This book is a Discussion Paper issued by the Ministry of Environment and Forests, Government of India, and issued earlier this year. Dr. Raina is an ex-Deputy Director of the Geological Survey of India, he spent many years studying the glaciers of the Himalayas, and has now produced a splendid over-view of the situation. He describes the history in investigation, details of glaciological studies, conclusions drawn from the studies, and finally a review of Global Warming and Glacier Retreat.
Here I shall refer to the main facts presented, and the basic conclusions, but put the emphasis on the last section, climate change, because the publication has already aroused vitriolic reactions from some quarters.
Himalayan glaciers show variable behaviour over the past hundred years. Most have retreated, some have stayed almost static, and some have a record of advance and retreat. This parallels the rest of the world, where most glaciers have been retreating since the end of the last glacial period. Many have shown alternating periods of advance and retreat.
Many people nowadays attribute the glacial retreat to anthropogenic global warming (AGW). But how can we tell when AGW started to affect the issue, or could all the changes we see be entirely natural, as they have to be for the pre-nineteenth century changes? Since glaciers have been retreating for thousands of years, why should the retreat of the past hundred years be attributed to a special cause? Can we find within the observed evidence some tests for deciding between natural and anthropogenic causes?
I believe that Dr Raina has provided the evidence, and interpreted it correctly.
Before getting into details, I shall explain the principles of glacier behaviour (which are also described in the book), so that the reader can understand the significance of the observations summarised later.
The Glacier Budget – why do glaciers advance or retreat
In general glaciers grow, flow and melt continuously with a budget of gains and losses. Snow falls on high ground, compacts, and becomes solid ice. More precipitation of snow forms another layer on the top, so the ice grows thicker by the addition of new layers at the surface. When the ice is thick enough it starts to flow under the force of gravity. A mountain glacier flows mainly downhill, but can flow uphill in places, as in the rotational flow that creates cup-shaped cirques. The flow of ice is generally slow, as expressed in the common metaphor "glacially slow", but the rate is variable. Some glaciers “surge”, meaning they have short periods of accelerated flow. Flow rate depends on stress and so on thickness of the ice, and a relatively small increase in thickness results in a large increase in flow rate.
When the ice reaches a lower altitude where temperature is higher it starts to melt and evaporate. (Evaporation and melting together are called ablation, but for simplicity I shall use 'melting' from now on). If growth and melting balance, the glacier appears to be 'stationary'. If precipitation exceeds melting the glacier grows and advances. If melting exceeds precipitation the glacier recedes. The position of the snout is the simplest indicator of where the balance lies, but does not indicate the cause.
Flow is mainly by a process called creep, essentially the movement of atoms from one crystal to another. Only the lower part of the glacier can flow plastically: the upper ice is brittle and cracks to form crevasses in the rigid ice carried along on the plastic lower ice.
Himalayan glaciers present yet another distinctive problem. Some mountain glaciers start from icecaps that flow at the edges, so there is continuous flow from the snow-collecting area to the glacier snout. In the Himalayas relief is great and the peaks are so sharp that snow falling on the peaks reaches the glaciers in the valleys via avalanches. So the growth of a glacier depends not just on the precipitation but on the frequency of avalanches. It could happen that increased temperature in the mountains caused increased avalanching, thickening the glaciers and cause increased flow of the glacier.
The Himalayan Glaciers
Raina divides the history of research into Himalayan glaciers into three phases. In the early phase from first exploration to 1957 there was the accumulation of much topographic detail. From 1957 to 1970 a more holistic approach was taken, ice thicknesses were observed, and understanding of glaciers improved. After 1970 a Hydrological Programme dominated the work on glaciers, which included measurements of glacier thickness, calculation of ice volumes for each basin, and mass balance assessment. It was found that the major factor for the retreat of glaciers is the relatively less snow precipitation during the winter, rather than enhanced melting in the summer. The glacier mass balance shows an inverse relation with monsoon precipitation.
There are thousands of glaciers in the Himalayas, and glaciers within the Himalayan region display different behaviour.
The main evidence for glacial retreat or advance is the changing position of the snout of glaciers. This is an easy and convenient thing to observe (though in the Himalayas it is harder than in most parts of the world), but as explained earlier the position of the snout depends on many factors and not just temperature.
Raina provides descriptions of large glaciers such as Siachen glacier (74 km long and the second longest outside Polar regions), and Gangotri, ( the largest in the central Himalayas and regarded as the source of Ganges) down to small ones just a few kilometres long.
The most fascinating accounts are of the growth and retreat of the different glaciers. Here is a sample:
• Gangotri was retreating at 20 m per year up to 2000, but then slowed, and since September 2007 has been at a standstill.
• Siachen glacier advanced 700 m between 1862 and 1909 and retreated 400 m between 1929 and 1958, since when retreat has been very small. The snout has retreated just 8-10 m since 1995.
• Other contrasts are that Sonapani glacier retreated 500 m in the last 100 yr, whereas Kangriz glacier shows virtually no retreat.
• The small (2km) Machoi Glacier has a continuous record of snout observations since 1875, and shows no major retreat in the last 50 years.
• Bhagirath glacier retreated 320 m between 1962 and 2005 (7.4 m/y), but only 1.5 m in 2006.
• In the early phase of Himalayan exploration glaciers were in general retreat, but even then Mirapin and Hassanabad advanced rapidly.
• In Kumaon three glaciers retreated, including Pindari (425 m in 57 years), but Poting glacier was stationary.
• Three surging glaciers of Kumdan behaved differently during 1958: Aktash and Chong Kumdan were advancing, but Kichik Kumdan was retreating.
• Chong Kumdan glacier has three limbs:
o The southern limb advanced 1.25 km up to 1990 and then retreated by 0.75