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AGU 2009: Humidity more than heat cause of vanishing glaciers

Harvey Leifert

The snows of Kilimanjaro, made internationally famous by Ernest Hemingway’s novel of that name, are fading away. Certainly, that is true of the glaciers that top the mountain’s summit plateau, some 5,900 metres above sea level.

Standing just three degrees south of the equator in Tanzania, Kilimanjaro is Africa’s tallest mountain. Its summit glaciers, covering 2.5 square kilometers, feature nearly vertical cliffs, ranging from three to over 40 metres in height. These cliffs are regressing, and the question is, why? At the American Geophysical Union’s annual meeting in San Francisco, Michael Winkler of the University of Innsbruck (Austria) provided some preliminary answers.

The ice cliffs face north and south and, depending on the season, receive sunlight all day long or not at all, in an alternating pattern. When in shadow, the cliffs cannot melt; only sublimation (direct change from solid to gaseous state) is possible, Winkler said in an interview. When in sunlight, the cliffs can melt as well as sublimate. Therefore, the retreat of the glacier occurs in two phases, slowly at the end in shadow and 20 to 30 times faster at the sunlit end.

Winkler and colleagues placed instruments at the base of a 25-metre cliff in 2005, to measure temperature and humidity. The humidity readings are particularly important, Winkler said, as the glaciers are mainly responsive to humidity. The air temperature is always below freezing.

The glaciers are, in fact, retreating because the climate is getting dryer, not warmer, he noted. The only way to replenish the glacier is through large, frequent snowfalls, and the big precipitation events stopped around 1850, he said. He attributed the decline to changes in the sea surface temperature and currents in the nearby Indian Ocean.


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