Recent variability of the climate and glaciers in China's monsoon region

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1 104 Water Resources Systems Water Availability and Global Change (Proceedings of symposium HS02a held during 1UGG2003 at Sapporo. July 2003). IAHS Publ. no Recent variability of the climate and glaciers in China's monsoon region HE YUANQING 1 ' 2, ZHANG ZHONGLIN 1, WILFRED H. THEAKSTONE 2, CHEN HO 1. YAO TANDONG 1, DAVID D. ZHANG 3 & PANG HONGXl' 1 Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou , China vqhe@ns.lzb.ac.cn 2 Department of Geography, University of Manchester, Manchester Ml 3 9PL, UK 3 Department of Geography, University of Hong Kong, Hong Kong, China Abstract Climatic data, ice core records, the tree-ring index and recorded glacier variations have been compared, and used to reconstruct a history of climatic and glacial changes in the monsoonal temperate-glacier region of southwestern China during the last 400 years. The results indicate that the region's temperature has increased in a fluctuating manner during the 20th century, after the two cold stages of the Little Ice Age of the 17th 19th centuries, with a corresponding retreat of most of the glaciers during the 20th century, against a background of global wanning. Rates of retreat accelerated after the 1980s. The few advancing glaciers that did exist have started to retreat in recent years. The amount, trend and amplitude of variation of precipitation have differed in different parts of the region. The climatic records in the Dasuopu ice core, from the Himalaya area in the western part of the region, show a decreasing trend in precipitation, the converse of the trend in temperature. However, in the Hengduan Mountains and other areas of the eastern part of the region, a rising trend in rainfall has accompanied increasing temperatures, a result of the variable atmospheric circulations from different sources. The data indicate that the Southwest Monsoon, which is the principal controlling factor in the Chinese monsoonal temperate-glacier region, can be classified into two parts. One is the Indian Monsoon from the Arabian Sea, passing across the Indian Peninsula. This transports the vapour for precipitation in the Himalaya area, the western part of the monsoonal temperate-glacier region. The other part is the Bengal Monsoon originating in the Bay of Bengal, passing over Bengal and Burma. This is the major source of precipitation in the Hengduan Mountains and other areas in the eastern part of the region. In addition, the eastern part is influenced by the Southeast Monsoon arriving from the western Pacific, whilst the western part is affected in winter by the southern branch of the westerly circulation. This complex atmospheric situation results in differing patterns of precipitation in the western and eastern zones. Although it is clear that both temperature and precipitation affect the glaciers, further work is needed to confirm which is the major factor influencing present glacier change. Key words climatic change; temperate glaciers; monsoon region; China INTRODUCTION The monsoonal temperate-glaciers in China are located in the region of the southeastern Tibet Plateau, including the Hengduan Mountains, the eastern part of the

2 Recent variability of the climate and glaciers in China's monsoon region D North regression line 20 Fig. 1 The monsoonal temperate-glacier region in China and the atmospheric circulations affecting the region. Himalaya Mountains, and the middle and eastern segments of the Nianqingtanggula Range, in the provinces of Tibet, Yunnan and Sichuan, southwestern China (Fig. 1). According to the Chinese glacier inventory, there are 8607 monsoonal temperate glaciers in China, covering an area of km 2 ; this is 18.6% of all the glaciers and 22.2%) of the total glacier area. The climate of the region is controlled by two major atmospheric circulations: the northern hemispheric westerly wind system, which carries little vapour in winter, and the South Asian Monsoon circulation system. The latter includes the main sources of precipitation, the Southwest Monsoon and the Southeast Monsoon, from the Indian and Pacific oceans, respectively. In winter, from November to March, the southern branch of the westerly wind, accompanied by very little precipitation, dominates the region. From April to May, the westerly retreats to the north of the region, beyond 30 N. Then the southern monsoon gradually arrives, bringing the onset of the rainy season. Of the annual precipitation, 60-90% occurs between April and October (Su & Shi, 2000; He et al, 2000a). The western part of the region is affected mainly by the Southwest Monsoon, but the eastern part is influenced by both the Southwest Monsoon and the Southeast Monsoon, which brings more precipitation (Hahn & Manabe, 1975; Lin & Wu, 1990; Ruddiman & Kutabach, 1989; Yao & Ding, 1991; Parthasarathy et al, 1992; Kripalani & Singh, 1993; Araguas & Roehlich, 1998; Duan et al, 2000; He et al, 2000b). The monsoonal temperate-glacier region is characterized by high precipitation ( mm) in the glacier-covered area, a low snow line ( m, which is about m lower than that of the polar glaciers in the western Tibet Plateau) (Shi et al, 1988; Li & Shu, 1996), and relatively high temperatures (equilibrium line mean annual value -6 C, summer value 1-5 C), resulting in very intensive glacial melting. The high accumulation rate of snow and temperatures at the glaciers result in rapid movement at the surface, up to several hundred metres per year, and in sliding at

3 106 He Yuanqing et al. the bottom. The most pronounced characteristic of monsoonal temperate glaciers is that they are very sensitive to climate, which means that a small increase or decrease of temperature can cause large-scale glacier retreat or advance (Shu & Shi, 2000; He et al., 2001). Thus, the glaciers are a very direct, clear indicator of climatic change. Handel-Mazzetti (1920), Ward (1924) and Wissmann (1932) described the Chinese monsoonal temperate glaciers in the 1920s and 1930s. Since the 1950s, Chinese researchers have studied the region from a variety of viewpoints (Ren et al., 1957; Luo & Yang, 1963; Xie & Cui, 1989; Li & Shu, 1996; Zhao et al, 1999; Shu & Shi, 2000; Zheng et al, 2000; He et al, 2000a,b, 2002). Here, information from the region, including climatic data, glacier change data, ice core records and tree-ring data, are examined against the background of global warming and general retreat of the glaciers since the Little Ice Age. More than 200 years of climate data from India is also used in order to improve understanding of recent climatic and glacial changes in China's monsoonal temperate-glacier region. This is of value in relation to wider studies of global change. RESULTS Temperature and precipitation variations in the last 50 years Annual average precipitation and temperature have been collected for 50 years at four stations, Lijiang, Zhongdian, Kangding and Yaan, respectively, located in the south, central, north and west of the Hengduan Mountains (Figs 1 and 2). The mean annual temperature at all four stations has been increasing since the 1970s, and the rate of change has accelerated since the 1990s, with a fluctuating cycle of 2-3 years. The annual precipitation at three stations (Lijiang, Zhongdian and Kangding) has increased gradually, but in a less marked fashion than temperature. In contrast, the mean annual precipitation at Yaan has been decreasing since the mid-1980s, and the change has been much larger than that at the other stations S : year Fig. 2 Variations of temperature and precipitation at Lijiang, Zhongdian, Kangting and Yaan in the eastern part of China's monsoonal temperate-glacier region during the last 50 years. year

4 Recent variability of the climate and glaciers in China's monsoon region 107 The data indicate that the climate in the Chinese monsoonal temperate-glacier region started wanning gradually in the 1970s, and that the warming trend intensified during the 1990s. Influenced by the Southwest Monsoon, precipitation in the Hengduan Mountains, in the eastern part of the region, started to increase in the 1980s, accompanying the global warming trend. At Yaan, the annual rainfall is higher, it's variability is greater, and the trend of change is different from those of other stations. In part, this is because the site is influenced by the Southeast Monsoon, as well as by the Southwest Monsoon. Dasuopu ice core records and Indian precipitation The record in the Dasuopu ice core (Figs 3 and 4), recovered from Xixiabangma Mountain in the Himalayas, in the western part of the study region, indicates changes of climate, including the intensity of the Southwest Monsoon (Yao et al, 2000; Thompson et al, 2000; Duan et al, 2001). The 8 I8 0 values in the ice core indicate that whilst the temperature was relatively stable between 1600 AD and 1750 AD, there was a cooling trend in the eighteenth century. The climate started to become warmer from 1880 AD, and this trend intensified after the middle of the 20th century. The variability of the accumulation rate in the core documents variations of precipitation and monsoonal activity at the site since 1600 AD. There were two cold, arid stages during the early half of the 17th century and the latter half of the eighteenth century, the period of the Little Ice Age. A negative relation between accumulation rate and dust content in the last 400 years suggests that higher precipitation corresponds to a lower dust content, and that the dust content is higher in arid climate conditions. As reported by Thompson et al. (2000), the dust arrives at Dasuopu primarily in winter and early spring, and most precipitation falls in summer. The Dasuopu record represents conditions at a single location, and a dating error of one year may affect the statistical correlation: a 3-year running mean applied to both records gives R = 4.72 (R~ : significance = 99.9%). The increase of dust content throughout the 20th century reflects the fact that the climate in the western part of the study region (the southern Tibet Plateau) has developed gradually into a warm, arid type. The values in the Dasuopu ice core started to rise in the 1930s, indicating an apparent warming period, during which there was a decreasing trend of precipitation. In contrast, precipitation in the eastern part of the region increased during this period. From 1850 AD, variations of precipitation in the central and northeastern parts of India were consistent with those of the accumulation rate in the Dasuopu ice core (Fig. 4). The relationship is statistically significant. The comparison of the climatic data and the ice core record suggests that, since the 1950s, the temperature has tended to rise in the whole of the study region, whilst precipitation has displayed varied patterns, increasing and decreasing in different parts. A decreasing trend in the western part, at Dasuopu and in northeastern India, has accompanied a rising trend in the eastern part, at Lijiang, Zhongdian and Kangding. This complex climatic situation in China's temperate-glacier region can be attributed to the different vapour sources and atmospheric circulation patterns within the area controlled by the Southwest Monsoon, which results from different geographical positions, altitudes and landforms.

5 108 He Yuanqing et al i Fig. 3 ô l8 0 values, accumulation rate and dust content in the Dasuopu ice core, 1600 AD to the present. Trend lines fitted to the data are fifth order polynomials. Tree-ring data from Mount Yulong and Zhongdian Tree-ring sampling has been undertaken recently between 3500 and 3750 m on Mount Yulong, the southernmost glacier-covered area in Eurasia. Samples for analysis were collected in 16 cores from eight trees in the fir belt, which has an upper limit close to glacier tongues. These are the only up-to-date tree-ring data from this region. Crossdating was carried out after the samples were dried, fixed and polished. Ring widths were measured with a precision of 0.01 mm. Using the method of Fritts (1976), the measured ring widths (Wi) were standardized and converted to ring-width indexes (/,), by dividing the width for year t by the expected growth (Yi): I, = WJY; Division by the expected growth both removes the trend in growth and scales the variance so that it is approximately the same throughout the entire length of the time series. The results (Fig. 5) confirm that the annual tree rings mainly represent variations of local temperature, related to sampling location and present climatic conditions. The values of the ring-width index were lower in the last part of the 17th century and the early half of the eighteenth century, and for much of the

6 Recent variability of the climate and glaciers in China's monsoon region Dasuopu ice core 1500 E 1000 o India 0.5 o Fig. 4 Variation of accumulation rate in the Dasuopu ice core, mean annual precipitation in northeastern India and the Indian temperature anomaly since 1850 AD. time between the mid-19th century and the 1930s. This reflects the colder conditions during the first and second stages of the Little Ice Age and in the early 20th century. The tree-ring data indicate that temperature has been rising progressively since the 1930s, except for two periods between 1950 and 2000, consistent with local glacier change. Variations of the tree-ring index since the 17th century in Mount Tianbao of the Zhongdian district, reported by Wu & Lin (1983), resemble the results from Mount Yulong, although the increasing amplitude in the Zhongdian profile for the period exceeded that of Mount Yulong (Fig. 5).

7 110 He Yuanqing et al. 2-5 r O-j i -- - i - -~ -f- i - - yea; Fig. 5 Variation of the tree-ring index from 1650 AD to the present, in samples from (a) Mount Yulong, near Lijiang and (b) Mount Tianbao, near Zhongdian. Recent changes of some typical temperate glaciers Data showing the advancing and retreating status of some typical temperate glaciers (Fig. 1), based on the location of new moraines and other glacial landforms, as well as on the Chinese glacier inventory, are presented in Tables 1 and 2. Table 1 shows the percentage area reduction between the Little Ice Age and the present for glaciers of different size in different parts of the study region (Su & Shi, 2000). Since the Little Ice Age of the 17th 19 th centuries, the total glacier area in the region has been reduced on average by 30%, about 3921 km 2. The glacier retreat rate differs as a function of the size of the glacier. The smaller the glaciers, the larger their rate of decrease; the larger the glaciers, the lower their rate of decrease. Table 1 Percentage area reduction of glaciers in different size classes in China's monsoonal temperate-glacier region since the Little Ice Age (LIA) (after Su Zhen et al, 2000). Glacier size <lkm km 2 5.0]-l0knr km 2 Sum Area LIA Present % LIA Present % LIA Present % LIA Present % LIA Present % Najiabawa S Palongzang ] 5 4 ] 9 Q 3 7 Q ] ) 4 1? g, 5 7 9, g 2 9 ], Q ] 2 fi g 6? 2 9 bu Chayuhe Yulong Gongga Queer Sum

8 Recent variability of the climate and glaciers in China's monsoon region 111 Table 2 Variation of some typical glaciers in the Chinese monsoonal temperate-glacier region since the Little Ice Age. Baishui Glacier no.l, Mount Yulong (area: 1.7 km", length: 2.5 km) Time Altitude of the glacier end (m"') Advance & retreat (m" 1 ) Melang Glacier, Mount Mainri (area: 13 km 2, length: 11.7 km) Time Altitude of the glacier end (m"') Advance & retreat (V) 17-19th C. (LIA) 19th C (1957) (1982) (1998) (2002) advance retreat 1250 advance 800 retreat 150 retreat (1971) (1982) (1998) (2002) retreat 2000 advance 800 advance 70 advance 40 retreat 30 Hailuogou Glacier, Mount Gongga (area: 23.7 km 2, length: 13.6 km) Time Altitude of the glacier end (nf') Advance & retreat (m" 1 ) Azha Glacier, southeastern Tibet (area: 29.5km 2, length: 20 km) Time Altitude of the glacier end (in" 1 ) Advance & retreat (actual distance) (nf 1 ) Early 20lh C s (1930) (1996) 2920 (1982) 2940 (1989) 2980 (1994) advance or retreat retreat 1150 retreat retreat 170 retreat (1973) 2600(1976) 2700 (1980) advance retreat 700 retreat 200 retreat 100 There are regional differences of glacier change as shown in Table 2 (He et al., 2000; Zheng et al, 1999). For instance, Baishui Glacier no. 1 on Mount Yulong (Fig. 1), the southernmost glacier of Eurasia, with a small area, is most sensitive to climate, and its area has decreased by 60% from the Little Ice Age to the present. It retreated about 1250 m between the Little Ice Age and the middle of the 20th century, and it has retreated again since the 1980s (Table 2). The retreat rate has increased in the last few years in response to rapid climatic warming; observed retreat from 1998 to 2002 was about 100 m. The Hailuogou Glacier and the Azha Glacier, located to the north and west, respectively (Fig. 1), also have retreated very quickly since the 1980s. Although the Melang Glacier on Mount Mainri was advancing before 1998, it began to retreat gradually after 1998, indicating that it, too, has responded to post-1980s climatic wanning. However, the mean rate of decrease of some large valley glaciers on Mount Gongga between the Little Ice Age and the present was only 27%. It is clear that the different geographical locations and scales of the glaciers, and their sensitivities to climate, have resulted in differences in the time and amplitude of their variations. DISCUSSION A variety of indices have been used to reconstruct the climatic and glacial changes in China's monsoonal temperate-glacier region during the last 400 years. Climatic data, tree-ring data and glacier variations provide a comprehensive picture of changes in the Hengduan Mountains during that period. The ice core record and the free-ring index clearly show the two cold stages of the Little Ice Age between the 17th and the 19th centuries, evidence of which also is provided by the moraines formed in front of the advancing glaciers at that time. There was another short cold stage in the early 20th

9 112 He Yuanqing et al. century, but both temperature and precipitation have been increasing irregularly since the 1980s. Since the early 20th century, especially from the 1930s, most of the glaciers have been retreating gradually in response to warming conditions. All observed and proxy climatic indices indicate a rising trend of temperature in the whole region from the 1950s, which has been intensified since the 1980s, during which period most glaciers have been retreating. However, there has been an asymmetrical change of precipitation in the region. The Dasuopu ice core data and precipitation data from India indicate that the western part of the region was wetter from the middle of the 19th century until the 1930s, and that annual precipitation decreased gradually from the mid-20th century to the present. Precipitation at Yaan, at the northeastern margin of the region, has been decreasing in the last 50 years, and the pattern of change has differed from that of temperature. In contrast, the 50-year record from Lijiang, Zhongdian and Kangting indicates a gradual rising trend of precipitation in the Hengduan Mountains, in the eastern part of this region, especially in recent years. These similarities and contrasts of climatic and glacial changes in China's monsoonal temperate-glacier region, which have taken place against a global background, are determined by the varied regional atmospheric circulation and precipitation sources. The highly developed industries in the world economy have resulted in a rapid increase of greenhouse gases in the atmosphere, which has caused global wanning (Fu & Wang, 1991). The data cited in this paper are evidence of a rising trend of temperatures in the whole monsoonal temperate-glacier region, and indicate that monsoonal activity has intensified. Global wanning has accelerated and intensified water circulation, leading to increased evaporation and precipitation. Changes in precipitation and temperature have differed, with variations of precipitation being much more complex than those of temperature. Many factors may be responsible for this (Su & Shi, 2000; Shi et al, 2002). Differences of regional precipitation have many causes (Lin & Wu, 1990; Yao et al, 1991; Thompson et al, 2000; Duan et al, 2001, 2002). Firstly, there are different sources and different patterns of atmospheric circulation. The Southwest Monsoon has two trajectories, one from the Bay of Bengal, by way of Bengal and Burma, to the Hengduan Mountains and then northwards, and the other from the Arabian Sea/Indian Ocean, crossing the Indian Peninsula and the Himalayas, to influence the southern part of the Tibet Plateau (Lin & Wu, 1990). As the distance between the sources and precipitation sites varies, the intensity and influence of the two water vapour sources differ. The Southwest Monsoon can be divided into the Indian Monsoon and the Bengal Monsoon (Fig. 1). The Indian Monsoon has had a principal effect on the Dasuopu ice core (eastern Himalayas) and the western part of China's monsoonal temperate-glacier region. The Bengal Monsoon controls the climate of Mount Hengduan and adjacent areas, the eastern part of the region. The Southeast Monsoon coming from the North Pacific crosses most of southeastern China and carries precipitation vapour to the study region. Both the Southeast Monsoon and the Bengal Monsoon influence the precipitation at Mount Hengduan and adjacent areas. The westerly circulation in the Northern Hemisphere mainly influences the winter precipitation of the region and its influence is obviously stronger in the west than in the east. Thus, it can be confirmed that the complex atmospheric circulation patterns influencing China's monsoonal temperate-glacier region necessarily result in non-

10 Recent variability of the climate and glaciers in China's monsoon region 113 uniform patterns of precipitation and glacier variations. In addition, other factors, such as topography and glacier size and orientation play a role. There is a positive correlation between precipitation and temperature in the east, and a negative correlation in the west. Since both temperature and precipitation influence glacier changes, further work is needed to clarify which is the major factor affecting glaciers. CONCLUSIONS Climatic data, the ice core record, the tree-ring index and recorded glacier variations indicate that the temperature in China's monsoonal temperate-glacier region has increased in a variable manner in the 20th century, resulting in the retreat of most of the glaciers, and there were two cold stages in the Little Ice Age of the 17th 19th centuries. The amount, variability and trend of precipitation has differed in different parts of the region. The climatic records in the Dasuopu ice core indicate a decreasing trend of precipitation in the Himalaya area and the western part of the region. There is an inverse relationship with temperature. In the Hengduan Mountains and adjacent areas, there has been a rising trend of precipitation, synchronous with temperature change, the result of the variable atmosphere circulation from different sources. The southwestern monsoon, which is the principal control of climate in the Chinese monsoonal temperate-glacier region, consists of two parts. One, the Indian Monsoon from the Arabian Sea, passes across the Indian Peninsula. It transports most of the vapour for precipitation in the Himalayas, the western part of the region. The other, the Bengal Monsoon from the Bay of Bengal, crosses Bengal and Bunna. It is the major source of precipitation in the Hengduan Mountains and other areas in the eastern part of the region. The eastern part of the region is also influenced by the Southeast Monsoon from the western Pacific, and by the westerly circulation in winter. This complex atmospheric situation results in a difference in the patterns of precipitation in the west and the east. The temperate glaciers in the region have been very sensitive to climate warming since the Little Ice Age, and most have been retreating continuously throughout the 20th century. The rate of retreat increased after the 1980s. The speed of the few advancing glaciers is decreasing. Acknowledgements This work was supported by the Hundred Talents Project (CAS ), Special Funds to the Famous Yung Scientists from the Chinese Natural Science Foundation ( ), the Knowledge-Innovation Programs (210506, and KZCZ2-301), and the Cold and Arid Regions Environmental and Engineering Institute, Chinese Academy of Sciences. REFERENCES Araguas, L & Froelilich, K. (1998) Stable isotopic composition of precipitation over Southeast Asia. J. Geophys. 103(D22), Res.

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