**2. Glaciers in the Tian Shan**

original term to describe 'an epoch of renewed but moderate glaciation that already has lasted about 4000 years' has been overtaken as '*neoglacial*', in this report, it was noted that '… the glacier-oscillations of the last few centuries have been among the greatest that have occurred during the 4000-year period…' [1]. The term has now been more formally and widely adopted to describe the period approximately from AD 1300 to 1850, characterized by lower temperature over most of the globe and growth of glaciers to a more advanced extent than that of prior

Following the warm period of the 'Medieval Climatic Optimum', a dramatic series of glacier advances and retreats symbolized the cooling events of the LIA. However, despite the evidence of similar events from many places around the world, the occurrence of the LIA cannot be assumed synchronous in time and uniform across space. A large amount of evidence of the LIA was assembled initially in the regions like Europe, Greenland and the Arctic, around the North Atlantic, e.g. [3–8]. The expanded stages of mountain glaciers, cycles of excessive cold, severe droughts, hot summers, or unusual rainfall were captured in abundant historical documents such as artistic paintings, ship logs and agricultural records or diary from Mediterranean, alpine Europe and further north. For examples, see Refs. [4, 6, 9]. An increased variability of the climate, as well as other LIA-type events induced significant impacts on human society, as evidenced in existing historical documents. As noted in Ref. [10], the beginning of the LIA is marked by the heavy rains, severe winters and harvest fails in 1315–1319 widespread in England. But, it needs to be noted that firstly, the timing of cold conditions occurred differently from region to region, and secondly, throughout the span of the LIA, the climate was never monotonically cold or always favourable to glacial expansion but instead with sometimes disastrous shifts between warmth and coldness at centennial, decadal, or even annual scales [11]. Although the observational record is less available outside of the North Atlantic region, it is well studied that mountain glaciers advanced far beyond their modern limits in highlands of Asia, the Andes of South America, New Zealand, western North America and other ranges, e.g. [12–18]. Grove's book [2] provides a summary of the

In this chapter, we focus on the Tian Shan range in central Asia, a less-studied region compared to other key mountains on the Earth, to synthesize (1) the critical regional settings of the Tian Shan glaciers; (2) current documentation and identification of modern glaciers and the LIA glacial events; and (3) the influence of climate and local factors to glacier change since the LIA. With LIA glacial chronologies established at a few sites across the Tian Shan, it makes the examination of the spatiotemporal pattern of the LIA glacial advances possible. The spatial variation of glacier changes reflects its response to climatic shifts as well as the local topography and geometry. What modifications in climate systems occurred here during the LIA? What local factors be attributed to explain the glacier variability? This chapter provides a holistic review of such questions based on current literature. At the end, we took an example of a dataset of 865 glaciers in the eastern Tian Shan to examine the significant local factors to glacier changes using random forest model, and we compared the results with

and post the period [2].

38 Glacier Evolution in a Changing World

previous findings.

LIA-type events from all major regions over the world.

As located in the arid and semi-arid environment, glaciers in high mountains in central Asia are sensitive indicators of climate change, as well as important freshwater storages in the region. Major glacier-covered mountains in central Asia include the Altay, the Urals, the Tian Shan, the Kunlun, the Karakoram and the Himalayas (**Figure 1**), and they represent a wide range of climatic and hydrological conditions. The Tian Shan is approximately located at 40.5°N to 43.5°N and 75.5°E to 94.8°E.

### **2.1. Geographical and climatic settings**

'Tian Shan' means 'Heavenly Mountains' (Shan = Mountains) in Chinese. Formed by the collision of the Indian and Eurasian continental plates about 40 to 50 million years ago, the Tian Shan is a ~2500 km long mountain series stretching from the western boundary of Kyrgyzstan across most of the Xinjiang Uyghur Autonomous Region, China (**Figure 1**). It is the largest mountain chain in the world's midlatitude arid region. Its highest peak is Tumur (other names: Victory Peak, Jengish Chokusu, or Pobeda), 7439 m above sea level (a.s.l.), on the border between easternmost Kyrgyzstan, Kazakhstan and China. Bounded by the Taklimakan Desert to the south and the Gurbantonggut Desert to the north, the Tian Shan indispensably serves

**Figure 1.** (a) Location of major mountains in central Asia and nearby area. (b) The Tian Shan with glacier data downloaded from the GLIMS Glacier Database (http://www.glims.org/download/); black dots represent the study sites with the LIA moraine identified by numerical dating methods [56–59]. (c) Study area of the eastern Chinese Tian Shan, with modern and LIA glacier data in three sub-regions.

as the source region of freshwater supply for the arid surroundings and nurtures abundant unique flora and fauna in its spectacular landscapes. Both the Western Tian Shan (Kyrgyzstan, Kazakhstan and Uzbekistan) and the Xinjiang Tian Shan (China) are listed as UNESCO World Heritage Site since 2013 and 2016, respectively (http://whc.unesco.org/en/list/).

The entire mountain system includes many individual mountain ranges shared across central Asia countries/regions. The western part of the system has relatively higher elevations, recognized as the Kyrgyz Tian Shan; whereas the Chinese Tian Shan (a.k.a. Xinjiang Tian Shan) is more characterized by smaller, lower ranges compared to the western section. The Lake Issyk-Kul (1608 m a.s.l.), situated in an intramountainous basin, dissects the Kyrgyz Tian Shan into a northern part (Zailiyskiy Alatau and Kungey Alatau Mountains) and a larger southern part (Terskey Alatau Mountains and Kok Shaal-tau Range) bordered with China. The altitude rises from about 700 m a.s.l. with steppe landscapes up to nearly 5000 m a.s.l. The central Tian Shan stretches from the eastern edge of Kyrgyzstan to China and is the highest mountain knot in the whole range where largest glaciers occupy. The Xinjiang Tian Shan in China covers the eastern portions of the Tian Shan, taking up about 1750 km, 2/3 of the total length. The Borohoro Range and the Tianger Range make up the northern branch of the Tian Shan.. Extending towards the east, there are the Bogeda Range located next to the regional capital city Urumqi and the Barkol-Karlik Range located to the easternmost end of the whole range (**Figure 1**).

The geographical location at the centre of the Earth's largest continent determines its typical continental, temperate climate in the Tian Shan. The confluence of major climate systems makes central Asia a transitional region, particularly sensitive to changes in the spatial pattern of climate systems. The overall continentality is characterized by sharp local differences. The elevation is the main factor influencing air temperature distribution rather than longitude or latitude. The contrasts in seasonal and diurnal temperature are high. The annual amplitude in monthly temperature of up to 38°C is common in the Kyrgyz Tian Shan, but regional differences exist [19]. Precipitation varies both longitudinally from west to east and from the northern slope to the southern slope. Such features are closely associated with two large-scale atmospheric circulations dominated in this area: midlatitude westerlies and the Siberian High. The moisture from the Atlantic Ocean and closed drainage basins such as the Aral, Black and Caspian seas is carried by the westerlies which delivers abundant precipitation to the western end of the mountains [20–22], while the orographic effect of the mountain barrier reduces it to the minimum amount at the eastern end. Sorg et al. [23] summarized that on the windward northwestern slopes, the annual precipitation can be up to 1500 to 2000 mm in high elevations, whereas to the east in the interior regions, it can be as low as 100 mm. In general, the Siberian anticyclonic circulation (high pressure) dominates the range in winter season with low temperatures helpful to maintain glaciers; after March, the westerlies associated with mid-latitude cyclones convey precipitation in form of snow to favour the growth of glaciers.

## **2.2. Status of current glaciers**

A high concentration of glaciers in the Tian Shan is known as the "Water Tower of Central Asia" [23]. They play an important role in water cycle in this arid environment. The meltwater runoff into transboundary rivers, such as the Syr Darya River, the Ili River, the Kaidu River and the Urumqi River, contributes substantially to the freshwater resource in downstream ecosystems and for over 50 million populations in the central Asia region [23–25]. While melting glaciers in a first phase release an increasing amount of water, the undergoing reduction of glacier volume will eventually reduce water availability and induce potential political disputes on water allocation issue [26]. Documenting the distribution of present-day glaciers and studying the history of glacier changes in the Tian Shan are fundamental and critical for current and future development in this region.

Existing records of glaciers in the Tian Shan have been derived from aerial photographs, topographic maps and more common nowadays, satellite images, e.g. [27, 28]. The key role of remote sensing in glacier monitoring has been widely recognized, especially in areas like remote mountain ranges in the Tian Shan where the field-based glaciological survey is difficult to conduct. Two main parameters of glaciers obtained from remote-sensing data are the surface area and elevation, which have been commonly used to derive changes of glacial extents and thickness through time. Two international projects, World Glacier Monitoring Service (WGMS) [29] and Global Land-Ice Measurements from Space (GLIMS) [30], have been long devoted to create worldwide glacier inventories and to provide easy access to standard data for the public.

In the Tian Shan, glacier data have been collected both from the Kyrgyz side and the Chinese side. Aizen et al. [31] mapped glaciers in the Kyrgyz Tian Shan based on remote-sensing data and identified 7590 glaciers with a total area of 13,271 km<sup>2</sup> and an estimated 1840 km3 volume of ice. The Chinese Academy of Sciences compiled the First Glacier Inventory of China (GIC) using topographic and aerial photographs acquired during the 1950s–1980s [32] and updated to the Second GIC in 2014 based on mostly Landsat images acquired between 2006 and 2010 [33]. This dataset is accessible via the Cold and Arid Regions Science Data Center at Lanzhou (http://card.westgis.ac.cn/), as well as through the GLIMS Glacier Database (http:// glims.colorado.edu/glacierdata/). The GIC data followed the GLIMS guidelines and contain attributes such as glacier name, glacier identifier, drainage code, glacier area, absolute and relative accuracies, debris-covered area and many other parameters. According to the Second GIC, the total number of glaciers in the Chinese Tian Shan is 7927, and they cover an area of 7256 km<sup>2</sup> and an ice volume of 781 km3 . Such inventory data sets provide great convenience to know the status of present glaciers in the Tian Shan. Due to the fact that glaciers are changing constantly, successive inventories are anticipated to keep tracking the status of glaciers, under the potential of better available data and methods in future.

## **2.3. Evidence of LIA glaciers**

as the source region of freshwater supply for the arid surroundings and nurtures abundant unique flora and fauna in its spectacular landscapes. Both the Western Tian Shan (Kyrgyzstan, Kazakhstan and Uzbekistan) and the Xinjiang Tian Shan (China) are listed as UNESCO World

The entire mountain system includes many individual mountain ranges shared across central Asia countries/regions. The western part of the system has relatively higher elevations, recognized as the Kyrgyz Tian Shan; whereas the Chinese Tian Shan (a.k.a. Xinjiang Tian Shan) is more characterized by smaller, lower ranges compared to the western section. The Lake Issyk-Kul (1608 m a.s.l.), situated in an intramountainous basin, dissects the Kyrgyz Tian Shan into a northern part (Zailiyskiy Alatau and Kungey Alatau Mountains) and a larger southern part (Terskey Alatau Mountains and Kok Shaal-tau Range) bordered with China. The altitude rises from about 700 m a.s.l. with steppe landscapes up to nearly 5000 m a.s.l. The central Tian Shan stretches from the eastern edge of Kyrgyzstan to China and is the highest mountain knot in the whole range where largest glaciers occupy. The Xinjiang Tian Shan in China covers the eastern portions of the Tian Shan, taking up about 1750 km, 2/3 of the total length. The Borohoro Range and the Tianger Range make up the northern branch of the Tian Shan.. Extending towards the east, there are the Bogeda Range located next to the regional capital city Urumqi and the

Heritage Site since 2013 and 2016, respectively (http://whc.unesco.org/en/list/).

Barkol-Karlik Range located to the easternmost end of the whole range (**Figure 1**).

**2.2. Status of current glaciers**

40 Glacier Evolution in a Changing World

The geographical location at the centre of the Earth's largest continent determines its typical continental, temperate climate in the Tian Shan. The confluence of major climate systems makes central Asia a transitional region, particularly sensitive to changes in the spatial pattern of climate systems. The overall continentality is characterized by sharp local differences. The elevation is the main factor influencing air temperature distribution rather than longitude or latitude. The contrasts in seasonal and diurnal temperature are high. The annual amplitude in monthly temperature of up to 38°C is common in the Kyrgyz Tian Shan, but regional differences exist [19]. Precipitation varies both longitudinally from west to east and from the northern slope to the southern slope. Such features are closely associated with two large-scale atmospheric circulations dominated in this area: midlatitude westerlies and the Siberian High. The moisture from the Atlantic Ocean and closed drainage basins such as the Aral, Black and Caspian seas is carried by the westerlies which delivers abundant precipitation to the western end of the mountains [20–22], while the orographic effect of the mountain barrier reduces it to the minimum amount at the eastern end. Sorg et al. [23] summarized that on the windward northwestern slopes, the annual precipitation can be up to 1500 to 2000 mm in high elevations, whereas to the east in the interior regions, it can be as low as 100 mm. In general, the Siberian anticyclonic circulation (high pressure) dominates the range in winter season with low temperatures helpful to maintain glaciers; after March, the westerlies associated with mid-latitude cyclones convey precipitation in form of snow to favour the growth of glaciers.

A high concentration of glaciers in the Tian Shan is known as the "Water Tower of Central Asia" [23]. They play an important role in water cycle in this arid environment. The meltwater runoff into transboundary rivers, such as the Syr Darya River, the Ili River, the Kaidu River and the Urumqi River, contributes substantially to the freshwater resource in downstream Two distinct time periods, the 'Medieval Warm Period' and the 'LIA', represent unique but opposite climate conditions during the last millennium, and they set contemporary glacier changes into a long-term context [6]. Arguments and inconsistency exist on the accurate definition of the two climate episodes. Relatively abundant records in Europe and North America help identify such millennial or centennial glacier activities, but meanwhile, reflect a bias in the spatial representation when limited data are available in other key regions like central Asia. Until recent decades, studies on reconstructing past climate and glacier variations in the Tian Shan started to sprout, thanks to the development of techniques and better accessibility to the place.

In the Tian Shan, the terminal moraine characterized with no or little vegetation cover, massive piling of loose tills, sharply-crested ridge and a location a few hundred meters apart from glacier front is often believed as the mark of the LIA maximum extent [17, 34–36] (**Figure 2**). Numerical dating using proxy records is the approach for assigning the formation ages of such moraines and thus the timing of glacier fluctuations. These proxy materials include erratic boulders, buried wood, trees and lichens. Lichenometry is the dating method that derives moraine age from measuring the diameter of the largest/oldest lichen and uses the lichen's growth rate to infer the time of the moraine's formation. Although this method has been adopted since the 1950s, the difficulties such as species identification, establishing accurate growth curve and lack of knowledge of its colonization time on boulders creates many uncertainties and limits its use, e.g. [37–40]. Radiocarbon dating of organic matters, such as buried tree trunks that have been incorporated in moraine sediments or transported tree logs that are deposited on till plain or outwash plain, can help identify the maximum age of moraine formation. The drawback of this method is the large uncertainties associated

**Figure 2.** Field photos displaying the fresh-looking marginal moraines. (a), (b) and (c) show locations of the Glacier No.1, the Bogeda range, and the Karlik range, respectively, as indicated in **Figure 1(c)**; (d) is located in the Haxilegen site, as marked in **Figure 1(b)**.

with the age [41–43]. In the case of the Tian Shan, lack of organic materials within or nearby moraines often prevents the application of radiocarbon dating. Dendrochronology, which uses tree-ring crossdating technique, is another method to date glacial moraines and can often provide precise age control [44–47]. The year of the innermost ring of living trees that grow on the moraine can indicate the minimum age of the moraine. Tree-ring dated outermost ring of glacially killed trees can be used to cross-validate radiocarbon results. But, the challenge is also the availability of the dating materials: trees are rarely present at margins of glaciers in the Tian Shan. Since the 1990s, cosmogenic exposure dating technique has allowed great improvement in dating glacial landforms. It measures the concentration of the targeted isotope that is only produced by the interaction of cosmic rays with minerals in rocks and then calculates the surface exposure time of the rock [48–51]. Nuclide isotope 10Be is the most widely used one in the application of reconstructing glacial chronology [52]. The advancement in techniques has made this method dating young event on timescales of 100 years possible, for examples, in New Zealand [16], Greenland [53], Switzerland [54] and other places including the Tian Shan [55–57].

In the Tian Shan, the terminal moraine characterized with no or little vegetation cover, massive piling of loose tills, sharply-crested ridge and a location a few hundred meters apart from glacier front is often believed as the mark of the LIA maximum extent [17, 34–36] (**Figure 2**). Numerical dating using proxy records is the approach for assigning the formation ages of such moraines and thus the timing of glacier fluctuations. These proxy materials include erratic boulders, buried wood, trees and lichens. Lichenometry is the dating method that derives moraine age from measuring the diameter of the largest/oldest lichen and uses the lichen's growth rate to infer the time of the moraine's formation. Although this method has been adopted since the 1950s, the difficulties such as species identification, establishing accurate growth curve and lack of knowledge of its colonization time on boulders creates many uncertainties and limits its use, e.g. [37–40]. Radiocarbon dating of organic matters, such as buried tree trunks that have been incorporated in moraine sediments or transported tree logs that are deposited on till plain or outwash plain, can help identify the maximum age of moraine formation. The drawback of this method is the large uncertainties associated

**Figure 2.** Field photos displaying the fresh-looking marginal moraines. (a), (b) and (c) show locations of the Glacier No.1, the Bogeda range, and the Karlik range, respectively, as indicated in **Figure 1(c)**; (d) is located in the Haxilegen site, as

marked in **Figure 1(b)**.

42 Glacier Evolution in a Changing World

Several studies focused on the direct dating of the LIA moraines in the Tian Shan are marked on **Figure 1**. As the climatic conditions shifted with cycles of coldness and warmth during the entire LIA, glaciers in the Tian Shan have been found with two or three moraine ridges that could be identified as LIA moraines [34], indicating different periods of glacial stagnations as glaciers respond to climate changes. In the Chinese Tian Shan, the Chinese Academy of Sciences established the Tian Shan Glaciological Station near the Urumqi Glacier No. 1 in 1959, and thereafter, the Glacier No.1 has become one of a few benchmark glaciers in central Asia (**Figures 1** and **2a**). The dating of the LIA moraines at this site has been conducted by different groups of researchers with various dating methods. Using lichenometry, Chen [58] dated three moraine ridges with minimum formation ages of 1538 ± 20 AD, 1777 ± 20 AD and 1871 ± 20 AD, respectively. Using AMS radiocarbon dating of inorganic carbonate coating of glacial boulders on the outermost moraine, Yi et al. [59] obtained two ages of 450 ± 120 yr and 480 ± 120 yr (calibrated in Ref. [17]), which are averaged and converted to 1535 ± 120 AD, consistent with Chen's results. Our group (authors and collaborators as mentioned in Acknowledgments) conducted cosmogenic exposure dating of the boulders from the fresh moraines at the same site and another site to the west, the Haxilegen Pass (**Figures 1b** and **2d**). The ages of seven samples from the Glacier No. 1 site clustered around 430 ± 110 yr, and the outer moraine at the Haxilegen site was dated to 430 ± 40 yr [57]. Similar ages from different methods provide reliable evidence of the LIA glacial extent, suggesting that the glacier retreated from its LIA maximum extent, 700–900 m from glacier front, approximately 430 years ago at sites in the Chinese Tian Shan. In the Kyrgyz Tian Shan, extensive work of dating LIA moraines has been done using lichenometry. Solomina et al. [36] studied the retreat of 293 glaciers across the Kyrgyz Tian Shan, and lichenometric dates of moraines indicated nearly identical maximum glacier advances occurred in the seventeenth to the mid-nineteenth centuries. Using cosmogenic exposure dating, Koppes et al. [56] measured two 10Be ages for one boulder from the outer moraine and one from the inner moraine in the Ala-Archa Valley, the Kyrgyz Front Range. They are recalculated in [17] to 612 ± 111 yr and 284 ± 75 yr which both belong to the LIA period.

The chronology evidence of the LIA maximum extents at the selected study sites better allows us to use morphologic appearance to interpret potential LIA extent in less-studied areas in the Tian Shan. Indeed, more moraine chronological data across different mountain ranges are needed to examine the timing and the extent of the LIA glaciers and to depict the spatial heterogeneity of LIA glacial events in the region.
