Global climate change has had a profound and lasting effect on the environment [1
]. The shrinkage of glacier ice caused by global warming has attracted a large amount of research interest, from the global scale to specific glaciers [2
]. Apart from polar ice [3
], most research is focused on glaciers on the third pole—the Asian high mountains [5
]. Called the Asian water tower, the Asian high mountains feed several major rivers by widespread glacier melt [10
]. Changing glacier mass there will have a far-reaching influence on the water supply of billions of people. Therefore, a good understanding of the glacier mass balance is important for planning and environmental adaptation.
The spatial scope of Asian high mountains mainly includes the Tibetan Plateau and the Tianshan. The Tianshan has some distinct features and is worth studying specifically. First, the Tianshan is the place most distant from the oceans in the Eurasian continent, so the atmospheric water vapor is usually blocked from reaching it [11
], and water resources are strongly dependent on meltwater from snow and glaciers; Second, the Tianshan is influenced by the mid-latitude westerly winds, quite different from the Himalayas, which are influenced by the Indian monsoon from the equator [9
Remote sensing observations are the only feasible approaches to estimating the overall mass balance of mountain glaciers in this region. Widely used techniques include space imagery, digital elevation models (DEMs), laser altimetry, and satellite gravimetry. Space-based imagery only constrains the area change in glaciers [12
], which is not enough to estimate the mass balance. Recently there have been results from the Pamir and Karakoram differencing two DEMs [14
]. ICESat, a type of laser altimetry, has a smaller footprint (70 m) compared with radar altimetry (several kilometers), so it is applicable in the estimation of the glacier mass balance. The implementation of ICESat in the Asian high mountains has been a popular method to constrain mass balance [7
]. Gravity changes observed by GRACE can directly constrain the overall glacier balance [2
], however, a special method is needed to restore the attenuated signals due to the coarse spatial resolution of GRACE (~300 km).
Recently, there has been a study in the Tianshan using three methods: satellite gravimetry, laser altimetry, and glaciological modeling [6
]. However, the mass change time series show great interannual variance (Figure 2a in Farinotti et al. [6
]), and the annual trend of glacier change even becomes positive around 2010 (Figure 6 in Farinotti et al. [6
]). This suggests that there is a large interannual variation in the glacier changes in the Tianshan. However, we have shown that there is a five-year fluctuation in the Pamirs in an earlier work [19
], and the large interannual variation in the Tianshan might be leakage from the signal in the Pamirs. One of the purposes of this work is to determine the spatial range of the large interannual fluctuation and check whether it affects estimates for glaciers in the Tianshan.
The Tianshan is mainly located in Xinjiang Province in western China. In the past decades, western China has been greatly developed as a result of its national policies. The geographical conditions in the Tianshan are diversified and are impacted jointly by climate change and anthropogenic activities. There are glaciers, snow, lakes, and burgeoning residential and agricultural districts in the area. These factors are intricately associated, and a synthetic study will aid in the understanding of the influencing and evolving mechanisms. This work features a systematic and comprehensive study of precipitation, snow, lakes, glaciers, and human activities in the Tianshan. Bosten Lake, a large lake located just at the foot of Tianshan and disturbed by intense anthropogenic activities is specifically discussed.
In the following sections, this work will be arranged in the following structure: First, changes in the total mass, glaciers, and snow coverage will be represented based on multiple data sources; second, we will discuss the spatial features of the large interannual variation introduced above and try to find common characteristics among different data sources; lastly, we will present the record of water level changes in Bosten Lake since 1955 and discuss its relationship with climate change and anthropogenic activies.
Glacier mass change in the Tianshan was estimated with GRACE data and ICESat data. The glaciers are divided into central, northern, and western parts according to spatial distribution. There are three main conclusions.
Firstly, there was a rapid transition from 2009, a dry year, to 2010, a wet year, and this phenomenon can be simultaneously found in the lake levels, snow coverage, precipitation, and total water mass. Furthermore, there is good consistency of spatial coverage in all these factors. The transition originated in the Pamir Plateau and extended eastward along the northern edge of Tianshan but is absent in central Tianshan. The change is likely caused by changes in the westerlies, and the AOI at the start of 2010 is the strongest over the last 65 years.
Secondly, glacier melting is concentrated in the central Tianshan, and the western part of the Tianshan demonstrates very little glacier mass change. After excluding the western and northern parts, which are polluted by the strong interannual variation caused by the varying westerlies, the central region of Tianshan shows a steady changing trend, which is −4.0 ± 0.7 Gt/year during 2003–2014 according to GRACE and −3.4 ± 0.8 Gt/year during 2003–2009 according to ICESat. Our results show a smaller mass loss and have better linearity compared with previous estimates.
Thirdly, we discussed how climate change and anthropogenic activities influence the water level of Bosten Lake. We demonstrate that the impact of climate change on the environment of Bosten Lake varies at different stages. Rising temperature benefited Bosten Lake during 1987–2002 along with the increasing precipitation, but has degraded its condition ever since due to the fast retreating snowline.