The mountain permafrost lower limit has been studied in the Khumbu Himal, the Nepal Himalayas, since 1973 (Fujii and Higuchi, 1976; Jakob, 1992; Barsch and Jakob, 1993). Fujii and Higuchi (1976) measured ground temperatures and found that the permafrost lower limit on slopes with a northern aspect was about 4900-5000 m in 1973. Barsch and Jakob (1993) conducted seismic refraction soundings and estimated that the lower limit on slopes with a southern aspect was 5400-5500 m in 1991. Fukui et al. (2007a) examined the permafrost lower limit in 2004 based on ground-temperature measurements. We summaries the changes in the permafrost lower limit over the last three decades based on these permafrost studies.
The Khumbu Himal is located in the eastern part of the Nepal Himalayas (Fig. 1). The permafrost studies focused on the upper Imja Valley (27°53-60'D, 86°48-50'E) located in the central part of the Khumbu Himal (Fig. 2). The Khumbu glacier extends from the south face of Mt. Everest (8850 m) to 4900 m ASL in this valley. The tree line limit forms at approximately 4000 m, above which alpine meadows extend up to glacier terminus (Fig. 3).
Automatic weather stations were established near Dingboche (4355 m) in 1987 and Lobuche (5050 m) in 1990. Mean annual air temperatures (MAATs) at Dingboche and Lobuche are -1.5 and -2.6°C, respectively (Grabs and Pokhrel, 1993; Tartari et al., 1998). Annual precipitation at Dingboche and Lobuche is 412 and 465 mm, respectively, and nearly 90% of the precipitation occurs during the summer monsoon season (June-September; Bollasina et al., 2002). Because little winter snow falls in this area (Fig. 3a), the permafrost distribution is unaffected by snow cover.
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