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

Abstract

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, and (ii) the information on an abrupt climate change to drier conditions in East Africa at the end of the 19th century, the following hypothesis is derived: owing 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, induced a differentially increased ablation that could not be compensated by mass advection on the mountains Baker and Speke.The results obtained from a combined radiation-terrain model, run for one more humid and one drier climatic scenario, confirm the hypothesis by quantifying the correlation between increased incoming shortwave radiation and glacier surface area loss. 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.

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.…”

“…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."…”

“…Much less significant, according to the two researchers, is the temperature-driven turbulent exchange of sensible heat, which they say "remains considerably smaller and of little importance." Molg and Hardy conclude "modern glacier retreat on Kilimanjaro and in East Africa in general [was] initiated by a drastic reduction in precipitation at the end of the 19th century (Hastenrath, 1984(Hastenrath, , 2001," and reduced accumulation and increased ablation have "maintained the retreat until the present (Molg et al, 2003a)." • 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.'"…”

“…• Molg et al (2003a) note all three glacier-bearing volcanic mountains in East Africa-Kilimanjaro (Tanzania, Kenya), Mount Kenya (Kenya), and Rwenzori (Zaire, Uganda)-have experienced strong ridges. The outer moraine has been dated by 10 Be, 26 Al, and 36 Cl at 11.75 ka and the inner moraine at 10.47 ka (Ivy-Ochs et al, 1996, 1999Kerschner et al, 1999).…”

Measuring and Modeling the Retreat of the Summit Ice Fields on Kilimanjaro, East Africa

“…Although the relative importance of both land-use change and global climate forcings are still being researched (Pepin et al, 2010;Fairman et al, 2011;Mölg et al, 2012), there is a growing consensus that whatever the cause(s), drying of the summit climate-resulting in lower humidity, less frequent cloud cover and enhanced daytime solar radiation input, increasing ablation on the one hand, and reduced precipitation decreasing accumulation on the other-has encouraged a long-term negative mass balance of the summit ice fields (Mölg et al, 2003b;Kaser et al, 2004;Mölg et al, 2008Mölg et al, , 2009aMölg et al, , 2009b, as is the case in other parts of tropical Africa (Mölg et al, 2003a).…”