Global Climate modelling
The use and development of Global Climate Models (GCM's) has greatly improved our understanding of the impacts of increased greenhouse gas concentrations on the future climate. However the current generation of GCM's is not able to properly simulate to South Asian Monsoon and GCM model run still give conflicting results about the impact of higher greenhouse gas concentration on Indian rainfall. Also important Land Atmosphere feedbacks are currently not incorporated in most GCM’s. For example, feedback between (disappearing) snow cover and precipitation patterns are currently weakly represented in the models.
Climate change impact and glacier melting
Rivers draining from Himalayan headwater basins, in which precipitation is enhanced orographically, deliver large quantities of runoff to the major tributaries of the Ganga river and hence make substantial contributions to the water resources of northern India. Flows in such Himalayan rivers are derived from both contemporary precipitation and melting of accumulated snow and ice from glaciers. Runoff from glacierised mountain basins is important as a water resource not just in quantity. Glaciers effectively moderate intra-annual variations in river flow, in cooler wetter years by runoff arising from precipitation over the ice-free areas offsetting reduced glacier melt, and in warmer drier summers through enhanced melt making up for reduced precipitation. Glacier meltwater runoff is a particularly useful resource where it provides water in places and at times when other sources are scarce, for example in downstream arid areas or during hot dry seasons.
Depressions in the westerlies deliver significant amounts of winter precipitation to the Himalayan region of north India. Under the influence of the Asian monsoon, however, most of the annual precipitation occurs in summer. Monsoon precipitation generally declines from east to west, and from south to north, southfacing windward slopes receiving greater amounts of precipitation than shadowed leeward west-east valleys (e.g. Bookhagen & Burbank 2006). At higher elevations monsoonal precipitation falls as snow, so that overall ice ablation on glaciers in the Indian Himalaya is reduced in July and August by comparison with June and September.
Runoff from a highly-glacierised (> c40%) Himalayan basin shows two maxima, a build up of snowmelt from April then icemelt from late May into June, then slightly reduced flow with cloud and snow cover, building to a secondary peak in late August-September (e.g. Collins & Hasnain 1995). Downstream, as basin size increases, decreasing percentage glacierisation and increasing the influence of monsoon precipitation, flow is enhanced in July and August as quickflow from rainfall is superimposed on the glacial signal (~ 10% glacierisation e.g. Singh & Bengttson (2004)). Glacier runoff provides most of the flow downstream into the Ganga in the spring and autumn ‘shoulders’ before the onset and during the retreat of the monsoon (Barnett and others 2005).
During the period of sustained climatic warming since the Little Ice maximum extent of glaciers (in ~1850 in the European Alps), runoff from glaciers might be expected first to increase as a result of enhanced melting of snow and ice. Meltwater discharge will not however continue to increase indefinitely as, during progressively warmer summers, declining glacier extent will reduce the surface area of ice over which energy exchange can occur. As glaciers recede, a component of flow in excess of that related to contemporary precipitation, a deglaciation discharge dividend, is added to basin runoff from depletion of the amount of water stored as ice. That dividend will decline, and ultimately cease, as glaciers disappear altogether. Basin runoff will then solely reflect whatever the future level of precipitation. In the European Alps, maximum discharge from highly-glacierised basins was reached in the late 1940s-1950s, after which flows have declined (Collins in press).
Future decline in glacier runoff will affect both mountain villages as small glaciers disappear and the four - five hundred million inhabitants of the entire Ganga basin as spring and autumn flows in particular decline, at differing timescales. Reviews from the region suggest that the timescales are short, may be the 2040s (World Wildlife Fund 2005) or the 2050s (Xu Jianchu and others 2007), although monsoon precipitation in the Indian and Nepal Himalaya appears to stave off glacier reduction in the central and eastern Himalaya by comparison with the Karakoram in the west (Rees & Collins 2006).
In the Ganges, Indus and Brahmaputra basins, which are in particular susceptible to climate change (Singh and Bengtsson, 2004) millions of people rely on freshwater supply originating from melt-water of snow and glaciers. Demands of industry, energy, agriculture and domestic use already make the economy show signs of water stress. Together with increasing freshwater demand from industry, agriculture and domestic use, a decrease in baseflow will cause water stress and a reduction in freshwater availability. Climate change will be another factor adding to the stress (WWF, 2005).
As part of the National Communication (NATCOM) project undertaken by the Ministry of Environment and Forests, Government of India, Gosain et al 2006 showed for 12 large river basins in India, that under the GHG scenario, severity of droughts and intensity of floods in various parts of the country may get deteriorated. Moreover, a general reduction in the quantity of the available runoff has been predicted under a scenarios with increase GHG concentrations/emissions.
For the Satlui River basin (Himalaya), the average annual contribution of snowmelt and glacier melt run-off is estimated to be 60 percent (Singh and Jain, 2002). With increasinge temperatures due to global warming there will be an earlier response of snowmelt run-off. In general there will be more run-off during spring and and less during late summer. Some lower zones will produce less melt run-off as more precipitation will come down as rain, but higher zones will initially produce more run-off because more snow will melt during the summer. The higher zones will counterbalance the decrease in run-off from the lower zones by glacial melt run-off, but this will only last for several decades.
Under a warmer climate, water availability tends to be reduced for all the seasons except spring (Singh and Bengtsson, 2004). The peak of water demand is in summer and due to higher temperatures demands for hydropower, irrigation and cooling will increase. So a decrease in water availability in summer will have serious implications for water resources in India.
The hydrological characteristics of the watersheds in the Himalayan region have undergone significant changes over the last decades. Urbanization, deforestation and more intensive agriculture have caused increased flooding, greater variability in precipitation and run-off and sedimentation of reservoirs. Climate change will also have an impact on the region, resulting in additional threats. Due to more frequent and severe extreme precipitation events more landslides are likely to occur. The hydrological system will also respond to changes in precipitation and higher temperatures (e.g. melting of glaciers and snow). Also erosion and sedimentation processes may change due to ....??(Mall et al., 2006). Flash flooding will seriously affect people’s life. Flash floods will cause fatalities and put the infrastructure at risk. The glacial lakes with unstable natural dams of debris (moraine ridges) will burst, flooding the area downstream. Not only glacial lake outbursts (GLOFs) can cause flash floods but also extreme precipitation, failures of dams and landslides. Besides fatalities flash floods will also cause severe damage to people’s assets. Houses, stables and animals will be washed away.
Kumar and Parikh (2001) studied the relationship between climate change and farm revenue. For an increase of 2 C in temperature and a increase of 7 percent in precipitation, farm revenue would decrease with 8.4 percent. Regional food supplies might be affected and be a cause for malnutrition (Patz et al., 2005).
With an increasing population and increasing water demand, water stress will further increase irrespective of climate change. Yield per hectare have to increase to meet the future Indian food demands. Grain yields have to increase from 1.7 ton per hectare in the 1990s to 2.2-3.5 ton per hectare (scenario IV- scenario I and III) in the 2050s (TERI).
In the past farmers lost faith in large irrigation projects and large irrigation canals and they switched to extracting groundwater which is a more reliable source. Pumping up groundwater also has been encouraged because it would check waterlogging in the Ganga plains which was said to be caused by the irrigation canals. However, pumping up groundwater has lowered groundwater tables and increased salinity of the Ganga and Indus plains.
For the next decades the growth of the economy is projected to remain at a high rate and the population growth rate will stabilize. The expectations foresee a higher standard of living due to economic growth and large investments in health care, water and sanitation. However, constraints to the growth are that investments in health care and education have to be done sufficiently and timely. Another constraint is food sufficiency and water quality. Access to sanitation and education is especially important for the poor rural population (MNP-IMP, 2007).
Land and water resources are already subject to severe conflicts from village to riverbasin level. “Irrigation systems based on a snow-fed Indo-Gangetic plain river are most vulnerable to global climate change, especially since much of its potential is already tapped. Any reduction in trans-boundary flows could increase water conflicts with neighbouring Bangladesh and possibly Pakistan.” The scope for increasing storage of surface water is limited and already there is growing opposition to developing new water resources due to issues of resettlement. Inter-state water disputes are likely to increase the pressure on existing water resources. Per capita water resource availability has already reached critical limits in many river basins.” (MNP-IMP, 2007).
Cases of poor water quality in the Himalayan region are related to poor sanitation facilities. In the rural areas the water is easily contaminated with pathogens because defecation happens in the open. Extreme dry periods due to climate change might limit the access to safe fresh water even further. This will increase the risk of water-borne diseases and of people using water of a poorer quality (Eriksson, 1996).