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TROPICAL
GLACIOLOGY GROUP INNSBRUCK UNIVERSITY |
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Kilimanjaro Glaciers (East Africa)
a brief overview of the research
Introduction
(Kaser et al., 2004)
Kilimanjaro, Africa's highest mountain, stands on the Kenya-Tanzania border ca. 370 km south of the equator and about the same distance from the Indian Ocean (3° 04' S / 37° 21' E). The huge stratovolcano (ca. 80 by 50 km) consists of three single peaks: Shira (4,005 m), Mawenzi (5,140 m), and Kibo (5,893 m) which is the only peak to retain glaciers. The summit region of Kibo has collapsed to form a caldera 1.9 by 2.4 km in diameter. Within this lies the Inner Cone, enclosing a crater ca. 800 m across (Reusch Crater) and harboring a third cone with a central crater. Total glacier surface area is currently 2.6 km2, as determined from aerial photographs taken in February 2000 by Ohio State University. These remaining glaciers are the ragged fringe of an ice cap which, it is believed, covered the entire summit of the mountain. The summit glaciers typically have vertical walls (see Fig. 1) mainly along their north and south margins, and ice bodies show a strong east-west orientation. An automated weather station (AWS) is operating since February 2000 on the summit's Northern Icefield (details on measured variables and data available at UMASS).
Fig. 1: The ~25 m high south-facing wall of the Northern Icefield in June 2001. Note the southeast facing portions being in the morning sun, where dripping melt water has removed the thin crater snow cover which fell two days prior. Melt dominates ablation on the respective illuminated vertical walls on the summit. Photograph: G. Kaser.
Since the scientific exploration of Kilimanjaro began in 1887, when Hans Meyer first ascended the mountain (not to the top this time, but to the crater rim), a central theme of published research has been the drastic recession of Kilimanjaro's glaciers. Early reports describe the formation of notches, splitting up and disconnection of ice bodies, and measurements of glacier snout retreat on single glaciers, while later books and papers advance to reconstructing glacier surface areas.
Research Goals and Results
The Tropical Glaciology Group's research on Kilimanjaro started in 2002 and is in progress. First results have been published and can be downloaded in the Publications menu. Central aspects of our research plan are:
1) Development of the working hypothesis: From a synopsis of (i) proxy data indicating changes in East African climate since ca. 1850, (ii) 20th century instrumental data (temperature and precipitation), and (iii) the observations and interpretations made during two periods of fieldwork (June 2001 and July 2002) a scenario of modern glacier retreat on Kibo is reconstructed. This scenario offers the working hypothesis for our project.
Results: Fundamental to the hypothesis is that investigating glacier recession on Kibo requires the definition of at least three different glacier regimes (Kaser et al., 2004), which differ in exchange of energy and mass with the atmospheric boundary layer: (1) the horizontal glacier surfaces on the summit plateau, (2) the vertical ice cliffs building the margin of the glaciers on the summit plateau, and (3) the slope glaciers below the summit. The onset of current glacier recession at ~1880 is most probably forced by an abrupt drop in atmospheric moisture, and maintained by a drier climate during the 20th century. This climate shift is indicated by several proxy sources (lake levels, lake sediments, other glaciers in east Africa, i.e., on Mount Kenya and Rwenzori).
2) Impact of local climate on the glaciers: This goal involves micrometeorological measurements on the glaciers (see below, "The new weather stations"), and the application of collected data to full glacier energy and mass balance models. These models quantify the impact of local climate on a glacier, based on pure physical system knowledge. Our models are validated by measured mass loss and surface temperature.
Results: Horizontal Glacier Surfaces. Precipitation amount, but also precipitation frequency, are the most important factors for this glacier regime (Mölg and Hardy, 2004), because they govern the key parameter in the energy budget (which determines the energy available for mass ablation): surface albedo. The albedo-precipitation feedback is much stronger than any feedback related to air temperature. ***** Vertical ice Walls. In a preliminary step, an experiment with an idealized ice cap model showed that retreat of the cliffs is governed by energy from direct solar radiation (Mölg et al., 2003), a direct implication of a dry climate with lack of snowfall. Data from the new station at the ice cliff confirm this finding: Ablation at the cliffs experiences a marked seasonal cycle, with more than 90% of mass loss during the time when the sun directly hits the wall (in the site-specific case: when apparent sun path is south of equator). ***** Slope Glaciers. Their mass balance and energy balance characteristics resemble the horizontal surface characteristics. By an extended model, it is shown that mass balance response is 2-4 times stronger to a change in 20% of snowfall amount than to a 1°C air temperature change (Mölget al., 2007). The albedo feedback is twice as strong as on a typical glacier of the Alps.
3) Latest Extent of the Kilimanjaro glaciers: Here, a satellite image was analyzed to derive the surface area and spatial distribution of glaciers on Kilimanjaro in February 2003. To validate this approach, an aerial flight was conducted in July 2005.
Results:
The glacier surface area resulting from this study (Cullen et al.,
2006) is 2.51 km2. Although the new figure demonstrates further
retreat of the glaciers on Kilimanjaro, retreat rates have decreased
compared to the early 20th century. Viewing the new extent together with former extents separately for glaciers on the slope (< 5700 m a.s.l.) and summit plateau (> 5700 m a.s.l) shows that slope glaciers were in strong imbalance in the early 20th century (resulting from an abrupt shift in climate), while plateau glaciers receded almost constantly and at a smaller mean rate. Since no abrupt climate change for East Africa is apparent in the 20th century, Kilimanjaro glaciers seem to be primarily remnants of a past climate.
4) Linking local climate to large-scale circulation: As glacier behavior on Kilimanjaro, a totally free-standing mountain, is likely to reflect changes in larger-scale climate, this goal explores the large-scale climate mechanisms driving local Kilimanjaro climate. Well known large-scale forcings of east African climate are sea surface temperature variations in the Pacific and, more important, in the Indian Ocean.
Results: Based on a global climate model simulation of the paleoclimate 1800-1990, the coupled atmosphere-ocean system over the Indian Ocean is found to have been most active between 1820-1880 (Mölg et al., 2006a), causing a higher frequency of extreme October-December rains in East Africa (which implies a wetter climate). This large-scale change is consistent with our microscale results in 2), telling us that precipitation is the governing climate variable for glacier health.
5) Regional modification of large-scale circulation: The regional precipitation response in East Africa due to large-scale forcing is not adequately resolved in a global climate model as used in 4). Thus, mesoscale model experiments with the numerical atmospheric model RAMS will be conducted within this goal. They are thought to reveal the modification of atmospheric flow by the Kilimanjaro massif on a regional scale.
Results: First idealized experiments have shown that the Kilimanjaro massif alters the structure of convective cells that are primarily responsible for precipitation in the tropics (Mölg et al., 2006b). Further RAMS studies are in progress ...
6) Practical aspects: Based on micro- and mesoscale results, (i) how much water is provided by glaciers, (ii) providing future projections of glacier behavior as basis for economic and societal studies (practical part), e.g., for studies on the impact of vanishing glaciers on Kibo's touristic appeal, and (iii) which impact does deforestation on the Kilimanjaro slopes have on summit climate?
Results: Our succesful energy balance model runs for the Northern Icefield (item 2), Mölg and Hardy, 2004) demonstrate that more than 75% of lost glacier mass directly evaporates into the atmosphere (this process is called sublimation). Further, our measurements show that mass exchange on the glaciers is very small compared to, e.g., Alpine glaciers. In total, meltwater runoff from the Kilimanjaro glaciers is minimal. A headline raised by some people in 2000 ("Retreating glaciers on Kilimanjaro will lead to a severe problem in water supply for the local population") can therefore be rejected. Still, the touristic potential of the glaciers, through their presence, must not be underestimated. Other sub-goals are in progress ...
The new Weather Stations
Referring to item 2), two new automatic weather stations have been installed in February 2005. They complete a station operated by Massachusetts University on the surface of the Northern Icefield since 2000. AWS 1 is placed in front of the Northern Icefield edge (5763 m a.s.l.), in order to get insight into ablation processes on the vertical ice cliffs (cf. Fig. 1). This AWS measures wind speed and direction, air temperature and humidity, all four components of the radiation balance (radiometer mounted vertically!), and distance to the ice wall by a sonic ranging sensor (pictures: 1 2). AWS 2 is found on the upper part of the Kersten Glacier (southern slope, Fig. 2, 5873 m a.s.l.) and shall record conditions representative for Kibo's slope glaciers. Here, wind speed and direction, air temperature and humidity at two levels in the surface boundary layer, the four components of the radiation balance (radiometer mounted horizontally!), and distance to the ice surface by a sonic ranging sensor are monitored. The latter is supported by ablation stakes placed close to the AWS (picture 1).
For transporting the components of the two AWS to the summit plateau, a team of 3 mountain guides and 43 (!) porters was necessary this time (heavy load!). The scientific team comprised 5 people. Together wit the University of Massachusetts AWS on the Northern Icefield horizontal surface, a total of three automatic weather stations is operating on the glaciers in the summit area.
Fig. 2: The Southern Icefield and its transition into the slope glacier (Kersten-Glacier). Note the automatic weather station in the upper part of the steep slope glacier. For a detail view of the AWS see the link provided above. Photograph: N. Cullen.
Collaborators
Research on Kilimanjaro is done in collaboration with the Climate System Research Center, University of Massachusetts within their Kilimanjaro-project (check it here). The local advisors are at the Tanzania Meteorological Agency, Dar es Salaam . Global climate model data were provided by the Physics Institute at the University of Berne.
Acknowledgements
This research is funded by the Austrian Science Foundation under grant no. P17415-N10, and the Tyrolean Science Foundation (see partners menu). Special thanks to our mountain guide Eric Masawe and his team for great help during field expeditions. Field trips are supported by Mountain Hardware mountain gear.
Document maintained by Thomas Mölg and Nicolas Cullen
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