Background
Like many other tropical glaciers, the ice field atop Kilimanjaro on the Tanzania-Kenya border in East Africa has retreated drastically throughout the 20th century, which retreat has prompted many people, including U.S. Senators John McCain and Hillary Clinton, to claim that its wasting away has been driven by 20th-century global warming (see our review of Kaser et al., 2004).

What was done
To further investigate this claim, Molg and Hardy derive an energy balance for the horizontal surface of the glacier that comprises the Northern Ice Field of Kibo -- the only one of the East African massif's three peaks that is presently glaciated -- based on data obtained from an automated weather station that was placed there in February 2000.

What was learned
The data and their analysis reveal, in the words of Molg and Hardy, that "the main energy exchange at the glacier-atmosphere interface results from the terms accounting for net radiation, governed by the variation in net shortwave radiation," which is controlled by surface albedo and, thus, precipitation variability, which determines the reflective characteristics of the glacier's surface.  Much less significant is the temperature-driven turbulent exchange of sensible heat, which the two scientists say "remains considerably smaller and of little importance."

What it means
Molg and Hardy conclude that "modern glacier retreat on Kilimanjaro and in East Africa in general [was] initiated by a drastic reduction in precipitation at the end of the nineteenth century (Hastenrath, 1984, 2001; Kaser et al., 2004)," and that reduced accumulation and increased ablation have "maintained the retreat until the present (Molg et al., 2003b)."  Buttressing their findings is the fact, as they report it, that "detailed analyses of glacier retreat in the global tropics uniformly reveal that changes in climate variables related to air humidity prevail in controlling the modern retreat [e.g., Kaser and Georges (1997) for the Peruvian Cordillera Blanca and Francou et al. (2003) 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)]."  Hence, the take-home message of their study is essentially the same as that of Kaser et al.: "positive air temperatures have not contributed to the recession process on the summit."

References
Francou, B., Vuille, M., Wagnon, P., Mendoza, J. and Sicart, J.E.  2003.  Tropical climate change recorded by a glacier in the central Andes during the last decades of the 20th century: Chacaltaya, Bolivia, 16°S.  Journal of Geophysical Research 108: 10.1029/2002JD002473.

Hastenrath, S.  1984.  The Glaciers of Equatorial East Africa.  D. Reidel, Norwell, MA, USA.

Hastenrath, S.  1995.  Glacier recession on Mount Kenya in the context of the global tropics.  Bull. Inst. Fr. Etud. Andines 24: 633-638.

Hastenrath, S.  2001.  Variations of East African climate during the past two centuries.  Climatic Change 50: 209-217.

Kaser, G. and Georges, C.  1997.  Changes in the equilibrium line altitude in the tropical Cordillera Blanca (Peru) between 1930 and 1950 and their spatial variations.  Annals of Glaciology 24: 344-349.

Kaser, G., Hardy, D.R., Molg, T., Bradley, R.S. and Hyera, T.M.  2004.  Modern glacier retreat on Kilimanjaro as evidence of climate change: Observations and facts.  International Journal of Climatology 24: 329-339.

Kruss, P.D.  1983.  Climate change in East Africa: A numerical simulation from the 100 years of terminus record at Lewis Glacier, Mount Kenya.  Z. Gletscherk, Glazialgeol. 19: 43-60.

Kruss, P.D. and Hastenrath, S.  1987.  The role of radiation geometry in the climate response of Mount Kenya's glaciers, part 1: Horizontal reference surfaces.  International Journal of Climatology 7: 493-505.

Molg, T., Georges, C. and Kaser, G.  2003a.  The contribution of increased incoming shortwave radiation to the retreat of the Rwenzori Glaciers, East Africa, during the 20th century.  International Journal of Climatology 23: 291-303.

Molg, T., Hardy, D.R. and Kaser, G.  2003b.  Solar-radiation-maintained glacier recession on Kilimanjaro drawn from combined ice-radiation geometry modeling.  Journal of Geophysical Research 108: 10.1029/2003JD003546.