1 Introduction

Observations have revealed that China’s glaciers have been shrinking over the past half-century (Yao et al. 2004; Ding et al. 2006; Xiao et al. 2007; Li and Cheng 2008). The growth or decline of glaciers is determined by their mass balance, which is influenced by climate variables such as air temperature and precipitation (Kang et al. 2002; Shi 2008a, b). A study of several high-mountain glaciers in Asia showed that the equilibrium line altitude (ELA) is sensitive to variations in summer mean air temperature and annual precipitation (Zhang et al. 1998). The ELA varied from 52 to 152 m in response to a mean summer air temperature variation of 1 °C. Similarly, with a change of 100 mm in mean annual precipitation, the ELA varied from 9 to 85 m (Zhang et al. 1998). Significant ablation occurs when air temperatures are high, even if the high temperatures coincide with high levels of precipitation over the accumulation zone (Liu et al. 1999b; Li et al. 2003). Shi and Liu (2000) estimated that by 2100, glacier volume in China may be only 30–67 % than that of the present day. Xie et al. (2006) simulated scenarios in which temperatures warmed by 0.01, 0.03, and 0.05 K a−1 and found that by the end of the twenty-first century, China’s glaciated areas would decrease by 14, 40, and 60 %, respectively.

The Tianshan range extends through China, Kyrgyzstan, and Kazakhstan and contains approximately 16,000 glaciers covering a total of 15,416 km2 (Liu 1995; Shi 2008a, b). These glaciers have been the target of much climate-related research in recent years (e.g., Li et al. 2006; Aizen et al. 2006; Bolch 2007; Kutuzov and Shahgedanova 2009; Narama et al. 2010). With global warming, most mountain glaciers, including those in the Tianshan, are in a state of rapid retreat (Haeberli et al. 2007; Hoelzle et al. 2007; Wu and Zhu 2008; Li et al. 2009; Bolch et al. 2010; Nie et al. 2010). In Xinjiang, located in China’s arid continental interior, glaciers are an important freshwater source and vital for industrial and agricultural development in the district (Yang et al. 2007). Recently, the climate in Xinjiang has switched from warm and dry to warm and relatively wetter. Both temperature and precipitation have increased and the drought index has declined over the past 50 years (Yao et al. 2009). The Cold and Arid Regions Environmental and Engineering Research Institute of the China Academy of Science estimated that at the northern foot of the Tianshan over the next 20–40 years, small glaciers of about 1 km2 or less are at great risk of melting completely. Glaciers covering more than 5 km2 will suffer a high degree of ablation and retreat. Rivers whose headwaters are dominated by the outwash from these small glaciers will probably experience severe fluctuations in their average annual flow volume because they are sensitive to variations in glacier growth, decay, and meltwater production. Rivers in the Xinjiang district have various headwater sources, and the proportion of water contributed by different glacial sources also varies. Therefore, the responses of rivers in the Xinjiang Uygar Autonomous Region to future climate change will also vary (Zhai and Zhou 1997).

The northern foot of the Tianshan is a vital ribbon of economic development in the Xinjiang region. The central district of northern Xinjiang in the Manas River basin is a large oasis-irrigated area of great socioeconomic importance in the region. The climate of the Manas River basin is essentially arid, and there is a great concern and uncertainty regarding current water allocation and the future of water resources under global warming. The reliability and quality of water resources are also crucial for the future development of the region and its economic and ecological welfare. Future plans for the Manas River basin are currently focused on economic development. However, whether water will be available for such development must be closely monitored, especially considering the possibility of continued climate warming. Remote sensing is a useful tool for monitoring headwater glaciers and other water sources in the Manas River basin. Remote sensing allows for monitoring of large areas at relatively low costs and can aid in understanding how fragile resources, such as glaciers, might be affected by ongoing and future climatic changes.

2 The Manas River basin

The Manas River basin (43°27′–45°21′N; 85°01′–86°32′E) is located between the northern foothills of the Tianshan and the southern margin of the Junggar Basin. The southern part of the Manas River basin starts at the Eren Habirga Range and lies adjacent to Hejing County. The northern margin of the Manas River basin abuts the Gurbantunggut Desert. To the east, the basin starts at the Taxi River and ends in the west at the Bayingou River. The basin is 198.7 km from east to west and 260.8 km from south to north, with a total area of 2.43 × 104 km2. The basin is made up of approximately 1.1 × 104 km2 of mountainous terrain, 9.6 × 104 km2 of plains, and 3.5 × 104 km2 of sand dunes. The land surface within the basin slopes from southeast to northeast. The highest height above sea level is 5,242.5 m, and the lowest is 256 m. Land higher than 3,600 m amsl is covered with accumulated snow year round. The area covered by glacial ice is 1,037.68 km2 (Fig. 1).

Fig. 1
figure 1

The figure of Manas River water system of Manas River basin (in accordance with Yuan Yujiang, Yu Shulong, 2005)

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3 Methodology

  1. (1)

    Glacier area: The glacier area is relatively simple to detect and measure via remote sensing. The size of a glacier is sensitive to the climate, and this is particularly true for the ratio of summer ablation (ice loss) to winter accumulation (growth) (Wang et al. 2008, 2011). We collected information regarding change in glaciated area in the Tianshan since 1960 (Table 1). To better understand variations on different spatial and temporal scales, it is important to identify the relationships between climatic variables and the mechanisms of changes in glacier size (Ren 1988; Hu 2004; Ding et al. 2006; Li et al. 2010a).

    Table 1 Sources of data regarding change in glacial area in the Chinese Tianshan
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Our primary information sources were published data of the annual percentage change in glaciated surface area (Ding et al. 2006; Wang et al. 2011). In China, most glaciers are in decline, and therefore, their annual change in surface area is negative. We modified a model that was originally established by Wang et al. (2011) to calculate an area-weighted mean growth rate. We used this area-weighted growth rate to assess the vulnerability of Tianshan glaciers. The area-weighted shrinking rate is

$$ \frac{\Updelta S}{{S_{0} }} = \sum\limits_{i = 0}^{n} {\left( {\frac{{S_{i} }}{{\sum\nolimits_{i = 0}^{n} {S_{i} } }}\frac{{\sum\nolimits_{j = 0}^{n} {\Updelta S_{ij} } }}{{\sum\nolimits_{j = 0}^{n} {S_{0ij} } }}} \right)} $$
(1)

where i is the order number of units in this study, j is the order number of references for each unit, S 0ij is the glacier area at the initial status (km2), ΔS ij is the variation of glacier area (km2), S i is the total area of glaciers in the unit which is acquired from the GIC (km2), i, j = 1, 2, 3,… Annual percentages of area changes of glacier’s acreage in any statistical unit since 1960 could be acquired with the method of interpolation:

$$ {\text{APAC}}_{i} = \frac{{\sum\nolimits_{j = 0}^{n} {\frac{{\Updelta S_{ij} }}{{\Updelta t_{ij} }}} }}{{\sum\nolimits_{j = 0}^{n} {\left[ {S_{0ij} + (t_{0ij} - 1960)\frac{{\Updelta S_{ij} }}{{\Updelta t_{ij} }}} \right]} }} $$
(2)

Annual percentages of area changes of glacier’s acreage in the entire Manas River:

$$ {\text{APAC}} = \frac{{\sum\nolimits_{i = 0}^{n} {\left\{ {S_{i} \frac{{\sum\nolimits_{j = 0}^{n} {\frac{{\Updelta S_{ij} }}{{\Updelta t_{ij} }}} }}{{\sum\nolimits_{j = 0}^{n} {\left[ {S_{oij} + (t_{0ij} - 1960)\frac{{\Updelta S_{ij} }}{{\Updelta t_{ij} }}} \right]} }}} \right\}} }}{{\sum\nolimits_{i = o}^{n} {S_{i} } }} $$
(3)
  1. (2)

    Single-element regression analysis was applied to climate data for the Xinjiang Uygur Autonomous Region obtained from the Climate Information Center. This analysis was conducted to examine the data for linear trends between runoff and precipitation (Li et al. 2011). Single-element analysis and areal interpolation are widely used in climatic research on mountain areas, such as for the Qilian (Jia et al. 2008) and Hengduan (Li et al. 2010b) mountain regions.

We used images taken at four different times for our remote-sensing analysis. The images were from the Landsat Multispectral Scanner (MSS) taken in 1964, the Landsat Thematic Mapper (TM) taken in 1983, the China-Brazil Earth Resources Satellite (CBERS) taken in 2006, and Landsat Enhanced Thematic Mapper taken in 2000. MSS (1964) used the artificial colors of 421 wavebands to make a compound image. The remote-sensing images taken in 1983, 2000, and 2006 used 432 waveband colors. Erdas 9.2, ArcGIS 9.3, and Photoshop 8.0 were used to digitize the images and examine topographic relationships.

4 Variations in glacier area in the Manas River basin during the past 50 years

Western China has become warmer over the past 50 years (from 1960 to 2010) as the Earth’s mean air temperature has increased by an average of 0.2 °C per decade. Approximately 82 % of the glaciers in the Manas River basin receded during this period (Yao et al. 2004; Ding et al. 2006; Wang et al. 2011). Nearly, all of the glaciers on the northern slopes of the Tianshan showed continual retreat, and 69 % of glaciers on southern slopes retreated. The area of glacier cover in the central Manas River basin decreased by 10 % (Shi and Liu 2000).

Presently, glaciers cover 608 km2 of the Manas River basin and the glacier volume is 3.9 × 108 m3. Over the 40-year period from 1960 to 2001, there has been a mean 0.41 % reduction in the area covered by glaciers. Annual glacier runoff was estimated to reach a maximum of 4.42 × 1010 m3 and is an essential water resource (Li et al. 2010a, b).

Figure 2 and Table 2 show that glacier mass balance remained negative over the 46-year study period. Annual mass balance between 1960 and 1985 was −84.5 mm year−1, whereas that between 1986 and 2006 was three times larger at −328.4 mm year−1. Glacier runoff depth also increased from approximately 478.4 mm year−1 between 1960 and 1985 to 836.6 mm year−1 between 1986 and 2006, a 74.8 % increase. Clearly, the rapid warming and increased melting have caused the volume of meltwater runoff to increase.

Fig. 2
figure 2

Variation of the annual glacial meltwater runoff in Manas River basin during 1960 to 2006

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Table 2 Melted percentage of glacier’s acreage in Manas River basin
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5 Changes in meltwater runoff in the Manas River basin from 1960 to 2010

During a warm and dry year, glacier melting increases and the increased glacier runoff can compensate for decreased rainfall in the headwater area. Conversely, during a wet and cold year, glacial runoff decreases and does not contribute as much to the total outflow from the river valley. However, in 1982/1983, there was an anomalous trend in glacier development. During that year, the average glacier retreat was -250 mm, but both the meltwater and river data show an obvious positive trend.

Sensitivity analysis showed that the annual average temperature variation was 1 °C and that the glacier mass balance varied by about 300 mm. This variation caused an 8 % fluctuation in flow volume in the Manas River. As melting intensifies, runoff increases (the increase is about 19 %), with a peak volume at around May to June. Because peak runoff increases during the summer, it is possible that glacial lake outbursts may have contributed to the river outflow.

6 Future glacier forecasts and their impact

6.1 Future climate change scenarios in the Manas River valley

Both temperature and rainfall have shown increasing trends in the Xinjiang region in recent 50 years. In the period from 1960 to 2010, both temperature and rainfall increased, with average temperature rising by 0.2 °C and rainfall by 15 mm (Li et al. 2011). These findings, combined with global climate model results from the Intergovernmental Panel on Climate Change (IPCC 2007), suggest that Xinjiang will have warmer temperatures and increased precipitation in the early twenty-first century. Drought and flooding are also expected to increase. Under the A1B scenario of the IPCC for the period 2007–2015 in the Manas River basin, temperature is predicted to rise by 1.19 °C and rainfall by 16.42 mm.

6.2 Future meltwater trend and its influence on water resources

By 2050, the temperature in the southern Junggar Basin is predicted to be 2.1 °C higher, with summer warming of about 1.3 °C. Of the glaciers in this area, 2,935 are less than 1 km2; these glaciers cover a total area of 807 km2 and hold ice reserves of 21.54 km3. With increased temperatures, these glaciers may melt completely (Bradley et al. 2009; Zhang et al. 2010). In the early twenty-first century, meltwater runoff of the Urumqi River is predicted to peak, reflecting the melting of most of the upland glaciers. Runoff will increase by 5–10 % in the early twenty-first century and by 8–15 % in the middle twenty-first century. With the increase in glacier melting in the early to mid-twenty-first century (Zhang et al. 2009), the Manas River outflow will likely be greater than at present with an incremental increase of 108 m3 year−1. The increased flow will be temporarily beneficial for economic development downstream. By 2050, however, the glacier runoff in the Manas River valleys will show decreasing trends (Table 3).

Table 3 The collection results of the future change trend of valley glacial meltwater in 3 kinds of climate scenarios (unit: 1 × 107 m3)
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6.3 Future melting trend and its response to rainfall

In future climate change scenarios, rainfall and temperature in the Tianshan mountain area will continue to increase. Rainfall is forecast to increase by 0.45–1.56 % and temperature by 0.49 to −4 °C by 2050. However, the flow volume will show large inter- and intra-annual variability of 25–50 %, which will exacerbate flood and drought events in the valley. Glaciers in the Manas River valley respond mainly to variations in rainfall. In a scenario in which future rainfall is reduced by 20 % (~100 mm year−1), the glacial area decreases by 1.5–8.6 %, glacier ice reserves decline by 1.8–9.8 %, and runoff increases by 6.6–17 %. In a scenario in which rainfall in 2050 decreases by 0.03 mm year−1, the glacier area shrinks by 2–12.5 %, volume is reduced by 3–13.3 %, and runoff is increased by 9.6–23.3 %.

6.4 Regional vulnerability in the case of retreating glaciers

We calculated water supply and demand and used single surrogate markers to calculate the vulnerability of water resources, agriculture, and ecosystems to changes caused by glacier retreat. Our climate change forecasts suggested that, as glacier melt, the Manas River basin region will become increasingly vulnerable to shortages in water resources, which will affect ecological and socioeconomic systems (Table 4).

Table 4 Vulnerability index
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6.5 Risk to planned socioeconomic development in the study area

6.5.1 Estimation of risk

To estimate the risk to planned socioeconomic development, we developed criteria for selecting risk factors. Factors were then selected and a risk evaluation index system was built, based on long-term changes in temperature, rainfall, and snowmelt in Xinjiang. We determined the degree, probability, and likely hazard of each climate variable’s influence on main elements of the economy and the structure of society. We then evaluated each individual factor and the degree of risk to the whole of Xinjiang region and key areas.

6.5.2 The technical scheme of evaluation estimate and selection of risk factor

See Tables 5 and 6.

Table 5 The technical scheme of evaluation estimate and selection of risk factor (10 aspects)
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Table 6 The grading standard of the degree of risk (4 grades)
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6.5.3 The evaluation of risk degree in Manas River valley

Because of global warming in Manas River valley, the risk brought by glacial melting is gradually increasing, but the risk level is not high. Before 1930s, it is a general risk, and after 1930s, it becomes bigger risk level (Table 7).

Table 7 Computation of the risk index in Manas River
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7 Conclusion

  1. (1)

    With rapid warming in this valley since the 1980s, runoff from melting glaciers has also increased rapidly. Glacial mass balance has been negative with the average mass balance from 1960 to 1985 being −84.5 mm year−1 and that from 1986 to 2006 being −328.4 mm year−1. The average meltwater runoff depth from glaciers in the Manas River basin was 478.4 mm year−1 from 1960 to 1985 and 836.6 mm year−1 from 1986 to 2006.

  2. (2)

    In the early and mid-twenty-first century, increased melting of glaciers will make the runoff from the Manas River 108 m3 greater than at present. This increase will be temporarily beneficial to economic development in downstream areas. However, by 2050, the glacier runoff in the Manas River valley will decrease and the river will become one of the smallest along the northern slopes of the Tianshan.

  3. (3)

    In a scenario for 2050 in which rainfall is reduced by 0.02 mm year−1 (20 % reduction), the glacier area is predicted to decrease by 1.5–8.6 %, glacier volume will decrease by 1.8–9.8 %, and runoff of meltwater will increase by 6.9–17.1 %. If rainfall decreases by 0.03 mm year−1 (30 % reduction), the glacier area may shrink by 2.0–12.5 %, with a volume reduction of 3–13.3 % and an increase in runoff of 9.6–23.3 %.

  4. (4)

    Global warming is increasing the risk of glacier melting in the Manas River valley, but the risk level is not yet high. Until the 2030s, a general level of risk will exist, but after that period, the risk will become greater.