likely to continue to increase in the Indo-Gangetic Plain and Himalayas. Unless these trends are stabilized or reversed, the impacts of greenhouse gases and black carbon on the rate of Himalayan glacial retreat will increase. That is, both the rate and volume of the glacial retreat will be relatively greater than it would be otherwise.

• The rate of retreat and growth of individual glaciers is highly dependent on glacier characteristics and location. The most vulnerable glaciers are small glaciers at low elevation and with little debris cover. These characteristics also make glaciers more susceptible to black carbon deposition, and model results indicate black carbon deposition may make them more vulnerable to retreat.

• 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.

• In the short term, climate change is likely to increase glacial wastage. In the longer term the impact of continued retreat of glaciers is not clear. The rate of glacial retreat depends not only on temperature, but also on precipitation changes associated with the summer monsoon in the central and eastern HKH and the winter westerlies in the western HKH. Black carbon aerosols, via atmospheric heating and deposition on snowpack and glaciers, may increase the rate of glacial wastage.

• 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 glacier wastage. Recent literature indicates that the importance of the glacial contribution to runoff has previously been overestimated.

• 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. For example, in the late summer when all snow has melted and the monsoon-rainfall contribution is declining or in the eastern HKH during times of drought.

• Although retreating glaciers will subsidize surface flow and mitigate immediate losses to discharge by retreating glaciers, the loss of glacier “insurance” becomes more problematic for flows in the upper reaches of the eastern HKH over the long term.

• In the western HKH where more of the surface-water flow is from higher elevations, the contribution of glacial wastage could be particularly important in affecting the timing and volume of surface-water discharge.

• Overall, retreating glaciers over the next several decades are unlikely to cause significant change in flows at lower elevations, which depend primarily on monsoon rain. However, for high-elevation areas, current glacial retreat rates, if they continue, appear to be sufficient to alter the seasonal and temporal streamflow in some basins. Removing water stored as glacial ice does not imply any a priori effect on average annual discharge in the long term, assuming annual precipitation remains the same. In the short term with constant annual precipitation, glacial wastage will augment the quantity of streamflow.

• Limited streamflow data in upper basin regions, along with government constraints on scientific access to international streamflow data, increase the uncertainties surrounding hydrological trends, variability, and extreme events in the region. Limited streamflow data also limit the understanding of the relative contributions of rain, snowmelt, and glacial meltwater, as well as groundwater recharge mechanisms in the region.

• Uncertainties in the role of groundwater in the overall hydrology of the region are even greater than those of surface water. Current understanding of groundwater in the region is confounded by a variety of limitations including knowledge gaps about the interaction between surface water and groundwater; difficult terrain; the fractured and variable nature of the underlying geological substrate; and the inability to easily distinguish the contributions of snowmelt, glacial meltwater, monsoonal precipitation, and human actions such as groundwater overdraft to flows. 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, GLOFs and LLOFs are the dominant physical hazard risk. For downstream populations in the central and eastern Himalayas, floods from changes in monsoon rainfall and cyclones are more likely to be important, along with changes in the timing of extreme events.



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