The Himalayan glacier that feeds the Ganga is receding at a worrying rate, the rate of retreat in the last three decades having been more than three times the rate during the preceding two centuries.
THE impact of global warming is perhaps already upon the Himalayas. The 30.2-km-long Gangotri glacier, the second largest among the 6,500-odd small and large glaciers in the Himalayas and which feeds the perennial Ganga, is receding at a rate that is wor rying. The rate of retreat in the last three decades has been found to be more than three times the rate during the earlier 200 years or so. From a detailed quantitative analysis of the geomorphological features of the Gangotri, a team of geologists from HNB Garhwal University has concluded that the retreat has become much faster than it was before 1971.
Glaciers are dynamic systems. Snowfalls in the highest part of a glacier, which is known as the accumulation area, add to its mass. As the snow slowly turns to ice, the glacier grows in weight, forcing glacial movement. Further down the glacier is the ab lation area where most of the melting and evaporation occur. Between these two areas a balance is reached where snowfall equals snowmelt; the glacier is then in equilibrium. Whenever this equilibrium is disturbed, either by increased snowfall or by exces sive melting, the glacier advances or retreats at more than its normal pace.
Glacial advances and retreats are thus natural cyclical phenomena that occur during glacial and inter-glacial periods respectively. The world is now in an inter-glacial warm phase, the previous glacial advance having occurred during the last phase of the last glacial period (15th-19th century), which has come to be called "The Little Ice Age" (LIA). Since then, as a result of the subsequent warming phase, all glaciers have been retreating.
The rate of retreat in recent times has, however, been much more rapid than the gradual retreat expected in an inter-glacial warming phase. This, glaciologists and climatologists believe, is due to global warming. This climatic change brought about by hu man or anthropogenic activity in the post-industrialisation period has already resulted in a global increase in the average surface temperature by 0.6 o C. A natural consequence of this is increased melt from ice caps and glaciers.
According to climatologists, mountain glaciers, such as those in the Himalayas, are particularly sensitive indicators of climate change. The volume of ice in a glacier - and correspondingly its surface area, thickness, and length - is determined by the b alance between input (accumulation of snow and ice) and output (melting, evaporation and calving). Temperature, precipitation, humidity, wind speed, and other factors such as slope and the reflectivity of the glacier surface, all affect the balance.
Most glaciers, however, are more sensitive to temperature than to other climatic factors. The central and eastern Himalayan glaciers are particularly so because of the monsoon-driven climatic conditions in the region. Explaining this property of Himalaya n glaciers, Syed Iqbal Hasnain of the School of Environmental Sciences, Jawaharlal Nehru University, says: "Unlike glaciers of Europe and other higher latitudes, both accumulation (because of precipitation in higher elevations as snow) and ablation in th ese parts occur during summer months, making them very sensitive to temperature changes. The Karakoram glaciers of the Western Himalayas (the Siachen glacier belongs to this class) are, however, more like the European glaciers with accumulation in winter and ablation in summer."
IT is not surprising, therefore, that a perceptible impact of global warming has been in evidence in the Himalayan glaciers over the last few decades. A 1999 report by the Working Group on Himalayan Glaciology (WGHG) of the International Commission for S now and Ice (ICSI), constituted in 1995, said: "Glaciers in the Himalayas are receding faster than in any other part of the world and, if the present rate continues, the likelihood of them disappearing by the year 2035 is very high."
The scientists, Ajay K. Naithani and associates, have, however, refrained from making any such statement about the possible cause of the swift recession seen in their research paper which appeared in the January 10, 2001 issue of the journal, Current Science. "Our findings clearly indicate that there is an increase in the rate of retreat. This is only a preliminary investigation and our objective here was limited," says Harish C. Nainwal, a member of the team. "There can be various causes for the retreat including tectonic movements as the glacier sits right on the Bhagirathi Thrust Plane and is north of the main central thrust (MCT) in the Himalayas. Global warming is only one of the factors. But a consistent trend of high retreat rate over the years, together with other morphological changes that are seen, is suggestive of a forcing like global warming. We can make any such conclusive statement only after 5-10 years based on a long-term data," says Nainwal.
The movement of a glacier leaves behind moraines, the dark bands of rocky debris pushed along by the glacier and deposited on its top and on its edges. The formation and the type of moraines, together with other landforms, provide the key morphological e vidence to the previous positions of a glacier, particularly its snout, and provide indications of its advance or retreat. The snout of a glacier is its terminus, the front of the region of melting and evaporation - that usually takes the shape of an ice cave in mountainous terrains like the Himalayas - where the ice-melt turns into a steady flow and the river begins.
A study of terminal moraines gives clues about the positions of the snout at various points in the glacier's past. The snout of the Gangotri is called Gaumukh, which is the source of the river Bhagirathi. Evidence from a study of terminal moraines of the Gangotri points to the snout having been nearly 3 km downstream from its present position near the place called Bhujbasa before the post-LIA retreat began around the turn of the 19th century.
The findings of Naithani and his associates are based on investigations over three and a half years, between May 1996 and October 1999. The aim of the Garhwal University group was to establish evidence for the increased rate of retreat seen in the earlie r data sets of other research groups in terms of the geomorphological characteristics of the glacier. They found that during this period the snout had retreated by 76 m. The extent of retreat will, of course, vary from year to year. To determine the tren d in the rate of retreat, data over a long period is required. From that perspective, the three-and-a half-year data on the retreat of the snout should be seen in conjunction with earlier data over a longer period to determine the trend.
Observations on the retreat of the Gangotri go back to 1842, and between 1842 and 1935 the snout of the Gangotri glacier was receding at an average rate of 7.3 m a year. According to data of the Geological Survey of India (GSI), between 1935 and 1996 it retreated by 1,147 m which amounts to an average rate of 19 m a year. This implies that the rate of retreat more than doubled during a good part of the last century.
But a dramatic increase in the rate seems to have occurred in the last three decades. In 1996, researchers L.A. Owen and M.C. Sharma showed, by studying the longitudinal profiles of the river, that between 1971 and 1996, the Gangotri glacier had retreate d by about 850 m. This would yield a post-1971 retreat rate of 34 m a year. For the post-1971 period, the 61-year (1935-1996) data set of the GSI too shows that the retreat rate is about 28 m a year, indicating a clear increase in the rate after 1971. Th e 1996-99 data of Naithani and associates too matches this general trend of an increased rate.
Faster retreat of the snout is only one of the features of a rapidly receding glacier. Many other changes also occur and it is these morphological changes that the present research work has investigated to get a better quantitative picture of the shrinka ge. "Within 1.0 km of the snout (where the snout stood in 1971 according to the GSI), a chaotic assemblage of hummocky (mounded) moraines and dead ice, with small ponds and lacustrine deposits, indicates rapid ice retreat since 1971," the researchers wri te in their Current Science paper.
According to their findings, while in the forefield of the Gangotri moraines that formed prior to 1971 are well developed, with sharp crested ridges that have strongly oriented fabrics, those formed after 1971 are mounded and have a weak fabric strength. This, they say, suggests that retreat was much slower before 1971, for only when a glacier is stationary for a long enough period can it produce well-defined moraines with strong fabric.
The effects of rapid recession are, however, the most prominent in the ablation zone, just above the snout, which undergoes significant morphological changes and rapid deterioration, points out Nainwal. These changes in turn, he says, influence the posit ion of the snout as the glacier retreats. One important feature is the deformation of the snout's structure with more active breaking of ice in the zone. "Big boulders of ice embedded in undulating surfaces are a characteristic feature," says Nainwal. Th e subsidence and fast degenerating nature of the glacier also leads to the formation of supraglacial lakes in the ablation zone formed by the melting of ice and damming by moraines. While supraglacial lakes are common in all glaciers, rapid retreat gives rise to a large number of these, a feature that is seen in the Gangotri, points out Nainwal.
Another feature of a rapid recession is the formation of almost vertical crevasses both in the longitudinal and transverse directions. Crevasses are giant cracks in the glacier ice caused by the deformation of ice under the force of its own movement. Nat urally, when the retreat is rapid the forces of deformation are greater, resulting in the formation of a larger number of crevasses. When the longitudinal and transverse crevasses intersect, they form huge blocks of ice which get detached from the main g lacier in short periods of time. Many such intersections have been found in the ablation zone of Gangotri, extending up to 400 m upstream, and these give yet another indication of a rapid retreat in the recent past. It is likely that these points of inte rsection become suitable spots for the formation of fresh snouts as the glacier retreats, says Nainwal.
All these processes lead to a thinning of ice in the ablation zone. This thinning can lead to an unusually long retreat of the snout. This, says the Current Science paper, was very much in evidence at the snout of the Gangotri. "During the month o f May and June in 1999, the snout changed its shape almost every day because huge blocks of ice got detached from the main body every day. We never saw this type of situation before," the researchers write.
Geomorphological evidence alone is not enough to establish rapid retreat, says Hasnain of the JNU. "Although retreat is accompanied by thinning of ice, it is more important to know how rapidly it is thinning," says Hasnain. "We need to know the net loss of glacial mass that occurs every year for a few years in order to assess the impact of global warming. For that, detailed mass balance studies have to be carried out and from that perspective there is the least amount of data on Himalayan glaciers even though they are the lifeline of northern India, and provide nearly 70 per cent of the water requirements of the north and the east," he says.
"Mass balance studies are only possible with small glaciers and they are being done. For massive ones like the Gangotri, which is fed by more than 18 tributary glaciers, mass balance studies become virtually impossible," says Deepak Srivastava, director of glaciology, GSI, Lucknow. The fact that accumulation and ablation occur simultaneously complicates mass balance studies. "Geomorphological investigations do provide reliable estimates of the retreat, and the observed retreat is significant. The effect of global warming is certainly there but there is nothing alarming about it," he says.
The immediate or short-term impact of a receding Gangotri (or any other large glacier) would be an increased risk of glacial hazards, such as incidents of landslips, changes in the courses of rivers and floods. Besides mass-wastings, in the form of lands lips and massive ice boulders hurtling down, the formation of glacial lakes on the main trunk of the glacier poses a hazard. One such instance occurred in Nepal in 1985 when a moraine-dammed glacial lake, Dig Tsho, burst in the Khumbu Himal area. The lak e emptied into the Lagmoche valley, and within a few hours flood waters 10-15 m high surged through the valley for nearly 100 m, causing enormous destruction.
The higher melt means more discharge into rivers and reservoirs. Already there is evidence to this effect. According to Srivastava, there have been instances of flooding in the Ganga basin during drought years and this is obviously because of increased m elt. This increased flow can also lead to landslips downstream by triggering unstable flow along the area evacuated by the receding glacier because the soil, rocks and the vegetation on them are loose and can give way to surging water easily, points out Hasnain.
Another important consequence of increased glacial melt is a higher silt load. This can affect reservoirs in river valley projects, points out Nainwal. Some experts feel that already the life span of two reservoirs downstream of the Bhagirathi, Tehri and Maneri Bali I, have been reduced - one from 250 years to 160 and the other from 200 years to 93.
The silt here contains hard quartz from the rocky bed of the glaciers. These are carried as colloidal suspension in the water which strike turbine blades and damage them. This is believed to be the chief reason why at the Triloth power station of the Man eri Bali I hydel project any one of the three turbines is forever under repair.
The rapid changes that the Gangotri is undergoing, and the likely long-term impact they may cause downstream, have induced the government to initiate long-term studies on the glacier. Interestingly, there are no time series data available on precipitatio n, snowfall and temperature in the region, let alone of snow melt, run-off and water flow, though the Gangotri is easily accessible and is a pilgrim centre.
Although the Earth Sciences wing of the Department of Science and Technology (DST) has an ongoing programme in Himalayan glaciology, no systematic study of Gangotri, which requires a well-conceived multi-institutional project over a long term, has been u ndertaken. Detailed studies of smaller glaciers that feed the Ganga, like the Dokriani, have been undertaken. The Garhwal University study happens to be one of the first studies on the Gangotri glacier funded by the DST. Only now, having realised the imp ortance of meteorological data on the Gangotri, an automatic weather station is being established at Gangotri by the Snow and Avalanche Study Establishment (SASE) of the Defence Research and Development Organisation (DRDO).
Hopefully, this will facilitate more detailed studies on the glacier, given the long-term impact it can have as a result of global warming.
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