4.5.3 Special Regional Features
Although reports on individual glaciers or limited glacier areas support the global picture of ongoing strong ice shrinkage in almost all regions, some exceptional results indicate the complexity of both regional- to local-scale climate and respective glacier regimes.
For glaciers in the dry and cold Taylor Valley, Antarctica, Fountain et al. (2004) hypothesised that an increase in average air temperature of 2°C alone can explain the observed glacier advance through ice softening.
Altimetric measurements in Svalbard suggested a small ice cap growth (Bamber et al., 2004), however, an alternative evaluation of mass balance processes indicates a slight sea level contribution of 0.01 mm yr–1 for the last three decades of the 20th century (Hagen et al., 2003). Svalbard glaciers were recently close to balance, which is exceptional for the Arctic.
In Scandinavia, Norwegian coastal glaciers, which advanced in the 1990s due to increased accumulation in response to a positive phase of the NAO (Nesje et al., 2000), started to shrink around 2000 as a result of a combination of reduced winter accumulation and greater summer melting (Kjøllmoen, 2005). Norwegian glacier tongues farther inland have retreated continuously at a moderate rate. Warming is also indicated by a change in temperature distribution in northern Sweden’s Storglaciären where, between 1989 and 2001, 8.3 m of the cold surface layer (or 22% of the long-term average thickness of this cold layer) warmed to the melting point. This is attributed primarily to increased winter temperatures yielding a longer melt season; summer ablation was normal (Pettersson et al., 2003). As with coastal Scandinavia, glaciers in the New Zealand Alps advanced during the 1990s, but have started to shrink since 2000. Increased precipitation may have caused the glacier growth (Chinn et al., 2005).
In the European Alps, exceptional mass loss during 2003 removed an average of 2,500 kg m–2 yr–1 over nine measured Alpine glaciers, almost 60% higher than the previous record of 1,600 kg m–2 yr–1 loss in 1996 and four times more than the mean loss from 1980 to 2001 (600 kg m–2 yr–1; Zemp et al., 2005). This was caused by extraordinarily high air temperatures over a long period, extremely low precipitation, and albedo feedback from Sahara dust depositions and a previous series of negative mass balance years (see Box 3.6.).
Whereas glaciers in the Asian high mountains have generally shrunk at varying rates (Su and Shi, 2002; Ren et al., 2004; Solomina et al., 2004; Dyurgerov and Meier, 2005), several high glaciers in the central Karakoram are reported to have advanced and/or thickened at their tongues (Hewitt, 2005), probably due to enhanced precipitation.
Figure 4.16. Changes in the surface area of tropical glaciers relative to their extent around 1900, grouped according to different glacier sizes. The sizes are given for 1990 or the closest available date to 1990. The broken red line highlights the retreat of Kilimanjaro glaciers. The insert shows the area change (km2) of the Kilimanjaro plateau (red) and slope (purple) glaciers as separated by the 5,700 m contour line (Kaser and Osmaston, 2002 (updated courtesy of S. Lieb); Mölg et al., 2003b; Georges, 2004; Hastenrath, 2005; Cullen et al., 2006; Klein and Kincaid, 2006).
Tropical glaciers have shrunk from a maximum in the mid-19th century, following the global trend (Figure 4.16). Strong shrinkage rates in the 1940s were followed by relatively stable extents that lasted into the 1970s. Since then, shrinkage has become stronger again; as in other mountain ranges, the smallest glaciers are more strongly affected. Since the publication of IPCC (2001), evidence has increased that changes in the mass balance of tropical glaciers are mainly driven by coupled changes in energy and mass fluxes related to interannual variations of regional-scale hygric seasonality (Wagnon et al., 2001; Francou et al., 2003, 2004). Variations in atmospheric moisture content affect incoming solar radiation, precipitation and albedo, atmospheric longwave emission, and sublimation (Wagnon et al., 2001; Kaser and Osmaston, 2002; Mölg et al., 2003a; Favier et al., 2004; Mölg and Hardy, 2004; Sicart et al., 2005). At a large scale, the mass balance of tropical glaciers strongly correlates with tropical sea surface temperature anomalies and related atmospheric circulation modes (Francou et al., 2003, 2004; Favier et al., 2004). Glaciers on Kilimanjaro behaved exceptionally throughout the 20th century (Figure 4.16). The geometry of the volcano and the dry climate above the freezing level maintain vertical ice walls around the tabular ice on the summit plateau and these retreat at about 0.9 m yr–1 (Thompson et al., 2002) forced by solar radiation (Mölg et al., 2003b). Their retreat is responsible for the steady shrinkage of the ice area on the summit plateau (Figure 4.16, insert) (Cullen et al., 2006). In contrast, the slope glaciers, which extend from the plateau rim onto the steep slopes of the volcano, decreased strongly at the beginning of the 20th century, but more slowly recently. This shrinkage is interpreted as an ongoing response to a dramatic change from a wetter to a drier regime, supposedly around 1880, and a subsequent negative trend in mid-troposphere atmospheric moisture content over East Africa (Cullen et al., 2006).