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TROPICAL
GLACIOLOGY GROUP INNSBRUCK UNIVERSITY |
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Rwenzori Glaciers (East Africa)
a brief overview of the research
Introduction
(Kaser and Noggler, 1996)
The Rwenzori Range is located at the equator on the Zaire-Uganda border, East Africa (0°, 30° E). The entire range consists of six ice-capped mountains: Mount Stanley, Speke, and Baker in the central part (see Fig. 1); Mount Luigi di Savoia, Emin, and Gessi in the outer parts. The mountain range culminates in Margherita Peak at 5,109 m, and several peaks exceed 4,500 m. H.M. Stanley was the first European to report on the snow-covered Rwenzori massif in 1888. Since then, numerous explorers attempted to reach these mountains. Finally, the duce of Abruzzi's expedition was succesful in 1906. First modern glaciological investigations were realized by the British Rwenzori Expedition in 1952. Continuous and detailed observations were made by the Makerere University Expeditions between 1958 and 1961. Due to political tensions between Uganda and Zaire, research has become difficult since then and was concentrated on single field trips.
The Tropical Glaciology Group's research foccused on two matters:
1) Reconstruction of the modern glacier fluctuations in the Central Rwenzori Range (research period: 1990-1996)
2) Climatological interpretation of these fluctuations (research period: 2001-2003)
Modern glacier fluctuations in the Central Rwenzori Range
(Kaser and Noggler, 1991, 1996; Kaser, 1999; Kaser and Osmaston, 2002)
To get an idea of glacier fluctuations in the Central Rwenzori Range, all material on the surface extend of the glaciers was collected (small impression!). Finally, three different dates during the 20th century provided the most reliable data: The 1906-extend is derived from the documents of the Duce of Abruzzi's expedition, the 1955-extend from a detailed map which was made from vertical air-photos, and the 1990-status is mainly based on two Innsbruck University expeditions. Data of glacier extents for the minor glaciated mountains in the Rwenzori Range, Mount Emin, Gessi, and Luigi di Savoia, are not exactly known and, thus, were excluded from this investigation. Fig. 1 shows the result of the reconstruction. According to their different area altitude distribution, the glaciers on the individual mountains of the Central Rwenzori Range have retreated diversely until the early 1990s: Mount Stanley could at least keep 35 % of its glacier surface area of 1906, Mount Speke 25 %, and Mount Baker only kept 8 %. Besides the generally strong recession, the complex retreat pattern, which is particularly conspicuous on the single glaciers Speke and Moore (emphasized in Fig. 1), is striking and initiated the idea presented in the next chapter.
Fig. 1: The reconstructed glacier surface areas of Mount Baker, Mount Speke, and Mount Stanley 1906, 1955, and 1990 (contours in metres, as calculated from the original values in feet / equidistance 125 m). base map by D.O.S.; (from Mölg et al., 2003, after Kaser and Osmaston, 2002) enlarge figure
Climatological interpretation of Rwenzori's glacier fluctuations
(Mölg, 2002, for technical details; Mölg et al., 2003)
Based on (i) the observation of spatially differential glacier retreats in the tropical Rwenzori Range (East Africa) during the 20th century, which are most striking on the mountains Baker and Speke (Fig. 1), and (ii) the information on an abrupt climate change to drier conditions in East Africa at the end of the 19th century, indicated by climatic proxy data, the following hypothesis was derived: due to a drier atmosphere than in a previous period both accumulation, possibly supported by increasing air temperatures, and convective cloud activity have decreased. Consequently, increased incoming shortwave radiation, especially during the morning hours, gave reason to a differentially increased ablation which could not been compensated by mass advection on the mountains Baker and Speke. To test this hypothesis incoming shortwave radiation was calculated for two different climatic scenarios. The results obtained from a combined terrain-radiation model which was run for these climatic scenarios confirm the hypothesis. A later onset of convective cloudiness in the morning hours leads to s spatially differential increse in incoming shortwave radiation which shows a direct proportionality to glacier recession. Fig. 2 gives a graphical presentation of the results: blue symbolizes glacier surface area loss in the period "1906 to 1955", red refers to the period "1955 to 1990". Light hues denote that glaciers retreated in areas of "no or little increase in incoming shortwave radiation" (category 1; 0 - < 5 W m-2), dark hues pick out glacier surface area loss in regions of "clearly increased incoming shortwave radiation" (category 2; 5 and more W m-2) as theme. Glacier surface area loss in the entire Central Rwenzori Range between 1906 and 1955 was twice as high in areas of a "clear increase in mean annual incoming shortwave radiation" (category 2) as in areas of "no or little increase in mean annual incoming shortwave radiation" (category 1). This plain picture remains in the second half of the 20th century between 1955 and 1990. Glacier surface area loss in regions of "clearly increased incoming shortwave radiation" is 1.6 times the amount of the loss in areas of "no or little increase in incoming shortwave radiation". Spatial distribution of the absolute amounts of mean annual incoming shortwave radiation during the 20th century cannot explain the differential glacier retreats, which is verifying the change in cloudiness.
In the context of modern climate fluctuations, the results are a further indicator for a drastic climatic dislocation in East Africa at the end of the 19th century, leaving a humid regime behind and leading to a relatively dry regime, which is forcing the recession of glaciers not only by less accumulation but also by less protection against shortwave radiation through clouds. This finding agrees with studies on the retreating glaciers of Mount Kenya, East Africa.
Fig. 2: Glacier surface area loss in areas of "no or little increase" (0 - < 5 W m-2 : category 1) and in areas of a "clear increase" (5 and more W m-2 : category 2) in mean annual incoming shortwave radiation in the Central Rwenzori Range in the periods "1906 to 1955" and "1955 to 1990" (contours in metres, as calculated from the original values in feet / equidistance 125 m). enlarge figure
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