systems. The following are more specific and detailed conclusions that relate to the questions in the Committee’s charge (the full charge can be found in Box 1.1 of the main text of the report).
How sensitive are the Himalayan glaciers to changes in temperature, precipitation, and the surface energy budget?
The climate of the Himalayas is not uniform and is strongly influenced by the South Asian monsoon in the east and the mid-latitude westerlies in the west. Evidence suggests that the eastern Himalayas and the Tibetan Plateau are warming, and this trend is more pronounced at higher elevations; however, the long-term significance of this trend is not clear. Absorbing aerosols such as desert dust and black carbon may contribute to the rapid warming of the atmosphere, and model results indicate this may in turn contribute to accelerated melting of snowpack and glacial retreat.
The rate of retreat and growth of individual glaciers is highly dependent on glacier characteristics and location. In the eastern and central Himalayas, there is evidence of glacial retreat with rates accelerating over the past century. Retreat rates are comparable to other areas of the world. Glaciers in the western Himalayas appear to be more stable overall, with evidence that some may even be advancing.
What does current glaciological and climatological knowledge imply about potential changes in climate on downstream flows? What are the likely major impacts on water supplies and flood regimes?
Surface water flow is highly seasonal and varies across the region, as does the relative importance of glacial meltwater. In most instances, the annual contribution of snowmelt and rainfall to streamflow exceeds that of glacial wastage. The contribution of glacial wastage can be more important when the glacial wastage acts as a buffer against hydrological impacts brought about by a changing climate. Overall, retreating glaciers over the next several decades are unlikely to cause significant change in water availability at lower elevations, which depend primarily on monsoon precipitation and snowmelt. However, for high-elevation areas, current rates of glacial retreat, if they continue, appear to be sufficient to alter the seasonal and temporal streamflow in some basins.
Uncertainties in the role of groundwater in the overall hydrology of the region are even greater than those of surface water. Evidence suggests that sizable and extensive overdraft in the central Ganges Basin is likely to have an earlier and larger impact on water supplies than foreseeable changes in glacial wastage. For upstream populations, glacial lake outburst floods and landslide lake outburst floods are the dominant physical hazard risks. For downstream populations in the central and eastern Himalayas, floods from changes in monsoon dynamics are more likely to be important, along with changes in the timing of extreme events.
What management systems (including water supply, water demand, land use, and other institutions and infrastructure) are in place to manage climate-induced changes in regional hydrology, and how might they be strengthened?
Water resources management and provision of clean water and sanitation are already a challenge in the HKH region. The adequacy and effectiveness of existing water management institutions is a reasonable, if coarse, indicator of how the region is likely to cope with changes in water supply. Changes in seasonal stream-flow could have significant impacts on the local populations by altering water availability patterns and affecting water management decisions and policies for irrigation, municipal, industrial, and environmental use.
Current efforts that focus on natural hazards and disaster reduction in the region can offer useful lessons when considering and addressing the potential for impacts resulting from glacial retreat and changes in snowmelt processes in the region. Water management assessments have advanced over the past 5 to 10 years, though their implementation in water policies and programs is less clear; to date, there is limited penetration to lower levels of governance or support for local water managers who are most at risk. Changes to the hydrological system are inevitable and adaptation is needed at all levels of governance and throughout societies from rural household to city level. Adaptation approaches need to be flexible enough to change with changing conditions, for example, smaller scale and lower cost water management systems, because of uncertainty in impacts and the dynamic nature of future changes.