(Note: This page will be updated from time to time. Eventually, I will add an online surface budget computation, allowing the reader to play with various parameters and see how they affect the ablation.)
The energy available for ablation of a glacier is determined by the energy budget of the glacier surface, illustrated in the accompanying figure. In the discussion below, the term specific humidity refers to the water vapor content of the air, as measured by the ratio of the mass of water in the air parcel to the total mass of the parcel. The term relative humidity refers to the ratio of the specific humidity to the specific humidity the air parcel would have if it were saturated (i.e. if it could not hold any more water without condensation). If relative humidity is held fixed while temperature increases, the specific humidity will increase.
Solar radiation (sunlight) carries energy to the glacier surface. A portion of the solar radiation is absorbed, and the rest is reflected. The portion reflected (called the albedo) is affected by precipitation and ablation, because fresh snow is more reflective than old snow or ablating ice. In fact, according to [Moelg and Hardy, 2004] precipitation aids net accumulation much more through the albedo affect, which reduces ablation, than through the mass added directly by snowfall. This means that precipitation effects are dependent on precipitation frequency as well as annual amount. Unfortunately, proxy data such as lake level, only give information about what the precipitation amount was in the past, and not about how it was distributed through the year.
In the Tropics, the seasonality of the solar radiation incident on the atmosphere is weak. At the Equator it has a peak-to-trough variation of only 13% of its mean value. The variation rises modestly to 29% at 15 degrees of latitude North or South of the Equator.
The glacier surface receives energy from solar radiation and from downward infrared. If it is colder than the air (as is often the case at night time) , it also receives energy by sensible heat transfer. If the glacier surface is very cold, it may also receive some energy by latent heat transfer, through frosting of atmospheric moisture onto the glacier. If the glacier surface is too cold, it won't get rid of all the heat that it is receiving, and therefore will warm. Warming of the glacier surface increases the upward infrared cooling and the upward latent heat flux; it increases the loss by sensible heat flux (or at least reduces the rate at which the glacier surface gains heat). A sufficient warming of the surface will increase the loss terms to the point that the energy budget balances. Because of the intense daytime solar radiation (particularly in the tropics), the glacier surface is typically warmer than the air during the daytime. As noted above, if the surface temperature reaches 0C, it can't increase further, and the energy budget is closed via melting. Because the glacier surface can be warmer than the air, melting can occur even if the air temperature remains below freezing throughout the day. This situation occurs on Kilimanjaro at present (see, e.g. [Moelg and Hardy,2004]) Note that sublimation and evaporation do not cease when melting sets in. They continue at a rate set by the wind and the moisture gradient, and rob the glacier of energy that would cause more ablation if used for melting.
Suppose now that the glacier is in equilibrium with a given air temperature. An increase in the air temperature will throw the system out of balance, through increasing the downward infrared flux and the downward sensible heat flux. If the relative humidity is held fixed as the air is warmed, the implied increase in specific humidity of the air will further increase the downward infrared flux, though it might somewhat decrease the latent flux. To achieve balance, then, the glacier surface must warm. The warming could, under certain circumstances, increase the sublimation, but the effect of this on ablation is generally small, because of the high energy required for sublimation. In fact, when ablation is dominated by sublimation, it takes a very large change in conditions (e.g. solar absorption) to appreciably affect ablation. The main way that a moderate change in the air temperature or any other term increases ablation is by increasing the proportion of the day during which melting can occur. When melting sets in, the amount of melt becomes very sensitive to changes in air temperature or absorbed solar radiation, since any increase in the energy input cannot be offset by a rise in the surface temperature; the whole increment goes directly into driving more melting.
A nice summary of the glacier energy balance and its implications, from the Innsbruck glacier group, can be found here. Other useful information, and an observational perspective, can be found in the following references.
Francou B, Vuille M, Wagnon P, Mendoza J and Sicart J-E 2003: Tropical climate change recorded by a glacier in the central Andes during the last decades of the twentieth century: Chacaltaya, Bolivia, 16S. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D5, 4154, doi:10.1029/2002JD002959.
Hastenrath S. 1984. The Glaciers of Equatorial East Africa. Reidel: Dordrecht.
Kaser G, Osmaston H. 2002. Tropical Glaciers. Cambridge University Press: Cambridge.
Wagnon PW, Ribstein P, Francou B, and Pouyaud B 1999: Annual cycle of energy balance of Zongo Glacier, Cordillera Real, Bolivia. Journal of Geophysical Research. 104D4, 3907-3923.
Mölg T, Hardy DR. 2004. Ablation and associated energy balance of a horizontal glacier surface on Kilimanjaro. Journal of Geophysical Research. 109, D16104, doi:10.1029/2003JD004338.