term (decadal time frame) will subsidize surface flows by melting water held in storage, mitigating immediate losses to discharge by retreating glaciers (Kaser et al., 2010).
As noted in Chapter 2, paleoclimate records suggest a mixed record of wetness and dryness during the 20th century in the monsoon-dominated eastern HKH and hydrological modeling indicates that glacial melt is not a major contributor to river systems in the east (i.e., the Ganges, Yangtze, and Yellow). Thus, for the eastern HKH, these factors could result in little change to annual surface-water discharge, but could result in the loss of “insurance” water that glacial melt provides for water security during times of drought. In the western HKH, paleoclimate records indicate a trend toward wetter conditions in the 20th century and hydrological models indicate that glacial melt is much more important in the west (i.e., the Indus Basin). Thus, the consequences of climate change to water security could be large if a reduction in available surface water either annually and/or seasonally occurs in the western HKH. However, the trend toward wetter conditions in the western HKH confounds this assessment.
During situations such as these, groundwater, a significant amount of which is supplied to the major river plains of the region by the Himalayas, will be looked to as a source to offset water scarcity. Thus, water security issues for lowland populations over the next decade are more likely to come from overdrafting of groundwater resources than changes in discharge from retreating glaciers.
Although a greater understanding of the glaciers of the HKH region will inform knowledge about water security in the region, improved understanding of the science of the glaciers is itself not sufficient to answer all questions about the relationship between the hydrology, the population, and the policies and politics of the region. As discussed in Chapter 3, social changes are affecting water use at a greater rate than environmental factors are affecting the availability of water. For example, rising standards of living, including improving and changing diets and greater energy use, will have a significant effect on water-use patterns over the coming decades. Even if streamflow remains relatively stable in the short term, human factors could lead to water scarcity. Changing standards of living could also influence vulnerability to natural hazards.
Although economic development could reduce adverse outcomes, including loss of life, monetary loss could increase.
When considering the link between humans and the environment in the context of water security in the HKH region, four themes emerge: (1) there is significant variability in the climate, hydrology, and glacier behavior as well as the demographics and water-use patterns of the region; (2) uncertainties exist and will continue to exist in both the physical and social systems; (3) to reduce and respond to this uncertainty there is a need for improved monitoring of both the physical and social systems; and (4) in the face of uncertainty, the most compelling need is to improve water management and hazards mitigation systems.
Theme 1: There is significant variability in the climate, hydrology, and glacier behavior in the region as well as the demographics and water-use patterns within the region. The retreat rates of Himalayan glaciers vary over time and space, with the rate of retreat being higher in the east than the west. There are confounding factors such as dust and black carbon that will affect glacial melt and in some cases increase glacial wastage. Changes in the monsoon will probably be more important than changes in glacial wastage at lower, downstream elevations. Rates of urbanization vary across the region, as does the portion of the population with access to improved water and sanitation.
Theme 2: Uncertainties exist and will continue to exist in both the physical and social systems. The impact of future climate change is uncertain but will probably accelerate rates of glacial retreat. Accelerated glacial retreat rates will have significant impacts in local, high-mountain areas but will probably not be very important downstream. As the region’s population becomes more urbanized and standards of living change, water-use patterns will also change in ways that will be difficult to predict. Existing demographic methods to not allow for projections at sufficient spatial resolution to determine whether, for example, certain basins and elevation zones will experience higher rates of population growth than others and how the demographic composition of those specific areas will change. In both the physical and social systems, stationarity—the assumption that