Glaciers as monitors of environmental change

Glaciers and ice sheets are commonly located in remote areas far from population centres. Despite this physical and mental distance, past and modern glacial environments provide an important key to our knowledge of past, present and future global environmental conditions. Glacial environments may at first look chaotic and complex. However, few other environments exhibit such rapid, dynamic and spatially variable changes of processes. Past and present glaciers and ice sheets have had a significant impact upon all aspects of Earth systems. Understanding of many aspects of glaciers and glacier processes still remains poor. As an example, the complex relationship between ice dynamics and mass balance fluctuations is not fully understood. Modelling of ice masses and mass balance studies have, and will, advance our understanding of global ice-sheet fluctuations in the past.

The effect of modern glaciers on a global scale can be looked upon at two levels. Firstly, they impact upon humans and habitats in their nearby surroundings. Meltwater outbursts and rapid ice advances resulting in the loss of pasture lands, property and human fatalities are well documented (e.g. Grove, 1988). Secondly, there is the large-scale impact on the global climate and sea-level. Related to this topic is the controversial question of ice-sheet stability and whether the large ice sheets are melting at an increased rate, injecting large volumes of cold fresh water into the polar oceans, and affecting oceans and near-shore habitats, currents, surface ocean water temperatures and global weather phenomena.

Over the past several decades the techniques of studying glaciers have greatly improved. For example, satellite images have improved the accuracy of measuring ice movement and mass balance. Ice cores retrieved from the Antarctic and Greenland ice sheets have greatly improved our knowledge of past environmental changes. Computer-generated ice-sheet models have increased our understanding of ice-sheet growth and potential stability/instability as a result of predictions of future ice-sheet variations. In addition, there is growing knowledge of the likely spatial and temporal development of the Pre-Pleistocene and Pleistocene ice sheets, and the causative mechanisms that may lead to global glaciation, carbon dioxide variations, and biomass and productivity changes.

International monitoring of glacier variations began in 1894. At present, the World Glacier Monitoring Service (WGMS) of the International Commission on Snow and Ice (ICSI/IAHS) collects standardized glacier information, as a contribution to the Global Environment Monitoring System (GEMS) of the United Nations Environment Programme (UNEP) and to the International Hydrological Programme (IHP) of the United Nations Educational, Scientific and Cultural Organization (UNESCO). The database includes observations on changes in length and, since 1945, mass balance. Most of the data come from the Alps and Scandinavia.

Two main categories of data - summary information and extensive information - are reported in the glacier mass balance bulletins published by IAHS. Summary information on specific balance, cumulative specific balance, accumulation area ratio (AAR) and equilibrium line altitude (ELA) is given for ca. or approximately 60 glaciers. This information provides a regional overview. In addition, extensive information such as balance maps, balance/altitude diagrams, relationships between accumulation area ratios, equilibrium line altitudes and balance, as well as a short explanatory text with a photograph, are presented for 11 selected glaciers with long and continuous glaciological measurements from different parts of the world. The long time series are based on high-density networks of stakes and firn pits. These data, most of which are now available on Internet, are useful for analysing processes of mass and energy exchange at the glacier-atmosphere interface and for interpreting climate/glacier relationships.

Glacier monitoring using satellites, based on 20 years of observations of glaciers by LANDSAT, SPOT, ERS and, in the future, EOS and Radarsat, has been developed to build a database covering most glaciers of the world and to monitor glacier changes on a periodic basis. Satellite monitoring of the world's glaciers should produce a uniform image data-set, monitor special events such as glacier surges, produce maps of the areal extent of glaciers and snow fields, give information about glacier surface velocities, advance and retreat, and an inventory of the glaciers, including mean surface speed, length, width, areal extent, snowline, and the temporal changes in these parameters. Adam et al. (1997) evaluated the effectiveness of ERS-1 synthetic aperture radar (SAR) imagery for mapping movement of the transient snowline in a temperate glacier basin during the ablation season. Despite localized confusion between glacier ice and wet snow, the wet snowline can be mapped reasonably well by using ERS-1 SAR imagery.

Studies show that most Arctic glaciers have experienced negative net surface mass balance over the last few decades (e.g. Dowdeswell et al., 1997; Pohjola and Rogers, 1997a,b). There is, however, no uniform recent trend in mass balance in the Arctic, although some regional trends are recognizable. In northern Alaska, for example, glaciers experience increased negative mass balance as a result of higher summer temperatures. This development may

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