Tropical Glacier Retreat

The reason there are tropical glaciers at all is that temperature decreases with altitude, as a consequence of the compressibility of air. As warm air rises, it expands and cools. Some of the cooling is offset by the heat released by condensation of water vapor, leading temperature to go down at a rate which is currently a little over 6.5 degrees C per kilometer of altitude. You may be slogging through the steamy lowlands at a temperature of 31C down in Tanga, but meanwhile, up atop Kilimanjaro at an altitude of 5892m, the temperature is a chilly -7 C.

The next ingredient needed to make a tropical glacier is precipitation, which at high, cold altitudes will fall as snow. The rate of snowfall needed to sustain a glacier depends on the rate of removal of glacier ice, called the ablation rate .

In contrast to the extratropics, the daily average Tropical temperature varies little throughout the course of the year. Most of the surface temperature variability is on the diurnal (day-night) time scale. On the other hand, the seasonal cycle of precipitation is strong. In the tropics, seasons are characterized by wet vs. dry, rather than cold vs. hot.

Another important feature of tropical climate is that horizontal temperature variations are weak once one goes above a relatively shallow layer near the Earth’s surface. This property arises from unassailable dynamic considerations having to do with the weak influence of the Earth’s rotation in the tropics, where the local vertical is nearly perpendicular to the Earth’s axis of rotation. Without strong Coriolis forces to balance the pressure differences that would be caused by temperature variations, the tropical mass redistributes itself until horizontal temperature gradients are nearly eliminated. [Pierrehumbert,1995; Sobel, et al.,2001] The horizontal homogeneity of temperature is something of an idealization, and becomes less valid towards the edge of the tropics, but temperature is much more uniform than other meteorological fields (precipitation, humidity, and cloudiness) which affect mountain glaciers. Because of the spatial homogeneity of tropical free-tropospheric temperature, when one sees tropical glaciers recede in concert, there is strong reason to presume that air temperature is playing a direct role, temperature being the one thing that is expected to change in lock-step throughout the tropics. For example, the uniform lowering of tropical snowline by about 900 meters during the Last Glacial Maximum is generally attributed to cooling [Porter 2001], and indeed provided the first indication that something was wrong with plankton-based estimates of tropical cooling during the glacial period.

Figure 3. The 1950-1995 climatological mean temperature along the equator at the 500mb level. This level is approximately at the altitude of the summit of Kilimanjaro. Results are based on the NCEP data set. Note that the annual mean equatorial temperature varies by only 1.5 degrees C over the entire globe.

There are two main ways that a body of snow or ice can lose mass: through melting (conversion of solid into liquid) or sublimation (conversion of solid into vapor). Both transformations require energy. It takes 8.5 times as much energy to convert a kilogram of ice into water vapor by sublimation as it does to convert the same kilogram into liquid water by melting. Therefore, if conditions allow the glacier surface to warm to 0 C, the amount of ablation that can be sustained by a given energy input increases dramatically. Sublimated water vapor is always carried away by the air, but the fate of meltwater has a strong effect on ablation by melting. Runoff from steep ice-cliffs, or through subglacial flow driven by water percolating through pores or fractures, will convert a high fraction of melting into ablation. If melt-water percolates into the glacier and re-freezes, the effect on ablation is more limited and indirect.

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