|Working Group I: The Scientific Basis|
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Glaciers and ice caps
For constant glacier area, from the AOGCM IS92a experiments including sulphate aerosol, predicted sea level rise from glacier melt over the hundred years 1990 to 2090 lies in the range 0.06 to 0.15 m. The variation is due to three factors. First, the global average temperature change varies between models. A larger temperature rise tends to give more melting, but they are not linearly related, since the total melt depends on the time-integrated temperature change. Second, the global mass balance sensitivity to temperature change varies among AOGCMs because of their different seasonal and regional distribution of temperature change. Third, the glaciers are already adjusting to climate change during the 20th century, and any such imbalance will persist during the 21st century, in addition to the further imbalance due to future climate change. The global average temperature change and glacier mass balance sensitivity may not be independent factors, since both are affected by regional climate feedbacks. The sensitivity and the present imbalance are related factors, because a larger sensitivity implies a greater present imbalance.
With glacier area contracting as the volume reduces, the estimated sea level rise contribution is in the range 0.05 to 0.11 m, about 25% less than if constant area is assumed, similar to the findings of Oerlemans et al. (1998) and Van de Wal and Wild (2001). The time-dependence of glacier area means the results can no longer be represented by a global glacier mass balance sensitivity.
Glaciers and ice caps on the margins of the Greenland and Antarctic ice sheets are omitted from these calculations, because they are included in the ice sheet projections below. These ice masses have a large area (Table 11.3), but experience little ablation on account of being in very cold climates. Van de Wal and Wild (2001) find that the Greenland marginal glaciers contribute an additional 6% to glacier melt in a scenario of CO2 doubling over 70 years. Similar calculations using the AOGCM IS92a results give a maximum contribution of 14 mm for 1990 to 2100. For the Antarctic marginal glaciers, the ambient temperatures are too low for there to be any significant surface runoff. Increasing temperatures will increase the runoff and enlarge the area experiencing ablation, but their contribution is very likely to remain small. For instance, Drewry and Morris (1992) calculate a contribution of 0.012 mm/yr/°C to the global glacier mass balance sensitivity from the glacier area of 20,000 km2 which currently experiences some melting on the Antarctic Peninsula.
Lack of information concerning glacier areas and precipitation over glaciers, together with uncertainty over the projected changes in glacier area, lead to uncertainty in the results. This is assessed as ± 40%, matching the uncertainty of the observed mass balance estimate of Dyurgerov and Meier (1997b).
Greenland and Antarctic ice sheets
For 1990 to 2090 in the AOGCM GS experiments, Greenland contributes 0.01 to
0.03 m and Antarctica –0.07 to –0.01 m to global average sea level (Table
11.13). Note that these sea level contributions result solely from recent
and projected future climate change; they do not include the response to past
climate change (discussed in Sections 184.108.40.206 and 11.3.1).
The use of a range of AOGCMs represents the uncertainty in modelling changing circulation patterns, which lead to both changes in temperature and precipitation, as noted by Kapsner et al. (1995) and Cuffey and Clow (1997) from the results from Greenland ice cores. The range of AOGCM thermodynamic and circulation responses gives a range of 4 to 8%/°C for Greenland precipitation increases, generally less than indicated by ice-cores for the glacial-interglacial transition, but more than for Holocene variability (Section 220.127.116.11). If precipitation did not increase at all with greenhouse warming, Greenland local sensitivities would be larger, by 0.05 to 0.1 mm/yr/°C (see also Table 11.7). Given that all AOGCMs agree on an increase, but differ on the strength of the relationship, we include an uncertainty of ±0.02 mm/yr/°C in Table 11.13 on the Greenland local sensitivities, being the product of the standard deviation of precipitation increase (1.5%/°C) and the current Greenland accumulation (1.4 mm/yr sea level equivalent, Table 11.5).
Estimates of Greenland runoff (Table 11.5) have
a standard error of about ± 10%. This reflects uncertainty in the degree-day
method (Braithwaite, 1995) and refreezing parametrization (Janssens and Huybrechts,
2000) used to calculate Greenland ablation. Given that a typical size of the
sensitivity of ablation to temperature change is 0.3 mm/yr/°C (Table
11.7), we adopt an additional uncertainty of ± 0.03 mm/yr/°C
for the local Greenland sensitivities in Table 11.13.
We include a separate uncertainty of the same size to reflect the possible sensitivity
to use of different high-resolution geographical patterns of temperature and
precipitation change (the T106 ECHAM4 pattern was the only one available). As
an estimate of the uncertainty related to changes in iceberg discharge and area-elevation
distribution, we ascribe an uncertainty of ± 10% to the net mass change,
on the basis of the magnitude of the dynamic response for Greenland described
in Section 18.104.22.168.
Warrick et al. included a positive term to allow for the possible instability of the WAIS. We have omitted this because it is now widely agreed that major loss of grounded ice and accelerated sea level rise are very unlikely during the 21st century (Section 22.214.171.124). The size of our range is an indication of the systematic uncertainty in modelling radiative forcing, climate and sea level changes. Uncertainties in modelling the carbon cycle and atmospheric chemistry are not covered by this range, because the AOGCMs are all given similar atmospheric concentrations as input.
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