Climate change, water resources and sustainable development in the arid and semi-arid lands of Central Asia in the past 30 years

The countries of Central Asia are collectively known as the five ''-stans'': Uzbekistan, Kyrgyzstan, Turkmenistan, Tajikistan and Kazakhstan. In recent times, the Central Asian region has been affected by the shrinkage of the Aral Sea, widespread desertification, soil salinization, biodiversity loss, frequent sand storms, and many other ecological disasters. This paper is a review article based upon the collection, identification and collation of previous studies of environmental changes and regional developments in Central Asia in the past 30 years. Most recent studies have reached a consensus that the temperature rise in Central Asia is occurring faster than the global average. This warming trend will not only result in a higher evaporation in the basin oases, but also to a significant retreat of glaciers in the mountainous areas. Water is the key to sustainable development in the arid and semi-arid regions in Central Asia. The uneven distribution, over consumption, and pollution of water resources in Central Asia have caused severe water supply problems, which have been affecting regional harmony and development for the past 30 years. The widespread and significant land use changes in the 1990s could be used to improve our understanding of natural variability and human interaction in the region. There has been a positive trend of trans-border cooperation among the Central Asian countries in recent years. International attention has grown and research projects have been initiated to provide water and ecosystem protection in Central Asia. However, the agreements that have been reached might not be able to deliver practical action in time to prevent severe ecological disasters. Water management should be based on hydrographic borders and ministries should be able to make timely decisions without political intervention. Fully integrated management of water resources, land use and industrial development is essential in Central Asia. The ecological crisis should provide sufficient motivation to reach a consensus on unified water management throughout the region.

ScienceDirect (Elsevier) database was consulted. The keywords used in searches were ''climate+Central Asia'', ''water+Central Asia'', ''land+Central Asia'', and ''social/economic development+Central Asia''. If the search words appeared in the title, abstract, or keywords of an article, then the article was preliminarily selected. A validation process was then conducted to check for double counting and erroneous articles. Finally, the total number of articles in each category was recorded. Any studies that used remote sensing were placed into a separate category. There were 1846 English language articles in total: 635 regarding climate, 510 regarding water, 295 regarding land, and 406 regarding social and economic development. Among them, 298 used remote sensing technology as the major research methodology. Most articles were written by Chinese, German and American authors, but this does not mean that these countries have taken the lead in geoscience research in Central Asia. Before the 1990s, Soviet scientists conducted more comprehensive studies (mostly published in Russian) of the meteorology, hydrology, geography and geology of "Middle Asia". In the past 30 years, there have been large advances in basic scientific research in this region. Scientists have still found it difficult to acquire data, which is partly the reason why remote sensing is a popular technique in studies of Central Asia (De Beurs and Henebry, 2004;Propastin et al., 2008;Zhang et al., 2010;Conrad et al., 2011;Zhou et al., 2013;Klein et al., 2014;Beurs et al., 2015).

Climate change
In the past 100 years, the global climate system has undergone a significant change characterized by greenhouse gas emissions and global warming. According to the IPCC fifth assessment report, the global warming trend is still continuing, and the average temperature of the global surface has increased by 0.85°C (Stocker et al., 2013). Central Asia is one of the largest arid and semi-arid areas in the world (Knorr et al., 2001;Xu et al., 2007;Gessner et al., 2013). The natural ecosystems of Central Asia are very sensitive and vulnerable to climate change (Chen, 2012;Yin et al., 2016). Hu et al. (2014) analyzed the near-surface air temperature changes in Central Asia from 1979 to 2011 using observations from 81 meteorological stations. They found that the rise in air temperature in Central Asia occurred at a faster rate than that in surrounding areas or than the global average. The warming trend is noticeable, especially in the northern part of the region and during the cold season. The surface air temperature over Central Asia will rise between 3°C-7°C on average in the future. The largest increase in near-surface air temperature occurred in spring. According to the projected emission scenarios for the period of 2071-2100, the warming trend in summer is expected to increase (Ozturk et al., 2017). Many climate models (Pollner et al., 2008;Lioubimtseva and Henebry, 2009;Hu et al., 2014) have been used to project the temperature rise in the 21 st century (Table 1). Most recent studies have reached a consensus that the temperature rise in Central Asia has occurred faster than the global average over the past 30 years. This warming trend will not only result in a higher evaporation in the basin oases, but also a significant retreat of glaciers in the mountainous areas (Sorg et al., 2012, Unger-Shayesteh et al., 2013. Spatial patterns of precipitation in Central Asia are strongly related to elevation (Klein et al., 2012;Chen et al., 2013). Annual precipitation in the mountainous areas is often more than 1000 mm, and even reaches 2000 mm in some southern parts of Central Asia. In the basin areas of Uzbekistan, Turkmenistan and Tajikistan and the southern parts of Kazakhstan, annual precipitation is less than 100 mm, but the summer temperature reaches more than 24°C on average. This has caused severe land desertification and water shortage problems.
Is there a significant response to the temperature rise in Central Asia under the background of global warming? Many researchers have studied the characteristics of climate change in Central Asia and their relationship with global warming from different perspectives. Some previous studies have shown that the convergence and divergence of atmospheric moisture will significantly increase the moisture content in the atmosphere, resulting in a pattern of "dry gets drier, wet gets wetter" (Allen and Ingram, 2002;Dore, 2005;Held and Soden, 2006). However, several recent studies have indicated the occurrence of the opposite pattern, namely "dry becomes wet, wet becomes dry" (Greve et al., 2014). The average precipitation in the arid regions has increased in both measured data and climate models over the past 30 years (Donat et al., 2016). In Central Asia, the annual precipitation has generally increased during the past 80 years, especially in winter (Chen et al., 2011). There was a significant increasing trend in precipitation in the southeast of Central Asia during 1960-2013 (Song and Bai, 2016).
Climate change in Central Asia is related to many factors, such as sea surface temperature, large-scale meridional circulation, and the El Niño-Southern Oscillation (ENSO). Some researchers have studied the mechanism of climate change in the arid and semi-arid regions of Central Asia. Lioubimtseva and Cole (2006) reported that the intensified westerly circulation and its southward movement may be the main reason for the increased precipitation over Central Asia in the context of global warming. Hu et al. (2017) used the Global Precipitation Climatology Centre (GPCC) V7 dataset to explore the spatiotemporal variations of annual precipitation. Their results indicate that annual precipitation over Central Asia is closely related to the ENSO. The ocean surface temperature anomaly has a significant effect on temperature over Central Asia. The winter temperature is much warmer when a positive sea surface temperature anomaly occurs in the equatorial central-eastern Pacific (ECEP) from autumn to winter, and vice versa. The summer temperature is much warmer (cooler) when there is a higher (lower) ocean surface temperature in the ECEP . Summer precipitation and temperature anomalies in Central Asia are closely related to a large-scale meridional circulation. Winter precipitation and temperature anomalies are closely connected to the singularity of the zonal circulation in the tropics. The Ural Mountains trough (ridge), western Pacific subtropical high and Arabia trough (ridge) are the main weather systems affecting the climate of Central Asia (Wang and Wei, 2012). The annual temperature and precipitation have shown a significant increasing trend, which are more rapid in the plains than in the mountainous areas of the Syr Darya Basin over Central Asia (Yao and Chen, 2015).

Water resources
There are more than 1×10 4 rivers and 1×10 4 natural lakes in Central Asia. The lake area is more than 5×10 5 km 2 , about 1/5 of the Earth's total lake area. However, most regions face water scarcity problems for two major reasons. First, there is a remarkably uneven distribution of water resources in the region, creating competition between up and downstream water users (Chalov et al., 2013). Most rivers originate from the mountainous regions of Tajikistan and Kyrgyzstan, providing relatively abundant fresh water for farmlands, forests, livestock, industry and domestic use. In contrast, Uzbekistan, Turkmenistan and Kazakhstan are facing severe water scarcity problems. Second, water quality problems are enhancing the shortage of fresh water. Salts, agrochemicals, organic pollutants and heavy metals are frequent water quality impairments in the downstream parts of most river catchments (Groll et al., 2015). Furthermore, many lake basins (including the Aral Sea, Caspian Sea, Balkhash Lake and Issyk-kul Basin) have experienced a reduction of water supply, lake area shrinkage and ecosystem degradation caused by water shortages and ecological deterioration (Table 2). Due to low precipitation in the basin area, nearly 98% of the river discharge originates from the mountainous regions. The Tianshan Mountains and Pamir Mountains are considered to be the fresh water tower of Central Asia. Based on statistical data for the past 30 years, glacier and snow meltwater contributes 20%-60% of the total runoff from the Tianshan Mountains, which is crucial for ensuring seasonal water availability. With higher rainfall amount in the mountains, forests also play an important role in fresh water storage and generation in the arid and semi-arid regions, but the ratios of rainfall to runoff are still lower than those in humid regions. Kyrgyzstan and Tajikistan are mostly located in the upper reaches of trans-border rivers, with an abundant water chinaXiv:201901.00114v1 supply from the mountains. In the lower reaches, water users from Uzbekistan, Turkmenistan and Kazakhstan often complain about insufficient river outflow into their countries (Fig. 2). There are two major characteristics of dryland rivers in Central Asia. First, the seasonal fluctuations of the hydrographs are often large (Yu et al., 2015). Peak flows normally occur in summer, and most rivers have a glacier period in winter. Intermittent streams and unstable flow channels are quite common. Second, water volumes are often much lower in the downstream, with less precipitation, higher evaporation, and infiltration losses, as well as greater water consumption. Temporary flow in dryland rivers has a special biological significance, enabling certain species to breed while eliminating others (Karthe, 2018). In the arid and semi-arid regions of Central Asia, the growth of most natural vegetation (also called Tugai vegetation) is highly reliant on groundwater conditions (Yu et al., 2018). Compared with surface water abstraction, groundwater exploitation and utilization rates are relatively low, and groundwater is mainly for irrigation and domestic use (Liu et al., 2018). Tugai vegetation has a strong tolerance to dry and saline soil (Thevs et al., 2012). In farmlands, crops often have to be grown under water-stressed conditions. An example of an ecological catastrophe in Central Asia is the shrinkage of the Aral Sea. Because of water withdrawal for irrigation and domestic use from both tributaries, i.e., the Amu Darya and Syr Darya (Erdinger et al., 2011), the size of the Aral Sea has declined dramatically from 4×10 5 km 2 in the early 1990s to approximately 1×10 5 km 2 in 2012 (Klein et al., 2014). Many leaders from Central Asian countries and the United Nations have addressed the crisis. However, the provision of a sustainable water management plan in the Aral Sea Basin (including the Amu Darya and Syr Darya oases) is currently proceeding too slowly to prevent further water losses and environmental deterioration. The Aral Sea may vanish, as happened to Lop Nor in Xinjiang of China.
Where there is sufficient data, hydrological models can be successfully applied at the large basin scale in the arid and semi-arid lands . However, in studies of Central Asia, there are often problems caused by the trans-border nature of water resources. According to the CAWater-Info (2018), approximately 30% of regional groundwater is stored within a trans-boundary aquifer in the Aral Sea Basin, and the figure is much higher in the mountainous regions (e.g., Tianshan Mountains). All of the large rivers in Central Asia are basically trans-border rivers, implying that the related regions share natural resources and responsibilities, which require interstate regulation and mutual protection measures. Unfortunately, political rivalries, economic competition and nationalism have seriously undermined efforts to solve trans-border water problems during the past 30 years (Howard and Howard, 2016).

Land use changes
From southeast to northwest, the elevation of Central Asia drops from 7495 to 132 m. The vast region between the Pamir Mountains and the Caspian Sea is mainly comprised of desert, semidesert and steppe. The main regions include Kazakhskiy and Melkosopochnik in the north, the Turan Plain in the middle, the Caspian Plain in the west, and the Karakum Desert in the south. In the arid and semi-arid regions of Kazakhstan, Uzbekistan and Turkmenistan, there are large irrigated areas with bare lands and sparse vegetation (Klein et al., 2012). The mountain landscapes of Kyrgyzstan and Tajikistan in the southeast of Central Asia contain glaciers, snow, coniferous forests and grasslands. Modern vegetation in the arid and semi-arid regions of Central Asia evolved from ancient Mediterranean species, which has a subtropical genetic background.
The prosperity of Central Asia, as an agrarian region since ancient times, has always been very closely linked to land use. Today, farming still employs about 60% of the rural population. In the Aral Sea Basin, of the 1.55×10 8 hm 2 total land area, 0.59×10 8 hm 2 is considered cultivable, of which only about 0.10×10 8 hm 2 is actually used. About half of the cultivated land is located in oases, with fertile soils (CAWater-Info, 2018). Since the disintegration of the Soviet Union, the most significant land use changes in Central Asia have been farmland abandonment and reclamation. Based on remote sensing technology, Chen et al. (2013) found that the agricultural area in Central Asia fell from 1.03×10 8 hm 2 in 1990 to 0.54×10 8 hm 2 in 2000, and then increased to 0.70×10 8 hm 2 in 2009 (Fig. 3). The area of steppe, forest and desert increased from 1990 to 2000, and then decreased slightly with the reclamation of farmlands. The Aral Sea shrank by 63.80% from 1990 to 2009, whereas there was no significant change in the area of Balkhash and Issyk Kul. Additionally, crop productivity dropped dramatically from 1992 to 1995, then steadily increased to its former level, and remained stable in the 21 st century (Chen et al., 2013). With the collapse of the Soviet Union in 1991, Central Asian countries experienced a transition from a centrally planned economy to a competitive social market economy, resulting in a profound restructuring of Central Asia's agricultural sector (Baumann et al., 2011). Markets were liberalized and foreign competition emerged (Ioffe et al., 2004). Massive landowners emerged and work forces migrated from the countryside to the cities (Lerman and Shagaida, 2007). A substantial number of farmlands were abandoned during the period that has been referred to as 'the most widespread and abrupt episode of land change in the 20 th century' . The farmlands were later reclaimed and the landscape was renewed. While the land use changes were regulated by humans, natural conditions (e.g., temperature rise, glacier melting, water availability, precipitation and evaporation) also played a role in land use changes.
Soil salinization is another problem in the arid and semi-arid lands of Central Asia. With high water losses, low precipitation and insufficient irrigation water, land salinization has affected over chinaXiv:201901.00114v1 47.5% of the irrigated land in Central Asia (33.0% in Kazakhstan,11.5% in Kyrgyzstan,16.0% in Tajikistan,95.9% in Turkmenistan and 50.1% in Uzbekistan) (Hamidov et al., 2016). Salinization is a serious threat to crop production in the arid and semi-arid regions (Kushiev et al., 2005). Sodic and saline soils are distributed throughout these regions. The formation and evolution of the soil patterns are linked to particular combinations of climatic and hydrological conditions in most parts of Central Asia.

Population and economic growth
The total population of Central Asia was 52.3×10 6 in 1993, which increased to 71.9×10 6 in 2017, with an annual growth rate of 0.81%. The population has been gradually and slowly increasing during the past three decades (Fig. 4). From 1993 to 2017, the following population increases were recorded: Kazakhstan from 16.4×10 6 to 18.4×10 6 , Kyrgyzstan from 4.52×10 6 to 6.13×10 6 , Tajikistan from 5.57×10 6 to 9.11×10 6 , Turkmenistan from 3.99×10 6 to 5.85×10 6 , and Uzbekistan from 21.9×10 6 to 32.4×10 6 . Considering the vast territory and low population density, the population growth in Central Asia should not generate much pressure on water resources and land use at the national level. During the past 30 years, Central Asia has experienced an economic boom. After a short period of economic recession from 1991 to 1995, the resurgence of the economy has been rapid and steady. The GDP per capita of Kazakhstan increased more than 30-fold from 1993 to 2017. The other countries in the region have also experienced rapid economic and productivity growth. However, industrial development has been uneven throughout the region and has potential weaknesses. Central Asian countries, especially Kazakhstan, Turkmenistan and Uzbekistan, possess a world-famous sedimentary basin that contains huge petroleum reserves (Gu and Yu, 2017). Therefore, foreign investment has been attracted to assist these countries to develop the secondary industries. In contrast, Kyrgyzstan and Tajikistan have developed hydropower due to their hypsographic advantages and abundance of water resources. These natural-resource-oriented developments have caused many practical problems. The competition between upstream hydropower and downstream irrigation has become a significant issue in recent years. Modern management measures have generally not been applied in agricultural production. Science and technology are slowly transforming traditional farming methods. The electricity industry, mining, mechanical engineering, and a number of other industries are struggling in most of the region. The overall economic situation is not balanced and powerful enough to resist an international financial crisis.

Discussion
Central Asia is one of the world's largest closed drainage basins with more than 1×10 4 rivers and 1×10 4 natural lakes. The vast areas of arid and semi-arid lands are highly dependent on rivers and lakes as sources of freshwater. Significant changes in water bodies have occurred in the past three decades, including large inland rivers and the Aral Sea. The shrinkage of the Aral Sea cannot be solely attributed to water abstraction over the past 30 years. Varis (2014) has proposed that irrigation-intensive industries in the former Soviet Union have drained water bodies in the Central Asian region. It is true that massive amounts of water consumption have resulted in a reduction in the water area of the Aral Sea since the 1960s (Saiko and Zonn, 2000), but the situation has not substantially changed in the last three decades. Water inflows are much lower than water losses following the breakup of the Soviet Union, and the Aral Sea has gradually dried up due to excessive irrigation water use in the upper tributaries. Today, the gradual drainage of the Aral Sea is not only a regional problem, but has also become a world famous trans-national ecological disaster (Cai et al., 2003;Kitamura et al., 2006;Qadir et al., 2009;Micklin, 2016). There is an urgent need to initiate integrated water resource management in the region to ensure ecosystem protection and socio-economic sustainability in Central Asia (Kubo et al., 2009).
Climate change and water resources have a high degree of ecological and economic importance in Central Asia, where the widespread and significant land use changes since the 1990s may also help improve our understanding of natural variability and human interaction (Chen et al., 2013). Regional precipitation mainly occurs in the Pamir Mountains and Tianshan Mountains on the southeastern edge of the region. Increasing evaporation and decreasing precipitation in the vast inland regions have resulted in a reduction of river runoff from the mountains to the plain areas. Frequent extreme natural events (such as prolonged drought and flooding) could have a significant impact on ecosystem structure and function in Central Asia Zerbe and Thevs, 2011). Despite a few watershed or basin scale studies, the ecosystem responses to water availability and land use changes in Central Asia are still unclear. Temperature changes and the alternation of precipitation may also have both positive and negative impacts on ecosystems. The potential impacts of climate change and the long-term sustainability of water resources need to be further investigated (Yang et al., 2013).
Ecological and environmental problems are the main concerns of the governments of Central Asian countries. The ecosystem is fragile in the arid and semi-arid regions. Climate change and human disturbance could easily cause significant ecosystem changes or even ecological disasters (Levintanus, 1992;Erdinger et al., 2011). Spatiotemporal variations in human-induced water availability and land use changes have resulted in regional differences in ecosystem services, namely supporting services, provisioning services, regulating services and cultural services. Local residents benefit from and rely on those ecosystem services, which have been affected by changes in a number of factors, including demographic, socio-economic, political, physical and biological conditions. Dryland ecosystems should be managed in a way that enables them to provide ecosystem services that meet human demands for social development. As well as the arid and semi-arid lands of Central Asia, the mountainous regions are also vulnerable to ecological disasters (Halvorson and Hamilton, 2007). It is therefore necessary to better understand the influences and functions of ecosystem services to support policy-making and integrated management measures in Central Asia. To understand the complexity of local socio-economic and water policies that are related to sustainable development, more comprehensive and systematic studies need to be conducted at varying scales.
Extensive land use changes and migrations have occurred from the transition of the Soviet planned economy to the current market economy. Poorly managed farmlands have led to food shortages, land degradation, biodiversity loss and desertification (Gintzburger et al., 2003). Regional networks for supplementary irrigation and improved water management, crop production and rangeland management are badly needed for the five Central Asian countries. A reduction in water supply will cause economic losses and have economy-wide repercussions on employment rates, as well as the degradation of irrigated lands in downstream countries (Bekchanov and Lamers, 2016). Researchers have demanded a change in policies towards the support of integrated crop production and livestock management (Suleimenov, 2014). Studies of natural resources and environmental problems are important for ecosystem protection and socio-economic development. Agricultural production and natural resource exploitation should be managed in a sustainable way that will benefit future generations. Given that water is the key factor affecting agriculture in Central Asia, water use efficiency and agricultural productivity should be emphasized in the future (Hojyaz et al., 2013;Devkota et al., 2015).
In general, there has been a tendency to reach water management agreements based on trans-border cooperation among the Central Asian countries in recent years (Ambrosio, 2008;Rakhimov, 2010). As international attention on the region has grown, and the number of research projects investigating water and ecosystem protection in Central Asia have also grown (Khamzayeva et al., 2009;Libert and Lipponen, 2012). However, this might not be transformed into practical actions in time to prevent ecological disasters, such as the continued shrinkage of the Aral Sea. There is intense competition for water resources among agriculture, hydropower, domestic uses, grazing, and industrial uses along the river oases. Water allocation conflicts are most likely to occur in international catchments shared by poorer, less democratic, and politically unstable countries, which are governed by weak international water management institutions (Bernauer and Siegfried, 2012). There is a lack of cooperation among national authorities and ministries responsible for water policies and environmental protection. Such water ministries should be based on hydrographic borders and be able to make timely decisions without political intervention. It is very difficult to reach a consensus on trans-border environmental and financial legal regulations. To provide stable and sustainable ecosystem services, the fully integrated management of water, land use and industrial development is essential in Central Asia within the framework of policy, finance, environmental regulations, relevant organizations and technologies.

Conclusions
This study conducted a review of climate change, water resources and regional developments in Central Asia over the past 30 years. The different research outcomes of international researchers were considered alongside reports from the Research Center for Ecology and Environment of Central Asia, Chinese Academy of Sciences. The following important conclusions were reached.
(1) Most recent studies have shown that the temperature rise in Central Asia has been higher than that of the global average over the past 30 years. The temperature change and alternation of precipitation may have both positive and negative impacts on the fragile ecosystem of arid and semi-arid regions.
(2) The Tianshan Mountains and Pamir Mountains are considered to be the fresh water towers of Central Asia. Most of the region is facing water scarcity problems for two major reasons: the uneven distribution of water resources and the water quality problems.
(3) Trans-border water bodies are poorly managed because of the different interests of the five Central Asian countries. The over-exploitation of water resources in trans-border rivers has caused the shrinkage of the Aral Sea, along with many other ecological disasters.
(4) Since the collapse of the Soviet Union in 1991, the most significant land use changes in Central Asia have been farmland abandonment and reclamation. The agricultural area reduced by nearly 50% from 1990 to 2000, but was steadily increased in the 21 st century. The area of steppe, forest and desert increased from 1990 to 2000, and then decreased slightly with the reclamation of farmlands.
(5) The population of the region has gradually increased over the past 30 years. Economic growth and industrial expansion have achieved varying degrees of success for all five Central Asian countries. However, the expansion has been mainly natural-resource-oriented and uneven. Environmental deterioration is becoming increasingly prominent with economic development in the Central Asian region.