The impact of climate change and human activities on the Aral Sea Basin over the past 50 years

Highlights

• Aral Sea has been retreating in recent 50 years, but the shrinking rate has slowed since 2005.

• The increase in the water area around the Aral Sea is due to agricultural drainage in the basin.

• The increase of upstream runoff and drop of water withdrawal led to the slowing down of Aral Sea shrinkage.

Abstract

The destruction of the Aral Sea area constitutes one of the world's most infamous ecological disaster. However, the retreat of the Aral Sea has slowed down in recent years and the underlying reasons are not reported. In this work, based on the extreme-point symmetric mode decomposition (ESMD) method and the multiple linear regression model, we analyzed the changing of the Aral Sea from 1960 to 2018, and detected the time for slowdown of retreat, then explored the driving forces. The results show that the Aral Sea retreated rapidly from 1960 to 2004, and the shrinking rates of water surface area, water volume and water level were 1087.00 km²/year, 25.07 km³/year, and 0.56 m/year, respectively; the retreat has slowed since 2005, with the shrinking rates being 760.00 km²/year, 2.86 km³/year, and 0.38 m/year, respectively. At the same time, the area of water bodies surrounding the Aral Sea increased due to the agricultural drainage water. The oscillation periods of water level in the Aral Sea are 2.1a, 7.6a and 29.5a, of which 29.5a is the main period of oscillation. The trend residual RES indicates that water level shows a non-linear downward trend, and the degree of fluctuation has decreased significantly after 2005. The impact of human activities on the Aral Sea is more significant than that of climate change. Overall, the increased upstream runoff, reduced water withdrawal, and rise in water delivery to the Aral Sea has led to a slowing down of the sea's notorious shrinkage. The findings provide a scientific reference for the management and protection of the Aral Sea.

Introduction

Lakes play a significant role in maintaining the survival and development of human habitations and the stability of the surrounding ecology, especially in arid regions with fragile environments (Klein et al., 2014). The Aral Sea is a typical inland lake located in the Central Asia. Over the past 50 years, the lake has experienced a sharp reduction in water, which has produced serious negative effects on water resources and the local ecology. This phenomenon grabbed world-wide attention in the 1990s and has since been dubbed “the Aral Sea crisis” (Chen et al., 2018; Micklin, 2016).

The Aral Sea Basin includes five Central Asian countries (Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan and Uzbekistan) as well as Afghanistan and Iran. Overall, the five Central Asian countries are the main water-consumers in the basin, as the economies of these countries depend mainly on irrigated agriculture (Awan et al., 2011). Since 1960, a large-scale reclamation and irrigation of the Soviet Union and post-Soviet Union countries have consumed an immense amount of water resources (Duan et al., 2019). However, compared with the past, the retreat of the Aral Sea has slowed in recent years, so it is very meaningful to analyze the changing trends of the Aral Sea since 1960 and the reasons for such changes.

In recent years, with the development and application of the remote sensing technology, many scholars have used remote sensing images to obtain information on dynamic changes of lakes, and to explore the driving forces from natural and human factors (Hanrahan et al., 2009; Yang and Lu, 2014; Jin et al., 2017; Tan et al., 2017; Jing et al., 2018; Zhang et al., 2018). Based on the water balance method, Lei et al. (2014) investigated the dynamics of inland lakes in the Tibetan Plateau and analyzed the effect of climate change on lake dynamics, proposing that the significant increase of regional precipitation is the main reason for lake growth. Li et al. (2017) analyzed the dynamic of Dalinor Lakes based on the Landsat images, and examined its relationship with climatic factors and vegetation changes by using the Grey Relational analysis. Liu et al. (2019) analyzed the interannual and seasonal changes of the lakes in Central Asia from 2001 to 2016 and the driving forces by using the Least Squares Method, and they found that the alpine lakes are mainly affected by climate change, while plains lakes are affected by climate change and human activities. In addition, ESMD is a new method that used to analyze the oscillation period and the nonlinear change of physical, meteorological and hydrological elements, which is approved to be more accurate than the traditional statistical methods (such as the empirical orthogonal functions (EOF) and the moving overages) (Wang et al., 2010; Qin et al., 2019; Zhou et al., 2019). However, few people have applied it to the studies of lake dynamics.

The shrinking of the lake in Central Asia is caused by climate change and human activities (Micklin, 2010; Chen et al., 2017). Rising temperature will increase the frequency of extreme hydrological events and exacerbate the temporal and spatial distribution of water resources (Hagg et al., 2013; Chen et al., 2016a, Chen et al., 2016b; Zhang et al., 2016a, Zhang et al., 2016; Li et al., 2019). White et al. (2014) found that the projected increases in summer temperatures of up to 5 °C by 2070–2099 in the Amu Darya Basin under a high-emission scenario, would lead to an increase in crop water consumptive demand of between 10.6 and 16%. Ozturk et al. (2017) revealed the future climate conditions of the Central Asia based on regional climate model simulations, they showed that the surface air temperatures will increase between 3 °C and about 7 °C, and the precipitation will decrease in almost parts of the region, this will make the ecosystems and social systems more vulnerable. Meanwhile, human activities have also been the main driving forces of the decline in the Aral Sea, especially the massive withdrawal of water from the transboundary rivers (Micklin, 2007). Chen et al. (2018) found that the main reasons for the water resources crisis in the Aral Sea Basin are the mismatched spatial distribution of land and water resources and excessive human activities.

Current research on the Aral Sea mainly focuses on the crisis of the lake's water resources, but the fact that the shrinking rate has slowed down in recent years is equally noteworthy. Thus, in our study, we comprehensively analyzed the change trends of the Aral Sea by using the ESMD method, and attempted to explain the factors causing the slowdown of the Aral Sea retreat. The findings will provide a decision-making reference for the management and protection of the Aral Sea and the formulation of water resources policies. It is also of great significance to improve the efficiency of water resources utilization and the reform of crop planting structure in the basin. Additionally, this study can provide a scientific reference for the construction of “Silk Road economic belt”, as the Aral Sea Basin serves as the core area of the Silk Road.