Tropical Glacier Retreat

A detailed description of the way the energy budget determines ablation can be found here, but a simplified version of the story goes as follows. In contrast to the midlatitude case, tropical glaciers do not have summertime melt seasons characterized by above-freezing air temperature. Lower altitude portions can be warmed directly by year-round exposure to above-freezing air, but at higher altitudes absorption of sunlight ultimately supplies all the energy which sustains ablation. However, the other terms in the energy balance directly or indirectly affect the amount of absorbed solar radiation which is available for ablation. These terms are sensitive to air temperature, atmospheric humidity, cloudiness, and wind. The daytime glacier surface temperature typically has to be greater than the air temperature in order to close the energy budget; in consequence, melting can occur even when the air temperature remains below freezing. Because melting is so much more energetically efficient than sublimation, the main way that moderate changes in atmospheric conditions — including air temperature– affect ablation is through changing the number of hours during which melting occurs, and the amount of energy available for melting. In particular, through infrared and turbulent heating effects, an increase in air temperature forces the glacier surface to warm, and makes it easier for melting to occur.

In addition to adding mass to a glacier, precipitation has an indirect effect on glacier mass balance by changing the amount of sunlight the glacier absorbs. This occurs because fresh snow is much more reflective than old snow or bare ice. The reflectivity effect can be almost ten times more important than the effect of mass directly added by precipitation [Moelg and Hardy, 2004]. Because a thin layer of snow is just as reflective as a thick layer, the reflectivity effect depends more on the seasonal distribution of snowfall than the annual average amount.

A healthy glacier has an accumulation zone at high elevations and an ablation zone at lower elevations; ice flow from the accumulation zone continually feeds glacier tongues that penetrate into the ablation zone. The altitude separating the accumulation zone from the ablation zone is known as the equilibrium line altitude. Glaciers shrink when climate change causes the equilibrium line to rise, but they stop at a new, smaller equilibrium size. However, if the equilibrium line rises to the summit of the mountain, the accumulation zone disappears altogether and the glacier is doomed. This has happened on Chacaltaya and, according to limited recent observations [Moelg and Hardy,2004], also on the summit glaciers of Kilimanjaro.


A glacier is like your bank account. Whether your wealth is growing or dwindling depends on how much money you deposit vs. how much you withdraw each year. The Kilimanjaro glaciers are nearing bankruptcy, but is this due to excessive withdrawals or insufficient savings? This, in essence, is the question raised (but not settled) in the paper by Kaser et al. [2004]. This paper has played a valuable role in calling attention to important work on the physics of tropical glaciers, that can help in teasing out the record of tropical climate change from glacier retreat data. It has also been widely misquoted and misinterpreted.

The aspect of the paper that has attracted the most attention is the claim that the retreat of the Kilimanjaro summit glaciers can be explained by precipitation reduction, without any compelling need to invoke a warming trend in local air temperature. The arguments are special to the high, cold glaciers of Kilimanjaro, and are not meant to generalize to other tropical glaciers. As the authors point out, even if the whole story comes down to precipitation changes which favor ablation, the persistence of these conditions throughout the 20th century still might be an indirect effect of global warming, via the remote effect of sea surface temperature on atmospheric circulation.

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