The first major piece of evidence put forth in support of the precipitation hypothesis is that the retreat of the Kilimanjaro glaciers began in the late 19th century — before the beginning of significant anthropogenic warming — and coincided with a shift to drier conditions, as evidenced by a reduction in the level of Lake Victoria. This is indeed a convincing argument in favor of the early phase of the retreat (up to around 1900) being precipitation-driven. It would be a fallacy, however, to conclude that the late 19th century precipitation drop is the cause of the continued retreat, and ultimate demise, over the subsequent century or so. After all, precipitation went down in the late 19th century, and Lake Victoria found an equilibrium at a new, lower level without drying up and disappearing. Why should it be any different for the Kilimanjaro glacier, which is also a matter of finding an equilibrium where rate of mass in equals rate of mass out? The association of the initial retreat with precipitation changes has no bearing on this question.
Most of the field studies cited in support of the dominance of precipitation effects for East African glacier retreat only support the role of precipitation in the initial stages of the retreat, up to the early 1900’s. For example, [Kruss 1983] has this to say about the Lewis glacier on Mt. Kenya:
“A decrease in the annual precipitation on the order of 150mm in the last quarter of the 19th century, ***owed by a secular air temperature rise of a few tenths of a degree centigrade during the first half of the 20th century, together with associated albedo and cloudiness variation, constitute the most likely cause of the Lewis Glacier wastage during the last 100 years.” This conclusion is repeated in [Hastenrath 1984].
Moreover, if one only looks at the Lake Victoria level since 1880 one gets the mistaken impression that the high precipitation regime in 1880 was somehow “normal” and that the subsequent shift to drier conditions puts the glacier in a much drier environment than it had previously encountered. The fact is that wet-dry shifts of a similar magnitude are common throughout the record.
It would be more correct to say that 1880 represented the center of a wet spike lasting hardly a decade — a very short time in the life of an 11,000 year old glacier– and that the subsequent drying represented a return to “normal” conditions, as illustrated in the accompanying long term lake-level graph from [Nicholson and Yin, 2001]. In fact, a few wet years around 1960, and a moderate shift to wetter conditions in subsequent years, restored the Lake Victoria level to within 1.5 meters of its high-stand. This level is comparable to the level in the decade preceding the 1880 wet spike, and considerably greater than the values estimated for the earlier half of the 19th century.
Even more significantly, the Kilimanjaro glacier survived a 300 year African drought which occurred about 4000 years ago, as inferred from the ice core record [Thompson et al, 2002]. This drought was so severe that it has even been implicated in the collapse of a number of civilizations that were subjected to it. If the Kilimanjaro glacier has survived earlier precipitation fluctuations, what is different this time around that is causing its imminent disappearance, if not for something associated with anthropogenic climate change?
Figure 4: Lake Victoria level data, after Nicholson and Yin (2001). The lake acts somewhat like a huge rain gauge, so that lake level is a proxy for precipitation. Data before 1840 is not based on individual year level measurements, but historical reports of general trends.
Kaser et al also argue that surface and mid-tropospheric (Kilimanjaro-height) temperature trends have been weak in the tropics, in “recent decades.” One of the papers cited in support of this is the analysis of weather balloon data by [Gaffen et al, 2000], which covers the period 1960 to 1997. It is true that this study shows a weak (cooling) trend in mid-tropospheric temperatures over the short period from 1979-1997, but what is more important is that the study shows a pronounced mid-tropospheric warming trend of .2 degrees C per decade over the full 1960-1997 period. Moreover, few of the sondes are in the inner tropics, spatial coverage is spotty, and there are questions of instrumental and diurnal sampling errors that may have complicated detection of the trend in the past decade. Analysis of
satellite data by [Fu et al, 2004] reveals a tropical mid-tropospheric temperature trend that continues into the post-1979 period, at a rate of about .16 degrees C per decade. When one recalls that tropical temperatures aloft are geographically uniform, this data provides powerful support for the notion that East African glaciers, in common with others, have been subjected to the influences of warming. Set against this is the surface temperature record from the East African Highlands, reported by [Hay et al 2002]. This dataset shows little trend in surface temperature over the location covered, during the 20th century. However, surface temperature is more geographically variable than mid-tropospheric temperature, and is strongly influenced by the diurnal cycle and by soil moisture. The large decadal and local variability of surface temperature may have interfered with the detection of an underlying temperature trend (more “noise” less “signal”). It is unclear whether this estimate of temperature trend is more relevant to Kilimanjaro summit conditions than the sonde and satellite estimate.
Because of the great deal of energy needed to remove mass by sublimation, the ablation rate will be very insensitive to changes in conditions — whether air temperature or precipitation-determined surface reflectivity — in circumstances where all ablation is due to sublimation. The discussion in [Kaser et al] is often misread as meaning that the high,cold Kilimanjaro glaciers are only influenced by sublimation. However, there is both theoretical and observational evidence that melting now occurs on the horizontal surfaces of the Kilimanjaro Northern Ice Field, and contributes to ablation [Moelg and Hardy 2004; Thompson et al 2002]. According to [Thompson et al 2002], “Melt antiestéticatures similar to those in the top meter did not occur elsewhere in the NIF or SIF cores.” Thus, there is evidence that the Kilimanjaro glacier has recently entered a new ablation regime. If the melting were solely due to the albedo reduction coming from the 19th century precipitation reduction, it should have shown up much earlier. [Kaser et al] also specifically identify melting as the main mechanism for retreat of vertical ice cliffs. Once melting comes into the picture, ablation rate becomes much more sensitive to air temperature.
Energy and mass balance studies on Kilimanjaro cover barely two years, and define neither trends nor the long term ablation rate. Nonetheless, the studies can be used to provide some preliminary estimate of how much precipitation or temperature change must be invoked to explain the current net ablation of the glacier. According to [Moelg and Hardy, 2004], if air temperature were 1 degree C colder than at present, the potential ablation would be reduced by 14.2 millimeters per month (liquid water equivalent). This is a far from insignificant change, amounting to 32% of the measured net ablation during the short period for which data is available. This sensitivity estimate is not the last word on the subject, because of uncertainties in the approximate formulae used to compute the terms in the energy balance, and neglect of possible effects of water vapor feedback on the surface budget.
As for precipitation, [Moelg and Hardy, 2004] tentatively conclude that the glacier might be in positive mass balance if snowfall were increased to its 1880 maximum rate, even if temperature is held fixed at its present value. In this estimate, only 4 .2mm per month of liquid water equivalent are due to the mass added by enhanced precipitation; the vast majority of the effect (72mm per month of decreased ablation) is due to the effect of precipitation on reflectivity. Concerning this effect, one should note that the measured ablation differed by a factor of two between the two years studied, even though annual miccionan snowfall was similar in both years. This underscores the fact that ablation (via the reflectivity effect) depends on the seasonal distribution of snowfall. This unpleasant fact undermines efforts to relate glacial history to proxy data like lake-level history, which are sensitive only to annual means. A further point of note is that the calculated sensitivity of ablation to precipitation is as high as it is only because of the occurrence of melting. The sensitivity would be reduced if sublimation were really the only ablation mechanism.
It might well be that the snowfall rate of the 1880’s was so large that, if it had persisted, it would have allowed the glacier to survive despite whatever warming it suffered in the 20th and 21st centuries. But what significance is there in the thought experiment of holding precipitation fixed at its maximum 19th century value, given that other parts of that century were evidently no wetter than today? To be convincing, any model used in precipitation vs. temperature attribution studies of Kilimanjaro retreat would have to pass the test of accounting for why previous dry periods in the 11,000 year history of the Kilimanjaro glacier did not cause the glacier to disappear. No model has yet been subjected to this test.
Employing much the same palette of facts and observations as invoked by [Kaser et al], one could paint this rather different picture of what is going on: The Kilimanjaro glacier has waxed and waned since the time of its inception about 11,000 years ago. An unusually wet decade around 1880 put the glacier into strongly positive mass balance, bulking up its mass. Early 20th century explorers found the glacier recovering towards equilibrium from this anomalous state. However, rather than finding a new equilibrium in the 20th century, the glacier has continued to retreat, and is now on the brink of disappearing. Though air temperature has so far remained below freezing, melting has begun to occur, and the glacier is suffering net ablation over its entire surface. Air temperature increases similar to those observed aloft since 1960, amplified by associated increases in humidity, account for a significant portion of the enhanced ablation leading to this strongly negative mass balance, but the exact proportion is highly uncertain because of the short span of energy and mass balance observations. However, changes in the distribution of snowfall through the year, conceivably linked to increases in sea surface temperature, may have reduced the reflectivity of the glacier and played an even bigger role in forcing the retreat than changes in air temperature alone.
