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Introduction

Many glaciers and snowpacks around the world are receding. This recession results from glacial ablation (melt and sublimation) rates that exceed the rates of glacial formation and accretion from precipitation over time. The rates and timing of glacial retreat vary across hemispheres, regions, and locales. The causes of such retreat are complex and although climatic warming is an important cause, care must be taken to recognize that in most instances there are multiple influences that interact in ways that are difficult to predict.

Initially, glacial wasting increases the volumes of glacial meltwater in downstream waterways. As a glacier continues to waste, a point will be reached at which the volumes of meltwater begin to decline and ultimately become zero when the glacier has disappeared (although precipitation falling on the land area previously occupied by the glacier will continue to contribute to downstream flows). Wherever glaciers are wasting continuously, there are concerns about the consequences for available water supplies.

Mountains are the water towers of the world and in many regions, the volume and timing of streamflow from glacial and snowmelt are critical for agricultural production, hydropower generation, water supply, and the functioning and health of ecosystems. In the Hindu Kush-Himalayan (HKH) region (Figure 1.1) in particular, the large populations of China and South Asia rely on both the water and electricity generation provided by the major rivers. Many basins in the HKH region are “water stressed” (Figure 1.2), and this stress is projected to increase due to large forecasted population growth. This has led to concern about potential negative impacts from changes in the availability of water supplies in the coming decades.

Although many glaciers across the globe have retreated over recent decades, the rates of retreat and mass loss can vary widely, indicating that a variety of regional-scale factors such as changes in circulation, cloudiness, precipitation, aerosol concentration, glacier geomorphology, tectonic activity, and debris cover, in addition to warming, can affect glaciers. A recent assessment of glaciers in the region using a combination of remote sensing and in situ data found different patterns of glacial retreat within the HKH region (Yao et al., 2012). Thus, a diverse range of factors affects the timing and amount of glacier-fed streamflow, with significant potential implications for water availability and regional stability. A recent Intelligence Community Assessment found that South Asia is one of three regions likely to be challenged by water scarcity in the coming decades (DNI, 2012).

In the HKH region, conventional wisdom is that glacial meltwater is an important supplement to naturally occurring runoff from precipitation and snowmelt. While correct, the watersheds of the area each exhibit complex hydrology, and the magnitude of the contribution of glacial meltwater to the total water supply in these rivers is less clear. The implications of accelerated rates of glacial wastage for downstream populations have not been precisely characterized.

Despite this fact, or perhaps because of it, there is confusion about how changes in the climate will affect the timing, character, and rates of glacial wastage in different parts of the HKH region. Scientific evidence indicates that glaciers in the eastern and central part of



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1 Introduction M any glaciers and snowpacks around the tive impacts from changes in the availability of water world are receding. This recession results supplies in the coming decades. from glacial ablation (melt and sublima- Although many glaciers across the globe have tion) rates that exceed the rates of glacial formation retreated over recent decades, the rates of retreat and and accretion from precipitation over time. The rates mass loss can vary widely, indicating that a variety of and timing of glacial retreat vary across hemispheres, regional-scale factors such as changes in circulation, regions, and locales. The causes of such retreat are com- cloudiness, precipitation, aerosol concentration, glacier plex and although climatic warming is an important geomorphology, tectonic activity, and debris cover, cause, care must be taken to recognize that in most in addition to warming, can affect glaciers. A recent instances there are multiple influences that interact in assessment of glaciers in the region using a combina- ways that are difficult to predict. tion of remote sensing and in situ data found different Initially, glacial wasting increases the volumes patterns of glacial retreat within the HKH region (Yao of glacial meltwater in downstream waterways. As a et al., 2012). Thus, a diverse range of factors affects glacier continues to waste, a point will be reached at the timing and amount of glacier-fed streamflow, with which the volumes of meltwater begin to decline and significant potential implications for water availability ultimately become zero when the glacier has disap- and regional stability. A recent Intelligence Commu- peared (although precipitation falling on the land area nity Assessment found that South Asia is one of three previously occupied by the glacier will continue to regions likely to be challenged by water scarcity in the contribute to downstream flows). Wherever glaciers coming decades (DNI, 2012). are wasting continuously, there are concerns about the In the HKH region, conventional wisdom is that consequences for available water supplies. glacial meltwater is an important supplement to natu- Mountains are the water towers of the world and rally occurring runoff from precipitation and snowmelt. in many regions, the volume and timing of streamflow While correct, the watersheds of the area each exhibit from glacial and snowmelt are critical for agricultural complex hydrology, and the magnitude of the contri- production, hydropower generation, water supply, bution of glacial meltwater to the total water supply in and the functioning and health of ecosystems. In the these rivers is less clear. The implications of accelerated Hindu Kush-Himalayan (HKH) region (Figure 1.1) rates of glacial wastage for downstream populations in particular, the large populations of China and South have not been precisely characterized. Asia rely on both the water and electricity generation Despite this fact, or perhaps because of it, there is provided by the major rivers. Many basins in the HKH confusion about how changes in the climate will affect region are "water stressed" (Figure 1.2), and this stress is the timing, character, and rates of glacial wastage in projected to increase due to large forecasted population different parts of the HKH region. Scientific evidence growth. This has led to concern about potential nega- indicates that glaciers in the eastern and central part of 7

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8 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY FIGURE 1.1 The Hindu-Kush Himalayan region extends over 2,000 km across South Asia and includes all or parts of Afghanistan, Bangladesh, Bhutan, China, India, Nepal, and Pakistan. The region is the source of many of Asia's major rivers, including the Indus, Ganges, and Brahmaputra. this region are retreating at rates comparable to those of accelerated glacial wastage lie with the Medieval in other parts of the world, but are relatively stable, and Warm Period1 that occurred over a thousand years ago. perhaps even advancing, in parts of the western Hima- Neither of these assertions is supported by scientific layas. These findings, the findings of recent reviews evidence, and they have served to create unnecessary by the U.S. Agency for International Development confusion. There is also confusion about the ultimate (USAID, 2010) and the International Centre for Inte- implications of these changes for domestic, industrial, grated Mountain Development (ICIMOD, 2011b), 1 The Medieval Warm Period was a period of warmer climate and evidence presented later in this report bring into in much of northern Europe, the North Atlantic, southern Green- question the results of several earlier studies: one sug- land, and Iceland. It lasted from roughly A.D. 950 to 1250. It is gested that the glaciers of the HKH would disappear the "Medieval" Warm Period because it coincided with Europe's altogether by 2035; another suggested that the origins Middle Ages (AMS, 2000).

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INTRODUCTION 9 FIGURE 1.2 Global map of water-stressed basins. The water stress indicator is the ratio of total water withdrawals to calculated in-stream flow requirements. Many basins in the HKH region have "high" levels of water stress. SOURCE: Smakhtin (2008). and agricultural water availability as well as for the Would the deglaciation of the HKH region various in-stream ecological uses. imply that the area is headed toward water scarcity In addition, in the context of incomplete science within the next few decades? and unresolved uncertainties, there are other important Over the next several decades,2 how will these questions about regional water security that need to be high-altitude changes in climate and hydrology com- addressed. From the beginning the Committee was pare, and interact with, other economic, social, and mindful that it would need to sort out these confusions demographic impacts on regional water supply and and resolve any resultant misunderstandings in order demand? to successfully address the statement of task (Box 1.1). Does the retreat or accelerated retreat of HKH Included among the questions examined by the Com- glaciers increase the likelihood of natural disasters mittee in the course of addressing their charge are the such as outburst floods from moraine dammed lakes following: or seasonal flooding? What are the rates of retreat of the Himalayan Available science does offer some important informa- glaciers over the last decades and how do they compare tion that leads to conclusions that are relevant to these with the rates of retreat of glaciers elsewhere in the questions. world? Are the rates of glacial retreat in the study region STUDY CONTEXT AND accelerating, decelerating, or remaining static? CHARGE TO THE COMMITTEE What proportion of the seasonal flows of the Indus and Ganges/Brahmaputra rivers are accounted This report was prepared by the Committee on for by glacier melt? Himalayan Glaciers, Hydrology, Climate Change, What has been the impact of recent climate change on both glacial wastage and streamflow in the 2 Wherever possible, the Committee has attempted to limit its region? projections to the order of three to four decades at most. However, in some cases this was not possible. For example, decadal-scale Has the relative contribution of glacier melt- climate projections are currently an area of active research, and the water to dry-season or wet-season flows increased, climate modeling community has more skill in projecting climate decreased, or remained static? over longer timescales.

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10 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY and Implications for Water Security, appointed by the BOX 1.1 Committee on National Research Council (NRC) in response to a Himalayan Glaciers, Hydrology, request from the intelligence community, to address Climate Change, and Implications the broad charge of identifying what is known scientifi- for Water Security Statement Of Task cally about the glaciers of the Himalayas, their likely The Committee was charged to explore the potential im- future, and the implications of that future on down- pacts of climate change on glacier-fed streamflow and regional stream water supplies and populations (see Box 1.1 water supplies in one region, the Himalayas. The glaciers in this for full Statement of Task). Part of its purpose was to region are the headwaters of several of Asia's great river systems, including the Ganges/Brahmaputra, Indus, Mekong, Yangtze, and clarify, where possible, many of the misunderstandings Yellow Rivers. These rivers are the sources of drinking water and surrounding the broad topic of glacial retreat and its irrigation supplies for billions of people. These rivers also gener- implications in the Himalayas and surrounding region. ate hydropower and support important ecological values, such as Recognition of the significance of potential impacts of fisheries. The Committee was asked to summarize the current state climate change on glacier-fed streamflow and regional of scientific understanding on questions such as water supplies in the Himalayas led to the commission- How sensitive are the Himalayan glaciers to changes in ing of this study. temperature, precipitation, and the surface energy budget? What does current glaciological and climatological knowl- edge imply about potential changes in climate on downstream GEOGRAPHIC SCOPE flows? What are the likely major impacts on water supplies and flood regimes? What additional observational and modeling The focus of this study is the HKH region, which resources are needed to improve knowledge of hydro-climate extends over 2,000 km from east to west across the trends and forecasts? Asian continent spanning several countries (Figure What management systems (including water supply, water 1.1): Afghanistan, Bangladesh, Bhutan, China, India, demand, land use, and other institutions and infrastructure) are in Nepal, and Pakistan. The eastern and western areas of place to manage climate-induced changes in regional hydrology, and how might they be strengthened? In addressing this question, the HKH region differ in climate, especially in timing the Committee will analyze the advantages and disadvantages of and type of precipitation, and in glacier behavior and various options for improving existing management systems, dynamics as well. For glaciers, there is no sharp dividing which could include consideration of new management systems, line between east and west; rather, conditions change but will not recommend specific options. gradually across the region (e.g., glacier mean elevation; What are some of the main vulnerabilities to adjusting Scherler et al., 2011a). The eastern end is dominated by to changes in water supply in these downstream areas? What are the prospects for increased competition, or improved cooperation, monsoonal activity in summer while the western end between different downstream water users? What are some of the is dominated by the mid-latitude westerlies3 in winter implications for national security? (Thayyen and Gergan, 2010). The monsoon declines To inform its analysis, the study Committee was supported in strength from east to west along the Himalayas, by information gathered at an interdisciplinary workshop, using while the westerlies weaken as they move from west both invited presentations and discussion to explore the issues to east. This gradient divides at about 78 E near the that may affect regional streamflow and water supplies in the face Sutlej valley (Bookhagen and Burbank, 2010). To the of a changing climate. The Committee examined a few selected west (Afghanistan; Kashmir and Jammu, India; western examples of changes in glacial melt and resultant streamflow for Nepal; Pakistan), there is a general pattern of winter a specified time horizon as a way of developing boundaries for the workshop discussion. The thinking at the workshop took the form accumulation and summer melt, similar to glaciers in of linking potential changes in temperature and precipitation to a North America and Europe. In contrast, to the east range of changes in glacial mass balance and regional hydrology, (Bhutan; Sikkim, India; eastern Nepal), glaciers are which in turn could lead to a range of outcomes for downstream summer accumulation types, where both maximum streamflow and water security, including water supplies and flooding regimes. 3 The mid-latitude westerlies (westerlies) are the dominant west-to-east motion of the atmosphere, centered over the middle latitudes of both hemispheres. At the Earth's surface, they extend from roughly 35 to 65 latitude (AMS, 2000).

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INTRODUCTION 11 accumulation and maximum ablation4 occur during cal boundaries are disputed, and boundary disputes are the summer. River discharge5 in the eastern end of the likely to remain a potential source of instability for the HKH arc appears to be driven by monsoonal activity. indefinite future. Large areas of the Indus river systems are dominated The glaciers of the region are found in the head by winter snowfall from the westerlies. Monsoon activ- waters of several of Asia's great river systems, including ity is lower and precipitation is dominated by winter the Indus, Ganges/Brahmaputra, Mekong, Yangtze, snow in the upper Indus, Jhelum, and Chenab basins. and Yellow Rivers. These rivers are the source of drink- Ladakh, which extends from Tibet to India, is a unique ing water and irrigation supplies for roughly 1.5 billion cold-arid region (Thayyen and Gergan, 2010). people. They also generate significant quantities of The political and hydrological boundaries within hydroelectric power and support important ecological the study areas are both important and add useful clar- and cultural amenities and services. The surface water ity to the study area definition. The political boundaries of these rivers and associated groundwater constitute in the region are shown in Figure 1.3a. The countries a significant strategic resource for all of Asia, which found in the study area include all or portions of is among the most water stressed regions of the world Afghanistan, Bangladesh, Bhutan, China, India, Nepal, (Smakhtin, 2008). and Pakistan. As the map indicates, some of the politi- The specific study area for this report includes the catchments of the Indus and Ganges/Brahmaputra rivers. The committee defined the primary scope of 4 Ablation includes any process that removes ice from a glacier. 5 River or stream discharge is the volumetric rate of flow (AMS, their report as the countries that include all or part of 2000). the Indus, Ganges, or Brahmaputra basins, including (a) FIGURE 1.3 The (a) political boundaries, (b) hydrology, and (c) elevation of the Hindu-Kush Himalayan region. Dashed lines are used to indicate disputed political boundaries, which follow the guidance issued by the U.S. State Department, and this Committee takes no position on these boundary disputes. continued

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12 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY (b) (c) FIGURE 1.3 Continued

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INTRODUCTION 13 the portion of the Tibetan Plateau included in those Although the Mekong originates on the Tibetan basins. According to the committee's charge, the areas Plateau and is the source of water for the populations of of major concern are those where the water supply for Cambodia, Laos, Thailand, and Vietnam, glaciers are a significant population could be influenced by changes a small component of water resources in the Mekong in the region's glaciers. Figure 1.4 shows the percentage (Figure 1.4). The Yangtze and Yellow Rivers, which of glaciated area within the river basins of the HKH provide water for large parts of China, also originate region. The Indus, Ganges, and Brahmaputra basins on the Plateau and have relatively small glacial coverage combined contain nearly three-quarters of the region's (Figure 1.4). Because the Mekong, Yangtze, and Yellow glaciated area. Except for the interior basins (labeled River basins contain a relatively small amount of glacier as Himalayan Endorheic basins in Figure 1.3c), the area, it follows that discharge into these rivers results other river basins each contain less than 5 percent of from snowmelt and rainfall. As discussed in Chapter 2, the region's glaciated area. The Committee includes in most basins, the contribution of rain far outweighs some discussion of the part of the region covered by the the contributions of snowmelt (e.g., Andermann et Endorheic basins in Chapter 2, Physical Geography, al., 2012). These three rivers are located in the eastern particularly in the section on ice core data (see Figure part of the HKH region, where annual precipitation 2.14). Because the population density of the Tibetan is dominated by the summer monsoon (Bolch et al., Plateau is very small (see Figure 3.1), it follows that 2012). On the basis of these considerations, discharge very few people depend on the interior basins for water into the Mekong, Yangtze, and Yellow Rivers is more supply, and any glacier-related changes to the water influenced by the monsoons, and in the future, climate supply in those basins would not affect a large number change effects on the monsoon will play a greater role of people. Thus the focus of Chapter 3, Human Geog- than any changes in the glaciers. raphy and Water Resources is on the Indus, Ganges, It is common to refer to the Tsangpo/Brahmaputra and Brahmaputra basins. to indicate that the upstream portion of the river on FIGURE 1.4 Glacier area and percentage of total HKH glaciated area in each of the region's river basins. Only the Indus, Ganges, Brahmaputra, and interior basins of the Tibetan Plateau contain more than 5 percent of the region's glacier area. SOURCE: ICIMOD (2011b).

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14 HIMALAYAN GLACIERS: CLIMATE CHANGE, WATER RESOURCES, AND WATER SECURITY the Tibetan Plateau is called Tsangpo, while the Brah- its analysis, the Committee organized an interdisci- maputra is the proper name of the river once it enters plinary workshop, using both invited presentations India. To avoid confusion, the river is referred to as the and discussion to explore the issues that may affect Brahmaputra throughout this report. In this report, regional streamflow and water supplies in the face of when discussing issues on a watershed basis, the Gan- a changing climate. The workshop, which was held in ges and Brahmaputra are grouped as a single watershed fall 2011 in Washington, D.C., was organized around because they eventually merge before draining into the four broad themes: (1) regional climate and meteorol- Bay of Bengal. However, when discussing hydrological ogy; (2) regional hydrology and water supply, use, and issues--for example, river hydrographs6--the Ganges management; (3) regional demography and security; and Brahmaputra are treated separately. The tributaries and (4) risk factors and vulnerabilities. The workshop of the Indus and the Ganges/Brahmaputra Rivers are agenda and participants are included in Appendix A, depicted in Figure 1.3b. and workshop presentation summaries are included The HKH region is geographically vast and com- in Appendix B. Workshop participants identified key plex both climatologically and hydrologically, and this concepts about the HKH region. Starting from those complexity is dynamic and possibly changing. This concepts, the Committee used its expert judgment, means that it is very difficult to generalize observations reviews of the literature, and deliberation to develop and findings over the entire region because spatial vari- conclusions about the physical geography, human ability is large. The story that emerges in this report is geography and water resources, and environmental risk also characterized by the fact that there remain many and security in the HKH region. These conclusions are open science questions that cannot be answered in listed at the end of each chapter. the absence of additional data and research. Although global temperatures are increasing generally, and these ORGANIZATION OF THE REPORT increases have been more rapid in recent decades, spatial variability and lack of local research data mean This report covers three broad areas of knowledge that the specific manifestations of climate change are about the Himalayan region: (1) physical geography, unclear in the HKH region. This includes how quickly (2) human geography and water resources, and (3) and regionally glaciers might retreat, and what the environmental risk and security. Chapter 2, Physical subsequent impacts on the hydrological system of the Geography, provides an overview of glaciers, followed HKH region might be. In recognition of the complexi- by a summary of the climate and meteorology of the ties of these issues, NRC formed the Committee on region within the context of paleoclimate patterns, Himalayan Glaciers, Hydrology, Climate Change, and and descriptions of the regional hydrology and physi- Implications for Water Security to begin to address cal hazards. Chapter 3, Human Geography and Water some of these important questions. Resources, covers population distribution, poverty and migration, and key natural resource issues of water use, STUDY APPROACH AND METHODOLOGY access to water, water scarcity, and water management. Chapter 4, Environmental Risk and Security, pre The Committee was formed in summer 2011 sents the Committee's further analyses of the linkages and completed its work over the course of the next 12 between physical and human systems, with an emphasis months. It held four meetings during which it reviewed on those that may pose potential instabilities for the relevant literature and other information. To inform region. Chapter 5 presents the Committee's synthe- sis of the range of physical and social changes facing 6 A hydrograph is a record and graphical representation of river the region, a summary of research questions and data or stream discharge as a function of time at a specific location needs, and options for adapting to the changes facing (AMS, 2000). the region.