Box 11.2: Mass balance terms for glaciers, ice caps and ice sheets
A glacier, ice cap or ice sheet gains mass by accumulation of snow (snowfall and deposition by wind-drift), which is gradually transformed to ice, and loses mass (ablation) mainly by melting at the surface or base with subsequent runoff or evaporation of the melt water. Some melt water may refreeze within the snow instead of being lost, and some snow may sublimate or be blown off the surface. Ice may also be removed by discharge into a floating ice shelf or glacier tongue, from which it is lost by basal melting and calving of icebergs. Net accumulation occurs at higher altitude, net ablation at lower altitude; to compensate for net accumulation and ablation, ice flows downhill by internal deformation of the ice and sliding and bed deformation at the base. The rate at which this occurs is mainly controlled by the surface slope, the ice thickness, the effective ice viscosity, and basal thermal and physical conditions. The mass balance for an individual body of ice is usually expressed as the rate of change of the equivalent volume of liquid water, in m3/yr; the mass balance is zero for a steady state. Mass balances are computed for both the whole year and individual seasons; the winter mass balance mostly measures accumulation, the summer, surface melting. The specific mass balance is the mass balance averaged over the surface area, in m/yr. A mass balance sensitivity is the derivative of the specific mass balance with respect to a climate parameter which affects it. For instance, a mass balance sensitivity to temperature is in m/yr/°C.
The water contained in glaciers and ice caps (excluding the ice sheets of Antarctica and Greenland) is equivalent to about 0.5 m of global sea level (Table 11.3). Glaciers and ice caps are rather sensitive to climate change; rapid changes in their mass are possible, and are capable of producing an important contribution to the rate of sea level rise. To evaluate this contribution, we need to know the rate of change of total glacier mass. Unfortunately sufficient measurements exist to determine the mass balance (see Box 11.2 for definition) for only a small minority of the world's 105 glaciers.
Figure 11.2: Cumulative mass balance for 1952-1998 for three glaciers in different climatic regimes: Hintereisferner (Austrian Alps), Nigardsbreen (Norway), Tuyuksu (Tien Shan, Kazakhstan).
A possible approximate approach to this problem is to group glaciers into climatic regions, assuming glaciers in the same region to have a similar specific mass balance. With this method, we need to know only the specific mass balance for a typical glacier in each region (Kuhn et al., 1999) and the total glacier area of the region. Multiplying these together gives the rate of change of glacier mass in the region. We then sum over all regions.
In the past decade, estimates of the regional totals of area and volume have been improved by the application of high resolution remote sensing and, to a lesser extent, by radio-echo-sounding. New glacier inventories have been published for central Asia and the former Soviet Union (Dolgushin and Osipova, 1989; Liu et al., 1992; Kuzmichenok, 1993; Shi et al., 1994; Liu and Xie, 2000; Qin et al., 2000), New Zealand (Chinn, 1991), India (Kaul, 1999) South America (Casassa, 1995; Hastenrath and Ames, 1995; Skvarca et al., Aniya et al, 1997; Kaser, 1999; 1995; Kaser et al., 1996; Rott et al., 1998), and new estimates made for glaciers in Antarctica and Greenland apart from the ice sheets (Weidick and Morris, 1996).
By contrast, specific mass balance is poorly known. Continuous mass balance records longer than 20 years exist for about forty glaciers worldwide, and about 100 have records of more than five years (Dyurgerov and Meier, 1997a). Very few have both winter and summer balances; these data are critical to relating glacier change to climatic elements (Dyurgerov and Meier, 1999). Although mass balance is being monitored on several dozen glaciers worldwide, these are mostly small (<20 km2) and not representative of the size class that contains the majority of the mass (>100 km2). The geographical coverage is also seriously deficient; in particular, we are lacking information on the most important maritime glacier areas. Specific mass balance exhibits wide variation geographically and over time (Figure 11.2). While glaciers in most parts of the world have had negative mass balance in the past 20 years, glaciers in New Zealand (Chinn, 1999; Lamont et al., 1999) and southern Scandinavia (Tvede and Laumann, 1997) have been advancing, presumably following changes in the regional climate.
|Table 11.4: Estimates of historical contribution of glaciers to global average sea level rise.|
|Reference||Period||Rate of sea-level rise (mm/yr)||Remarks|
|Meier (1984)||1900 to 1961|
|Trupin et al. (1992)||1965 to 1984|
|Meier (1993)||1900 to 1961|
|Zuo and Oerlemans (1997), Oerlemans (1999)||1865 to 1990||0.22 0.07a||Observeed temperature changes with mass balance|
|1961 to 1990||0.3a|
|Dyurgeov and Meier (1997b)||1961 to 1990||0.25 0.10||Area-weighted mean of observed mass balance for seven regions|
|Dowdeswell et al. (1997)||1945 to 1995 approx||0.13||Observed mass balance, Arctic only|
|Gregory and Oerlemans (1998)||1860 to 1990||0.15a||General Circulation Model (GCM) temperature changes with mass balance sensitivities from Zuo and Oerlemans (1997)|
|1960 to 1990||0.26a|
|a These papers give the change in sea level over the period indicated, from which we have calculated the rate of sea level rise.|
Estimates of the historical global glacier contribution to sea level rise are shown in Table 11.4. Dyurgerov and Meier (1997a) obtained their estimate by dividing a large sample of measured glaciers into seven major regions and finding the mass balance for each region, including the glaciers around the ice sheets. Their area-weighted average for 1961 to 1990 was equivalent to 0.25 ± 0.10 mm/yr of sea level rise. Cogley and Adams (1998) estimated a lower rate for 1961 to 1990. However, their results may be not be representative of the global average because they do not make a correction for the regional biases in the sample of well investigated glaciers (Oerlemans, 1999). When evaluating data based on observed mass balance, one should note a worldwide glacier retreat following the high stand of the middle 19th century and subsequent small regional readvances around 1920 and 1980.
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