Climate change and sea level: ice-dynamics and mass-balance studies on the Greenland ice sheet

Summary of the work packages perfomed by AWI:(1) Field work on Nioghalvfjerdsfjorden glacier:The AWI glaciology group joined the Danish glaciologists working on the floating tongue of Nioghalvfjerdsfjorden Glacier during thesummers of 1997 and 1998. The emphasis was on (i) seismic measurements to derive ice thickness and water depth below the ice shelf,(ii) tilt meter measurements at the margin, (iii) CTD profiling underneath the glacier and the sea ice, and (iv) GPS measurements. Themeasurements revealed a strong ice thickness gradient near to the grounding line, the existence of a large cavity below the ice shelf , andthe existence of a thick fresh water layer near to the ice front. These observations support the large basal melting rates below the ice shelfrevealed by the Danish field work. It is postulated that relatively warm oceanic water which enters the cavity through Dijmpna Sundprovides the energy for melting.(2) Observations on Storstroemmen glacier:Stakes planted on Storstroemmen in 1995 were revisited at the end of the summer season of 1997. Only three stakes were still standing.Velocity determinations revealed that the overall flow pattern is still governed by post-surge behaviour.(3) Flowline model studies:A flowline model was developed for the lower section of the Nioghalvfjerdsfjorden flowline. This line was embedded in a 3-D model ofthe entire Greenland ice sheet which provided the boundary conditions at the upper end of the section. Input parameters were provided bythe recent field data collected on the glacier. First results showed that the flowline model is able to produce quite realistic results and thatthe coupling did not introduce unwanted numerical effects or discontinuities. The meltrates required below the ice shelf to fit the modelledgeometry to the seismic measurements match very well with the field observations.(4) 3-D model studies:A 3-D thermomechanical ice-sheet model was used to refine estimates of the past, present, and future contribution of the Greenland icesheet to global sea-level changes. Datasets for ice thickness, bedrock elevation, surface elevation, and precipitation rate were first updatedto take into account the wealth of new data that became available during the last decade. Also the mass-balance treatment was refined andrecalibrated to obtain a best fit to available mass-balance observations. New calculations were performed over the last two glacial cycles toobtain the current evolution of the ice sheet. It was found that the ice sheet is presently close to a stationary state, although that largespatial variations occurred. Patterns and scenarios of future climatic change were downscaled from output of the Hamburg (ECHAM)climate model for the period 1985-2084. A major result was that the total sea-level rise from the Greenland ice sheet (around 5 cm after100 years) would be substantially less than obtained from earlier model studies. That is due to the small warming predicted for theice-sheet margin in southern Greenland, where it matters most for melting. An extensive sensitivity study highlighted the role of icedynamics and the height/ mass balance feedback on the future behaviour of the ice sheet. It was found that ice dynamics cannot beneglected, not even on a century time scale, and that it produces a counteracting effect. Even when greenhouse gas concentrations wouldstabilize by the early 22nd century, Greenland melt-down is found to be irreversible for equivalent CO2 concentrations more than twicethe present value, which would produce a 7 m sea-level rise after a few thousand years

Accumulation rates in Dronning Maud Land, Antarctica, as revealed by dielectric-profiling measurements of shallow firn cores

The European Programme for Ice Coring in Antarctica includes a comprehensive pre-site survey on the inland ice plateau of Dronning Maud Land, Antarctica. The German glaciological programme during the 1997/98 field season was carried out along a 1200 km traverse on Amundsenisen and involved sampling the snow cover in pits and by shallow firn cores. This paper focuses on the accumulation studies. The cores were dated by dielectric-profiling and continuous-flow analysis. Distinct volcanogenic peaks and seasonal signals in the profiles served to establish a depth time-scale. The eruptions of Krakatoa, Tambora, an unknown volcano, Kuwae and El Chichon are well-documented in the ice. Variations of the accumulation rates over different times were inferred from the depth time-scales. A composite record of accumulation rates for the last 200 years was produced by stacking 12 annually resolved records. According to this, accumulation rates decreased in the 19th century and increased in the 20th century. The recent values are by no means extraordinary, as they do not exceed the values at the beginning of the 19th century. Variations in accumulation rates are most probably linked to temperature variations indicated in δ18O records from Amundsenisen