Working Group I: The Scientific Basis

Other reports in this collection Evolution of area

The above calculations all neglect the change of area that will accompany loss of volume. Hence they are inaccurate because reduction of area will restrict the rate of melting. A detailed computation of transient response with dynamic adjustment to decreasing glacier sizes is not feasible at present, since the required information is not available for most glaciers. Oerlemans et al. (1998) undertook such detailed modelling of twelve individual glaciers and ice caps with an assumed rate of temperature change for the next hundred years. They found that neglecting the contraction of glacier area could lead to an overestimate of net mass loss of about 25% by 2100.

Table 11.5: Current state of balance of the Greenland ice sheet (1012 kg/yr).
Source and remarks A B C D E F
  Accumulation Runoff Net accumulation Iceberg production Bottom melting Balance
Benson (1962) 500 272 228 215   +13
Bauer (1968) 500 330 170 280   110
Weidick (1984) 500 295 205 205   ± 0
Ohmura and Reeh (1991): New accumulation map 535          
Huybrechts et al. (1991): Degree-day model on 20 km grid 539 256 283      
Robasky and Bromwich (1994): Atmospheric moisture budget analysis from radiosonde data, 1963-1989 545          
Giovinetto and Zwally (1995a): Passive microwave data of dry snow 461a          
Van de Wal (1996): Energy-Balance model on 20 km grid 539 316 223      
Jung-Rothenhäusler (1998): Updated accumulation map 510          
Reeh et al. (1999) 547 276 271 239 32 ± 0
Ohmura et al. (1999): Updated accumulation map with GCM data; runoff from ablation-summer temperature parametrization 516 347 169      
Janssens and Huybrechts (2000): recalibrated degree-day model on 5 km grid; updated precipitation and surface elevation maps 542 281 261      
Zwally and Giovinetto (2000): Updated calculation on 50 km grid     216b      
Mean and standard devation 520 ± 26 297 ± 32 225 ± 41 235 ± 33 32 ± 3c -44 ± 53d
a Normalised to ice sheet area of 1.67610 6 km 2 (Ohmura and Reeh, 1991).
b Difference between net accumulation above the equilibrium line and net ablation below the equilibrium line.
c Melting below the fringing ice shelves in north and northeast Greenland (Rignot et al., 1997).
d Including the ice shelves, but nearly identical to the grounded ice sheet balance because the absolute magnitudes of the other ice-shelf balance terms
(accumulation, runoff, ice-dynamic imbalance) are very small compared to those of the ice sheet (F=ABDE).

Dynamic adjustment of glaciers to a new climate occurs over tens to hundreds of years (Jóhannesson et al., 1989), the time-scale being proportional to the mean glacier thickness divided by the specific mass balance at the terminus. Since both quantities are related to the size of the glacier, the time-scale is not necessarily longer for larger glaciers (Raper et al., 1996; Bahr et al., 1998), but it tends to be longer for glaciers in continental climates with low mass turnover (Jóhannesson et al., 1989; Raper et al., 2000).

Table 11.6: Current state of balance of the Antarctic ice sheet (10 12 kg/yr).
Source and remarks A B C D E F
  Accumulation over grounded ice Accumulation over all ice sheet Ice shelf melting Runoff Iceberg production Flux across grounding line
Kotlyakov et al. (1978)   2000 320 60 2400  
Budd and Smith (1985) 1800 2000     1800 1620
Jacobs et al. (1992). Ice shelf melting from observations of melt water outflow, glaciological field studies and ocean modelling. 1528 2144 544 53 2016  
Giovinetto and Zwally (1995a). Passive microwave data of dry snow. 1752a 2279a        
Budd et al. (1995). Atmospheric moisture budget analysis from GASP data, 1989 to 1992.   2190b        
Jacobs et al. (1996). Updated ice-shelf melting assessment.     756      
Bromwich et al. (1998). Atmospheric moisture budget analysis from ECMWF reanalysis and evaporation/ sublimation forecasts, 1985 to 1993.   2190b        
Turner et al. (1999). Atmospheric moisture budget analysis from ECMWF reanalysis, 1979 to 1993.   2106        
Vaughan et al. (1999). 1800 in situ measurements interpolated using passive microwave control field. 1811 2288        
Huybrechts et al. (2000). Updated accumulation map. 1924 2344        
Giovinetto and Zwally (2000). Updated map on 50 km grid. 1883c 2326c        
Mean and standard deviation. 1843 ± 76d 2246 ± 86d 540 ± 218 10 ± 10e 2072 ± 304  
a Normalised to include the Antarctic Peninsula.
b Specific net accumulation multiplied by total area of 13.9510 6 km 2 (Fox and Cooper, 1994).
c Normalised to include the Antarctic Peninsula, and without applying a combined deflation and ablation adjustment.
d Mean and standard deviation based only on accumulation studies published since 1995.
e Estimate by the authors.
The mass balance of the ice sheet including ice shelves can be estimated as BCDE=376 ±38410 12 kg/yr, which is 16.7 ±17.1% of the total input B.
Assuming the ice shelves are in balance (and noting that the runoff derives from the grounded ice, not the ice shelves) would imply that 0=F+(B A) C E, in which case
the flux across the grounding line would be F=A B+C+E =2209 ±39110 12 kg/yr.

Meier and Bahr (1996) and Bahr et al. (1997), following previous workers, proposed that for a glacier or an ice sheet in a steady state there may exist scaling relationships of the form V µ Ac between the volume V and area A, where c is a constant. Such relationships seem well supported by the increasing sample of glacier volumes measured by radio-echo-sounding and other techniques, despite the fact that climate change may be occurring on time-scales similar to those of dynamic adjustment. If one assumes that the volume-area relationship always holds, one can use it to deduce the area as the volume decreases. This idea can be extended to a glacier covered region if one knows the distribution of total glacier area among individual glaciers, which can be estimated using empirical functions (Meier and Bahr, 1996; Bahr, 1997). Using these methods, Van de Wal and Wild (2001) found that contraction of area reduces the estimated glacier net mass loss over the next 70 years by 15 to 20% (see also Section

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