The role of radiation geometry in the climate response of Mount Kenya's glaciers, part I: Horizontal reference surfaces

Abstract

Model calculations of incident shortwave radiation were performed for the Mount Kenya massif with the objectives of: (i) exploring the major factors controlling the ice distribution in the peak region: and (ii) determining the primary forcings that led to the differential retreat of the various glaciers.The ice distribution on Mount Kenya, characterized by largest glaciers to the south and east, is primarily controlled by the precipitation pattern. By contrast, the radiation pattern is unconducive to maintaining this ice distribution, as insolation tends to be higher on these largest glaciers. Spatial variability in the incident radiation is mostly due to shading effects, with cloudiness however being more important in reducing absolute values. Steep slopes, essentially above 5,100 m, and higher temperatures below 4,600 m are further limiting factors.A drastic glacier recession is borne out by mappings ofthe ice extent in 1899 and 1963. Over this time span, the number of glaciers diminished from 18 to 11, and another disappeared since 1978. Of the eight defunct ice entities, six were small. By far the largest glacier to disappear since the turn of the century was the Kolbe, in a precipitation-abundant but radiationally very exposed location on the eastern flank of the mountain. The largest and now easternmost ice body, the Lewis, which occupies the radiationally most exposed location of all the glaciers, has experienced the greatest ice wastage.The model calculations indicate that the drastic ice recession since the late 1800s is largely due to an increase in absorbed shortwave radiation accompanying, and in part brought about by, changes in cloudiness, precipitation, and temperature. In addition to the consequences for the overall ice amount, the spatial distribution of radiation totals, as influenced by topographical shading, appears to account for the remarkably diverse shrinkage of the glaciers on Mount Kenya.

Observations: Cryosphere

“…In support of the findings of Molg et al (2003a, b), for Africa, analyses of glacier retreat throughout the tropics uniformly suggest that changes in air humidity have been dominant in controlling modern retreat where it has occurred [e.g., for the Peruvian Cordillera Blanca and for the Bolivian Cordillera Real (both South American Andes); Kruss (1983), Kruss and Hastenrath (1987), and Hastenrath (1995) for Mount Kenya (East Africa); and Molg et al (2003a) for the Rwenzori massif (East Africa)]. conclude "changes in air humidity and atmospheric moisture content (e.g.…”

“…• Molg et al (2003b) applied a radiation model to an idealized representation of the 1880 icecap of Kilimanjaro, concluding "modern glacier retreat on Kilimanjaro is much more complex than simply attributable to 'global warming only.'" Instead, and as reported by many other authors, the ice retreat has been "a process driven by a complex combination of changes in several different climatic parameters [e.g., Kruss, 1983;Kruss and Hastenrath, 1987;Hastenrath and Kruss, 1992;Wagnon et al, 2001;Kaser and Osmaston, 2002;Molg et al, 2003b], with humidity-related variables dominating this combination." report "all ice bodies on Kilimanjaro have retreated drastically between 1912-2003," but they add the highest glacial recession rates on Kilimanjaro "occurred in the first part of the twentieth century, with the most recent retreat rates (1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) smaller than in any other interval."…”

“…Citing "historical accounts of lake levels (Hastenrath, 1984;Nicholson and Yin, 2001), wind and current observations in the Indian Ocean and their relationship to East African rainfall (Hastenrath, 2001), water balance models of lakes (Nicholson and Yin, 2001), and paleolimnological data (Verschuren et al, 2000)," Molg et al (2003a, b) say "all data indicate that modern East African climate experienced an abrupt and marked drop in air humidity around 1880," and the resultant "strong reduction in precipitation at the end of the 19th century is the main reason for modern glacier recession in East Africa," as it considerably reduces glacier mass balance accumulation, as demonstrated for the region by Kruss (1983) and Hastenrath (1984). In addition, they note "increased incoming shortwave radiation due to decreases in cloudiness-both effects of the drier climatic conditions-plays a decisive role for glacier retreat by increasing ablation, as demonstrated for Mount Kenya and Rwenzori (Kruss and Hastenrath, 1987;Molg et al, 2003a)." conclude all relevant "observations and facts" clearly indicate "climatological processes other than air temperature control the ice recession in a direct manner" on Kilimanjaro, and "positive air temperatures have not contributed to the recession process on the summit."…”

“…Soden and Schroeder, 2000) seem to play an underestimated key role in tropical high-mountain climate (Broecker, 1997)." Regarding East African montane glaciers, it is important to remember "the dominant reasons for this strong recession in modern times are reduced precipitation (Kruss, 1983;Hastenrath, 1984;Kruss and Hastenrath, 1987;Kaser and Noggler, 1996) and increased availability of shortwave radiation due to decreases in cloudiness (Kruss and Hastenrath, 1987;Molg et al, 2003b)." These factors are related to a drying of the regional atmosphere that commenced around 1880 and lasted through the twentieth century.…”

The contribution of increased incoming shortwave radiation to the retreat of the Rwenzori Glaciers, East Africa, during the 20th century

“…For the three glaciated massifs in East Africa (Mount Kenya, Rwenzori, 1 and Kilimanjaro), this was comprehensively reported by Hastenrath (1984). Specific details for Mount Kenya are provided by Hastenrath and coworkers (Kruss and Hastenrath, 1987;Hastenrath, 1989;Hastenrath et al, 1989), for the Rwenzori by Kaser and coworkers Noggler, 1991, 1996;Kaser and Osmaston, 2002), and for Kilimanjaro by Hastenrath and Greischar (1997) and Thompson et al (2002). Decreased precipitation and increasing temperature, and, therefore, a higher position of the lower snow fall limit, are regarded as dominating reasons for the generally strong glacier recession in Equatorial East Africa (Kruss and Hastenrath, 1987;Kaser and Noggler, 1996).…”