A method of dealing with the lack of mass balance measurements is to estimate the changes in mass balance as a function of climate, using mass balance sensitivities (see Box 11.2 for definition) and observed or modelled climate change for glacier covered regions. Mass-balance modelling of all glaciers individually is not practical because no detailed description exists for the great majority of them, and because local climate data are not available; even regional climate models do not have sufficient resolution, while downscaling methods cannot generally be used because local climate measurements have not been made (see Section 10.7). A number of authors have estimated past glacier net mass loss using past temperature change with present day glacier covered areas and mass balance sensitivities (Table 11.4). In this report, we project future mass balance changes using regional mass balance sensitivities which take account of regional and seasonal climatic information, instead of using the heuristic model of Wigley and Raper (1995) employed by Warrick et al. (1996).
Meier (1984) intuitively scaled specific mass balance according to mass balance amplitude (half the difference between winter and summer specific mass balance). Braithwaite and Zhang (1999) demonstrated a dependence of mass balance sensitivity on mass balance amplitude. Oerlemans and Fortuin (1992) derived an empirical relationship between the mass balance sensitivity of a glacier to temperature change and the local average precipitation, which is the principal factor determining its mass turnover rate. Zuo and Oerlemans (1997) extended this idea by distinguishing the effects of temperature changes in summer and outside summer; the former have a stronger influence on mass loss, in general. They made a calculation of glacier net mass loss since 1865. For 1961 to 1990, they obtained a rate of 0.3 mm/yr of sea level rise (i.e., a total of 8 mm, Oerlemans, 1999), very similar to the result of Dyurgerov and Meier (1997b). Gregory and Oerlemans (1998) applied local seasonal temperature changes over 1860 to 1990 calculated by the HadCM2 AOGCM forced by changing greenhouse gases and aerosols (HadCM2 GS in Table 9.1) to the glacier model of Zuo and Oerlemans.
Zuo and Oerlemans (1997), Gregory and Oerlemans (1998) and Van de Wal and Wild (2001) all stress that the global average glacier mass balance depends markedly on the regional and seasonal distribution of temperature change. For instance, Gregory and Oerlemans (1998) find that projected future glacier net mass loss is 20% greater if local seasonal variation is neglected, and 20% less if regional variation is not included. The first difference arises because annual average temperature change is greater than summer temperature change at high latitudes, but the mass balance sensitivity is greater to summer change. The second is because the global average temperature change is less than the change at high latitudes, where most glaciers are found (Section 9.3.2).
Both the observations of mass balance and the estimates based on temperature changes (Table 11.4) indicate a reduction of mass of glaciers and ice caps in the recent past, giving a contribution to global-average sea level of 0.2 to 0.4 mm/yr over the last hundred years.
Precipitation and accumulation changes also influence glacier mass balance, and may sometimes be dominant (e.g. Raper et al., 1996). Generally, glaciers in maritime climates are more sensitive to winter accumulation than to summer conditions (Kuhn, 1984). AOGCM experiments suggest that global-average annual mean precipitation will increase on average by 1 to 3%/°C under the enhanced greenhouse effect (Figure 9.18). Glacier mass balance modelling indicates that to compensate for the increased ablation from a temperature rise of 1°C a precipitation increase of 20% (Oerlemans, 1981) or 35% (Raper et al., 2000) would be required. Van de Wal and Wild (2001) find that the effect of precipitation changes on calculated global-average glacier mass changes in the 21st century is only 5% of the temperature effect. Such results suggest that the evolution of the global glacier mass is controlled principally by temperature changes rather than precipitation changes. Precipitation changes might be significant in particular localities, especially where precipitation is affected by atmospheric circulation changes, as seems recently to have been the case with southern Scandinavian glaciers (Oerlemans, 1999).
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