 2000-2004. Glacier acceleration is generally brutal, probably once the glacier reached a threshold, and sustained. In the northwest, most glaciers are largely out of balance. Jakobshavn accelerated significantly in 2002, and glaciers in its immediate vicinity accelerated more than 50 percent in 2000-2004. Less is known about southwest Greenland glaciers due to a lack of ice thickness data but the glaciers have accelerated there as well and are likely to be strongly out of balance despite thickening of the interior. Overall, I estimate the mass balance of the Greenland ice sheet to be about -80 +/-10 cubic km of ice per year in 2000 and -110 +/-15 cubic km of ice per year in 2004, i.e. more negative than based on partial altimetry surveys of the outlet glaciers. As climate continues to warm, more glaciers will accelerate, and the mass balance will become increasingly negative, regardless of the evolution of the ice sheet interior.

C41A-03

Rapid Changes of Large Tidewater Glaciers in SE Greenland

* Stearns, L A (leigh.stearns@maine.edu) , Climate Change Institute, University of Maine, 5790 BGSC, Orono, ME 04469 United States
Hamilton, G S (gordon.hamilton@maine.edu) , Climate Change Institute, University of Maine, 5790 BGSC, Orono, ME 04469 United States

New field and satellite remote sensing measurements show that Kangerdlugssuaq Glacier and Helheim Glacier, two fast-flowing tidewater glaciers in South-East Greenland, accelerated 40-300% between 2001 and 2005 and retreated 3-5 km since July 2003. Together, the catchment basins of these two glaciers encompass ~10% of the area of the Greenland ice sheet. Previous studies observed rates of surface lowering on the main trunks of both glaciers that were too large to be caused by enhanced surface melting or decreased snow fall alone. One hypothesis to explain the thinning rates is a change in ice dynamics. We use repeat satellite imagery and published reports to reconstruct the last ~decade of flow histories for both glaciers and compare the results with velocities derived from field GPS surveys in the summer 2005. Helheim Glacier was flowing at ~8 km/yr in 1995 and 2001. In 2005, flow speeds were ~11.7 km/yr, a ~40% increase. The acceleration of Kangerdlugssuaq Glacier was more substantial. Portions of the main trunk that were flowing at ~5 km/yr in 1988, 1996 and 2001 were flowing at ~14 km/yr in summer 2005, an almost threefold increase. The accelerations in flow speeds were accompanied by other changes, including the rapid retreat of calving fronts that had maintained quasi-stable positions for the previous ~40 years, and a lowering of the ice surface by about 100 m, leaving stranded ice on adjacent ridges. The rapid thinning, acceleration and retreat of these two relatively nearby glaciers suggests a common triggering mechanism, such as enhanced surface melting due to regional climate warming. The current flow speeds, ~11 - 14 km/yr at the terminus, are too fast to be caused solely by internal deformation of the ice, implying that an increase in basal sliding forced by additional meltwater production is the probable cause of the velocity increases. The new observations and the hypothesized cause highlight the sensitivity of large outlet glaciers to local climate changes, and have implications for ice sheet mass balance and sea level rise predictions.

C41A-04

Dramatic Changes in the Flow Velocity of Helheim Glacier, SE Greenland: a Result of Climate Forcing?

* de Lange, R (geordl@leeds.ac.uk) , School of Geography, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT United Kingdom
* de Lange, R (geordl@leeds.ac.uk) , Glaciology Group, School of Environment and Society, Swansea University, Singleton Park, Swansea, SA2 8PP United Kingdom
Murray, T (T.Murray@swansea.ac.uk) , Glaciology Group, School of Environment and Society, Swansea University, Singleton Park, Swansea, SA2 8PP United Kingdom
Luckman, A (A.Luckman@swansea.ac.uk) , Glaciology Group, School of Environment and Society, Swansea University, Singleton Park, Swansea, SA2 8PP United Kingdom
Hanna, E (E.Hanna@Sheffield.ac.uk) , Department of Geography, University of Sheffield, Winter Street, Sheffield, S10 2TN United Kingdom

Outlet glaciers in Southeast Greenland have recently been speeding up dramatically; including Helheim Glacier which accelerated 25% between 2002/2003 in comparison to the late-1990's. During the 1990's NASA observed the margins of the Greenland Ice Sheet have been thinning at rates exceeding that expected from melt. The recent observed speed-Up event seems to confirm that dynamic thinning is occurring in this part of Greenland. We investigate the reasons behind the speed-Up of Helheim Glacier. For Helheim's drainage basin, monthly modeled climate data, such as temperature, surface runoff and accumulation, are derived from ECMWF reanalyses. These data show that the surface runoff was anomalously high during the years of 1995 and 1998, but not during either 2002 or 2003. The flow velocity of Helheim Glacier, derived from intensity tracking of 35 days repeat pass period ERS SAR data, was 25% higher in 2002/2003 than the rather stable flow during the period of 1995-1998. Throughout this period velocities of up to 20 md$^{-1}$ were observed with no consistent seasonal pattern a few kilometres up-glacier from the front. Although observations are sparse, ice velocities within the years of 1992 and 1993 were also similar. Our results show a dramatic increase in velocity during 2002/2003 but none during the 1990's, although thinning rates observed by NASA of around 1.5 ma$^{-1}$. If dynamic thinning has been occurring during the 1990's, it happened gradually, or during data gaps which in 1993-1994. These two years were anomalously low melt years, while the flow regime remained robust against the high runoff during 1995 and 1998. The recent speed-Up does not coincide with high runoff. Our results suggest no direct link between surface runoff and velocity during the 1990's or the recent speed-Up event.

C41A-05

Recent Flow Velocities at the Front of Jakobshavn Isbrae/Greenland from 2004 Field Observations

* Dietrich, R (dietrich@ipg.geo.tu-dresden.de) , TU Dresden Institut für Planetare Geodäsie, Helmholtzstr. 10, Dresden, 01062 Germany
Maas, H (hans-gerd.maas@tu-dresden.de) , TU Dresden Institut für Photogrammetrie und Fernerkundung, Helmholtzstr. 10, Dresden, 01062 Germany
Baessler, M (baessler@ipg.geo.tu-dresden.de) , TU Dresden Institut für Planetare Geodäsie, Helmholtzstr. 10, Dresden, 01062 Germany
Rülke, A (ruelke@ipg.geo.tu-dresden.de) , TU Dresden Institut für Planetare Geodäsie, Helmholtzstr. 10, Dresden, 01062 Germany
Schwalbe, E (ellen.schwalbe@tu-dresden.de) , TU Dresden Institut für Photogrammetrie und Fernerkundung, Helmholtzstr. 10, Dresden, 01062 Germany

The Jakobshavn Isbrae is one of the most dynamic tidewater glaciers of the Greenland ice sheet. For the recent time a remarkable retreat of the ice front and an increasing flow velocity have been reported. In August 2004, geodetic field work was carried out next to the present front position of the Jakobshavn Isbrae. With ground-based observations the flow velocity field for the area of the ice front up to 5 km upstream was determined. The observation concept and the data analysis will be described in detail. The obtained velocities range up to 40 m/day, which supports recently published findings based on satellite remote sensing data. Compared to observations performed 20 years ago the flow velocities increased almost by a factor of 2. The consequences for the mass balance of the Jakobshavn Isbrae and its catchment area will be discussed.

C41A-06

The Rapid Retreat of Jakobshavns Isbrae, West Greenland: New Field Observations of 2005 and Structural Analysis of its Evolution

* Mayer, H (mayerh@mechanik.tu-darmstadt.de) , Institut fuer Mechanik, Technische Universitaet Darmstadt, Hochschulstr. 1, Darmstadt, 64289 Germany
Herzfeld, U C (herzfeld@iceberg.colorado.edu) , Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309-0449 United States

Jakobshavns Isbrae in West Greenland, a major outlet glacier of the Greenland Ice Sheet and a continuously fast-moving ice stream, has long been the fastest moving and one of the most productive glaciers on Earth. It had been moving continuously at speeds of over 20 m/day with a stable front position throughout most of the latter half of the 20th century. Changes in front position that did occur during this time period were relatively small. In 2002, this ice stream, also a tidewater glacier with a 12 km long floating tongue, apparently suddenly entered a phase of rapid retreat. The ice front started to break up, the floating tongue disintegrated, the production of icebergs increased. In July 2005, we conducted an extensive aerial survey of Jakobshavns Isbrae to measure and document the present state of retreat compared to our previous field observations since 1996. We use an approach that combines structural analysis of deformation features with continuum mechanics to assess the kinematics and dynamics of glaciers, based on aerial imagery, satellite data and GPS measurements. Results from interpretation of ERS-SAR and ASTER data from 1995 to 2005 in combination with aerial imagery from 1996 to 2005 shed light on the question of changes versus stability and their causes in the Jakobshavns Isbrae dynamical system.

C41A-07

West Greenland ice sheet melt lake observations and modeling

Bryzgis, G (brizgys.2@osu.edu) , Department of Geography, 1036 Derby Hall 154 N Oval Mall The Ohio State University, Columbus, OH 43210-1002 United States
* Box, J E (box.11@osu.edu) , Byrd Polar Research Center, 1090 Carmack Rd, Scott Hall rm 108 The Ohio State University, Columbus, OH 43210-1002 United States

This study examines the spatial and temporal variability of supraglacial melt lakes over the western ablation zone of the Greenland ice sheet. Based on mid-lake automatic weather station surface energy budget measurements, automatic camera imagery, and inflatable boat measurements, we derive lake volume estimates from daily 250 m MODIS imagery for this region. We investigate the correlation of the timing and location of lake water volume and correlation with local-scale climate anomalies from Polar MM5 regional climate model output. Implications for water supply to melt-induced ice sheet acceleration are discussed.

C41A-08

Intermittent Thinning of Jakobshavns Isbr' Since the Little Ice Age, Reconstructed From Photogrammetry, Remote Sensing and Glacial Geologic Evidence

* Csatho, B (csatho.1@osu.edu) , Byrd Polar Research Center, The Ohio State University, 1090 Carmack Rd., Columbus, OH 43210 United States
van der Veen, C (vanderveen.1@osu.edu) , Department of Geological Sciences, The Ohio State University, 125 South Oval Mall, Columbus, OH 43210 United States
Schenk, T (schenk.2@osu.edu) , CEEGS Department, The Ohio State University, 2070 Neil Av., Columbus, OH 43210 United States
Thomas, R (robert_thomas@hotmail.com) , EG&Amp;G Services, NASA Wallops Flight Facility, Building N-159, Wallops Island, VA 23337 United States

Rapid thinning and velocity increase on major Greenland outlet glaciers (Jakobshavn Isbr', west Greenland; Kangerlussuaq and Helheim glaciers in southeast Greenland) during the last two decades may indicate that these glaciers became unstable as a consequence of the Jakobshavn effect, with terminus retreat leading to increased discharge from the interior and consequent further thinning and retreat. Potentially, such behavior could have serious implications for global sea level. However, the current thinning may simply be a manifestation of longer-term behavior of the ice sheet as it responds to the general warming following the Little Ice Age (LIA). Although Greenland outlet glaciers have been comprehensively monitored since the 1980s, studies of long-term changes mostly rely on records of the calving front position. Such records can be misleading because the glacier terminus, particularly if it is afloat, can either advance or retreat as ice further upstream thins and accelerates. To address this issue, we compiled a history of surface elevation changes of Jakobshavn Isbrae since the LIA. We first combined data from historical records, ground surveys, airborne laser altimetry, and field mapping of lateral moraines and trimlines. This record shows two periods of rapid thinning by about 70 meters, in the early 1950s and since 1997. Observed changes in glacier behavior during these two events are markedly different. The recent thinning, which involved several episodes of retreat followed by large thinning, resulted in a rapid retreat of the calving front toward grounding line. Thinning in the 1950s occurred during a period when the calving front was stationary with only minor annual fluctuations. Nevertheless, aerial photographs collected in the 1940s and 50s indicate that thinning extended far inland.