Glaciers in many parts of the Himalayas have undergone significant shrinkage in the last century in response to climatic warming, which in some areas is occurring faster than the global average. Some of this warming is part of a natural climatic cycle, although over the last 50 years or so probably about half of the warming is attributable to human sources (greenhouse gases). A particular problem associated with glacier shrinkage is the formation and catastrophic drainage of large moraine-dammed lakes, causing significant environmental hazards in many Himalayan valleys. In recent decades, there have been several Glacier Lake Outburst Floods (GLOFs), which have caused widespread damage to homes, roads, bridges and industry, destruction of farmland, and loss of life. Many more potentially unstable lakes have been identified in populated regions, and some (such as Tsho Rolpa in the Rolwaling Himalaya, Nepal) are the subject of engineering projects to lower their levels and mitigate GLOF hazards. Mitigation works, however, are both expensive and logistically difficult in these unstable, high altitude environments.
In the Himalayas, large ice-contact lakes form in association with debris-covered glaciers, which behave differently to normal, clean glaciers. First, debris-covered glaciers deposit massive ridges of debris - moraines - around their snouts, which are capable of ponding large volumes of meltwater, but lack the mechanical strength to impound the water for more than a few decades. Furthermore, by insulating ice from solar radiation and daily temperature fluctuations, a thick debris cover inhibits melting, thus delaying the response of the glacier to global warming. During periods of climatic warming, debris-covered glaciers slowly melt down, until closed hollows and lakes begin to form on the glacier surface. These lakes can rapidly expand and join up as icebergs break from the surrounding cliffs, so once lakes begin to form, the disintegration of the glacier and the formation of a large, potentially unstable lake can occur within a few years. It is as if the glaciers have ‘stored up’ their response to climate change, and are only now catching up with over a century of rising temperatures. It is possible that many large Himalayan valley glaciers are currently so far out of equilibrium with climate that they may cross the threshold into rapid lake growth in the coming decades even if the recent warming trend halts or reverses.
In the coming decades, potentially dangerous lakes are expected to form on many glaciers, and resources in Himalayan countries may be insufficient for the necessary mitigation works. There is therefore a growing need for a long term view of evolving glacier hazards in the region, based on a sound understanding of the life cycle of these glaciers, and their response to global warming.
The Ngozumpa Glacier Project aims to study the response of a large, debris-mantled glacier to climate change, and to obtain data which will allow the prediction and mitigation of potential hazards, especially lake outbursts and slope failures. This glacier is of particular interest because there are several supraglacial lakes on its ablation area, some of which are believed to lie close to the threshold for rapid lake growth. Additionally, the western lateral moraine of the glacier dams up a series of lakes in side valleys, which may be destabilised by glacier downwasting and moraine degradation.
The specific objectives of the research are to:
Work has been conducted on the glacier in October 1998, April 1999, October - November 1999, October - November 2000, and September - November 2001, and annual or twice-annual research expeditions to the glacier will continue. Field research to date has focused on three main themes:
One of the studied lakes (Spillway Lake) does not occupy a perched basin, but overflows through the western lateral moraine of the glacier about 1 km from the terminus, and receives water from subaerial and subaqueous portals. Spillway Lake underwent only limited growth 1998 - 2001, except in areas of bare ice cliffs near the north-western and eastern margins. Growth of the lake was more rapid during 2000-2001 than in previous years. Additionally, widespread subsidence of the glacier surface to the north of the lake, due to the collapse of englacial conduits, has resulted in the formation of a chain of new lakes, which are expected to join up with Spillway Lake in the near future. Because the level of Spillway Lake is controlled by the altitude of the terminal moraine, it will experience continued growth unchecked by lake drainage unless the moraine dam itself is reduced in height or fails.
Energy Balance and Melting of the Glacier Surface
Predictive models of glacier behaviour require functions relating climatic inputs to glacier response (accumulation and ablation). Such functions are largely unknown for high altitude Himalayan glaciers, and are likely to be complex because of the influence of avalanching on accumulation and debris cover on ablation. Detailed studies of ice ablation beneath debris cover have been conducted at several sites on the glacier. The data have allowed the individual components of the surface energy balance of the glacier to be identified and quantified for selected study periods. During the autumn ablation period, incoming shortwave radiation provides over 90% of the energy for ablation, with surface albedo exerting an important modulating influence. In September 2001, sets of thermistors were installed within the debris layer, and results up to November 2001 are being analyised. The thermistor arrays were left in place, and combined with data from an automatic weather station installed on the glacier, will provide continuous records of energy fluxes to and through the debris layer.
An important new finding is that local variations in debris thickness across the glacier exert a strong control on overall melting rate. Uneven melting and topographic development are linked by a series of feedbacks, which transmit small-scale processes to large scale downwasting patterns. A numerical model of this process is currently under development.
Stable Isotope Analysis of Snow and Ice
The accumulation areas of the Ngozumpa Glacier extend up to over 8,000 metres and are guarded by highly unstable icefalls, hindering attempts to quantify inputs of snow and ice to the glacier. Insights into accumulation processes can, however, be gained through analysis of stable isotopes (O and H) in glacier ice in the ablation zone. The isotopic composition of glacier ice in the Himalayas varies with both altitude (5,000 - 8,000m in the Ngozumpa catchment) and season of snowfall (winter or monsoon). A large number of samples have been collected from the Ngozumpa and adjacent glaciers, which will allow the contribution of winter and monsoon snow to be identified within the ice stratigraphy, and possibly allow the relative importance of distinct snow accumulation areas to be determined.
Work planned for the 2002 autumn season includes continuation of the lake monitoring program and energy balance studies and Ground Penertrating Radar surveys of the glacier subsurface.
Seonaid Wiseman (BSc, Aberdeen):
The origin and life-cycle of supraglacial lakes
Kathryn Hands (BSc, Aberdeen): The consequences of glacier downwasting: moraine degradation and supraglacial lake formation
Lindsey Nicholson (BSc, Edinburgh): Ice ablation, isotope studies and energy-balance modelling
Reynolds Geo-Sciences Glacier hazard assessment and mitigation
Pyramid Group, Milan: meterorology and climate change
in the Khumbu Himal
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