February 17, 2006

By Dr. AB Thapa

The last ice age, the Pleistocene, consisted of several periods of glaciations separated by interglacial periods of mild climates. During the past two million years there have been five major glacial advances and five glacial retreats, the last of these being our present period. It is said that Earth is now in a warm interglacial period. At present the ice covers only about 10 percent of the land surface. During the last ice age, however, ice covered nearly 30 percent of the land. At its peak about 18,000 years ago ice sheets a kilometer thick covered most of Northern Hemisphere. When the ice melted sea level rose by tens of meters, flooding large areas including the Bering land bridge that had served as a migration corridor for people moving into North America from Asia . During the present warm interglacial period these large ice sheets have disappeared and glaciers worldwide have generally shrunk.

Origin of Glaciers

Historical records on climate generally do not go back more than 2,000 years. Fortunately the past climates can be traced from many different sources of evidence. Tree rings, for example, can provide information on climate during the past 1,000 years; ice cores can cover the past 100,000 years; lake sediments furnish evidence stretching back as much as a million years; and marine sediments can yield data covering the past 10 million years. Scientists have used a combination of this evidence to determine that ice ages, or cold periods, when Earth’s temperature is about 8°C colder than during the warm, so-called interglacial periods, occur at roughly 100,000-year intervals. Some scientists believe that cycles of changes in the distribution of sunlight due to long-term variations in Earth’s orbit and the inclination of its spin axis to the Sun cause ice ages. These cycles are known as Milankovich cycles, named after the Serbian mathematician who first computed them. It is also a widely held belief that the changes in atmosphere, such as the decrease in carbon dioxide content that allows a faster rate of heat loss to outer space or an increase in atmospheric dust due to volcanic eruptions that prevents the warming effect of some of the sun’s rays from reaching the earth account for the origin of the ice ages.

Annual Snow Line

Most glaciers have two parts, an accumulation area and an ablation or wastage area. In the accumulation area snowfall exceeds melting in each year. In the ablation area melting exceeds snowfall. The boundary between the two areas is called the annual snowline or sometimes the fern limit. In winter most glaciers are entirely snow-covered. In spring the snow cover begins to melt in the lower reaches, exposing the ice surface. As temperatures increase, the melting moves up the glacier. The snowline is the highest position the melting reaches during the year. Fern is old granular snow. The fern limit may not exactly coincide with the annual snowline since in some years rapid melting leaves behind fern patches below the snowline.

Some glaciers exhibit features called ice streams and icefalls. Ice streams are valley glaciers that form tributaries to a common compound glacier that fills a valley. The tributary glaciers do not intermix but maintain their individual streams of ice, despite compression and extension as they move along side by side. The streams can easily be recognized as individual ice streams by the deposits of boulders, gravel, sand, and mud that separate them. Icefalls occur where a glacier flows over very steep terrain that accelerates the flow. The ice is stretched and fractures into large blocks and a maze of ice pinnacles. Icefalls are spectacular features that can extend over the entire width of the glacier and over a height of up to a kilometer

Glacier Movement

As glaciers move over bedrock they scrape and abrade its surface, producing fine-grained rock flour. Glaciers can also pluck away rocks up to boulder size and transport and deposit them along the margins of the glacier down in the valleys. The glaciers deposit these materials as till, a sediment consisting of mud, sand, gravel, and boulders. Much of this material is deposited in long mounds called moraines. Lateral moraines are formed on each side of a valley glacier where abraded sediment and plucked rocks are deposited. These moraines are often preserved when glaciers melt and can indicate previous glacier heights. Medial moraines separate tributary glaciers that flow into a compound valley glacier. Terminal or end moraines mark the farthest distance down a valley that a glacier has reached in its advance. Recessional moraines indicate to where glaciers advanced and remained stationary for some time in the past. Both terminal and recessional moraines can dam melt water streams, forming glacial lakes. Glaciers also deposit a blanket of till that forms a ground moraine on the surfaces over which the glacier flowed

Climatic Changes

Glaciers are very sensitive to climate change. Their size, life span, and history of growth and retreat all depend strongly on climate conditions. Since they are so sensitive to climatic changes they also serve as good indicators of such change. A glacier’s accumulation and ablation, or gain and loss of mass, are primarily dependent on temperature and precipitation, but also on solar radiation, humidity, and wind speed. Location, orientation, and exposure of the glacier are also important, particularly for the smaller valley glaciers. The energy budget or balance of a glacier’s surface reflects how much heat energy is received or lost from a glacier and whether evaporation or melting can occur. The energy budget explains in quantitative terms what is termed the microclimate of a glacier.

The large ice sheets can provide information about climate conditions over the past several hundred thousand years. Cores drilled deep down into the ice in Greenland and Antarctica allow the reconstruction of past climates since the analysis of successively deeper layers of ice yields information such as the atmospheric temperature at the time the ice was first deposited as snow. Dust layers from known volcanic eruptions provide reliable age determinations; ice that lies beneath a known dust layer is older, while dust that lies above is younger. Analysis of the ice itself and of the air bubbles trapped in the ice allows deductions about the composition of the atmosphere at the time when the ice was deposited.

Himalayan Glacier Study

About two decades ago the Royal Nepal Academy of Science and Technology (RONAST) had carried out extensive exercise to set up a Regional Center on snow and ice study in Nepal. The objective of the proposed CENTER was to develop cooperation among the countries of the Himalayan region for glaciological research in the mountain range of the Himalaya. The CENTER was expected to promote sustainable economic and social development studies. . As such, it would have consisted largely of application oriented research with both scientifically and socially valid objectives. RONAST, to take this idea a step further, even established relationship with Italian National Research Center (CNR) to carry out jointly Himalayan studies. A big research center equipped with modern facilities has already been set up near the base camp of the Mount Everest at Lobouche.

Snow and ice, representing both valuable resources and natural hazards are significant elements of the world hydrological systems, which occur subject to tremendous variations in space and time. Nowhere change is more significant than in the advancing and retreating tides of snow and ice. The RONAST was hoping that the proposed Regional Center on Snow and Ice would be engaged in scientific studies of the snow and ice balance of individual catchments and of regional groupings of catchments forming the headwaters of major Himalayan rivers.

The proposed regional center was also to promote sustainable economic and social development. As such, it would have consisted largely of applications oriented studies with both scientifically and socially valid objectives. Hydropower development has an enormous potential for the Himalayan region. Effective site analysis, as well as decisions on scale of capital installation, depend on determination of annual water supply and its seasonal variation. In addition, glacier-fed rivers are notorious for their very high sediment load. This, of course, relate to the rate of reservoir sedimentation and rate of cavitation damage to turbines. Response to these problems can be made through dam, reservoir, and penstock intake design, which in turn will be influenced by detailed glacio-hydrological studies.

Glacier Lake Outburst Floods

At present glaciers are retreating in the Himalayan region, as a result, glaciers lakes are being formed. Such ephemeral lakes disrupt communication systems and various infrastructures like hydropower directly, or indirectly subjecting the mainstream to periodic catastrophic floods. Glacier lake outburst floods also produce peaks in sediment transfer.

On August 4 th 1985 the nearly completed Namche hydropower plant was completely destroyed by the Dig Tsho glacier lake outburst flood( GLOF). The Dig Tsho glacier was on the terminus of the Langmoche Glacier. The GLOF damaged not only the entire Namche Hydropower station but also all the bridges, trails, cultivation fields, houses, livestock along its path to the confluence of the Dudh-Kosi and the Sun-Kosi rivers at a distance of 90 km from the Dig Tsho glacier.

In 1988 for the first time a joint team of Sino-Nepalese conducted the studies of the glaciers and glacier lakes in the Arun and the Sun-Kosi basins primarily in the Tibetan region of China. The Lanzhou Institute of Glaciology and Geocryology took part in the study from the Chinese side, similarly the Water and Energy Commission took part from the Nepalese side. The field investigation team used satellite imagery data to locate the lakes and to estimate their dimensions as well as their morphological characteristics. In Arun basin there are 737 glaciers in Tibet , whose total water storage is estimated to be 121 billion cu. m. It was found that there are 229 glacier lakes with a total storage volume of 1.23 billion cu. m out such glacier lakes 24 are potentially dangerous. Similarly there are 45 glacier lakes in the Sun-Kosi basin with a total storage volume of 388 million cu. m out of them 10 are potentially dangerous.

In 1990s Dr. Tomomi Yamada of Japan and Dr. B.P. Upadhyay, Professor of TU were involved in the study of glacier lakes within Nepal The study was conducted under the Water and Energy Commission. Their study covered Lower Barun , Chamlang Tsho, Naulekh, Sabai Tsho, Dudh Kund, Mojang, Tsho Rolpa, Duwo, Thulagi, Khyimjung and Kang Guru glacier lakes.

(Dr. Thapa writes on water resources)