FEATURE ARTICLE
The Shrinking Glaciers of Kilimanjaro: Can Global Warming Be Blamed?
The Kibo ice cap, a "poster child" of global climate change, is being starved of snowfall and depleted by solar radiation
Phillip W. Mote, Georg Kaser
Stuck in the Freezer
Another important observation is that the air temperatures measured at the altitude of the glaciers and ice cap on Kilimanjaro are almost always substantially below freezing (rarely above -3 degrees). Thus the air by itself cannot warm ice to melting by sensible-heat or infrared-heat flux: On the occasions when melting takes place, it is produced by solar radiation in conditions of very light wind, which allows a warm layer of air to develop just next to the ice.
A related line of evidence concerns the shape and evolution of ice. Stunning vertical walls of astonishing height (greater than 40 meters in places) tower over the visitor to Kibo's summit. These edges cannot grow horizontally but lose mass constantly to ablation (primarily to sublimation and intermittently to melting) when they are exposed to the sun—even when the air temperature is below freezing. Once developed, the near-vertical edges will retreat until the ice is gone, since no snow can accumulate on these walls.
The careful observer notes another striking fact about these walls: They are predominantly oriented in the east-west direction. This too implicates solar radiation, whose intensity is modulated by a seasonal and daily pattern of cloudiness: The daily cycle of deep convection over central Africa means that afternoons, when the Sun is to the west, are typically cloudy. The equinox seasons when the Sun is overhead are also cloudy, whereas when the Sun is to the south or north (solstices), the summit is typically cloud-free. For the same reason, the edges of the ice are retreating more slowly on the west, southwest and northwest sides.
The role of solar radiation in shaping the ice edges is evident in other features as well. As the ice retreats horizontally, it can leave behind knife-thin vertical remnants that eventually become so thin that they fall over and disintegrate. Like other explorers who came before them, Kaser and Hardy also noted the sculpted features called penitentes in the Kibo ice cap on several occasions. Penitentes are seen also in many places in the Andes and the Himalaya, where they are sometimes much larger. These finger-like features arise when initial irregularities in a flat surface result in the collection of dust in pockets, which accelerates melting in those places by enhancing absorption of solar radiation. The cups between the penitentes are protected from ventilation even as wind brushing the peaks of the developing spires enhances sublimation, which cools the surface.
If infrared radiation and sensible heat transfer were the dominant factors, these sculpted features would not long survive. Solar radiation and sublimation are sculptors; infrared radiation and sensible heat transfer are diffuse, coming equally from all directions, and so they are smoothers. The prevalence of sculpted features on Kilimanjaro's peak provides strong evidence against the role of smoothers, which are energetically closely related to air temperature.
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