Vicki Parry (1), Peter Nienow (1), Douglas Mair (2), Julian Scott (3), Duncan Wingham (4)

1. 600 JAR 3 323 m 500 JAR 2 568 m 400 300 JAR 1 962 m 200 100 Swiss Camp 1149 m 0 1996 1998 2000 2002 2004 2006 Years Vicki Parry (1), Peter Nienow (1), Douglas Mair (2), Julian Scott (3), Duncan Wingham (4) 1 – School of Geosciences, University of Edinburgh, UK, 2 – School of Geography and the Environment, University of Aberdeen, UK, 3 – British Antarctic Survey, Cambridge, UK, 4 –Centre for Polar Observation and Modelling, University College London, UK. RESULTS Variations in mass balance and snow and firn densities along a transect in the percolation zone of the Greenland Ice Sheet INTRODUCTION Accurate elevation changes over large areas of the polar ice sheets can be measured using satellite radar altimetry, from which mass balance can be derived. In the percolation zone seasonal changes in snow pack density ensure that changes in surface elevation cannot be directly correlated with changes in mass. We aim to determine the influences of summer densification along a 57 km transect in the percolation zone of the Greenland Ice Sheet (GIS). Fig 2. • The average Core 2005 (post-melt) density is 20% greater than Pit 2006 (pre-melt) (Fig. 2) from T1 to T7. • Pre-melt (Pit 2006) density shows no change with elevation. Post-melt (Core 2005) density decreases by 36 kg m-3 for every 100 m elevation increase from T1 to T7 (Fig. 2). Fig. 2 Fig. 3 • Density decreases with increasing elevation from T4 to T6 for Cores 2003, 2004 and 2005 (Fig. 3). • The gradients of densification with elevation for Cores 2003, 2004 and 2005 are all statistically significantly different at or above 97.5% (Fig. 3). FIELD SITES Fig. 1 The field sites are located along a 57 km transect on the EGIG line in the percolation zone on the west of the GIS (Fig 1.). DISCUSSION: Implications for mass balance measurements derived from elevation changes measured by satellite radar altimetry: If densification due to meltwater percolation and refreezing is not accounted for, changes in elevation may be misinterpreted as a change in mass. E.G. Assuming no mass change, and applying the average densification from the transect, 120 kg m-3, there is a surface lowering of 0.24 m in a 1 m thick snowpack. Fig 4. However, if temperatures change so will densification rates and elevation changes will occur without any change in mass. The transect extends from T1 (69o 44 N, 48o 7 W) at 1680 m elevation to T7 (69o 56 N, 46o 48 W) at 2050 m elevation (Fig 1). r=404 kg m-3 WE=0.4 m 1 m r=524 kg m-3 WE=0.4 m 0.76 m Fig 4. shows the number of Positive Degree Days (PDD) at 4 AWS stations (Steffen and others 1996) down glacier from T1 from 1996 to 2005. There is a modest positive correlation showing that since 1996 the number of PDDs per year have been increasing by 17 PDDs per year at 323 m, and 6 PDDs per year approximately at the ELA (1149 m). METHODS Density measurements were made by measuring volume and mass of samples from visually identified stratigraphic layers in snowpits and shallow cores. Using a lapse rate of 1 oC for every 142 m (Hanna and others 2005) for Core 2005 results, there is an increase in density of 51 kg m-3 for every 1 oC warming. With predicted increases in temperature in the Arctic of 2 to 3 oC by 2040 (ACIA 2005), for a 1 m snowpack, pre-melt density of 404 kg m-3, there will be an increase in post-melt density of 102 – 204 kg m-3 and a decrease in elevation of 0.21 – 0.34 m, with no mass loss. Thus, changing temperatures in the Arctic also need to be considered. CONCLUSION In order for accurate mass balance measurements to be derived from accurate elevation change measurements, future changes in rates of densification must be considered. We have found that in the percolation zone of the Greenland ice sheet, elevation changes may not directly relate to mass change due to seasonal variations in densification of the snowpack associated with processes of surface melt, percolation and refreezing. As expected, densification decreases with increasing elevation, but this gradient changes annually. Firn density is related to annual melt and accumulation (Braithwaite 1994), both of which vary. With projected rising temperatures, rates of densification will increase with a subsequent impact on elevation change but not necessarily on mass balance at higher elevations in the percolation zone. The shallow cores may extend through more than one years accumulation, so the bottom part may include part of the previous years accumulation. ACKNOWLEDGEMENTS This work is a contribution to the validation of the ESA CryoSat. The work is funded by NERC through grant NER/O/S/2003/00620. REFERENCES Braithwaite, R., M. Laternser, W. Pfeffer. 1994. Variations of near-surface firn density in the lower accumulation area of the Greenland ice sheet, Patkitsoq, West Greenland. J. Glaciol.,40(136), 477-485. Pfeffer, W., M. Meier, and T. Illangasekare. 1991. Retention of Greenland runoff by refreezing: Implications for projected future sea level change. J Geophys. Res.96(C12), 22,117-22,124. Steffen, K., J. Box, and W. Abdalati, 1996 Greenland LCimate Network: GC-Net, in Colbeck, S. C. Ed. CREEL 96-27 Special Report on Glaciers, Ice Sheets and Volcanoes, trib. To M. Meier, pp. 98-103. ACIA. 2005: Arctic Climate Impact Assesment. Cambridge university press, New York, pp.27 Our thanks for help in the field go to Yenz, and for logistical support go to Kristian Keller and Rene Forsberg of KMS and DNSC, Copenhagen, Malcolm Davidson of the European Space Agency, Robin Abbot of VECO Polar Resources, Kate Bar Friis of Kangerlussuaq International Science Support and the Danish Polar Centre.