Permafrost warming in the Tien Shan Mountains, Central Asia

https://doi.org/10.1016/j.gloplacha.2006.07.023Get rights and content

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.

Introduction

The alpine permafrost zone in the Tien Shan Mountains (69–95°E, 40–44°N) belongs to the Asian high-mountain permafrost region, the largest in the world (Fig. 1). The occurrence and evolution of alpine permafrost in the middle latitudes directly relates to the tectonic history of the Earth. The facts collected recently provide information about the Pre-Quaternary age of permafrost in the Tien Shan Mountains. Permafrost first formed about 1.6 million years ago because of mountain elevation (Aubekerov and Gorbunov, 1999). Since then and up to the present day, the alpine permafrost of the Tien Shan never disappeared completely. During this time, the extent of mountain permafrost area in the Tien Shan changed many times. These changes were caused by the mountain continuously rising and by the unfolding planetary climate events. The glacial and periglacial features evidently show that during some time intervals the ancient permafrost occurred at a much lower elevation than the present day permafrost (Gorbunov, 1985, Aubekerov, 1990, Marchenko and Gorbunov, 1997). The maximum glacial expansion in the Tien Shan Mountains happened during the Middle Pleistocene time (Aubekerov and Gorbunov, 1999). However, the maximum extension of the permafrost area in the Tien Shan and adjacent foothills and plains occurred in the Late Pleistocene when the combination of low air humidity and cold temperatures created a more favorable condition for permafrost formation and expansion. At that time, the lower boundary of permafrost was located at about 900–1000 m a.s.l., which is at least 1500–1700 m lower than the modern lower altitudinal permafrost boundary. This means that during the Late Pleistocene the alpine permafrost of Tien Shan Mountains merged with the Siberian permafrost area via a sporadic permafrost zone that occurred on the foothill plains of Djungar Alatau, Saur-Tarbagatai and Altai.

Ground temperatures in the Tien Shan permafrost area have been subjected to repeated fluctuations during the Holocene brought about by the general planetary changes in climate. The altitudinal oscillations of the mean annual air temperature (MAAT) zero Centigrade isotherm had a range of about 500 m during the Holocene. Early Holocene (approximately between 9000 and 7000 yr ago) was the most unfavorable time for the existence of alpine permafrost in the Tien Shan (Marchenko and Gorbunov, 1997). There was a period of permafrost degradation in the Tien Shan Mountains. A Middle Holocene cooling was replaced by a short phase of warming in the Late Holocene, after which ground temperatures again significantly decreased. During the Little Ice Age, there was a downward shift of the lower boundary of permafrost distribution by 200–300 m of altitude. Since the second part of the nineteenth century, permafrost in the Tien Shan Mountains is experiencing a warming period, which continues up to the present. This article considers the recent (last century) permafrost changes in the Tien Shan Mountains, which were studied using geothermal measurements in boreholes, the analysis of climatic data, and numerical modeling of permafrost temperature field dynamics. The major aim of this work is to evaluate changes in Tien Shan's permafrost during the last century using both observed data and a modeling approach.

Section snippets

General features of permafrost distribution in the Tien Shan

The first information about the presence of permafrost in the Tien Shan appeared in 1914 (Bezsonov, 1914). General features of permafrost distribution in the Tien Shan Mountains are resulting from latitudinal and altitudinal zonality, and from changes in climatic and topographic factors. The systematic investigations of mountain permafrost in the Tien Shan began in the mid-1950s (Gorbunov, 1967, Gorbunov, 1970). The regional patterns of permafrost distribution depend on elevation, slope and

Recent changes in climate in the Tien Shan Mountains

Many components of the cryosphere, particularly glaciers and permafrost, are very sensitive to climate change. Climatic changes and changes in permafrost were reported recently from many mountain regions. In Asia, analyses of temperature data from 49 stations in Nepal for the period 1971–1994 revealed warming trends after 1977 ranging from 0.06 to 0.12 °C/yr in most of the Middle Mountain and Himalayan regions (Shrestha et al., 1999). In the western Mongolian sector of the Altai Mountains, the

Permafrost temperature and active-layer change in the Tien Shan Mountains

There are 24 active thermometric boreholes with depths ranging from 3 m to 300 m in different landscape settings and at varying altitudes available for measurements near the two permafrost stations (“Main Station” and “Cosmostation”) in the Northern Tien Shan (Fig. 2). Ground temperature measurements are carried out by using thermistor sensors (MMT-4 and TSM-50) with a sensitivity of 0.02 °C and an accuracy not less than 0.05 °C. There are three sites equipped with temperature data loggers

Modeling of permafrost thermal dynamics

The main objectives of the modeling process were to estimate the permafrost thermal regime and assess the area where permafrost disappeared since the second part of the nineteen century. A one-dimensional numerical model of heat transfer for a multi-layered medium (Marchenko, 2001, Tipenko and Romanovsky, 2001, Romanovsky et al., 2002) was used for this purpose. The model takes into account the latent heat of water freezing/thawing and has the capability for computations of a convective heat

Changes in spatial permafrost distribution

To evaluate the changes in the area of permafrost distribution during the last 120 yr, two data-rich basins of the Bolshaya and Malaya Almatinka Rivers (Fig. 2) with different morphological, glaciological and periglacial characteristics were selected. A wealth of material on permafrost conditions (permafrost temperatures, cryogenic structures, and periglacial landforms distribution) within the limits of these basins was collected during the last 30 yr. Currently, we developed the Geographical

Conclusions

Both geothermal observations and modeling of permafrost thermal state show significant changes in permafrost temperature and extent during the 20th century in the Tien Shan Mountains. Geothermal observations during the last 30 yr indicate an increase in permafrost temperatures in a range from 0.3 °C up to 0.6 °C. The average active-layer thickness increased by 23% in comparison with the early 1970s. As a result of a deep thawing penetration of up to 5 m and more, a residual thaw layer (talik)

Acknowledgements

This research was been funded by the Polar Earth Science Program, Office of Polar Programs, National Science Foundation (OPP-0327664), and by the State of Alaska. The temperature data used in this study are available to other researchers through the GCOS/GTN-P site: http://www.gtnp.org, CALM site database: http://www.udel.edu/Geography/calm/, and from NSIDC (http://nsidc.org). The authors thank their collaborators in Kazakhstan for help in geothermal observations. We would like to thank

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