Long-term glacier observations which were coordinated internationally began in 1894 with the establishment of the International Glacier Commission in Zurich, Switzerland.
The goal of this worldwide monitoring programme was to provide information on mechanisms of modern climate and glacier variations. At present, glacier variations are recognized as summer temperature and winter precipitation indicators used in the early detection of possible human-induced climate change (IPCC, 1992, 1995).
Glacier fluctuations contribute information about natural climate variability and rates of change with respect to short- and long-term energy fluxes at the glacier surface. Historical and Holocene glacier fluctuations, reconstructed from direct measurements, paintings, written sources and moraines, indicate that the glacier extent in many mountain regions has fluctuated considerably over the past centuries and millennia. The range of variability is defined by the early Holocene climate optimum and today's reduced stages, and the maximum Little Ice Age glacier extent.
Glacier margins advance or retreat, with variable time lags, in response to variations in glacier mass balance. Ablation removes ice from the glacier and the horizontal velocity component carries ice forward. Boulton (1986) demonstrated that a glacier margin will remain in the same position when the horizontal velocity component is equal to the horizontal component of ablation. Although the frontal position is stationary, the ice is in motion, but is removed from the glacier at a rate equal to the velocity. Frontal retreat takes place when the horizontal velocity component is less than the horizontal ablation component, whereas glacier advance occurs when the horizontal velocity component is larger than the horizontal ablation component. During the winter season, glacier sliding velocities tend to be low due to a lack of lubricating meltwater at the glacier base. Because ablation rates are generally negligible during the winter compared with the summer, however, the relative magnitudes of the horizontal velocity component and the ablation rate over the year tend to cause winter advance and summer retreat. Commonly, winter advances start late in the ablation season when melting at the margin does not exceed the forward flow of glacier ice. Normally, the horizontal ablation component is low in late winter, causing the small winter flow velocities to produce small glacier advances. Despite higher summer flow velocities than in winter, high summer ablation rates cause net retreat of the glacier (e.g. Benn and Evans, 1998).
On longer time-scales, glacier fronts are subject to advance and retreat as a result of climate change or internal instabilities. Climatic influences on the frontal response can be divided into factors causing changes in ablation and accumulation. Debris-covered glacier fronts are, however, rather insensitive to changes in mass balance. Varying amounts of ice flowing through a glacier causes changes in ice thickness and gradients and thereby influences the driving stresses. Advance and retreat of the glacier front normally lags behind the climate forcing because the signal must be transferred from the accumulation area to the snout. This is referred to as the time lag or the response time, which is longest for long, low-gradient and slow-moving glaciers, and shortest on short, steep and fast-flowing glaciers (e.g. Johannesson et al., 1989; Paterson, 1994). Kinematic wave theory has been applied to calculating response times (Nye, 1960; Paterson, 1994). However, physically-based flow models may help to determine the response times more precisely (van de Wal and Oerlemans, 1995).
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