Tropical Glaciers The Cordillera Blanca Peru

We normally think of glaciers as occupying only cold places so it may come as a bit of a surprise to learn that tropical glaciers occur near the Equator on the mountains of East Africa, Indonesia and in the Andes. On a global scale, the total area of these tropical glaciers is small (^2500 km2) and they make up only 4% of the area of all

2.6 Tropical Glaciers: The Cordillera Blanca, Peru 27

Earth's mountain glaciers which is equivalent to about 0.15% of the Earth's total glacier area. More than 99% of all tropical glacier area is in the South America Andes between Bolivia and Venezuela, with more than 70% in the Peruvian Cordilleras alone. The tropical climate is very different to the mid-latitude or polar environments so glaciers can exist at these low latitudes only where there are mountains of sufficient altitude to allow snow accumulation. Climatically, these mountains are characterised by a homogeneous atmosphere without frontal activity, a lack of thermal seasonality and by very pronounced precipitation seasons. Around the tropical glaciers of the South American Andes, for example, the year can be divided into three climatic periods: the dry season (May-August), the wet season (January-April) and a transition period (September-December), where there is a gradual build up in precipitation towards the wet season. As a result of this our normal concepts of simple accumulation and ablation seasons (see Section 3.1) do not hold for tropical glaciers. In many locations the period of maximum precipitation, and therefore snow accumulation, coincides with the period when air temperatures are also highest, and therefore ablation is highest, so that mass balance relationships are complex on these glaciers.

The glaciers of the Cordillera Blanca in Peru provide a good example of tropical glaciation (Figure 2.9). Here there is more than 720 km2 of high-altitude glaciers, making it the largest glaciated area in the tropics, equivalent to ^25% of the area of tropical glaciers globally. Glaciers exist in the mountains for 120 km between 8.5° and 10°S along a northwest-southeast strike fault, dividing stream runoff between the Pacific and Atlantic oceans. Between 70 and 90% of the annual precipitation falls between October and March in these regions. The glaciers are important socially and economically because they act as water stores and therefore influence water supplies. River water for drinking and sanitation comes exclusively from glacier melt during the dry season when there is little or no precipitation. The rivers are also increasingly used for hydroelectric power generation.

Glaciers exist here because the air temperatures are so cold at high elevations (up to 6000 m above sea level). In contrast to mid-latitude glaciers, tropical glaciers do not have summer time melt seasons characterised by widespread above-freezing air temperature. Their lower altitude portions are warmed directly by year-round exposure to above-freezing air but at higher altitudes absorption of sunlight ultimately supplies all the energy that sustains ablation. Ablation is sensitive to the amount of absorbed solar radiation, air temperature, atmospheric humidity, cloud cover and wind. Tropical glaciers therefore react faster to changes in ablation than those in the mid-latitudes, such that a number of glaciers have receded dramatically or even disappeared in recent years. The tropical glaciers of the Cordillera Blanca have been receding rapidly in recent years, with a consequent reduction in glacier volume throughout the nineteenth and twentieth centuries. Larger-scale climate features, such as the El Nino-Southern Oscillation (ENSO) have been shown to have an impact on interann-ual variability of temperature, precipitation and stream discharge in this region. The mostly widely cited explanation for this recession is a reduction in air humidity, with all the consequent changes in energy and mass balance. Rising air temperatures explain only part of the glacier recession.

Figure 2.9 Mountain glacier in the Cordillera Blanca, Peru. Note the progressive change from clean ice in the steep icefall to debris cover near the snout where there is a much lower surface gradient. [Photograph: N.F. Glasser.]

Figure 2.9 Mountain glacier in the Cordillera Blanca, Peru. Note the progressive change from clean ice in the steep icefall to debris cover near the snout where there is a much lower surface gradient. [Photograph: N.F. Glasser.]

The sediments and landforms created by tropical glaciers are varied. Most glaciers in the tropical Andes are receding and undergoing down-wasting in their terminal parts. As this debris becomes concentrated on the ice surface, the glaciers become progressively more debris-mantled down-glacier. The debris forms an irregular cover of hummocky, coarse angular material interspersed with supragla-cial ponds, much like those developed on Himalayan glaciers (see Section 2.5). In front of some glaciers are very large (up to 100 m high) lateral and frontal moraines, dating from the Little Ice Age or earlier (Figure 2.10). The moraines often have vegetated outer faces and an unvegetated, loose, collapsing inner face composed of sandy boulder-gravel derived from a mixture of rockfall debris and material from the zone of traction along the valley sides. Like their Himalayan counterparts, these terminal moraines often impound large supraglacial or proglacial lakes that can drain catastrophically if the moraine dam fails. The moraines that dam these proglacial lakes therefore pose an increasing hazard to communities in the Andes.

Figure 2.10 Cirque glacier in the Cordillera Blanca, Peru. The glacier has receded behind the large moraines in the foreground during the twentieth and twenty-first centuries. The moraines are now being dissected by the proglacial stream fed by the glacier. [Photograph: N.F. Glasser]

The moraines are prone to failure through collapse, overtopping by lake waters or the effect of displacement waves resulting from ice and rock avalanches.

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