C11A-0070
Late-Glacial and Holocene Relative Sea-Level Change, Scoresby Sund, East Greenland
The age of the Milne Land Stade (MLS) moraines in East Greenland is key for testing the hypothesis that abrupt cooling events seen in Greenland ice cores are characterized by extreme seasonality. In recent papers, the moraines have been assumed to be of Younger Dryas age, allowing direct comparison of snowline lowering estimates with coeval temperature depressions from the nearby ice cores. It is a mismatch of as much as 10 degrees C between these two climate proxies that led to the idea of extreme seasonality. Thus, accurately determining the age of the moraines is a prerequisite to testing this hypothesis. In the Scoresby Sund region, retreat from the MLS moraines was accompanied by marine inundation and isostatic rebound. Here, we present more than 150 radiocarbon dates of shells in raised beaches and deltas from Kjove Land and nearby Schuchert Dal that formed after deglaciation from adjacent MLS moraines. Together, these samples are the basis for two curves, each of which shows very high rates of relative sea-level change typical of recently deglaciated regions. The curves are similar, with both indicating total uplift (including areas now drowned by rising sea level) of slightly less than 200 m. The marine limit (134 m) dates to 12,200 cal yr B.P. We suggest that this is a close minimum age for moraine formation and indicates that the outer MLS moraines likely date to earliest Younger Dryas time. The younger, inner moraine set grades to a former sea level, now at 101 m elevation, and dates to 10,600-10,900 cal yr B.P. The Younger Dryas age for the older set is consistent with the idea that there was extreme seasonality in Greenland during late-glacial time.
C11A-0071
Sea-Level and Ice Sheet History of West Greenland During the Last Millenia
Ice core and other palaeoenvironmental archives from terrestrial and marine settings record several major climate perturbations during the last millennia in Greenland that include the "Medieval Warm Period" and the "Little Ice Age". These changes were associated with variations in ice sheet mass balance and associated changes in ice margin position and ice load. For example, in Disko Bugt (West Greenland), the ice sheet margin is thought to have retreated between AD 600 and AD 1300, before readvancing to a maximum position at c. AD 1750 and then retreating to present. In this poster we present a new methodology for reconstructing high resolution records of RSL change from West Greenland, from which we may infer variations in ice sheet mass balance over decadal to centennial time scales. Our challenge has been to minimise age and altitude uncertainties, which are routinely high in conventional RSL approaches in Greenland and other Arctic areas that relie on shell and isolation basin chronologies. To achieve this, we target thin (0.5 m) salt marsh sediment sequences to provide multiple RSL data points based on AMS radiocarbon dating of terrestrial plant macrofossils and a transfer function based water level reconstruction that uses diatoms and other sedimentological criteria. The transfer function has a typical height uncertainty for each data point of +/- 0.2 m, whilst high precision AMS radiocarbon dates reduce age uncertainty to a little as +/-20 years. We present and compare two salt marsh- based RSL records from West Greenland (Disko Bugt and Kangerlussuaq) that together span the last millennia. Existing models of RSL change during this interval suggest a rise in RSL of up to 8 m since AD 1100. In contrast, our new work indicates a slower and more modest rate of RSL rise during this interval, which at Kangerlussuaq is c. 1.3 mm yr-1. New, high resolution RSL records, such as these, provide targets for geophysical models of ice sheet mass balance change and a long term context for short term geodetic records of crustal motions and ice sheet dynamics.
C11A-0072
Shelf ice glaciation in the Arctic Ocean? New results from northernmost Greenland
Bounding on the last remaining patch of permanent sea ice and capped by an ice sheet with meltwater sufficient to disrupt the thermohaline circulation, North Greenland is strategically located for contributing to the understanding of the climate system. The coastal plain, which faces the Arctic Ocean, more than 100 km long and 15 km wide, is covered by a continuous blanket of Quaternary sediment that spans at least the period since the last deglaciation c. 9000 years ago, and is capped by an array of glacial and marine landforms. This area therefore contains an unsurpassed source for recording marine and glacial activities along the world's northernmost coast – a source which, owing to its inaccessibility, has largely remained untapped. Preliminary results from the ‘LongTerm Project', which ended this summer, show that at least two major glacial events hit the coasts by the end of the last ice age. One of them was possibly a large scale expansion of the Inland Ice resulting in formation of a 100,000 km2 ice shelf in the Arctic Ocean – a type of glaciation, which has usually been thought to be an Antarctic speciality. Even more significantly, abundant accumulations of glacio- fluvial and -lacustrine sediments show that heat transfer to these extreme latitudes by the end of the last ice age was sufficient to allow massive melting of land-based ice. Finally, among the summer's surprises was the discovery of thick piles of raised marine sediments along the coast, allowing a detailed record of sea level history and faunal change, which can be correlated with a terrestrial record from cores, obtained from two lakes on the coastal plain.
C11A-0073
Modeling the Deposition of Deglaciation Moraines
Moraine ages figure prominently in reconstructions of the history of the Greenland ice sheet and of other ice masses, and in testing of hypotheses such as that of Denton et al. (2005, QSR) that north Atlantic millennial oscillations including the Younger Dryas were primarily wintertime events. We present a fast, coupled ice- flow/moraine-deposition model to aid in such hypothesis-testing. We find that not only ages and positions of moraines, but also number of moraines and their volumes, can help in interpretation. The model allows for multiple sediment-transport mechanisms and climate forcings, in a simple but flexible flowline configuration fast enough that inversions may be practical in the future. The temperature forcing for the model was the GISP2 ice core record spanning the time period between LGM and modern. We decoupled the orbital deglacial signal and the millennial-scale signal using a band-pass filter with cut-off at 10,000 years. We applied the ice core records as the forcing to the glacier-sediment model while systematically varying the strength of the millennial scale signal. Our results are examples of moraines deposited with varying degree of strength of millennial-scale signals, some of which show similarity to moraine sets observed around Greenland and the northern hemisphere.
C11A-0074
Quantification of Holocene Climates Around Greenland Using Pollen Records
A series of 12 pollen diagrams from all areas of coastal Greenland, as well as several from the adjacent Arctic Islands, can be used to document the spatial patterns of climate during the Holocene. We used a newly available modern pollen data set for calibration of the climate-pollen relation across the Arctic. Both the eastern and western sides of northern Greenland were relatively warm in the early Holocene, whereas southern Greenland had warmest conditions in the late Holocene. Temperature variations were on the order of a couple of degrees during the Holocene. Questions remain, due to the low density of available modern calibration data, relatively low temporal resolution of many of the records and issues in dating of the sediments.
C11A-0075
Systematic Correlation Sets for Greenland Ice Methane-Temperature During Last 50 kyr
Based on studies of Greenland and Antarctic ice cores, it is conventionally thought that the time series of atmospheric methane concentrations [CH4] extracted from occluded bubbles closely track Greenland temperatures, as calculated from δ18Oice. In this classical view, considerable debate continues regarding the lead-lag relationships of temperature, methane and carbon dioxide concentrations. In the Greenland GISP2 record, this apparent synchronous relationship between ice [CH4] and temperature is generally inferred only through visual inspection of the time series. In this study, we examined the true correlations between GISP2 [CH4] and δ18Oice for the last 50 kyr BP. We do observe the expected trend towards increasing methane mixing ratios with increasing local temperatures. The linear regression through the entire data set reveals an approximate 20 ppbv increase in [CH4] with each 1 °C rise. However, our closer inspection of the GISP2 [CH4]- δ18Oice correlation reveals clearly discernable variations in the magnitude of this response during the late Pleistocene (<50 kyr BP). Our study shows distinct temporal sets of responses that range from 10 to 30 ppbv per 1 °C rise. Our tests show that these different correlations are insensitive to 1) uncertainties in aligning the gas age scales with the ice ages, 2) factors other than temperature that affect δ18Oice, e.g., seasonality and proximity of precipitation, sea surface conditions and atmospheric circulation, 3) errors in the calculation of temperature from δ18Oice measurements (oxygen isotope temperature sensitivity (α), i.e., δδ18Oice/δT). During the Holocene, this [CH4]-δ18Oice relationship decouples, in fact. This indicates that other factors have modulated the methane budget in the past 10 kyr BP than during the Pleistocene. These warmer Holocene data (ca. -30 °C) have no apparent relationship to temperature despite a ca. 200 ppbv range in [CH4]. The change in the [CH4]-δ18Oice correlations indicate a variable teleconnection between the climate in Greenland and that of methane producing regions, over relatively short geologic time spans. These are likely governed by different climate conditions within and since the last ice age. Caution is required when reconstructing latitudinal climate connections from [CH4]-δ18Oice relationships. However, these variations may also provide clues to understanding the interdependence of North Atlantic and tropical climate.
C11A-0076
Greenland Permafrost Temperature Simulations
Permafrost is a crucial component in landscape formation and functioning. The presence of permafrost is reflected in geomorphology, hydrology, ecology, but often much less in human made infrastructure and the climate modeling community. Predictions of permafrost stability over the next hundred years will add to the understanding of landscape changes, and will determine the need for infrastructure improvements. Very little research has been done to integrate long term climate predictions and high resolution permafrost temperature predictions for future climate. The goal of this research is to utilize high resolution climate simulations to estimate the consequences of rising air temperatures on the permafrost. Our research domain is the non-glaciated land mass of Greenland, where we have climate simulations at a 25 kilometer grid from 1950 till 2100. The model is calibrated using ground based observations and well data. We found that many areas that currently have permafrost will be losing ground ice over the next hundred years, while some areas will lose all permafrost. The results of these predictions will be used to make an assessment of vulnerability of infrastructure in Greenland.
C11A-0077
Was there a 1930's Meltdown of Greenland Glaciers?
While climate warming is suggested to have caused acceleration of Greenland Ice Sheet outlet glaciers, recent warming, however, is not without precedent. Examination of Greenland's meteorological stations confirms that 1920s through 1940s surface-air temperature anomalies were at least as equal in magnitude to the recent "global warming" decade, yet observations have not yet been compiled to confirm a similar glacier response. Mining The Ohio State University Libraries, this work compiles historical observations of glacier front positions and surface velocities from maps, photographs and other documentary evidence from mid 19th century Arctic expeditions. Of the glaciers reviewed, an acceleration and retreat indeed occurred between 1920 and 1940. The research put forth is thus consistent with a dynamical response in Greenland's outlet glaciers to the "roaring twenties" warming.
C11A-0078
Atmospheric Conditions Associated With a Significant Greenland Melting Event
The Greenland melt event in late June 2002 is widely identified as being the most significant melt event, in terms of area extent, that has occurred on the Greenland ice sheet over the period 1979-2003. A mesoscale forecast model (Polar MM5) is used to document the large and regional scale atmospheric circulations that gave rise to this melting event. Model hourly forecasts of downward short wave radiation, 2-m temperature, 10-m wind speed, wind direction, are compared to observations at seventeen Greenland automatic weather stations. The modeled vertical structures of temperature, wind speed and wind direction are verified as well with the sounding data at Aasiaat, Narsarsuaq and Tasiilaq. The model simulation shows a high degree of skill for the basic meteorology variables. Temperature advection and adiabatic heating play are shown to play different roles in southern and northern Greenland. In southern Greenland, the temperature advection resulting from a high-pressure ridge was responsible for the higher temperatures that lead to melting in the region. In contrast, the adiabatic heating resulted from the lee side subsistence was responsible for the abrupt temperature rise and melting over northeastern Greenland. The results provide insight into the atmospheric conditions that result in melting over Greenland.
C11A-0079
Field Measurement of Past and Present Meltwater Infiltration in the Percolation Zone of the Greenland Ice Sheet
We present results from a field campaign focused on meltwater infiltration and horizontal water transport processes in the percolation zone of the Greenland Ice Sheet. Field data were collected during a period of heavy melt in June/July 2007 along a ~50 km transect (from ~2000 m to ~1600 m elevation) of the EGIG line of west Greenland. Snow and firn stratigraphies of the upper 10 m were documented with snowpit measurements, core analysis of 21 firn cores drilled to 10+ m, and with over 60 km of constant offset radar profiles collected at a variety of frequencies. We also acquired 15 constant midpoint profiles to characterize depth-density relationships in the upper 80 meters of the firn column. Dye tracing experiments were used to identify meltwater migration pathways and to quantify the relative rates of horizontal and vertical water movement. Five thermister strings with 33 channels and a 30 min time base were installed for long term monitoring of the thermal signature of meltwater migration and ice layer formation in the upper 10 m of firn. Two meteorological stations were installed to provide information on surface boundary conditions. Our work shows massive ice layers (up to 0.4 m thick) form at depth under conditions of heavy surface melt. The ice layers, however, lack spatial coherence over meter length scales and therefore allow vertical meltwater infiltration. Hence, we found no evidence of significant horizontal water transport along internal ice layers within this elevation band of the GIS percolation zone. http://www.umt.edu/geosciences/faculty/harper
C11A-0080
Increased rates of surface elevation change driven by the surface temperature (1982-2006) over the Greenland Ice Sheet
The surface elevation change due to the changes in firn densification rate is an important component among the total changes as observed from the satellite altimetry in the mass-balance studies. In particular, the elevation change caused by the temperature variations alone does not involve any ice sheet mass exchange. This part of the change must be separated from the total changes as a correction for the accurate mass balance estimations. In this study, we use a time-dependent firn densification model driven by the continuous monthly surface air temperature (1981-2006) from AVHRR over Greenland to derive the corresponding surface elevation change over the accumulation region (> ELA) of the ice sheet. The model is characterized by the higher temperature sensitivity according to the grain growth and ice deformation rates, with the consideration of the melt effect. The results show that the continuous surface warming during the last two decades has caused the strong decrease of the surface elevation at the average rate of 7 cm a-1 (2003-2006) in comparison with 1 cm year-1 over the last decade (1992-2002).
C11A-0081
The rate of ice sheet mass-loss from southeast Greenland from combined GLAS and ASTER observations
Repeat laser altimetry from the Geoscience Laser Altimeter System (GLAS) aboard the Ice, Cloud and Elevation Satellite (ICESat) provides direct measurements of changes in ice sheet volume at unprecedented temporal and spatial scale. However, recent observations show that the rate of loss from many of Greenland's large outlet glaciers has substantially increased over the past several years and there is concern that GLAS may not capture this contribution to mass-loss due to the spacing of altimeter tracks. Furthermore, GLAS measurements are much poorer at lower elevations where the ice slopes are high and the track-offset errors are larger. To test the importance of these sources of uncertainty in GLAS estimates of Greenland mass-loss, we combine these measurements with concurrent elevation changes obtained from repeat digital elevation models (DEMs) from the Advanced Spaceborne Thermal Emissivity and Radiation Radiometer (ASTER). In complement to GLAS, ASTER DEMs provide complete spatial coverage at high resolution over limited areas and work best below the equilibrium line because the parallax image generation relies on correlation of surface features. We constructed repeat DEM's for the outlet glaciers of the southeast Greenland coast between 2002 and 2005, with some imagery from 2001 and 2006, and combined them with higher-elevation GLAS point measurements using a simple linear interpolation to produce a map of elevation change rate from the coast to the divide. Our estimates for total mass-loss agree closely with other regional estimates from the same time period, but emphasize the importance of the period of observation for these rapidly changing glaciers. We also find that the added constraint provided by ASTER DEM's do not substantially impact the estimates for total mass-loss provided by GLAS alone. While the outlet glaciers are thinning at rates of 10's of m/yr, the area of this thinning (100's of sq. km) is small compared to the 1000's of sq. km of thinning at 1-10 m/yr captured by GLAS. This suggests that GLAS repeat altimetry is an effective tool for measuring Greenland mass-loss, even during times of rapid outlet glacier change. However, the high spatial resolution of the combined GLAS-ASTER maps provide detailed information into the distribution of mass-loss and will aid process-oriented studies of ice sheet change.
C11A-0082
Greenland Ice Sheet Surface Temperature, Melt, and Mass Loss - 2000 to 2006
A daily-time series of clear-sky surface temperature has been compiled of the Greenland Ice Sheet (GIS) using the 1-km resolution, clear-sky land-surface temperature (LST) standard-data product from the Moderate- Resolution Imaging Spectroradiometer (MODIS), from 2000 to 2006. Surface temperature is important because it is a sensitive indicator of surface-melt timing and duration. We also use the Gravity Recovery and Climate Experiment (GRACE) mass concentration, or mascon, solution data to study mass change on the GIS in relationship to surface melt from 2003 to 2006. The mean LST of the GIS increased during the last six years of the study period by ~0.27 deg C per yr. The LST increase was especially notable in the northern half of the ice sheet during the winter months and at higher elevations. Over the course of the study period, most of the six major drainage basins of the GIS experienced longer melt seasons, and a stable or later start and end of the melt season; however basins 4 and 5, in southern Greenland, showed dramatically earlier initiation of the melt season by up to 18 and 22 days, respectively. While none of the observed trends is statistically significant in part due to the brevity of the record, they are consistent with trends seen using a variety of other observations. End-of- melt-season data for 2007 will also be presented. Furthermore, we show that when less than 1 percent of the ice-sheet experienced surface melt, rapid (less than 15 days) and sustained mass loss below 2000-m elevation is triggered in 2004 and 2005 as recorded by GRACE. Below 2000 m in 2004 and 2005, the ice sheet lost about 321 and 444 Gt of mass, respectively, and experienced more melting in 2005 as compared to 2004. After cessation of surface melt (in 2003, 2004 and 2005), GRACE data show an increase in mass from accumulation, with a delay of less than 30 days. The result that initiation of large-scale surface melt is followed rapidly by mass loss supports other work showing that surface meltwater is flowing rapidly to the base of the ice sheet causing acceleration of outlet glaciers. This result highlights the metastability of parts of the GIS and the vulnerability of the ice sheet to air-temperature increases. If air temperatures continue to rise over Greenland, increased surface melt will play a large role in ice-sheet mass loss.
C11A-0083
A new Programme for Monitoring the Mass Loss of the Greenland Ice Sheet
Recent years has seen a dramatic increase in the mass loss from the Greenland ice sheet, as documented from satellite remote sensing, airborne surveys and in situ measurements. In recognition of these changes, the Danish Ministry of the Environment has now launched the new Programme for Monitoring of the Greenland Ice Sheet (PROMICE), designed and operated by the Geological Survey of Denmark and Greenland (GEUS). The programme aims at estimating the annual mass loss from surface melting as well as from iceberg calving, combining modeling with in situ data collection. During the programme initiation phase from 2007-2010, seven transects, each with two automatic mass-balance stations, will be established in the ablation zone of the Greenland ice sheet. The final station network, intended to operate from 2011 and onwards, will complement the Greenland Climate Network (GC-Net) of the US and the Dutch K-transect, so that ice sheet surface melting and its climatic causes can be monitored in all geographical regions of the Greenland ice sheet. An airborne survey crossing the entire marginal zone of the Greenland ice sheet was carried out in August 2007, measuring ice sheet elevation with a scanning laser altimeter and ice sheet thickness with a 60 MHz radar. The intention is to repeat this survey every 2-3 years, first in 2010, to monitor temporal changes in ice sheet elevation. The comprehensive ice-sheet cross section be combined with ice-sheet surface velocities derived from radar- satellite to yield the ice flux and thus eventually the mass loss by ice sheet calving. The programme will also deliver glacier and ice sheet outlines to the Global Land Ice Measurements from Space (GLIMS) project, with the long term aim of compiling a complete inventory of Greenland glaciers and ice caps as well as the ice sheet margin. Here we present the status after the first season, with six automatic mass-balance stations established in four transects, the first airborne survey completed and a Greenland glacier inventory in its making.
C11A-0084
A Long-Term Network of Automatic Weather and Ice Monitoring Stations in the Ablation Zone of the Greenland Ice Sheet
This August the first five of in total 14 automatic weather and ice monitoring stations were placed in the ablation zone of the Greenland Ice Sheet. In the coming years the network will be completed, covering seven regions of the ablation zone around the ice sheet. This effort is part of the Programme for Monitoring of the Greenland Ice Sheet Margin (Promice). The Promice stations measure all basic meteorological parameters, supplemented by GPS readings, potentially enabling us to correlate ice dynamics to surface melt water production. However, the most prominent feature of the stations is that surface-height variability is monitored in detail using two sonic rangers as well as pressure tubing drilled 30+ m into the ice. Observations are forwarded through satellite communication four times a day. We discuss the potential of the monitoring network and show early results for one of the Promice stations placed on the ice in south-east Greenland. Surface-height variability since installation is interpreted in terms of atmospheric conditions. Furthermore, we discuss expected regional differences using climate data as recorded by pre-Promice stations at three locations on the ice-sheet margin over the period 2001-2007.
C11A-0085
Examining the Recent Peripheral Thinning of the Greenland Ice Sheet Using PARCA Laser Altimetry Data
Recent observations have shown that the periphery of the Greenland ice sheet is thinning at an anomalously high rate and that this thinning is largest around outlet glaciers. We present thinning rates from PARCA laser altimetry surveys for 14 and 11 of the largest outlet glaciers in Greenland from 1993 to 1998 and 1998 to 2006 respectively. Outlet glacier thinning rates are compared with concurrent surveys over inland ice to examine the influence of ice velocity on surface thinning rates. Thinning rates of land terminating and tidewater outlets are also compared from 1993 to 1998 and 1998 to 2006. In general land terminating glaciers have thinned less rapidly during both time periods. From 1998 to 2006 the difference in thinning rates increased, with several tidewater glaciers retreating and thinning significantly. Possible causes of tidewater outlet thinning are investigated by a simple statistical analysis of thinning rates with outlet glacier geometry and velocity, fjord characteristics and records of sea-ice cover.
C11A-0086
Annual accumulation for Greenland updated using ice core data developed during 2000-2006 and analysis of daily coastal data
An updated accumulation map was prepared using ice-core and snow-pit data from 250 historical locations on the ice sheet, plus 39 new ice cores strategically placed to fill gaps in the previous accumulation map. In addition data from 26 coastal weather stations were used in the analysis. Daily coastal data were analyzed, gaps filled using interpolation from nearby stations, and wind corrections made. Interpolation was carried out using kriging with semivariogram fitting and cross-validation analysis. A third-order trend was removed from the data with trend analysis. Kriging was performed on the residuals and the trend added back to the kriged residuals to estimate the final accumulation. Results were very sensitive to small changes in corrections to the coastal data. Using only solid precipitation from coastal stations for the interpolation, the average accumulation over Greenland was found to be about 30 g cm-2 yr-1, similar to the average found previously. Using total precipitation from coastal stations in the interpolation gives a Greenland-wide average of about 35 g cm-2 yr-1. There are five areas with differences compared to the previous map, published in 2001. Our current estimate of accumulation is lower in the southwest, northwest and east, but higher in the southeast and northeast.
C11A-0087
Annual layer mapping and net snowfall measurements across the southern Greenland ice sheet using shallow radar and ice cores
Ground penetrating radar was used to detect internal layers along a 200 km, east-west snowmobile traverse across the Greenland ice sheet at 66oN latitude. The traverse also included 5 shallow to intermediate depth ice cores and extends from a region of recent strong thickening of the ice sheet in the west to a region of recent strong thinning in the east. Density and high resolution glaciochemistry measurements from the ice cores were used to establish precise depth-age relationships along the traverse. Radar events corresponding to annual layers were identified using one ice core and then mapped across the entire radar profile. Independent comparisons to the depth-age relationships at the remaining ice core sites show that uncertainty in the annual layers determined from the radar traverse are on the order of 0.1 years. The result is an accurate, continuous ~1950 to 2002 record of annual net snowfall along the 200 km traverse. Elevation measurements from global positioning system surveys show that accumulation rates correlate highly with surface undulations. After accommodating for surface undulations, results show that temporal changes in accumulation rate are spatially coherent and likely reflect changes in annual snowfall resulting from large scale changes in atmospheric circulation.
C11A-0088
Interpretation of the Shallow Firn Layer Temperature Data from GC-Net AWS Stations
Measurements of temperature and other climate variables for the Greenland ice sheet are continuously recorded by the Greenland Climate Network (GC-Net) surface meteorological stations. All GC-Net stations are equipped with a string of thermocouple temperature sensors to provide a record of the firn temperature profile as well as surface temperature and snow height change (dH/dt) sensors. Thermocouple temperature measurements are accurate to 0.1C. A subset of six automatic weather stations is chosen that exhibits good spatial and temporal coverage of the extensive, high elevation (>2000m) accumulation regions of the interior of the ice sheet. The procedure to remove simultaneous temperature variations (STVs) in the thermocouples follows a newly developed, physically based data cleaning technique. This method allows more physical process and parameter information to be extracted from the existing firn temperature data. To account for densification in the firn, and thus movement of the sensors relative to one another, a temperature-dependent model of firn densification is used which reproduces the strong seasonal signal in the densification rate of the upper firn. Harmonic trend analysis reveals the cycle of sub-surface temperatures, as well as a significant upward trend in all study sites after the year 2000. The firn temperature increase observed at various depths in the firn reveals an air temperature increase that is masked by the large seasonal and inter-annual variability in the record. The temperature increase experienced at 10m depth is as great as .3C/year. Further, the in situ time series of constant-depth temperatures at various levels are compared with modeled temperatures to validate the use of GC-Net data to estimate heat transfer into the snowpack. Finally, a new set of firn-temperature distribution maps is presented.
C11A-0089
Greenland Ice Sheet Outlet Glacier Calf Ice Production Reconstruction: 1958-2006
Several Greenland outlet glaciers have been observed in a state of significant thinning, acceleration and retreat in the recent decade. Accompanying with glacier acceleration and retreat, the rate of the ice loss to the ocean also increased. Ice discharge variability could be related to many factors, among which climate variability seems of greatest concern. Glaciers south of 70 degrees N have been shown to be sensitive to melt with higher sensitivity from those at southwest. A melt sensitivity model is used to reconstruct long term total ice discharge from Greenland ice sheet. Glacier surface melt rates are found to explain up to 60-90% of ice discharge variance depending on the location of the glacier. The flow-temperature sensitivity is used to estimate that Greenland outlet glaciers contributed calf ice to global sea level rise 0.6 mm/yr during 1958-2006, which accounts for two thirds of the "unexplained" sea level contribution of 1 mm/yr during last 50 years.
C11A-0090
How to Model Water Flow in Moulins?
The development of large melt ponds on the Greenland Ice Sheet (GIS) and their drainage system indicate that moulins are a major contributor to the englacial water system. Here we review the current state of knowledge and the history of moulin research. In the late 19th century glaciologists led by Vallot climbed and studied the Grand Moulin on Mont Blanc. Despite being considered mystic due to their size and water drainage they have been studied by a few scientists such as Holmlund and Hooke (1980) or Puccini and Badino (1990). We develop a qualitative model of geometry as well as of the driving forces in the life cycle of moulins using data, photos, sketches, and climbing reports by ice speleologists and climbers. The GIS is temperate for the first 10 km at its margin and consists of cold ice further inland. The recent increase in melt water leads to an increase in basal water availability. The observed increase in ice velocity might be caused by the lubrication at the bed combined with a possible temperature rise in the cold part of the GIS. The raise of englacial water flow increases the volume of the conduits thus reducing the timing of water to reach the ice sheet bed. Our initial model starts with a narrow englacial non-arborescent channel network. We anticipate the development of the englacial hydrology system by using the 'Roethlisberger' conduit model. In addition we will show first model results on temperature fluctuations in the ice due to the hydrologic system.
C11A-0091
Modeled Increase of ice Drainage of Jacobshavn Isbrae due to ice bed Uncoupling
Increased ice drainage of major ice sheets will lead to a rise in sea level. The Greenland Ice Sheet seems to react faster to global warming than originally was thought. Three major outlet glaciers in Southern Greenland, increased their ice drainage significantly since 1997. Most of the draining takes place through fast moving calving glaciers called ice streams. A not well understood condition for ice streams to occur is the presence of basal water. We present a possible mechanism for the dynamics of ice streams by splitting up each column of ice along a flow line into a floating part and a grounded part. The floating part of the ice column is balanced by a tensile force and the grounded part is balanced by basal drag. Using this model, the center flow line of Jacobshavn Isbrae in West Greenland is monitored from 1985 to 2003. A gradual uncoupling of ice from it's bed is observed from 1993 to 2003. One observation is that the small ice shelf of Jacobshavn Isbrae was simply pushed forward by the increasingly uncoupled ice before the shelf collapsed. The uncoupling pinches off some sixty kilometers upstream from the grounding line. This uncoupling is then quantified as a force perturbation that is used in a time dependent model of ice surface lowering. The initial reaction to the perturbation is a strong increase in ice draining velocity that dies out after several years until a new steady state is accomplished.
C11A-0092
Toward estimating sea level rise contribution from the observed Greenland Ice Sheet volume changes
The Greenland ice sheet is a major contributor to observed and projected global sea level rise. Ice sheet surface elevation changes have been used to estimate this input to sea level change. However, the process of snow surface layer densification, which causes changes in volume (elevation) but not necessarily mass, represents a recognized but poorly quantified uncertainty in estimating true mass losses from surface elevation changes. Our project is using remote sensing, field observations and Polar MM5 climate model data, together with a melt/refreezing densification model to help quantify the magnitude of this effect across the Greenland Ice Sheet. Preliminary results for melt model inputs will be presented. Annual precipitation maps are derived from firn cores and coastal meteorological station data. Surface melt intensity and duration is estimated from microwave remote sensing datasets and compared with Polar MM5.
C11A-0093
Recent Climate in Greenland Through Ice Cores and Self-organizing Maps
Recent years have seen a large increase in the availability of high-quality ice cores from the Greenland Ice Sheet (GIS) and the subsequent development of many new high-resolution proxy climate records. Here we apply a relatively new, nonlinear approach using self-organizing maps (SOMs) to study the spatial and temporal variability seen in accumulation records from 50-plus GIS sites covering part or all of the period 1957-2002. SOMs provide an unsupervised classification of complex geophysical data sets, e.g., time series of the atmospheric circulation or sea-ice extent, into a fixed number of distinct generalized patterns, or modes, that represent the probability density function (PDF) of the input data. These patterns collectively provide a nonlinear classification of the continuum of the PDF into a two-dimensional, spatially organized grid form. In contrast to principal component analysis, SOMs do not force orthogonality or require subjective rotations to produce interpretable patterns. Results from analyses of annual accumulation show that the SOM readily captures the high spatial diversity seen in climate records from the GIS, including nonlinear gradients in latitude and elevation. For example, sites in the northern and central regions (e.g. Humboldt, NASA-U) tend to be unrelated to sites in the south and east (e.g., Das1, STUNUA). Sites within the south/southeast also show richer patterns of variability than simply above or below average accumulation. Understanding these relationships, and the spatial complexity of Greenland's climate, is key to improving our ice core-based reconstructions of past climate and projecting possible future changes in the GIS. The SOM algorithm is widely held to be robust in the presence of incomplete input data, a characteristic typical of multi-site/project ice core analyses. Here we examine the sensitivity of the SOM-derived accumulation patterns and reconstructed site records to changes in the number of sites and record lengths. This explores the information available from this climate dataset and gives insight to which sites "matter" the most in understanding ice sheet history (as well as testing the analysis technique itself). When combined with ongoing SOM-based analyses of the atmospheric circulation, we anticipate new insights into the complex climate of this region, including relationships with phenomena such as the North Atlantic Oscillation/Arctic Oscillation.
C11A-0094
Estimation of Greenland's Ice Sheet Mass Balance Using ICESat and GRACE Data
Data of the GRACE gravity mission and the ICESat laser altimetry mission are used to create two independent estimates of Greenland's ice sheet mass balance over the full measurement period. For ICESat data, a processing strategy is developed using the elevation differences of geometrically overlapping footprints of both crossing and repeated tracks. The dataset is cleaned using quality flags defined by the GLAS science team. The cleaned dataset reveals some strong, spatially correlated signals that are shown to be related to physical phenomena. Different processing strategies are used to convert the observed temporal height differences to mass changes for 6 different drainage systems, further divided into a region above and below 2000 meter elevation. The results are compared with other altimetry based mass balance estimates. In general, the obtained results confirm trends discovered by others, but we also show that the choice of processing strategy strongly influences our results, especially for the areas below 2000 meter. Furthermore, GRACE based monthly variations of the Earth's gravity field as processed by CNES, CSR, GFZ and DEOS are used to estimate the mass balance change for North and South Greenland. It is shown that our results are comparable with recently published GRACE estimates (mascon solutions). On the other hand, the estimates based on GRACE data are only partly confirmed by the ICESat estimates. Possible explanations for the obvious differences will be discussed.