Permafrost Degradation Between 1973 and 2004

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The lapse rate change in 2004 at GT50 indicated that the permafrost lower limit lies between 5400 and 5500 m on south-facing slopes of the Khumbu Himal. The frozen layer was found below 45 cm at P1 located at 5540 m. Because ground freezing had already started by 20-21 October, this frozen layer had likely survived from the preceding summer and could be considered permafrost. Judging from the above results, the permafrost lower limit on slopes with a southern aspect in the Khumbu Himal lay between approximately 5400 and 5500 m in 2004.

The permafrost lower limit on slopes with a southern aspect in the Khumbu Himal in 1973 was estimated as 5200 to 5300 m based on the ground temperature data collected by

Fujii and Higuchi (1976). These findings indicate that the permafrost lower limit rose 100300 m between 1973 and 2004.

Barsch and Jakob (1993) and Jakob (1992) examined the permafrost lower limit using seismic reflection soundings and estimated the permafrost lower limit of 5400 to 5500 m on slopes with a southern aspect in the Khumbu Himal in 1991. Their estimation corresponds with our finding on the permafrost lower limit in 2004. Thus, it is possible that the permafrost lower limit has risen 100-300 m between 1973 and 1991, followed by a stable limit of 5400 to 5500 m over the last decade (Fig. 7).

Figure. 7 Schematic diagrams of changes in the lower limit of permafrost in the Khmbu Himal.

Figure 8 shows the variations in average air temperatures during winter (January-February-March) and those during summer (June-July-August) at weather stations in Namche Bazar (3450 m) in 1971-1982 and Dingboche (4355 m) in 1987-2001. Namche Bazar is located in approximately 2.5 km south-west of Dingboche. The data were collected and published by the Department of Hydrology and Meteorology (DHM), His Majesty's Government (HMG) of Nepal and its Snow and Glacier Hydrology Unit (SGHU). Because these data were much fragmentary, we could not calculate annual means. In 1971-1982, variations in the air temperatures in summer were almost stable, however those in winter had clear warming trend. In 1987-2001, variations in the air temperatures in summer were stable and those in winter had large variety year to year and no warming trend. These should be indicate that the air temperature variations in the Khumbu Himal have a warming trend in 1971-1982 and have no warming trend in 1987-2001. These trends correspond with our finding on the changes in the permafrost lower limit over the last three decades.

It has been reported that the permafrost area of the Tibetan Plateau has experienced the most substantial climate warming (Jin et al., 2000). Wang et al. (2000) reported that the permafrost lower limit rose 40-80 m from the 1970s to 1990s in the Tibetan Plateau, according to an increase in MAAT of approximately 0.2 to 0.4°C. However, because the rise in the mountain permafrost lower limit in the Khumbu Himal is larger than that in the Tibetan Plateau, it is possible that climate warming in the Khumbu Himal has been more severe than that in the Tibetan Plateau.

1973

1991

2004

Figure. 7 Schematic diagrams of changes in the lower limit of permafrost in the Khmbu Himal.

Figure. 8 Variations in average air temperatures during winter (January-February-March) and those during summer (June-July-August) at weather stations in Namche Bazar (3450 m) in 1971-1982 and Dingboche (4355 m) in 1987-2001 (modified Fukui et al., 2007a).

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