Permafrost Warming in the Tien Shan Mountains, Central Asia ⁎ S.S
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Global and Planetary Change 56 (2007) 311–327 www.elsevier.com/locate/gloplacha Permafrost warming in the Tien Shan Mountains, Central Asia ⁎ S.S. Marchenko a, , A.P. Gorbunov b, V.E. Romanovsky a a Geophysical Institute, University of Alaska Fairbanks, AK 99775-7320, USA b Institute of Geography, Almaty, Kazakhstan Received 11 May 2005; accepted 19 July 2006 Available online 5 October 2006 Abstract The general features of alpine permafrost such as spatial distribution, temperatures, ice content, permafrost and active-layer thickness within the Tien Shan Mountains, Central Asia are described. The modern thermal state of permafrost reflects climatic processes during the twentieth century when the average rise in mean annual air temperature was 0.006–0.032 °C/yr for the different parts of the Tien Shan. Geothermal observations during the last 30 yr indicate an increase in permafrost temperatures from 0.3 °C up to 0.6 °C. At the same time, the average active-layer thickness increased by 23% in comparison to the early 1970s. The long-term records of air temperature and snow cover from the Tien Shan's high-mountain weather stations allow reconstruction of the thermal state of permafrost dynamics during the last century. The modeling estimation shows that the altitudinal lower boundary of permafrost distribution has shifted by about 150–200 m upward during the twentieth century. During the same period, the area of permafrost distribution within two river basins in the Northern Tien Shan decreased approximately by 18%. Both geothermal observations and modeling indicate more favorable conditions for permafrost occurrences and preservation in the coarse blocky material, where the ice-rich permafrost could still be stable even when the mean annual air temperatures exceeds 0 °C. © 2006 Elsevier B.V. All rights reserved. Keywords: climate warming; alpine permafrost; active layer; modeling 1. Introduction elevation (Aubekerov and Gorbunov, 1999). Since then and up to the present day, the alpine permafrost of the The alpine permafrost zone in the Tien Shan Tien Shan never disappeared completely. During this Mountains (69–95°E, 40–44°N) belongs to the Asian time, the extent of mountain permafrost area in the Tien high-mountain permafrost region, the largest in the Shan changed many times. These changes were caused world (Fig. 1). The occurrence and evolution of alpine by the mountain continuously rising and by the un- permafrost in the middle latitudes directly relates to the folding planetary climate events. The glacial and tectonic history of the Earth. The facts collected recently periglacial features evidently show that during some provide information about the Pre-Quaternary age of time intervals the ancient permafrost occurred at a much permafrost in the Tien Shan Mountains. Permafrost first lower elevation than the present day permafrost formed about 1.6 million years ago because of mountain (Gorbunov, 1985; Aubekerov, 1990; Marchenko and Gorbunov, 1997). The maximum glacial expansion in ⁎ Corresponding author. Tel.: +1 907 474 7698; fax: +1 907 474 the Tien Shan Mountains happened during the Middle 7290. Pleistocene time (Aubekerov and Gorbunov, 1999). E-mail address: [email protected] (S.S. Marchenko). However, the maximum extension of the permafrost 0921-8181/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2006.07.023 312 S.S. Marchenko et al. / Global and Planetary Change 56 (2007) 311–327 Fig. 1. Location of the Tien Shan Mountains, Central Asia and alpine permafrost distribution (Brown et al., 1998; Qiu et al., 2002; Marchenko et al., 2005) in the Asian high mountains. area in the Tien Shan and adjacent foothills and plains cene. Early Holocene (approximately between 9000 and occurred in the Late Pleistocene when the combination 7000 yr ago) was the most unfavorable time for the of low air humidity and cold temperatures created a existence of alpine permafrost in the Tien Shan more favorable condition for permafrost formation and (Marchenko and Gorbunov, 1997). There was a period expansion. At that time, the lower boundary of perma- of permafrost degradation in the Tien Shan Mountains. frost was located at about 900–1000 m a.s.l., which is at A Middle Holocene cooling was replaced by a short least 1500–1700 m lower than the modern lower alti- phase of warming in the Late Holocene, after which tudinal permafrost boundary. This means that during the ground temperatures again significantly decreased. Late Pleistocene the alpine permafrost of Tien Shan During the Little Ice Age, there was a downward shift Mountains merged with the Siberian permafrost area via of the lower boundary of permafrost distribution by a sporadic permafrost zone that occurred on the foothill 200–300 m of altitude. Since the second part of the plains of Djungar Alatau, Saur-Tarbagatai and Altai. nineteenth century, permafrost in the Tien Shan Moun- Ground temperatures in the Tien Shan permafrost tains is experiencing a warming period, which continues area have been subjected to repeated fluctuations during up to the present. This article considers the recent (last the Holocene brought about by the general planetary century) permafrost changes in the Tien Shan Moun- changes in climate. The altitudinal oscillations of the tains, which were studied using geothermal measure- mean annual air temperature (MAAT) zero Centigrade ments in boreholes, the analysis of climatic data, and isotherm had a range of about 500 m during the Holo- numerical modeling of permafrost temperature field S.S. Marchenko et al. / Global and Planetary Change 56 (2007) 311–327 313 dynamics. The major aim of this work is to evaluate Mountains. This altitudinal shift accounts for about changes in Tien Shan's permafrost during the last cen- 140 m with a decrease in latitude by 1° (Gorbunov et al., tury using both observed data and a modeling approach. 1996). Because of the differences in surface energy balance, the lower limits of permafrost altitudinal zones 2. Results from the previous permafrost research in on the south-facing slopes are about 400–800 m higher the Tien Shan Mountains than on the north-oriented ones (Cheng, 1983; Gorbu- nov, 1988; Map of snow, ice and frozen ground in 2.1. General features of permafrost distribution in the China, 1988; Gorbunov et al., 1996). We found isolated Tien Shan patches of permafrost beneath coarse debris at an ele- vation of 3250 m a.s.l., which were exposed during road The first information about the presence of perma- construction in 2000 along the route section between frost in the Tien Shan appeared in 1914 (Bezsonov, Kazakhstan and Kyrgyzstan through the Zailiysky and 1914). General features of permafrost distribution in the Kungei Alatau Mountain Ranges of the Northern Tien Tien Shan Mountains are resulting from latitudinal and Shan (Fig. 2). This finding indicates the lowest known altitudinal zonality, and from changes in climatic and boundary of sporadic permafrost for the south-facing topographic factors. The systematic investigations of slopes in the central Northern Tien Shan. mountain permafrost in the Tien Shan began in the mid- On the north-facing slopes in the Northern Tien 1950s (Gorbunov, 1967, 1970). The regional patterns of Shan, sporadic permafrost occurs at elevations above permafrost distribution depend on elevation, slope and 2700 m a.s.l. This elevation approximately coincides aspect, which have a major influence on incoming short- with the tree line and the MAAT isotherm of 0 °C wave radiation to the ground surface. Vegetation and (Gorbunov et al., 1996). However, small isolated snow cover, ground texture and moisture content, winter patches of permafrost can be found much lower than air temperature inversion, surface and groundwater 2700 m a.s.l. These patches occur at the feet of north- presence and movement, and climatic and geothermal facing or shaded slopes inside the coarse blocky debris conditions are also among the most important para- or beneath a mossy cover even at 1800 m a.s.l. where the meters that shape the mountain permafrost distribution. MAAT is 3.0–4.0 °C (Gorbunov, 1993). Traditionally, the alpine permafrost area of the Tien Coarse blocky debris of various origins is widespread Shan Mountains is divided into altitudinal sub-zones of in the Tien Shan and occupies a large area of high- continuous, discontinuous and sporadic (sometimes mountain territory. Convective mass and heat transfer, called islands) permafrost (Gorbunov, 1978, 1988). especially during the cold period, are very typical for the Table 1 shows the general characteristics of permafrost blocky material because of its high porosity (Haeberli distribution and the altitudinal boundaries of continuous et al., 1992; Lieb, 1996; Harris, 1996; Wakonigg, 1996; and discontinuous permafrost sub-zones in the different Humlum, 1997; Harris and Pedersen, 1998; Delaloye parts of the Tien Shan Mountains. The altitudinal et al., 2003; Goering, 2003; Hertz et al., 2003; Sawada boundaries of these sub-zones move upwards from the et al., 2003; Gude et al., 2003; Gorbunov et al., 2004). northern part towards the southern part of the Tien Shan Our measurements in the Zailiysky Alatau Range (the Northern Tien Shan) (Fig. 2) during 1974–1987 show that the temperatures inside the coarse debris are Table 1 – The altitudinal zonality of permafrost distribution in the Tien Shan typically 2.5 4.0 °C colder than the MAAT (Gorbunov Mountains (after Gorbunov et al., 1996) et al., 2004). For this reason the altitudinal distribution Part of the Tien Shan Continuous Discontinuous Sporadic of rock glaciers are a few hundreds meters lower than Mountains that of open glaciers. .Altitudinal sub-zones of permafrost (m a.s.l.) Western (41°30′N) Higher 3800–3600 3600– 2.2. Ground ice 3800 3000 Northern and Eastern Higher 3500–3200 3200– Mountain permafrost and associated periglacial – (42 43°N) 3500 2700 landforms contain large quantities of stored fresh Inner (40°30′–42°N) Higher 3600–3300 3300– 3600 2800 water in the form of ice.