Glacial retreat on the Tibetan Plateau and surrounding regions is characteristic since the 1960s and has intensified in the past 10 yr. The magnitude of glacial retreat is relatively small in the interior of the Tibetan Plateau and increases to the margins of the plateau, with the greatest retreat around the edges. Glacial retreat in this region is impacting the hydrological processes in the Tibetan Plateau and surrounding regions. The glacial retreat has caused an increase of more than 5.5% in river runoff from the plateau. In some areas, such as the Tarim River basin, the increase in river runoff is greater. Glacial retreat has also caused rising lake levels in the areas with large coverage of glaciers, such as the Nam Co Lake and Selin Co Lake areas. Rising lake levels are devastating grasslands and villages near the lakes.
The Tibetan Plateau and surrounding regions contain about 46,300 glaciers, amounting to a total glacial area of about 59,400 km2, and a total glacial volume of about 5600 km3 (Table 1). These glaciers mainly concentrate around the Himalayan Mountains, Nyainqentanglha Mountains, Kunlun Mountains, Karakoram Mountains, and Tian Shan Mountains (Fig. 1, Table 1). The Tibetan Plateau itself contains 36,800 glaciers, with a total glacial area of 49,873 km2, and a total glacial volume of 4561 km3
Glaciers in mountain ranges in the Tibetan Plateau and surrounding regions based on the data in the Glacial Inventory of China (Shi, 2005).
The figures of glaciers and glacial area given above are based mainly on data obtained in the 1970s and 1980s referenced in the Glacier Inventory of China (Wang et al., 1991; Shi, 2005). Glacier volumes were estimated in the inventory from area and average depth for each glacier. The average depth was estimated by: H = −11.32 + 53.21S0.3 (where H is the average depth and S the area for the given glacier) based on field observations (Wang et al., 1991; Shi, 2005). In the 1980s, most of these glaciers retreated severely because of climatic warming. Some previously advancing glaciers shifted to retreat with rapid climatic warming. In the 1990s, glaciers retreated even more rapidly (Yao et al., 2004). At the same time, the runoff of some rivers in these regions had increased, largely due to glacial melting (Shi, 2001). This paper reports the most recent studies on glacial retreat in the Tibetan Plateau and surrounding regions and its impact on the hydrological processes on the Tibetan Plateau and surrounding regions.
The Characteristics of Glacial Retreat in the Tibetan Plateau and Surrounding Regions
The glacial retreat in the Tibetan Plateau and surrounding regions started in the early 20th century and became more and more intensive with climatic warming. Before the 1950s, most glaciers advanced, although some glaciers shifted from advance to retreat. After the 1950s, a large percentage of glaciers retreated, which continued to the 1970s. Studies suggested that more than half of the glaciers were retreating and about one third advancing at that time (Zhang et al., 1981; Ren, 1988; Shi et al., 2002), and the remainder were stable. Between the 1970s and 1980s, glacial mass balances were somewhat positive and snowlines dropped. With more advancing glaciers, fewer glaciers retreated during this period. From the 1980s to the 1990s, glaciers retreated rapidly again and only 10% of glaciers were advancing. From the 1990s on, glacial retreat was more extensive than during any other period in the 20th century. There are still some glaciers advancing (Table 2) as inferred from some satellite images (Shi et al., 2006). However, all the field expeditions in recent years reported few advancing glaciers. A glaciological expedition in 1989 reported that glaciers of the southeast Tibetan Plateau were retreating rapidly (Yao et al., 1991). The Zepu Glacier and Kaqing Glacier were examples of retreating glaciers. But some glaciers, such as the Large Dongkemadi Glacier and the Small Dongkemadi Glacier, were still advancing in 1989. Detailed study of the Large Dongkemadi Glacier and the Small Dongkemadi Glacier in the Tanggula Mountains and the Meikuang Glacier in the Kunlun Mountains (Fig. 1) showed that these glaciers continued to advance during those years (Pu et al., 1998; Yao, 2002). However, all these glaciers have shifted from advance to retreat since the 1990s and all these glaciers are currently retreating.
Proportions of advancing and retreating glaciers on the Tibetan Plateau and surrounding regions at different times.
Glacial retreat amplitude appears to be accelerating after the 1990s. There are two types of examples. The first example is more dramatic retreating of original retreating glaciers such as the Glacier No. 1, in the Urumqi River basin in the Tian Shan Mountains (Fig. 1). This glacier consists of two branches (east and west branches). Since the 1960s, Glacier No. 1 has been mapped nine times using large-scale cartography and plane table mapping (in 1962), ground photography (in 1973, 1980, 1986, and 1994) and GPS (in 2000 and 2002) by the Tian Shan Glaciological Station (Jiao et al., 2004), and using aerial photography (in 1964 and 1992) by Chen et al. (1996). Moreover, since 1986 the location of the glacier terminus has been measured every year. The ice tongues of east branch and west branch joined together in 1962, but the confluence area was thinning continuously as the two branches retreated. In 1993, the two branches were totally separated from each other and by 2001, the distance between the two branches exceeded 100 m. Glacier No. 1 retreated most rapidly from the early 1960s through the early 1970s (Fig. 2), with a retreat rate of 6.5 m yr−1. The rate of glacial retreat decreased significantly in the mid-1970s and reached a minimum in the early 1980s. Afterwards the retreat rate increased again, and reached the maximum of 6.6 m yr−1 between 1990 and 1991.
The second example is the Large and Small Dongkemadi Glaciers in the Tanggula Mountains (Yao, 2002). These two glaciers were both advancing when they were first observed in 1991 (Fig. 3). The area of the Large Dongkemadi Glacier was about 14.6 km2, and the area of the Small Dongkemadi Glacier was about 1.8 km2. Based on glaciological theory, there is a time lag for a glacier to respond to climatic change that depends on glacial size: the larger the glacier, the longer the lag-time. Therefore, the Small Dongkemadi Glacier should shift from advance to retreat earlier than the Large Dongkemadi Glacier. The Small Dongkemadi Glacier advanced about 4 m in the summer of 1994, and then shifted to retreat in 1995, with a retreat rate of only 0.2 m yr−1 that year. After 1995, the Small Dongkemadi Glacier kept retreating at an increasing rate, reaching 2.9 m yr−1 in 2000. The Large Dongkemadi Glacier advanced 15.7 m yr−1 between 1989 and early 1994, then shifted to retreat after the summer of 1997. The annual retreat of the Large Dongkemadi Glacier also increased continuously and reached 4.6 m yr−1 in 2001.
More and more studies are confirming the accelerating glacial retreat in the Tibetan Plateau, Ren et al. (1998, 2004) has reported glacial retreat of the Rongbuk Glacier in Mt. Qomolanma which was interpreted as an impact of global warming. Others not only reported glacial retreat (Pu et al., 1998, 2001), but also mass-balance fluctuations (Pu et al., 1999; Liu et al., 2000). Glacial retreat varies from region to region (Chen et al., 1996; Su et al., 1996, 1999; Liu et al., 2000, 2002; Pu et al., 2001; Wang and Liu, 2001; Jing et al., 2002; Lu et al., 2002; Yao et al., 2004). Figure 4a demonstrates the retreat of the observed glaciers in different regions. These observations show that the magnitude of glacial retreat generally decreases from the margin to the interior of the plateau, and reaches its minimum in the central Tibetan Plateau. The magnitude of glacial retreat was the largest in the Karakorum Mountains and the southeastern Tibetan Plateau. The annual retreat rate is nearly 50 m yr−1 for the Basu Glacier in the Karakorum Mountains (Figs. 1, 4b). However, the glacial retreat in the interior of the Tibetan Plateau was small, no more than 10 m yr−1. For example, the annual retreat rates of the Puruogangri Glacier and the Malan Ice Cap were less than 10 m yr−1. As a lot of studies (Yao et al., 1990; Yang., 1991, 1995; Yang and Hu, 1992; Liu et al., 1999; Ye et al., 1999; Shen, 2003, Shen and Liu, 2003; Lu et al., 2005) pointed out, glacial melting water increase caused by more intensive glacial retreat has important influence on water resources or hydrological processes.
The Characteristics of Mass Balance in the Tibetan Plateau and Surrounding Regions
The fundamental cause of glacial behavior is mass balance, which is directly related to climatic change. Mass balance is the algebraic sum of glacial mass gain (precipitation, avalanches, etc.) minus mass losses (melting, evaporation, sublimation, calving) over a year. Glacier mass balance is determined using contour maps of accumulation and ablation data. Ablation is measured with stakes and accumulation is measured with snow pits on the glacier surface. A positive algebraic sum means more mass input and a negative algebraic sum means more mass output.
Glacial mass-balance observations are not many in the past because of its difficulty of measurement on the Tibetan Plateau and surrounding regions. A few observed sites include the Glacier No. 1 in the Urumqi River basin (1956–2001), Small Dongkemadi Glacier in the Tanggula Mountains (1990–2001), and Meikuang Glacier (1990–2001) in the Kunlun Mountains (Pu et al., 1998, 1999; Jiao et al., 2004; Wang et al., 2004). The data from Glacier No. 1 and Small Dongkemadi Glacier have been included in the Glacier Mass Balance Bulletin of the World Glacier Monitoring Service. Obviously, the limited monitoring sites of mass balance can not represent the pattern of the whole Tibetan Plateau and surrounding regions, but help us to understand the regional behavior of glaciers. To further understand the nature of glacial retreat on the Tibetan Plateau in terms of mass balance, more sites have been monitored in the past 1 to 2 yr. The new sites include the Gurenhekou Glacier and the Zhangdang Glacier (Fig. 1).
From Figure 5a, it is clear that the net mass balance of Glacier No. 1 in the Tian Shan Mountains was strongly negative during most of the observation period, that the mass balances of the glaciers in the central and northern Tibetan Plateau shifted recently from positive to negative with different magnitudes. For example, the mass balances of the Small Dongkemadi Glacier and Meikuang Glacier became negative in the mid-1990s, characterizing the general retreat of glaciers in the central and northern Tibetan Plateau. However, the mass balance of the Meikuang Glacier in the Kunlun Mountains was less negative than that of the Small Dongkemadi Glacier in the Tanggula Mountains, and their glacial mass balances were mostly positive before the 1990s (Fig. 5a). If enough new sites added, we can further study the spatial pattern of glacial mass balance in the Tibetan Plateau, which is not possible so far. However, with existing mass balance sites in the Tibetan Plateau, we can see that the most negative mass balance is found in the southern Tibetan Plateau as shown in Figure 5b.
Precipitation in most parts of the Tibetan Plateau and surrounding regions is increasing, probably adding more snow to the accumulation zone of glaciers. However, the additional mass gains have not been reflected as positive mass balances. The reason is that along with precipitation increasing, temperatures are rising (Shrestha et al., 1999; Liu and Chen, 2000; Yao et al., 2000). Increased melt due to rising temperatures outweighs any increased snowfall on glaciers. Therefore, the key cause of more and more negative glacial mass balance and general glacial retreat on the Tibetan Plateau and surrounding regions is climatic warming.
The Impact of Glacial Retreat on Hydrological Process on the Tibetan Plateau
The glacial retreating on the Tibetan Plateau is impacting hydrological process in many ways including increased discharge, lake level rising, more frequent glacial lake outburst flood (GLOF), glacial debris flow, and so on.
The annual glacial meltwater runoff in China is about 56 km3 (Yang and Hu, 1992; Yang, 1995), and is equal to the total annual runoff of the Yellow River. It is therefore important for the Tibetan Plateau and surrounding regions, particularly, northwest China because glacial melting water supplies the desert oases. In northwest China, which includes Gansu, Qianghai, and Xinjiang provinces, there are 24,752 glaciers, with a total area of 31,351 km2, and a glacial volume of 3108 km3 (Wang and Yang, 1991; Yang et al., 1996). Glacial meltwater accounts for more than 20% of the water resource in Xijiang, Gansu, and Qinghai. The rivers in these provinces are mostly inland rivers disappearing in the arid or desert area. The large outflow rivers such as the Yangtze River, the Yellow River, and the Brahmaputra River are also impacted in their headwaters.
Yang (1995) estimated that the total annual glacial meltwater runoff in Northwest China is about 22 km3. Yao et al. (2004) recently pointed out that the supply of glacial meltwater runoff to the rivers in Northwest China has increased 5.5% in the past 27 yr. Obvious climate warming appeared in the 1980s and became more intensive in the 1990s (Yao et al., 1996). It would therefore be reasonable to speculate that glacial meltwater runoff contributed even more than 5.5% in the 1990s. Ren et al. (2004) predict that glacial retreat will speed up in the central Himalayas if climatic warming and drying continues. Shi and Liu (2000) forecasted that small glaciers would be retreating more intensively responding to global warming in the future. All these studies mean that, in the future, global warming will result in more contribution of glacial meltwater to the river discharge in Northwest China.
A runoff increase of 13% in the 1990s was estimated in the Tarim River basin and glacial melting caused by climatic warming is the major cause (Shi et al., 2002). Shen and Liu (2003) suggested that the Tailan Glacier, which is at the upper reaches of the Tarim River basin had thinned 1.26 m with an average thinning of 0.29 m yr−1 between 1957 and 2000, is a major source. The supply of glacial melting water from the Tailan Glacier had increased by 13% between 1957 and 1986 and by 23% between 1987 and 2000.
Lake level rising caused by more input of glacial melting water is now a particularly serious problem in the Tibetan Plateau because grassland and villages near these rising lakes are devastated (Yang et al., 2003). To investigate quantitatively the impact of the glacial melting water on lake level rising, several specific areas occupied by large lakes including Xingxinghai Lake, Nam Co Lake, and Selin Co Lake on the Tibetan Plateau (Fig. 1) have studied based on aerial photos in 1970 and TM satellite remote-sensing data in 1990 and 2000.
In the Xingxinghai region, glaciers are small in both numbers and areas, with a total of 57 glaciers and an area of 103.8 km2 in 2000 (Table 3). Our study shows that, except three advancing glaciers and two stable glaciers, the remainder of the 57 glaciers had retreated in 1970–2000. The total glacial area in the region had decreased 17.3% between 1970 and 2000 (Table 3). The largest retreat glacier was the Yehelong Glacier. The glacier retreated 1950 m in length, about 76.8% in about 30 yr.
Glacial retreats around the Xingxinghai Lake in 1970, 1990, and 2000.
The lake area change in the Xingxinghai region is shown in Table 4. Five lakes are located in this region with a total area of 1262 km2 in 1970, of 1245 km2 in 1990, and 1215 km2 in 2000 (also see Fig. 6). It means that the water supply from the lost of glacial mass balance did not help the water evaporation because of global warming. The reason is that glacial area is small and scattered in different places and some of them do not supply the lakes.
Lake area changes in the Xingxinghai region in 1970, 1990 and 2000.
Glacial retreat in the Nam Co region was studied by Lu et al. (2005) and by Wu et al. (2007). Unlike the Xingxinghai region, the glaciers in the Nam Co region have greater area, are rather concentrated in distribution, and supply Nam Co Lake (Table 5). The glacial area decrease in the Nam Co region is dramatic, with a retreat rate of 15.4% in glacial area in about 30 yr.
Glacial area changes in the Nam Co region in 1970, 1990 and 2000.
Based on the remote-sensing data, the areas of all the lakes in the Nam Co region had expanded between 1970 and 1990 (Table 6). The areas of the Nam Co Lake, Baimalangmu Co Lake, Tong Co Lake, and Shen Co Lake are expanding continuously between 1990 and 2000. However, Cuojiana Lake shrank during this period.
Lake area changes in the Nam Co region in 1970, 1990 and 2000.
The remote-sensing analyses of glaciers around the Selin Co region indicated that there were 6 advancing glaciers, 26 stable glaciers, and the remainder were generally retreating between 1970 and 2000. The total glacial area decreased 1.7% between 1970 and 2000 (Table 7), and the largest retreat in glacier length is the Qianggudiru Glacier with a 1288 m retreat. However, the largest retreat glacier in area is Glacier No. 5K451F3 with a decrease of 19.4%.
Glacial retreats in the Selin Co region in 1970, 1990, and 2000.
Area changes of eight lakes in the Selin Co region were analyzed based on the aerial photos in 1970, and the TM satellite remote-sensing data in 1990 and 2000 (Table 8). It showed that the area of the eight lakes had expanded at a rate of 10.9% from 1970 to 2000. The exception is two small lakes, Xiaqiong Co and Gejialing Co, with area only about 4 km2 which once disappeared in 1990.
Lake area changes in the Selin Co region in 1970, 1990, and 2000.
The Selin Co region has a large glacial melt water supply, and larger number and area of glaciers compared to the Xingxinghai region, or even the Nam Co region. The lake area increase in the Selin Co region is evident in the past 30 yr as shown in Figure 8.
Glaciers in the Tibetan Plateau and surrounding regions are retreating rapidly. The glacial retreat has resulted from negative glacial mass balance. Many glaciers that previously had positive glacial mass balance also shifted to negative balance. The glacial retreat in the 1990s was the most intensive of any period during the 20th century.
The general glacial retreat in the Tibetan Plateau and surrounding regions in the 1990s caused glacial runoff to increase significantly, more than 5.5% in Northwest China. In the Tarim River basin where glaciers are most concentrated, the net increase of water from glacial retreat reached 13%.
Glacial retreat also caused lake level rising in the areas with larger coverage of glaciers in the Tibetan Plateau, such as the Nam Co region and Selin Co region. In the region where glacial area is small and precipitation dominates, lake level is dropping.
With continued global warming in the future, stronger glacial retreat is anticipated (Shi and Liu, 2000) and its more intensive impact on hydrological processes is expected. More studies should be focused on the mechanism of the interaction between glaciers and lakes in the future study.
This study has been supported by the project from MOST (2005CB422004), the Innovation Group Fund of the National Natural Science Foundation of China (40121101), and the program BRAHMATWINN (FP6-036952).