There is no doubt that people and property for considerable distances downstream from the unstable lakes are facing a serious threat; the problem, however, is how to determine the degree of probability of such an event. Analysis of the rapidly growing worldwide literature, including field and theoretical knowledge, on the outburst of glacial lakes, led a recent ICIMOD commission on GLOFs in Nepal to conclude that it is not feasible to make a reliable prediction of a specific occurrence on the basis of existing knowledge ( ICIMOD, 2011a). Because direct predictions cannot be made, a careful selection of prioritized lakes needs to be monitored on a regular basis.
GLOFs are not the only outburst lake hazards in the HKH region. Another is the landslide lake outburst flood (LLOF), which is a catastrophic release of impounded water from behind a natural dam formed by a landslide. In the steep mountainous Himalayas, landslides are a common event, whether they are triggered by normal weathering and erosion processes, extreme rainfall events, or earthquakes. The release potential of water by LLOFs can exceed that for GLOFs (Hewitt, 1982). This is because landslide dams can be very large. Dunning et al. (2006) describe a landslide dam that formed in Bhutan in 2003 and subsequently impounded 4 × 106 to 7 × 106 m3 of water before its failure in 2004. Landslide lakes can also occur at much lower elevations than glacial meltwater lakes, where they can impound runoff from larger upstream catchment areas compared with the areas contributing to glacial meltwater lakes. Hewitt (1982) provides a detailed history of outburst floods in the Karakoram, including some massive LLOFs, and Gupta and Sah (2008) present an example of a LLOF in the Satluj catchment in Himachal Pradesh, India, well below the termini of any glaciers above it.
Like GLOFs, LLOFs pose a serious hazard to people, property, and infrastructure downstream from the landslide dams. However, compared with the large number of glacial meltwater lakes forming today because of climate warming and glacial retreat, landslide lakes are less commonly formed in the high Himalayas (e.g., Hewitt, 1982). Therefore, the risk posed by future LLOFs is likely to be significantly less than that posed by GLOFs in the HKH region. In the lower trans-Himalayan regions where landslide lakes replace meltwater lakes as hazards (e.g., Gupta and Sah, 2008), LLOFs will be a greater future risk. That being said, the development of landslide lakes is almost certainly less predictable than meltwater lakes because landslides are more random and where they will occur is harder to predict than glacial retreats. Thus, LLOFs are even less predictable than GLOFs.
Key features of the physical geography of the HKH region were identified at the workshop by the breakout groups on Climate and Meteorology and on Hydrology, Water Supply, Use, and Management. Starting from those concepts, the Committee used its expert judgment, reviews of the literature, and deliberation to develop the following conclusions:
• The climate of the Himalayas is not uniform and is strongly influenced by the South Asian monsoon and the mid-latitude westerlies. Projecting impacts of climate change in the Himalayas is challenging because of complex interactions between global, regional, and local forcing and responses.
• Evidence suggests that the eastern Himalayas and the Tibetan Plateau are warming, and this trend is more pronounced at higher elevations. However, a lack of sufficient paleoclimate data makes assessing the long-term significance of this warming trend a challenge.
• There are sparse historical climate data in the region, but scientists are fairly confident about projections of future temperature increases. There is more uncertainty in projections of amounts and timing of precipitation.
• Aerosols from the combustion of fossil fuels, wood, and other sources are increasing in the Indo-Gangetic Plain and the foothills of the Himalayas. Absorbing aerosols such as desert dust and black carbon may contribute to the rapid warming of the atmosphere, and model results indicate that this may in turn contribute to accelerated melting of snowpack and retreat of glaciers. Black carbon deposited directly on non-debris-covered glaciers and snowpack could increase the rate of retreat by reduction of surface albedo.
• Over the next few decades, atmospheric concentrations of greenhouse gases are projected to continue to increase globally, while black carbon aerosols are