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
The fact that glaciers exist in the tropics at all takes some explaining. Atmospheric temperatures drop about 6.5 degrees Celsius per kilometer of altitude, so the air atop a 5,000-meter mountain can be 32.5 degrees colder than the air at sea level; thus, even in the tropics, high-mountain temperatures are generally below freezing. The climber ascending such a mountain passes first through lush tropical vegetation that gradually gives way to low shrubs, then grasses and finally a zone that is nearly devoid of vegetation because water is not available in liquid form. Tropical mountaintop temperatures vary only a little from season to season, since the sun is high in the sky at midday throughout the year. With temperatures this low, snow accumulates in ice layers and glaciers on Kilimanjaro, Mount Kenya and the Rwenzori range in East Africa, on Irian Jaya in Indonesia and especially in the Andean cordillera in South America, where 99.7 percent of the ice in tropical glaciers is found.
A simple, physically accurate way to understand the processes creating and controlling these and other glaciers is to think in terms of their energy balance and mass balance.
Mass balance is merely the difference between accumulation (mass added) and ablation (mass subtracted); in this case mass refers to water in its solid, liquid or vapor form. A glacier's mass is closely related to its volume, which can be calculated by multiplying its area by its average depth. When a glacier's volume changes, a change in length is usually the most obvious and well-documented evidence. Alaska's vanishing Muir Glacier, an extreme case, shrank more than 2 kilometers in length over the past half-century.
Glaciers never quite achieve "balance" but rather wobble like a novice tightrope walker. Sometimes a change in climate throws the glacier substantially out of balance, and its mass can take decades to reach a new equilibrium.
Added mass comes largely from the atmosphere, generally as snowfall but also as rainfall that freezes; in rare cases mass is added by riming, in which wind carries water droplets that are so cold that they freeze on contact.
The most obvious subtractive process is the runoff of melted water from a glacier surface. Another process that reduces glacial mass is sublimation, that is, the conversion of ice directly to water vapor, which can take place at temperatures well below the melting point but which requires about eight times as much energy as melting. Sublimation occurs when the moisture in the air is less than the moisture delivered from the ice surface. It is the process responsible for "freezer burn," when improperly sealed food loses moisture.