International monitoring of glacier variations started in 1894. At present, the World Glacier Monitoring Service (WGMS) collects standardized glacier information. The data basis includes observations on specific balance, cumulative specific balance, accumulation area ratio (AAR), the equilibrium line altitude (ELA), and changes in length. Most of the data and the longest records come from the Alps and Scandinavia.
About 90 years of observations reveal a general shrinkage of mountain glaciers on a global scale after the Little Ice Age (ca. ad 1650-1930). This was most pronounced during the first half of the twentieth century. More recently, however, glaciers have begun to grow in several regions, for example, in maritime western Scandinavia and in New Zealand. Important empirical information has started to become available on the complex relationship between climate and glaciers (e.g. Haeberli et ah, 1989; Oerlemans, 1989).
Records of glacier fluctuations compiled by the WGMS have been used by Oerlemans (1994) to derive an independent estimate of global warming during the last 100 years. The retreat of glaciers during the last 100 years appears, with a few exceptions, to be coherent over the globe. Modelling of the climate sensitivity of glaciers reveals that the observed glacier retreat can be explained by a linear warming trend of 0.66 Kelvin per century (Oerlemans, 1994).
During the last 25 years, several remote sensing techniques have been applied to the study of glaciers. These techniques include measurements of ice thickness by radio-echo sounding from surface and airborne platforms, changes in surface elevation over time with aerial photogrammetric methods and by geodetic airborne and spacebome radar and laser altimetry, declination of the surface expression of glacier facies with satellite sensors, and measurements of the fluctuations in the fronts of valley glaciers and outlet glacier margins at ice fields, ice caps and ice sheets.
Airborne scanning laser altimetry is a relatively new technique for remote sensing of ground elevation (e.g. Kennett and Eiken, 1997). A laser ranger scans across a swath beneath the aircraft, giving a 2D distribution of altitude when combined with data on position and orientation of the aircraft. Smooth snow-covered glaciers are the best for laser scanning altimetry since they are highly reflective. Results from Hardangerjokulen, central southern Norway (Kennett and Eiken, 1997) show that noise levels are very low (ca. 2 cm). Overlapping swaths show repeatability of ±10 cm. The high accuracy and coverage (about 20,000 points per km2) enables reliable measurement of glacier volume changes. Scanning laser altimetry has many advantages compared with photogrammetry.
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