Mass balance studies
Is the volume of ice on earth growing or decreasing?
That is a question closely related to climate research and one of the main issues of glaciology. There are different scales to deal with this issue, looking to small alpine glaciers or to large polar ice sheets.
The mass balance describes the behaviour of a glacier or ice sheet. The mass balance is the sum of gain and loss of ice within a certain time. The process of gaining ice mass is called accumulation and comprises mainly precipitation (snowfall), the process of loosing ice mass is called ablation and comprises mainly melting at the surface or subglacial melting and calving of icebergs. Condensation and evaporation are additional processes with minor importance, partly levelling out each other. The glacial mass balance is one important parameter used to describe climatic changes.
Mass balance studies at AWI are focussed on the Greenland ice sheet and the Antarctic ice sheet. Both ice sheets can be treated similarly with respect to accumulation. One main difference is that annual accumulation rates over Greenland are much higher then over Antarctica. Antarctica is a real desert. But around their margins the two ice sheets behave is very different. Around the whole Greenland ice sheet we see a wide ablation zone, where surface melting occurs during the summer months. Almost the same amount of ice as lost due to surface melting leaves the Greenland ice sheet due to iceberg calving. A minor role, but still existing is subglacial melting under the huge floating ice tongues, especially in Northeast and North Greenland. In the South ice shelves edge approximately half of the perimeter of Antartica. Surface melting is very rarely found in Antarctica, if, then mostly in the northern part of the Antarctic Peninsula. The gain of mass by accumulation is balanced by calving of huge tabular icebergs from the ice shelves and to a certain amount by subglacial melting at the hinge zone of the large ice shelves and at the front of ice shelves. To estimate the mass balance of Antarctica one has to know the outflow of ice mass across the hinge zone of the ice shelves and compare it to the amount of accumulation over the grounded ice sheet. It is important to consider ice dynamics as well.
AWI is involved in mass balance studies with active fieldwork, measurements on ice cores, and numeric modelling in Greenland (NGT-Traverse) and in Dronning Maud Land, Antarctica (EPICA pre site survey, ITASE). Investigations on ablation at the Greenland ice margin are resting at the moment.
Important means for mass balance studies are the direct measurements of either accumulation or ablation by stake farms, snow pit studies and the analysis of firn and ice cores. Airborne radio or ground based radio-echo sounding (RES) is used to measure ice thickness, especially in the hinge zone of ice shelves. Ground penetrating radar (GPR) is also used to calculate the spatial variation of snow accumulation. Ice flow velocities are determined directly by repeated GPS measurements or analysed with the aid of satellite images. Ice dynamics is described by numeric modelling. An important task is to keep up a good database and work with the data by means of a geographic information system (GIS). Changing ice masses may also been detected by means of gravity measurements.
Projects
Cooperating institutes
TU Dresden, Institut für Planetare Geodäsie Prof. Dr.-Ing. habil. Reinhard Dietrich
University of Canterbury Dr. Wolfgang Rack
