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
Air, Ice and Equilibrium
Melting, sublimation and the warming of ice require energy. Energy in the high-mountain environment comes from a variety of energy fluxes that interact in complex ways. The Sun is the primary energy source, but its direct effect is limited to daytime; other limiting factors are shading and the ability of snow to reflect visible light. Energy can nevertheless reach the glacier through sensible-heat flux—the exchange of heat between a surface and the air in contact with it, in this case heat taken directly from the air in contact with the ice—and via infrared emission from the atmosphere and land surface. Energy can also leave glacier ice in several ways: sensible-heat flux from the glacier to cold air, infrared emission from snow and ice surfaces, and the "latent heat" required for water to undergo a phase change from solid to liquid (melting) or gas (sublimation).
Mountain glaciers accumulate snow at high altitudes, slide downhill—some at speeds approaching 2 meters a day—and melt at low altitudes in summertime. Some midlatitude glaciers reach sea level in part because of copious snowfall, exceeding the liquid equivalent of 3 meters per year.
Somewhere between the top and bottom of a glacier on a mountain slope, there is an elevation above which accumulation exceeds ablation and below which ablation exceeds accumulation. This is called the equilibrium line altitude or ELA. Rising air temperatures increase the sensible-heat flux from the air to the glacier surface and the infrared radiation absorbed by the glacier, so that melting is faster and is taking place over a larger portion of the glacier.
Thus rising temperatures also raise the equilibrium-line altitude. In latitudes with pronounced seasons, this expands the portion of the glacier that melts each summer and may even, in some cases, reduce the portion of the glacier that can retain mass accumulated in the winter. Virtually all glaciers in the world have receded substantially during the past 150 years, and some small ones have disappeared. Warming appears to be the primary culprit in these changes, and indeed glacial-length records have been used as a proxy for past temperatures, agreeing well with data from tree rings and other proxies.
In many respects, however, conditions are quite different for glaciers in the tropics, where temperature varies far more from morning to afternoon than from the coldest month to the warmest month. The most pronounced seasonal pattern in the tropics is the existence of one or two wet seasons, when glacial accumulation is greater and, owing to cloud cover, solar radiation is less.
Because there is almost no seasonal fluctuation in the ELA of tropical glaciers, a much smaller portion of the glacier lies below the ELA. That is, because the processes causing depletion of the glaciers operate almost every day of the year, they are effective over a much smaller area. This smaller area also means that the terminus or bottom edge of tropical glaciers tends to respond more quickly to changes in the mass balance.
An additional important distinction among tropical glaciers divides wet and dry regimes. In wet regimes, changes in air temperature are important in mass-balance calculations, but for dry regimes like East Africa, changes in atmospheric moisture are more important. Connections between such changes and global increases in greenhouse gases are more tenuous in tropical regimes. Year-to-year variability and longer-term trends in the seasonal distribution of moisture are influenced by the surface temperatures of the tropical oceans, which, in turn, are influenced by global climate. On many tropical glaciers, both the direct impact of global warming and the indirect one—changes in atmospheric moisture concentration—are responsible for the observed mass losses. The mere fact that ice is disappearing sheds no light on which mechanism is responsible. For most glaciers, detailed observations and measurements are missing, adding to the difficulty of distinguishing between the two agents.
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