The methods introduced above are applicable to larger-scale ice masses and continental to global-scale climate change scenarios, but few ice masses are as well-studied and thoroughly understood as Storglaciaren or the Peyto Glacier. Mass balance records have been made on a total of roughly 280 glaciers over the past 60yr (Dyurgerov, 2002), out of an estimated 160,000 glaciers worldwide. Long-term records are available from less than 25% of these sites, with data from only 44 glaciers available for the 1964-1995 global mass-balance assessment of Dyurgerov (2002). In addition, and of greater importance for decade- and century-scale sea-level forecasts, large-scale icefields and ice sheets have too great an area to fully assess with mass-balance field campaigns. Satellite techniques (e.g. Zwally & Giovinetto, 1995; Abdalati & Steffen, 2001) and laser altimetry (e.g. Arendt et al., 2002) are creating new possibilities to quantify interannual mass balance variability, and show promise for glacier-change monitoring.
For model-based estimates of global-scale glacier and icefield response to climate change, two principal methods are possible. One approach that has been insightful involves sensitivity tests of different glacial systems to a hypothetical step change in climate. A 'reference' climatology representing present-day conditions can be taken from observational reconstructions (e.g. Legates & Willmott, 1990a,b), reanalysis fields (Kalney et al., 1996) or AGCM simulations. It is common to take the reference state as a 30-yr average of observed, reconstructed or simulated climate, such as the 1961-1990 climate normals. Global-scale glacier mass-balance fields (accumulation and ablation rates) are then estimated for this reference climatology, as illustrated in Plate 32.1 for the western Arctic.
Using the same schemes for mass balance calculation, climate sensitivity can then be tested through a step change in the reference climatology, such as a 1°C warming or a 10% increase in precipitation (e.g. Braithwaite & Zhang, 2000). This kind of experiment illuminates the sensitivity of different glacial systems to temperature versus precipitation shifts, and it provides quantitative insight into the mass balance (hence, ice volume) response to climate change in different geographical regions (Gregory & Oerlemans, 1997). The seasonality of potential climate change is also important and is variable in different glacial environments (Oerlemans & Reichert, 2000). Numerous ice-sheet-scale investigations of mass-balance sensitivity have also been carried out using AGCM-derived climate change scenarios. For instance, Ohmura et al. (1996) and Thompson & Pollard (1997) assessed the impacts of doubled cO2 on mass balance in Greenland and Antarctica, without regard to ice dynamical feed-backs.
At present, global-scale studies of the mass-balance impacts of a given climate change scenario can only be explicitly coupled with ice dynamics models for large-scale ice caps and ice sheets, such as those in Iceland, the Arctic islands, Greenland and Antarctica. As discussed in section 32.5, the complex relief in mountain regions is too poorly resolved in global gridded observational, reanalysed or modelled climatology to provide meaningful massbalance information for individual glaciers or alpine icefields. General insights can be attained, however, through large-scale hypsometric characterization of glacierized areas (e.g. Marshall & clarke, 1999). Knowing the distribution of areas in different elevation bands, generalized regional-scale mass balance impacts can be assessed for seasonal or annual climate perturbations (Marshall, 2002).
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