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
Mass in the Balance
What is known about the mass balance of Kibo's ice? Detailed studies of mass and energy fluxes have shown that the mass balance on Kibo's horizontal surfaces is driven by the occurrence or lack of frequent and abundant snowfall. On Kilimanjaro, Hardy has measured the annual layering of snow directly since 2000 using snow stakes. These measurements show that the horizontal surface of the mountain's Northern Ice Field has experienced two years of near-neutral mass balance. The largest net gain observed was in 2006 when the calendar year over East Africa ended with exceptional heavy and extended rains, associated with sea-surface temperature anomalies over the Indian Ocean, and snow blanketed much of the summit of Kilimanjaro for several months.
Obviously snowfall is the main way to increase the mass of ice, but snowfall also has a role in the energy balance, one made even more important by the prominent role of solar radiation. The loss side of the balance is very much affected by the amount and even more by the frequency of snowfall: The surface of aged or polluted snow is dark and absorbs considerably more energy from solar radiation than does a white surface of fresh snow. When there is more energy available to a glacier's surface, sublimation increases. But even in below-freezing air temperatures, the same energy can increase melting if there is no wind. Meltwater from the surface is thought to be refrozen in lower ice layers; thus such melting does not necessarily constitute a loss for the ice cap as a whole. Indeed, an observer watching a slope glacier will rarely see more than a trickle of meltwater from the toe.
Comparison of historic photographs indicates that over the past century the thinning of the plateau ice has amounted to perhaps 10 meters—a rate of loss that can be explained by snowfall insufficient to balance sublimation. The observed reduction of the ice's surface area has taken place mainly at the vertical edges, however, which is not explained by snowfall patterns.
The mass balance of the slope glaciers is somewhat different from that of the plateau ice. Retreating midlatitude glaciers typically lose most mass below the ELA and little or none above. The Kibo slope glaciers, though, show shrinkage at both top and bottom. Their history suggests that in 1900 they were already far from equilibrium, but their retreat appears to be slowing; that and their convex shape suggests that they are approaching a new smaller equilibrium between the (relatively constant) loss term and the smaller accumulation term.
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