Nature: Climate-driven changes in glacier-fed streamflow regimes have direct implications on freshwater supply, irrigation and hydropower potential. Reliable information about current and future glaciation and runoff is crucial for water allocation, a complex task in Central Asia, where the collapse of the Soviet Union has transformed previously interdependent republics into autonomous upstream and downstream countries.
Photo: Location map of the Tien Shan mountains and seasonal distribution of precipitation in Central Asia.
Although the impacts of climate change on glaciation and runoff have been addressed in previous work undertaken in the Tien Shan (known as the ‘water tower of Central Asia’), a coherent, regional perspective of these findings has not been presented until now. Here we show that glacier shrinkage is most pronounced in peripheral, lower-elevation ranges near the densely populated forelands, where summers are dry and where snow and glacial meltwater is essential for water availability. Shifts of seasonal runoff maxima have already been observed in some rivers, and it is suggested that summer runoff will further decrease in these rivers if precipitation and discharge from thawing permafrost bodies do not compensate sufficiently for water shortfalls.
In regions with little summer precipitation, glaciers play an important role in streamflow regimes, as meltwater from the ice is released when other sources such as snowmelt are depleted1, 2, 3. This situation is well reflected in the Tien Shan (Chinese for ‘Celestial Mountains’), where glaciers contribute considerably to freshwater supply during summer in the densely populated, arid lowlands in Kyrgyzstan, Kazakhstan, Uzbekistan, Turkmenistan and Xinjiang/China4, 5.
As in many other parts of the world, glaciers in the Tien Shan have been retreating since the end of the Little Ice Age (LIA) in the mid-nineteenth century6, 7, 8 — a tendency that has accelerated since the 1970s9, 10. Intensified glacier melt strongly affects the quantity and seasonal distribution of runoff in Central Asia’s glacier-fed watersheds11, 12. Although in the first instance shrinking glaciers supply ample quantities of water in the form of increased glacial runoff, reduced glacier volume will ultimately result in a decrease in both glacier-fed and total runoff, if there are no increases in water amount from other sources, for example precipitation and/or thawing of ice-rich permafrost, to offset water deficiency from reduced glacier melt. As a consequence, continued glacier shrinkage will eventually transform glacial–nival runoff regimes in the Tien Shan into nival–pluvial regimes, with a much larger year-to-year variability in water yields13. Such an alteration in runoff may not only intensify ecological problems such as the drying of the Aral Sea14, 15, 16 but also add to political instability in Central Asia17.
Only a limited number of studies currently address the timing and evolution of expected glacier shrinkage18, 19 and related changes in runoff20, 21. In this Review, we explore the range of changes in glaciation and related discharge in different climatic regions of the Tien Shan. Based on existing data, we present a comprehensive perspective by addressing the following key questions: (1) How does climate change affect the Tien Shan mountains and what responses of glaciers and rivers have been observed? (2) Which alterations in glaciers and runoff can be expected based on future climate scenarios and what are the uncertainties? (3) What are possible impacts of altered water availability on social and political stability in Central Asia?
The Tien Shan mountains cover a large fraction of Central Asia, spanning regions from Uzbekistan to Kyrgyzstan and from southeastern Kazakhstan to Xinjiang/China (Fig. 1a). The range constitutes the first montane barrier for northern and western air masses travelling from Siberia and the Kazakh steppes to Central Asia22 (Fig. 1b). The resulting barrier effects lead to a distinct continentality gradient with decreasing precipitation rates and mean temperatures from northwest to southeast23. Three main climatic subregions are identified (Fig. 1c and Supplementary Figure S1), namely (1) the outer ranges in Western and Northern Tien Shan with a relatively moist climate, as well as (2) the inner ranges in Central Tien Shan and (3) the eastern ranges in Eastern Tien Shan with a pronounced continental climate7. Maximum precipitation occurs earlier in the outer and eastern ranges (spring and early summer in Northern and Eastern Tien Shan, late winter to early spring in Western Tien Shan) than in the inner ranges (summer in Central Tien Shan)24 (Fig. 1c,d). With increasing altitude, precipitation maxima occur later in the season, and average annual precipitation sums are higher25. The mean annual precipitation (MAP) measured at the highest meteorological station with long-term measurements for Central Tien Shan (3,614 metres above mean sea level, masl), however, is only slightly more than 300 mm.
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