Climate Change and Himalayan Glaciers: Role of Remote Sensing

I.M.Bahuguna and Ajai

Space Applications Centre, Ahmedabad-380015

E-mail: imbahuguna@sac.isro.gov.in

Extended Abstract

Glaciers are formed due to recrystallization and metamorphism of naturally fallen snow on land surface. Snow is a type of precipitation in the form of crystalline ice, consisting of a multitude of snowflakes that fall from clouds. Snow is composed of small ice particles. It is a granular material. The process of this precipitation is called snowfall. The density of snow when it is fresh is 30-50 kg/m3. When it becomes firn the density becomes about 400-830 kg/m3. Snow becomes glacier ice when density is 830-910 kg/m3. Snow becomes firn when it survives for minimum one summer and becomes glacier ice in many years. Density increases due to remelting and recrystallization and reduction in air spaces within the ice crystals. The required atmospheric conditions for snow fall are met at higher latitudes and altitudes of the earth.

There are three major classes of snow cover i.e. temporary, seasonal and permanent. Snow covers almost 40 per cent of the Earth’s land surface during Northern Hemisphere winter. This makes albedo and areal extent of snow as important component of the Earth’s radiation balance. Monitoring an accumulation and ablation of seasonal snow cover is an important requirement for various applications. In addition, large areas in the Himalayas are also covered by snow during winter. Area of snow can change significantly during winter and spring. This can affect stream flow for rivers originating in the higher Himalayas. All the rivers originating from higher Himalayas receive almost 30-50 % of annual flow from snow and glacier melt run off. In addition, snow pack ablation is highly sensitive to climatic variation. Increase in atmospheric temperature can influence snowmelt and stream runoff pattern. Therefore, mapping of the areal extent and reflectance of snow are important parameter for various climatological and hydrological applications. In addition, extent of snow cover can also be used as input for numerous other applications. It is also needed for strategic application, as arrival of snow can significantly affect mobility of man and machine.

Mapping and monitoring of seasonal snow cover can be best done by remote sensing because a large area is covered, high temporal frequency data are available and snow has distinct signatures in optical remote sensing data which makes it easily identifiable and mappable. Therefore remote sensing can provide faster information on accumulation or ablation of snow cover than any other conventional means. This even becomes much more useful in a terrain like Himalayas where accessibility to remote areas is highly difficult and hazardous.

It is permanent snow cover which gives rise to formation of glaciers. Glaciers are formed on the earth when rate of accumulation of snow is higher than rate of ablation and falling snow gets enough time and space to get metamorphosed to form ice. Nonetheless the glacier ice must move down under the influence of gravity to be called as glacier. The Glaciers are mass of snow, ice, and water and rock debris slowly moving down a gradient. Presently, glaciers are distributed either in Polar Regions of earth or in high mountainous regions. The glaciers in Polar Regions of the earth cover the topography and appear on the surface as ice sheets or ice caps. The glaciers in the mountainous regions are constrained by topography and the shape of valley influences their flow and such glaciers are classified as valley glaciers, cirque glaciers and ice fields. The part of the glacier at its lowest altitude is called the terminus or snout of glacier. The distribution of glaciers as what we see today is the result of last glaciation. Glaciation and deglaciation are the alternate cycles of cold and warm climate of earth. During Pleistocene, the earth’s surface had experienced repeated glaciations over a large land mass. The most recent glaciations reached its maximum advance about 20,000 years ago due to fall of temperatures by 5º to 8º C. A Little ice age has been recognized during 1650-1850 AD. During peak of glaciations approximately 47 million km2 area was covered by glaciers, three times more than the present ice cover of the earth.

Glaciers are very vital to human kind as these natural resources are (i) reservoirs of freshwater (ii) control global climate as the albedo over snow and glaciers is very high, (iii) sensitive indicators of climatic variations. Since Glaciers of Himalaya constitutes the largest concentration of freshwater reserve outside the polar region, a great significance is attached to the fact that these natural resources are the source of fresh water to almost all minor and major rivers of northern India and sustain the civilization for irrigation, hydroelectricity and drinking water.

There is a pertinent relationship between retreat and advance of glaciers and variations in the mass balance of glaciers. It is the climate which is the driving forces controlling the mass balance of glacier in space and time and resulting in recession and advancement of glacier. Climatic ice fluctuations cause variation in the amount of snow and ice lost by melting. Such changes in the mass initiate a complex series of change in the flow of glacier that ultimately results in a change of the position of terminus. The adjustment of a glacier to a change in its mass balance continues for many years. Thus a glacier may advance or retreat as a result of past balance changes even though it’s net balance for the current year might be zero. However if the net balance remains zero for many years the dimensions of the glacier will eventually remain constant. The glacier is then said to be in a steady state. If the glacier experiences a change in input (usually as an increase or decrease in the amount of snow accumulating on the surface) it must respond accordingly. But because glacier stores material, there is always a lag before the response becomes easily discernible at the snout. This is called response time. The flow of ice through a glacier system varies according to how much material is being added or lost.

But glaciological studies in high altitude terrains and under inclement weather conditions as in higher Himalayas become difficult by conventional means. Thus remote sensing techniques play much greater role in mapping and monitoring of permanent snowfields and glaciers. False Color Composites (FCCs) prepared from three bands in visible and near infrared region have been used successfully to map glacier boundary, snow/equilibrium line, and accumulation and ablation area. Application of remote sensing is now well established in the glacier studies and has been used extensively in the glacier mapping. Based on this potential remote sensing data has also been successfully utilized for monitoring the retreat/advance of glaciers.

Studies on advance/retreat have been estimated for glaciers of 13 sub-basins of Himalaya. The oldest information about glacial extent is available from Survey of India topographic maps, surveyed in 1962. For long term change monitoring, Survey of India (SOI) topographical maps of 1962 at 1:50 000 scales have been used as reference maps. In some glaciers fragmentations have also been observed and therefore the numbers of glaciers in a specific range of area are also changing. Retreat/advance has also been estimated based on glacier extent mapped from IRS data available in between 1997 to 2008 and Landsat TM data of 1989-90 time frame. In the field one glacier in each basin has been visited and snout reading has been taken using high precision GPS. Many Himalayas glaciers do not have clean surfaces as these are covered with varying amounts of moraine cover, consisting of dust, silts sands, gravel, cobbles and boulders. Moraine cover is one of the most important components of a glacier system in view of the control it exercises on rate of glacier melting. Its areal cover and thickness should be known in order to estimate effect of climate on retreat of glaciers. Himalayan glaciers show considerable changes over a period of time but the changes differ from one glacier to other and one geographical region to other. There are many glaciers which do not indicate any change in the area. Some glaciers in Karakoram ranges also show a little advancement. The response time of glaciers to adjust to changes in its mass balance varies from one glacier to other and therefore the climatic variations are never reflected on retreat or advance of glaciers immediately. In order to conclude on the human induced climate variations based on changes taking place on the snow and ice of Himalayas, a regular monitoring of snow and ice is required. Modeling effect of terrain parameters on the retreat or advance of glaciers is essential to predict any future changes for a large number of glaciers of Himalayas. The study must culminate in finding discharges of rivers originating from glaciers and our preparedness for such changes.