Box 11 Retreat of Chacaltaya and its effects case study of a small disappearing glacier in Bolivia

The observed general glacier retreat in the warming tropical Andes has increased significantly in recent decades (Francou et al., 2005). Small-sized glaciers are particularly vulnerable in warmer climates, with many of them having already disappeared in several parts of the world during the last century. The Chacaltaya Glacier in Bolivia (16°S) is a typical example of a disappearing small glacier, whose area in 1940 was 0.22 km2, and which has currently reduced (in 2005) to less than 0.01 km2 (Figure 1.1) (Ramirez et al., 2001; Francou et al., 2003; Berger et al., 2005), with current estimates showing that it may disappear completely before 2010. In the period 1992 to 2005, the glacier suffered a loss of 90% of its surface area, and 97% of its volume of ice (Berger et al., 2005). Although, in the tropics, glacier mass balance responds sensitively to changes in precipitation and humidity (see Lemke et al., 2007, Section 4.5.3), the fast glacier shrinkage of Chacaltaya is consistent with an ascent of the 0°C isotherm of about 50 m/decade in the tropical Andes since the 1980s (Vuille et al., 2003), resulting in a corresponding rise in the equilibrium line of glaciers in the region (Coudrain et al., 2005).

Ice melt from Chacaltaya Glacier, located in Choqueyapu Basin, provides part of the water resources for the nearby city of La Paz, allowing the release of water stored as ice throughout the long, dry winter season (April-September). Many basins in the tropical Andes have experienced an increase in runoff in recent decades, while precipitation has remained almost constant or has shown a tendency to decrease (Coudrain et al., 2005). This short-term increase in runoff is interpreted as the consequence of glacier retreat, but in the long term there will be a reduction in water supply as the glaciers shrink beyond a critical limit (Jansson et al., 2003).

Chacaltaya Glacier, with a mean altitude of 5,260 m above sea level, was the highest skiing station in the world until a very few years ago. After the accelerated shrinkage of the glacier during the 1990s, enhanced by the warm 1997/98 El Niño, Bolivia lost its only ski area (Figure 1.1), directly affecting the development of snow sports and recreation in this part of the Andes, where glaciers are an important part of the cultural heritage.

Figure 1.1. Areal extent of Chacaltaya Glacier, Bolivia, from 1940 to 2005. By 2005, the glacier had separated into three distinct small bodies. The position of the ski hut, which did not exist in 1940, is indicated with a red cross. The ski lift, which had a length of about 800 m in 1940 and about 600 m in 1996, was normally installed during the summer months (precipitation season in the tropics) and covered a major portion of the glacier, as indicated with a continuous line. The original location of the ski lift in 1940 is indicated with a segmented line in subsequent epochs. After 2004, skiing was no longer possible. Photo credits: Francou and Vincent (2006) and Jordan (1991).

Figure 1.1. Areal extent of Chacaltaya Glacier, Bolivia, from 1940 to 2005. By 2005, the glacier had separated into three distinct small bodies. The position of the ski hut, which did not exist in 1940, is indicated with a red cross. The ski lift, which had a length of about 800 m in 1940 and about 600 m in 1996, was normally installed during the summer months (precipitation season in the tropics) and covered a major portion of the glacier, as indicated with a continuous line. The original location of the ski lift in 1940 is indicated with a segmented line in subsequent epochs. After 2004, skiing was no longer possible. Photo credits: Francou and Vincent (2006) and Jordan (1991).

now either solidly dammed or drained, but continued vigilance is needed since many tens of potentially dangerous glacial lakes still exist in the Himalayas (Yamada, 1998) and the Andes (Ames, 1998), together with several more in other mountain ranges of the world. The temporary increase in glacier melt can also produce enhanced GLOFs, as has been reported in Chile (Peña and Escobar, 1985), although these have not been linked with any long-term climate trends.

Enhanced colonisation of plants and animals in deglaciated terrain is a direct effect of glacier and snow retreat (e.g., Jones and Henry, 2003). Although changes due to other causes such as introduction by human activities, increased UV radiation, contaminants and habitat loss might be important (e.g., Frenot et al., 2005), 'greening' has been reported in relation to warming in the Arctic and also in the Antarctic Peninsula. Tundra areas in the northern circumpolar high latitudes derived from a 22-year satellite record show greening trends, while forest areas show declines in photosynthetic activity (Bunn and Goetz, 2006). Ice-water microbial habitats have contracted in the Canadian High Arctic (Vincent et al., 2001).

Glacier retreat causes striking changes in the landscape, which has affected living conditions and local tourism in many mountain regions around the world (Watson and Haeberli, 2004; Molg et al., 2005). Warming produces an enhanced spring-summer melting of glaciers, particularly in areas of ablation, with a corresponding loss of seasonal snow cover that results in an increased exposure of surface crevasses, which can in turn affect, for example, snow runway operations, as has been reported in the Antarctic Peninsula (Rivera et al., 2005). The retreat, enhanced flow and collapse of glaciers, ice streams and ice shelves can lead to increased production of iceberg calving, which can in turn affect sea navigation, although no evidence for this exists as yet.

Snow cover

Spring peak river flows have been occurring 1-2 weeks earlier during the last 65 years in North America and northern Eurasia. There is also evidence for an increase in winter base flow in northern Eurasia and North America. These changes in river runoff are described in detail in Section 1.3.2 and Table 1.3. There is also a measured trend towards less snow at low altitudes, which is affecting skiing areas (Table 1.2).

Frozen ground

Degradation of seasonally frozen ground and permafrost, and an increase in active-layer thickness, should result in an increased importance of surface water (McNamara et al., 1999), with an initial but temporary phase of lake expansion due to melting, followed by their disappearance due to draining within the permafrost, as has been detected in Alaska (Yoshikawa and Hinzman, 2003) and in Siberia (Smith et al., 2005).

Permafrost and frozen ground degradation are resulting in an increased areal extent of wetlands in the Arctic, with an associated 'greening', i.e., plant colonisation (see above). Wetland changes also affect the fauna. Permafrost degradation and wetland increase might produce an increased release of carbon in the form of methane to the atmosphere in the future (e.g., Lawrence and Slater, 2005; Zimov et al., 2006), but this has not been documented.

The observed permafrost warming and degradation, together with an increasing depth of the active layer, should result in mechanical weakening of the ground, and ground subsidence and formation of thermokarst will have a weakening effect on existing infrastructure such as buildings, roads, airfields and pipelines (Couture et al., 2000; Nelson, 2003), but there is no solid evidence for this yet. There is evidence for a decrease in potential travel days of vehicles over frozen roads in Alaska (Table 1.2). Permafrost melting has produced increased coastal erosion in the Arctic (e.g., Beaulieu and Allard, 2003); this is detailed in Section 1.3.3.

Thawing and deepening of the active layer in high-mountain areas can produce slope instability and rock falls (Watson and Haeberli, 2004), which in turn can trigger outburst floods (Casassa and Marangunic, 1993; Carey, 2005), but there is no evidence for trends. A reported case linked to warming is the exceptional rock-fall activity in the Alps during the 2003 summer heatwave (Table 1.2).

Sea ice

Nutritional stresses related to longer ice-free seasons in the Beaufort Sea may be inducing declining survival rates, smaller size, and cannibalism among polar bears (Amstrup et al., 2006; Regehr et al., 2006). Polar bears are entirely dependent on sea ice as a platform to access the marine mammals that provide their nutritional needs (Amstrup, 2003). Reduced sea ice in the Arctic will probably result in increased navigation, partial evidence of which has already been found (Eagles, 2004), and possibly also a rise in offshore oil operations, with positive effects such as enhanced trade, and negative ones such as increased pollution (Chapter 15; ACIA, 2005), but there are no quantitative data to support this.

Increased navigability in the Arctic should also raise issues of water sovereignty versus international access for shipping through the North-west and North-east Passages. Previously uncharted islands and seamounts have been discovered due to a reduction in sea ice cover (Mohr and Forsberg, 2002), which can be relevant for territorial and ocean claims.

Ocean freshening, circulation and ecosystems

There is evidence for freshening in the North Atlantic and in the Ross Sea, which is probably linked to glacier melt (Bindoff et al., 2007). There is no significant evidence of changes in the Meridional Overturning Circulation at high latitudes in the North Atlantic Ocean or in the Southern Ocean, although important changes in interannual to decadal scales have been observed in the North Atlantic (Bindoff et al., 2007). Ocean ecosystem impacts such as a reduction of krill biomass and an increase in salps in Antarctica, decline of marine algae in the Arctic due to their replacement by freshwater species, and impacts on Arctic mammals, are described in Section 1.3.4.2.

Lake and river ice

Seasonal and multi-annual variations in lake and river ice are relevant in terms of freshwater hydrology and for human activities such as winter transportation, bridge and pipeline crossings, but no quantitative evidence of observed effects exists yet. Shortening of the freezing period of lake and river ice by an

Table 1.3. Observed changes in runoff/streamflow, lake levels and floods/droughts.

Environmental Observed changes

Time period

Location

Selected references

factor

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