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Glaciers are becoming 'warmer', responding faster to ongoing change in climate, contributing more to the global water cycle and receding at an accelerating rate. Glacier sensitivity is the key parameter which links changes in climate, glacier volume and the eustatic component of sea-level rise. To show the relation between these changes in the Northern Hemisphere, where such data are more complete and accurate, I used observational data of air temperature and glacier mass balance over the years 1961-1998. The last decade of the twentieth century was the warmest over the previous hundreds of years (Mann & Jones, 2003). The global water cycle has accelerated in terms of increases in precipitation rate and rise in sea level (RSL) (Church et al., 2001b). The eustatic component of RSL has increased and the source of this must be continental (Miller & Douglas, 2003). We have attributed this continental source to the ongoing process of volume losses by mountain and subpolar glaciers (Meier et al., 2003). We need to gain better knowledge on what has caused a large increase in glacier wastage on a hemispheric scale.

The annual-balance sensitivity to temperature (db/dT: where b is the annual mass balance and T is the air temperature in °C) is used for most projections of glacier wastage. Mass balance sensitivity calculated here uses all available time series for Northern Hemisphere glaciers in respect to observed Northern Hemisphere annual air temperature. Glacier sensitivity to climate depends on mass turnover (Oerlemans & Reichert, 2000). In regions with dry climate conditions and small mass turnover, summer temperature is the major driver of glacier mass balance change. In wetter climates with larger mass turnover, glaciers' mass balance is very sensitive to change in the amount of precipitation. This suggests a different glacier response by regions with climate warming, such as:

1 in the high Arctic, and the Canadian Archipelago, ice caps are disappearing fast because they are very sensitive to change in equilibrium line altitude (ELA), due to a glacier's surface topography—i.e. a small increase in ELA results in an enormous decrease in accumulation area;

2 everywhere in continental mountains a small warming results in a decrease in the snow/rain ratio, thus there is a decrease in surface albedo, and the increased ablation rate causes more negative mass balance—glaciers with a summer peak of precipitation are more sensitive to air temperature change (Naito et al., 2001);

3 large tidewater glaciers in Alaska and the Arctic, and individual ice caps in Greenland and other parts of the Arctic, may have been especially sensitive to an increase in sea-level and the direct impact of oceanic water—i.e. as a result of wave erosion, propagation of the grounding line towards the land and acceleration of bottom melting (Pfeffer et al., 2000; Zwally et al., 2002a; Thomas et al., 2003; Steffen et al., 2004).

Different responses resulted in huge variability in glacier mass balance sensitivity over 1961-1987 (Fig. 25.1). This has changed sharply. Since 1988 db/dT decreased (more negative mass balance) enormously from 0.017m°C-1yr-1 in 1961-1987 to -0.734m °C-1yr-1 in 1988-1998 (the warmest period); less change in summer temperature is required to cause the same glacier wastage. Variability in sensitivity reduced enormously (Fig. 25.1).

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Figure 25.1 Northern hemisphere glacier mass balance sensitivity to annual air temperature. Long-term annual mass-balance time series averaged for about the same 40 bench mark glaciers have been used to calculate averages (Dyurgerov, 2001). Note, this is a different measure of sensitivity than that used by the IPCC (Church et al., 2001b), which involves a change between two steady states.

Such changes in db/dT have never previously been observed. These hemispheric-scale changes remain unexplained. They may be due to the complex combination of large-scale atmospheric circulation patterns, which requires further analyses. Before this is accomplished, the simple hypothesis is that these changes are due to a hemispheric increase in air temperature.

Realistic knowledge of glacier regimes and their involvement in global processes depends on data from direct observations. This is more true now than it was 10-20 yr ago. Direct observations cannot be substituted by modelling results. Observational data have revealed previously unknown processes and changes, such as changes in sensitivity and its variability. Knowledge of these two is crucial in order to predict glacier response to climate change and glacier contribution to sea-level rise.

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