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Journal of Geographical Sciences >

2021 , Vol. 31 >Issue 9: 1283 - 1298

DOI: https://doi.org/10.1007/s11442-021-1898-9

Research Articles

Recent signal and impact of wet-to-dry climatic shift in Xinjiang, China

  • YAO Junqiang ,
  • MAO Weiyi ,
  • CHEN Jing ,
  • DILINUER Tuoliewubieke
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  • Institute of Desert Meteorology, China Meteorological Administration, Urumqi 830002, China

Yao Junqiang (1987-), Associate Professor, specialized in climate change and water cycle. E-mail: ;

Received date: 2021-05-06

  Accepted date: 2021-07-16

  Online published: 2021-11-25

Supported by

National Key Research and Development Program of China(2019YFA0606902)

National Natural Science Foundation of China(41975146)

National Natural Science Foundation of China(41971023)

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Copyright reserved © 2021. Office of Journal of Geographical Sciences All articles published represent the opinions of the authors, and do not reflect the official policy of the Chinese Medical Association or the Editorial Board, unless this is clearly specified.
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Abstract

The Xinjiang region of China is among the most sensitive regions to global warming. Based on the meteorological and hydrological observation data, the regional wet-to-dry climate regime shifts in Xinjiang were analyzed and the impacts of climatic shift on the eco-hydrological environment of Xinjiang were assessed in this study. The results showed that temperature and precipitation in Xinjiang have increased since the mid-1980s, showing a warming-wetting trend. However, drought frequency and severity significantly increased after 1997. The climate of Xinjiang experienced an obvious shift from a warm-wet to a warm-dry regime in 1997. Since the beginning of the 21st century, extreme temperatures and the number of high temperature days have significantly increased, the start date of high temperature has advanced, and the end date of high temperature has delayed in Xinjiang. In addition, the intensity and frequency of extreme precipitation have significantly increased. Consequently, regional ecology and water resources have been impacted by climatic shift and extreme climate in Xinjiang. In response, satellite-based normalized difference vegetation index showed that, since the 1980s, most regions of Xinjiang experienced a greening trend and vegetation browning after 1997. The soil moisture in Xinjiang has significantly decreased since the late 1990s, resulting in adverse ecological effects. Moreover, the response of river runoff to climatic shift is complex and controlled by the proportion of snowmelt to the runoff. Runoff originating from the Tianshan Mountains showed a positive response to the regional wet-to-dry shift, whereas that originating from the Kunlun Mountains showed no obvious response. Both climatic shift and increased climate extremes in Xinjiang have led to intensification of drought and aggravation of instability of water circulation systems and ecosystem. This study provides a scientific basis to meet the challenges of water resource utilization and ecological risk management in the Xinjiang region of China.

Key words： climatic shift; wet-to-dry; water resources; drought; Xinjiang

Cite this article

YAO Junqiang , MAO Weiyi , CHEN Jing , DILINUER Tuoliewubieke . Recent signal and impact of wet-to-dry climatic shift in Xinjiang, China[J]. Journal of Geographical Sciences, 2021 , 31(9) : 1283 -1298 . DOI: 10.1007/s11442-021-1898-9

1 Introduction

Drought as one of the most serious meteorological disasters is characterized by high frequency, long duration, and extensive-affected scope, hence has attracted great attention from the scientific community and society (He et al., 2011; Yang et al., 2017; Ma et al., 2018; Zhang and Shen, 2019; Zhang et al., 2019). With global warming and the resultant acceleration of water cycle process, the climate of Xinjiang has changed obviously, giving rise to widespread concern (Chen et al., 2015, 2020; Huang et al., 2015, 2017; Yao et al., 2016, 2020; Zhang et al., 2012). At the beginning of the 21st century, Shi et al. (2003, 2007) put forward the warming-wetting climatic shift taking place in the northwestern arid region of China, especially more distinctive in Xinjiang. Since the beginning of the 21st century, the climate of Xinjiang has significantly changed. The temperature increased abruptly and subsequently remained high and afterward fluctuated since the late 1990s. Meanwhile, the precipitation decreased slightly (Yao et al., 2018b). These changes will inevitably impact the regional wet-to-dry climatic shift and subsequently affect the water resources and ecological security of Xinjiang.
Currently, most monitoring indexes have been used to quantitatively characterize droughts worldwide; drought monitoring technology is being developed in the fields of information synthesis and technology integration (Zhang et al., 2011). Variations in regional drought are affected by both water and energy regimes, that is, precipitation and potential evapotranspiration (PET). Variations in precipitation and PET are the two main driving factors of arid climate formation (Vicente-Serrano et al., 2010a; Yao et al., 2018b). The standardized precipitation evapotranspiration index (SPEI) considers both precipitation and PET and can reasonably evaluate drought variations at different timescales (Beguería et al., 2004; Tao et al., 2014; Vicente-Serrano et al., 2010b, 2015). In addition, SPEI can directly reflect the distribution and variation trends of regional humidity.
Under global warming and increased human activities, the threat of a dry-wet climatic shift to regional water resources, ecological security, and socio-economic sustainable development has become increasingly prominent (Chen et al., 2015, 2020). As an important arid area of Central Asia, Xinjiang has less water resources and is sensitive to global climate change. Moreover, it has the most vulnerable ecological environment (Yao et al., 2019). Vegetation is the link between atmosphere, water, and soil. Therefore, analyzing the dynamic changes in vegetation coverage to study the influence of dry-wet climatic shifts on vegetation ecosystems is of great significance. Xinjiang has various forms of water resources, with widely developed mountain glaciers, and significantly varying proportion of snowmelt to the total runoff of each basin (Chen et al., 2015). Climate change can affect snow cover and glacier variations, which can subsequently affect runoff and water resources. Due to global warming, glaciers in mountainous areas are rapidly shrinking, which has significantly impacted water resources, ecological environment, and socio-economic development (Wang et al., 2011; Chen et al., 2017). Therefore, finding ways to monitor drought variation and evaluate the impacts of dry-wet climatic shift on water resources and ecological environment objectively and accurately are important scientific issues to address.
Thus, in this study, the spatio-temporal variation characteristics of the dry-wet climate of Xinjiang were systematically analyzed, impacts of wet-to-dry climatic shift on regional water resources and vegetation growth were comprehensively evaluated, and possible ways through which the wet-to-dry climatic shift affects water cycle system and ecosystem were discussed. The results can provide a valuable decision-making reference for preventing and reducing regional drought disasters and managing water resources.

2 Study area, data, and method

2.1 Study area

Located in the hinterland of Eurasia and western part of arid northwestern China, Xinjiang is an important arid area of Central Asia. It is dominated by continental arid climate, with scarce precipitation and frequent droughts. The topography shows a pattern of “three mountains with two basins situated in between”, forming a unique mountain-oasis- desert ecosystem. The unique water cycle, fragile ecological environment, and extreme sensitivity to climate change make it a typical arid area and a key area for global climate change research.

2.2 Data and method

2.2.1 Data
The meteorological data used in this study included daily air temperature, precipitation, relative humidity, wind speed, and surface pressure observed at 89 meteorological stations located in Xinjiang from 1961 to 2018. The data were provided by the Meteorological Information Center of Xinjiang Uygur Autonomous Region. After strict data quality control, the data from 55 evenly distributed weather stations with complete observation data and strong representativeness were finally selected to analyze the climate elements and dry-wet climatic shift. Subsequently, based on the monthly meteorological data, the monthly SPEI time series on the scales of 1-24 months for 1961-2018 were established. SPEI was used as the drought index in this study, which reflected the regional dry-wet distribution and its variation trend. SPEI has multiple timescales and is sensitive to temperature variation, thus, is of great advantage to be used for dry-wet analysis with global warming as the background (Vicente-Serrano et al., 2010a, 2015).
The hydrological data of five typical river basins of Xinjiang from 1961 to 2017 obtained from the Hydrological Bureau of Xinjiang Uygur Autonomous Region were selected to analyze the correlations between climate elements and runoff variations. These basins included Ebinur Lake Basin situated on the northern slope of the Tianshan Mountains, Kaidu and Aksu river basins situated on the southern slope of the Tianshan Mountains, and Hotan and Yarkant river basins situated on the northern slope of the Kunlun Mountains. The meteorological observation data of the runoff forming region were used to represent the climate change of the river basins. Normalized difference vegetation index (NDVI) is widely used to characterize vegetation coverage (Yao et al., 2018a). NDVI data were obtained from the Global Inventory Modeling and Mapping Studies (GIMMS) NDVI dataset released by the National Aeronautics and Space Administration (NASA) (Zhu et al., 2013). Its spatial resolution was 8 km × 8 km, with temporal resolution being 15 d. To reflect the characteristics of surface vegetation coverage, NDVI data of the vegetation growth season (April-October) were selected, and subsequently, the GIMMS NDVI datasets for each month from April to October and for the vegetation growth season were obtained by using the maximum synthesis method.
Terrestrial water storage (TWS) data were retrieved from the time-varying gravity field of the Gravity Recovery and Climate Experiment (GRACE) satellites. They can be used to monitor regional droughts (Cao et al., 2015). The monthly data have a spatial resolution of 1° × 1°, covering the period from August 2002 to July 2013. There were some missing data as well, including June 2003, January 2011, June 2011, May 2012, and October 2012 and 2013, which were treated as vacancies.
2.2.2 Applicability of SPEI to monitor the drought in Xinjiang
GRACE data have been widely used for regional drought monitoring. Cao et al. (2015) confirmed that GRACE satellites can effectively monitor drought events in arid regions, and drought events monitored in Xinjiang were consistent with the records. Therefore, the relative water storage index (RWSI) retrieved from the GRACE satellite data was selected to verify the applicability of SPEI and standardized precipitation index (SPI) in monitoring the drought in Xinjiang.
An SPEI threshold value of -1.0 was used to determine the drought condition (Table 1). The results showed that GRACE satellites successfully captured the drought events during the summers of 2007 and 2008-2009, which were consistent with the records of the disaster bulletin. SPEI also monitored two drought events with the SPEI values of less than -1.5, indicating that they became severe drought (Figure 1). However, SPI was unable to monitor the drought events in Xinjiang, indicating that it seriously underestimated the drought disasters. Although some deviations existed, both RWSI and SPEI could monitor the drought events in Xinjiang. Therefore, SPEI, which considers the effects of both precipitation and evaporation, was considered more effective for monitoring the drought variations in Xinjiang than SPI, which only considers precipitation. In addition, SPEI at different timescales can monitor different types of droughts. For example, SPEI at 6- and 12-month timescales represents agricultural and hydrological droughts, respectively.
Table 1 Drought classifications of SPEI
Category Index value
Extremely wet SPEI≥2
Moderately wet 1.5≤SPEI<1.99
Slightly wet 1≤SPEI<1.49
Near normal -0.99<SPEI<0.99
Mild drought -1.49<SPEI≤-1
Moderate drought -1.99<SPEI≤-1.5
Extreme drought SPEI≤-2
Figure 1 Comparison of the GRACE-TWS and 12-month SPI (a) and SPEI (b) over Xinjiang, China (The green line represents the SPEI or SPI value that was -1, and a threshold value of -1 was used to determine drought condition)

3 Results and analysis

3.1 Change in temperature and precipitation in Xinjiang

The annual mean temperature of Xinjiang showed a significant increase during 1961-2018, with a warming rate of 0.30℃/10a (p < 0.01), higher than that during 1951-2018 in China (0.24℃/10a) (Figure 2a). The results of Mann-Kendall (M-K) mutation test showed that temperature in Xinjiang was mutated in 1997, after which it significantly increased and fluctuated at a high level. The multi-year mean temperature after 1997 was found to increase by 1.1℃ when compared with that before 1997. The variation trend showed that temperature increased sharply during 1987-1997, with the warming rate being as high as 0.79℃/10a (p < 0.01). However, the warming trend stagnated after 1997, with the warming rate being only 0.11℃/10a (p > 0.05), but still fluctuated at a high level.
Figure 2 Changes of mean annual temperature (a) and annual total precipitation (b) during 1961-2018 over Xinjiang, China
Precipitation in Xinjiang changed simultaneously with temperature during 1961-2018, increasing at a rate of 10 mm/10a (p < 0.01) (Figure 2b). The results of M-K test showed that precipitation in Xinjiang was mutated in 1986. The annual precipitation was relatively stable before 1986 and significantly increased afterwards (with an increase of 35.3 mm), and the interannual variation in precipitation increased after 1997.
The recalculated results of climate change for the past 300 years based on the tree-ring proxy data of Xinjiang showed that the main historical climate types in northern Xinjiang and Tianshan Mountains were cold-dry and warm-wet climate regimes, accounting for 32% and 30% of the climate regimes, respectively, whereas warm-dry and cold-wet climate regimes accounted for 12% and 10%, respectively. Since the mid-1980s, temperature and precipitation in Xinjiang have significantly increased, showing an obvious transition signal to a warm-wet climate regime. However, after 1997, the temperature fluctuated at a high level but precipitation showed a slight reduction.

3.2 Change in extreme climatic events over Xinjiang

Under the background of global warming, the frequency and intensity of extreme climates in Xinjiang have been gradually increasing. The extreme maximum temperature was found to increase during 1961-2018, with a warming rate of 0.13℃/10a (p < 0.05). Specifically, the number of high temperature days greater than or equal to 35℃ have significantly increased, with the first day of high temperature advancing and the end day of high temperature delaying (Figure 3). With global warming, the frequency and intensity of extreme high temperatures have also increased, especially since the beginning of the 21st century. The extreme minimum temperature was observed to evidently increase, with a warming rate of 0.67℃/10a (p < 0.01), significantly higher than that of the extreme maximum temperature. Warming has become more evident since the 1980s, with the extreme minimum temperature significantly fluctuating since the beginning of the 21st century (Figure 3).
Figure 3 Changes of the Max Tmax (a. ℃), annual count when daily max temperature ≥35℃ (b. days), Min Tmin (c. ℃), annual count when daily minimum temperature ≤-30℃ (d. days) during 1961-2018 over Xinjiang, China
The extreme and average daily maximum precipitations in Xinjiang significantly increased from 1961 to 2018, with the rate of 0.9 d/10a and 0.86 mm/10a (p < 0.05), respectively (Figure 4). No noticeable increasing trend was observed in the maximum number of persistent precipitation days (0.06 d/10a, p > 0.05); however, the maximum number of consecutive dry days significantly decreased (0.97 d/10a, p < 0.05). The annual rainfall of rainstorm and rainstorm days showed an obvious increasing trend, with the increasing rates of 1.82 mm/10a and 0.05 d/10a (p < 0.01), respectively. Moreover, the annual snowstorm amount and days also showed an obvious increasing trend with rates of 0.47 mm/10a and 0.03 d/10a (p < 0.01), respectively.
Figure 4 Changes of the R24 (a. days), RX1day (b. mm), number of extreme rainfall days (c. days), annual total extreme rainfall (d. mm), number of extreme snowfall days (e. days), and annual total extreme snowfall (f. mm) during 1961-2018 over Xinjiang, China

3.3 Wet-to-dry climatic shift over Xinjiang

From 1961 to 2018, the 12-month SPEI in Xinjiang showed a decreasing trend and evident interdecadal characteristics with a variation trend of -0.0135/a (p < 0.05). In 1997, the climate in Xinjiang experienced an obvious abrupt transition from a warm-wet climate to a warm-dry climate (p < 0.05) (Figure 5), that is, from a relatively wet period during 1961-1996 (with a variation rate of 0.0269/a) to a drought period during 1997-2018 (with a variation rate of -0.0199/a) (Figure 5). From the mid-late 1980s to the 1990s, the warm-wet climate was distinct, with a significantly increased temperature, precipitation, and drought index. As the temperature increased abruptly, PET also increased, but precipitation showed a slower increase, indicating significant fluctuations in the interannual variation. This resulted in an obvious warming and drying regime, that is, the wet-to-dry climatic shift that occurred since the late 1990s.
Figure 5 Hovmoller-type diagrams for the temporal variability of the SPEI at different timescales (1 to 24 months) during 1961-2018 over Xinjiang, China
The year 1997 was a turning point for the dry-wet climate of Xinjiang. During 1961-1996, SPEI showed that 72.5% of the stations had an obvious warming-wetting trend, mainly in southwestern Xinjiang, Tianshan Mountains, and northern Xinjiang, but Tacheng and southeastern Xinjiang showed a slight drying trend. However, SPEI during 1997-2018 showed significant regional differences. More than 70% of the stations showed a gradual drying trend, mainly in the Tianshan Mountains, Ili River Valley, the northern slope of Kunlun Mountains, and eastern Xinjiang, but wetting was predominant in the Tacheng and Altay regions of northwestern Xinjiang, western part of southern Xinjiang, and the Pamirs Plateau. According to the multi-source data, such as precipitation data, self-calibrating Palmer Drought Index (sc_PDSI), and TWS data retrieved from GRACE, Ma et al. (2018) also concluded that Xinjiang has been experiencing an aridification trend since the beginning of the 21st century.
From 1961 to 2018, the annual average drought months (SPEI ≤ -1) in Xinjiang showed an obvious increasing trend (0.5 month/10a, p < 0.01). Before 1997, two or fewer drought months per year were observed, however, the number of drought months has gradually increased thereafter. Since the beginning of the 21st century, the average number of annual drought months has exceeded four. During 1997-2018, the drought frequency significantly increased; the greater the drought intensity, the more obvious the increase in frequency. The extreme drought frequency increased from 0.6 times/a during 1961-1996 to 2.65 times/a during 1997-2018. In addition, Xinjiang mainly suffered from regional drought during 1961-1996, whereas the ratio of stations reporting drought increased noticeably during 1997-2018; the arid land areas in Xinjiang obviously expanded after 1997.

3.4 Impact of climatic shift on Xinjiang

3.4.1 Impact of climatic shift on changes of vegetation coverage
NDVI showed an obvious increasing trend, with an increasing rate of 0.004/10a (p < 0.05), indicating vegetation greening during 1982-1997. However, it decreased significantly at a rate of 0.003/10a (p < 0.05) after 1997, indicating that vegetation degraded significantly after 1997 and ecological reversal occurred (Figure 6). Vegetation degraded areas were mainly situated in the natural vegetation covered areas, such as Ili River Valley, Tacheng, and Altay, constituting more than 25% of Xinjiang, while vegetation coverage mainly increased in areas along the Tarim River, northern slope of Tianshan Mountains, northern slope of Kunlun Mountains, and the Pamir Plateau, constituting approximately 20% of Xinjiang. This was mainly related to oasis expansion and ecological water transfer projects. It should be noted that vegetation index increased with fluctuations after 2008, but the increasing trend was not obvious.
Figure 6 Temporal variation of the average NDVI over Xinjiang from 1982 to 2015 (The dotted represents the trends of different periods. Blue line represents the lowpass filter result)
Climate is a dominant factor in vegetation coverage variation in Xinjiang. In general, vegetation coverage was usually positively correlated with precipitation and negatively correlated with PET. During 1982-2015, the correlation coefficients between NDVI and precipitation, PET, and SPEI were 0.34 (p < 0.05), -0.52 (p < 0.01), and 0.36 (p < 0.05), respectively. After 1997, the relationship between NDVI and climatic factors became more significant. The negative correlation between NDVI and PET significantly increased, with the correlation coefficient reaching -0.58 (p < 0.01), and the correlation coefficient between NDVI and precipitation increased to 0.38 (p < 0.05). Therefore, the decrease in vegetation coverage was mainly controlled by PET. In arid areas, an increase in PET can promote soil water evaporation, induce drought, and result in vegetation degradation.
Since the 1980s, soil moisture in Xinjiang decreased at a rate of -3.8%/10a (p < 0.01), especially in the shallow layer (Table 2). The results of M-K test showed that soil moisture decreased abruptly and significantly in 1994 (p < 0.01). After mutation, the average soil moisture decreased by 42.2%, which was significant during 1994-1997 and stabilized after 1997. The rapid increase in temperature expanded the demand for PET and increased soil evaporation, resulting in an abrupt decrease in soil moisture.
Table 2 Mean value and tendency of soil moisture at each depth (0-50 cm) over Xinjiang from 1961 to 2010
Depth (cm) 0-50 0-10 10-20 20-30 30-40 40-50
Mean value (%) 11.3 9.5 10.7 11.5 12.0 12.9
Trend (%/10a) -3.8 -3.6 -4.0 -3.5 -3.0 -2.3
3.4.2 Impact of climatic shift on hydrologic regime and water resources
From 1961 to 2017, the main river runoff in Xinjiang showed an obvious increasing trend, which was consistent with the variations in temperature and precipitation. Since the begin-ning of the 21st century, obvious regional differences have appeared in the variation of river runoff in Xinjiang. From 1961 to 2017, the runoff of Tarim River Basin showed an increasing trend, with the runoff of Aksu River increasing significantly at a rate of 2.36 × 108 m3/ 10a (p < 0.01) (Figure 7a). The runoff of Yarkant River showed an insignificant increasing trend (2.31 × 108 m3/10a, p > 0.05) (Figure 7b). The runoff of Hotan River increased slightly (1.40 ×108 m3/10a), but it did not pass the significance test (p > 0.05) (Figure 7c). The runoff of Kaidu and Aksu rivers situated on the southern slope of the Tianshan Mountains increased and subsequently decreased, whereas the runoff showed an increasing trend on the northern slope of the Kunlun Mountains. For example, the runoff of Hotan River showed an obvious increasing trend after the mid-1990s, with an increasing rate of 0.659 × 108 m3/a.
Figure 7 Changes of the annual runoff anomaly over the Tarim River Basin during 1961-2017 (a. Aksu River; b. Hotan River; c. Yarkant River; d. Mainstream)
Based on the analyses of the relationships between the runoff variations of seven representative rivers and SPEI of the corresponding river basins of Xinjiang from 1961 to 2018, it was found that the runoff variations of rivers originating from the Tianshan Mountains were positively correlated with SPEI, with the correlation coefficients ranging between 0.16 and 0.53. Among these rivers, the correlation coefficient of Ebinur Lake Basin passed the significance test at a confidence level of 99%. The variations in the runoffs of Hotan and Yarkant rivers originating from the Kunlun Mountains were negatively correlated with SPEI, with the correlation coefficients of -0.35 and -0.29, respectively. This implied that SPEI has a certain significance in indicating the variations in runoffs, which was similar to the conclusions made for six major river basins of China (Zhai et al., 2010). In addition to the influence of precipitation and wet-to-dry shift on the confluence area, the obvious differences in river runoff variations in different regions of Xinjiang were also affected by the proportion of runoff recharge from glacial meltwater and snow meltwater, which are discussed in detail in the discussion section.

4 Discussion

4.1 Possible mechanisms of climatic shift in Xinjiang

Since the mid-1980s, the climate of Xinjiang has shown a warming-wetting trend, with the summer precipitation showing an increasing trend at the interdecadal scale (Shi et al., 2007). The interdecadal increase in precipitation in Xinjiang is jointly affected by circulation systems at high-middle-low latitudes and low-pressure system in Central Asia, with the westerly jet in West Asia being the link connecting the circulation systems in high-middle-low latitudes (Yang et al., 2018). With global warming, water vapor transport from higher latitudes in the Northern Hemisphere increases (Dai et al., 2006), and the tropical Indian Ocean and Arabian Sea also become the important sources of water vapor supplementation for interdecadal humidification (Zhao et al., 2006; Yang and Zhang, 2008). The warming of the Indian Ocean increases the summer precipitation in Central Asia (Xinjiang) by affecting the northward transport of water vapor (Zhao and Zhang, 2016).
Some studies have been conducted on the mechanisms of aridification in Xinjiang since the beginning of the 21st century. A significant negative correlation has been observed between the Atlantic Multi-decadal Oscillation (AMO) and SPEI in Xinjiang. The positive (negative) phase of the AMO corresponds to the relatively dry (wet) period of Xinjiang, especially after 1997 (Figure 8). When the AMO is in the positive (negative) phase, the Indian summer monsoon strengthens (weakens), Indian summer precipitation increases (decreases), and Xinjiang summer precipitation decreases (increases) (Goswami et al., 2006; Feng and Hu, 2008). Variations in the South Asian high, westerly jet, and Iran subtropical high are essential to the AMO and dry-wet variations in Xinjiang (Yang et al., 2018).
Figure 8 Relationship between the 12-month SPEI (a. red line indicates the 121-month smoothed SPEI index) and the Atlantic Multidecadal Oscillation (AMO) index (b. blue line indicates the 121-month smoothed AMO index) for 1962-2015
The dry-wet variation in Xinjiang was significantly negatively correlated with the El Niño-Southern Oscillation (ENSO). In northern Xinjiang, drought events were closely related with ENSO, which occurs 12 months after the sea surface temperature anomaly. The dry (wet) period corresponded to the negative (positive) anomaly of the sea surface temperature, that is, La Niña (El Niño) can cause severe drought (humidification).

4.2 Impact of climatic shift and climate extremes on adverse ecological consequences

Global warming enhances dynamic variation in vegetation in the Northern Hemisphere (Shen et al., 2015). Under the background of global warming, vegetation coverage in Xinjiang has experienced a decreasing trend. It is important to note that in the warm-wet areas with more obvious warming and humidification in Xinjiang, vegetation degradation was more noticeable. It was mainly affected by extreme climate and climatic shifts, with the frequent occurrence of extreme climate events playing a key role (Yao et al., 2018). The relationship between extreme climate index and NDVI became more significant after 1997. For example, the extreme rainstorm days (R24) and maximum consecutive dry days were positively correlated with NDVI, and the annual average daily minimum temperature (Tnav) and warm-night days (Rwn) were also closely correlated with NDVI, among which Rwn was negatively correlated with NDVI.
Extreme temperature can affect plant physiology and ecology through respiration and photosynthesis, which in turn affects the variations in vegetation coverage (Piao et al., 2016). The increase in nighttime temperature can increase vegetation productivity by enhancing its autotrophic respiration, which can be confirmed by the negative correlation between the increase of Rwn and decrease in NDVI. However, the increase in Tnav was not consistent with the decrease in NDVI, which indicated that the mechanism of extreme temperature affecting the vegetation coverage variation needs to be further confirmed. In southwestern Tibetan Plateau, a decrease in NDVI growth period was related to a delay in vegetation regeneration (Shen et al., 2015).
In arid areas, precipitation can promote vegetation growth. However, some studies have shown that an increase in precipitation can inhibit vegetation growth and promote ecological reversal. This is because an increase in precipitation in Xinjiang is mainly caused by an increase in extreme precipitation, which accounts for approximately 50% of the total precipitation. The ecological environment of Xinjiang is extremely fragile, and extreme precipitation aggravates soil and water loss in Xinjiang. Based on the data, the area of soil and water loss in Xinjiang accounts for nearly 1/3 of that of China, and is still expanding. In the oasis area of nearly 7 × 104 km2, areas as large as 2 × 104 km2 show different degrees of soil and water loss. With the sudden increase in temperature and its fluctuation at a high level, vegetation transpiration and soil water dissipation in the plain desert area increase, with some shallow-rooted desert plants dying from drought, thus, reducing species diversity and vegetation coverage (Li et al., 2017).
Figure 9 describes the mechanisms for impacts of climatic regime shift and climate extreme on vegetation growth change. The above analyses showed that extreme temperature and precipitation can greatly impact vegetation coverage variation in Xinjiang. The increased air temperature can increase evaporation capacity, accelerate regional water cycle, intensify the frequency and intensity of extreme precipitation and temperature events. This will create the spatio-temporal heterogeneity of water resources at different scales, thus, affecting the variation in vegetation coverage. Therefore, vegetation degradation in Xinjiang since the beginning of the 21st century may have been caused by the wet-to-dry climatic shift and frequent extreme climate.
Figure 9 Schematic representation of processes for impacts of climatic regime shift and climate extreme on vegetation growth change in Xinjiang. Circled symbols of ‘+’ represent a positive sign of impact, respectively.

4.3 Impact of glacier change on regional water resources

Climate change can affect glacial meltwater and further runoff from glaciers, which significantly contributes to the total amount of water. Runoff in the arid regions of Central Asia mainly depends on glacier melting (Chen et al., 2017). Glacial meltwater accounts for 31.8%, 33.7%, and 56.3% of the total river runoff on the northern slope of Tianshan Mountains, southern slope of Tianshan Mountains, and northern slope of Kunlun Mountains, respectively (Chen et al., 2017).
In the past 50 years, the Tianshan glaciers have retreated, and the glacier area and mass balance have decreased by 18 ± 6% and 27 ± 15%, respectively (Farinotti et al., 2015). The glacier retreat was accelerated in the 1990s, but slowed down or remained stable since the beginning of the 21st century, especially in the central and western parts of the Tianshan Mountains (Chen et al., 2017). This was consistent with the increasing trend observed in the river runoff originating from the Tianshan Mountains in the 1990s and a decreasing trend observed since the beginning of the 21st century. The continuous decrease in the runoff since the beginning of the 21st century was closely related to the decrease in the glacier area of the basins, thinning of glaciers, and the rise of the equilibrium line.
The proportion of glacial meltwater to total runoff was mainly affected by precipitation and temperature variations in the mountainous areas. On the Pamirs Plateau, precipitation continues to increase, but the summer temperature abnormally decreases, which leads to the glaciers being stably maintained or slightly increasing, and positive balances of some glacier materials (Hewitt et al., 2005; Bolch et al., 2012; Kapnick et al., 2014). On the northern slope of the Kunlun Mountains, runoff has a positive (negative) correlation with temperature (precipitation) (Chen et al., 2017). A good correlation was observed between runoff and SPEI in Xinjiang, which reflected the joint influence of temperature, precipitation, and evaporation on the glaciers.

5 Conclusions

Based on the meteorological and hydrological observation data, the regional wet-to-dry climatic shift in Xinjiang and its impact from 1961 to 2018 were analyzed. The main conclusions are as follows.
From the mid-late 1980s to the end of the 20th century, Xinjiang experienced a warm-wet climate regime. However, after 1997, desiccation took place in more than 70% of the area where temperature rose significantly, potential evaporation intensified, and precipitation slightly decreased. The drought frequency in Xinjiang significantly increased. An obvious signal of climatic shift from a warm-wet climate regime to a warm-dry climate regime was observed in Xinjiang, that is, a wet-to-dry climatic shift occurred since the late 1990s.
From 1961 to 2018, the number of extreme maximum temperature, extreme minimum temperature, and high temperature days significantly increased. The start date of high temperature was found to be advanced, and the end date of high temperature was observed to be delayed. The intensity and frequency of extreme precipitation events, rainstorms, and snowstorms have significantly increased since the beginning of the 21st century.
NDVI in Xinjiang first increased and subsequently decreased. It significantly increased during 1982-1997, with vegetation tending to “turn green”. However, after 1997, the vegetation growth became stagnant, and soil moisture significantly decreased, causing an ecological reversal. The more frequent occurrence of extreme climate events and wet-to-dry climatic shift resulted in a prominently negative ecological effect in Xinjiang.
Affected by climate change and human activities, the surface water resources in Xinjiang have significantly changed, with obvious regional differences. The responses to wet-to-dry climatic shifts were complex. Influenced by the replenishing proportion of snowmelt to runoff, the river runoff originating from the Tianshan Mountains had a positive response to the wet-to-dry climatic shift, whereas that originating from the Kunlun Mountains had no obvious response.
Water resource scarcity and ecological problems are major issues that restrict the sustainable socio-economic development in Xinjiang. Global warming has significantly changed the water cycle structure in Xinjiang, resulting in a sharp increase in aridification. With the frequent occurrence and recurrence of extreme climate events, the instability in the water cycle system and ecosystem has intensified, creating severe challenges to water resources and ecological security in Xinjiang. Therefore, it is necessary to carry out observations and research on the elements of regional atmospheric water cycle of Xinjiang and strengthen research on the facts, mechanisms, and impact assessment of regional wet-to-dry shifts. To build a beautiful Xinjiang and promote high-quality development, we should utilize the opportunities provided by the climate of Xinjiang and improve the ecological environment.

References

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[3]
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[4]
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[5]
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[9]
Feng S, Hu Q, 2008. How the North Atlantic multidecadal oscillation may have influenced the Indian summer monsoon during the past two millennia. Geophysical Research Letters, 35: L01707.

[10]
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[12]
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[13]
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[14]
Huang W, Feng S, Chen J H et al., 2015. Physical mechanisms of summer precipitation variations in the Tarim Basin in northwestern China. J. Clim., 28(9): 3579-3591.

[15]
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[16]
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[17]
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[19]
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Tao H, Borth H, Fraedrich K et al., 2014. Drought and wetness variability in the Tarim River Basin and connection to large-scale atmospheric circulation. International Journal of Climatology, 34(8): 2678-2684.

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[23]
Vicente-Serrano S M, Beguería S, López-Moreno J I, 2010a. A multiscalar drought index sensitive to global warming: The standardized precipitation evapotranspiration index. Journal of Climate, 23(7): 1696-1718.

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Vicente-Serrano S M, Beguería S, López-Moreno J I et al., 2010b. A new global 0.5 gridded dataset (1901-2006) of a multiscale drought index: Comparison with current drought index datasets based on the Palmer Drought Severity Index. Journal of Hydrometeorology, 11: 1033-1043.

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[25]
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[26]
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[28]
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[29]
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[30]
Yao J, Chen Y, Zhao Y et al., 2020. Climatic and associated atmospheric water cycle changes over Xinjiang, China. Journal of Hydrology, 585: 124823. DOI: 10.1016/j.jhydrol.2020.124823.

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[31]
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[32]
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[33]
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[34]
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[35]
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[36]
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[37]
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[39]
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[40]
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[41]
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