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
Glaciers and Global Climate
The observations described above point to a combination of factors other than warming air—chiefly a drying of the surrounding air that reduced accumulation and increased ablation—as responsible for the decline of the ice on Kilimanjaro since the first observations in the 1880s. The mass balance is dominated by sublimation, which requires much more energy per unit mass than melting; this energy is supplied by solar radiation.
These processes are fairly insensitive to temperature and hence to global warming. If air temperatures were eventually to rise above freezing, sensible-heat flux and atmospheric long-wave emission would take the lead from sublimation and solar radiation. Since the summit glaciers do not experience shading, all sharp-edged features would soon disappear. But the sharp-edged features have persisted for more than a century. By the time the 19th-century explorers reached Kilimanjaro's summit, vertical walls had already developed, setting in motion the loss processes that have continued to this day.
An additional clue about the pacing of ice loss comes from the water levels in nearby Lake Victoria. Long-term records and proxy evidence of lake levels indicate a substantial decline in regional precipitation at the end of the 19th century after some considerably wetter decades. Overall, the historical records available suggest that the large ice cap described by Victorian-era explorers was more likely the product of an unusually wet period than of cooler global temperatures.
If human-induced global warming has played any role in the shrinkage of Kilimanjaro's ice, it could only have joined the game quite late, after the result was already clearly decided, acting at most as an accessory, influencing the outcome indirectly. The detection and attribution studies indicating that human influence on global climate emerged some time after 1950 reach the same conclusion about East African temperature far below the peak.
The fact that the loss of ice on Mount Kilimanjaro cannot be used as proof of global warming does not mean that the Earth is not warming. There is ample and conclusive evidence that Earth's average temperature has increased in the past 100 years, and the decline of mid- and high-latitude glaciers is a major piece of evidence. But the special conditions on Kilimanjaro make it unlike the higher-latitude mountains, whose glaciers are shrinking because of rising atmospheric temperatures. Mass- and energy-balance considerations and the shapes of features all point in the same direction, suggesting an insignificant role for atmospheric temperature in the fluctuations of Kilimanjaro's ice.
It is possible, though, that there is an indirect connection between the accumulation of greenhouse gases and Kilimanjaro's disappearing ice: There is strong evidence of an association over the past 200 years or so between Indian Ocean surface temperatures and the atmospheric circulation and precipitation patterns that either feed or starve the ice on Kilimanjaro. These patterns have been starving the ice since the late 19th century—or perhaps it would be more accurate to say simply reversing the binge of ice growth in the third quarter of the 19th century. Any contribution of rising greenhouse gases to this circulation pattern necessarily emerged only in the last few decades; hence it is responsible for at most a fraction of the recent decline in ice and a much smaller fraction of the total decline.
Is Kilimanjaro's ice cap doomed? It may be. The high vertical edges of the remaining ice make a horizontal expansion of the ice cap more difficult. Although new snowfall on the ice can accumulate over the course of months or years, new snowfall on the rocky plateau usually sublimates or melts in a matter of days (with the notable exception of the period of several months of continuous snow cover in late 2006 and into 2007), partly because thin snow above dark rock cannot long survive as the loss processes reduce the reflective snow and expose the sunlight-absorbing rock. If the cap ice were much thicker and shaped in a way that allowed ice to creep outward, gentle slopes could develop along the edges; new snow would be buffered against loss and would accumulate. But steep edges do not allow such expansion.
Imagine, though, a scenario in which the atmosphere around Kilimanjaro were to warm occasionally above 0 degrees. Sensible and infrared heating of the ice surface would gradually erode the sharp corners of the ice cap; gentler slopes would quickly develop. If, in addition, precipitation increased, snow could accumulate on the slopes and permit the ice cap to grow. Ironically, substantial global warming accompanied by an increase in precipitation might be one way to save Kilimanjaro's ice. Or substantially increased snowfall, like the 2006-07 snows, could blanket the dark ash surface so thickly that the snow would not sublimate entirely before the next wet season. Once initiated, such a change could allow the ice sheet to grow. If the Kibo ice cap is vanishing or growing, reshaping itself into something different as you read this, glaciology tells us that it's unlikely to be the first or the last time.
- Cullen, N. J., T. Mölg, G. Kaser, K. Hussein, K. Steffen and D. R. Hardy. 2006. Kilimanjaro Glaciers: Recent areal extent from satellite data and new interpretation of observed 20th century retreat rates. Geophysical Research Letters 33:L16502. doi:10.1029/2006GL027084
- Gaffen, D. J., B. D. Santer, J. S. Boyle, J. R. Christy, N. E. Graham and R. J. Ross. 2000. Multidecadal changes in the vertical temperature structure of the tropical troposphere. Science 287:1242-1245.
- Kaser, G. 1999: A review of modern fluctuations of tropical glaciers. Global and Planetary Change 22 (1-4):93-103.
- Kaser, G., D. R. Hardy, T. Mölg, R. S. Bradley and T. M. Hyera. 2004. Modern glacier retreat on Kilimanjaro as evidence of climate change: observations and facts. International Journal of Climatology 24:329-339. doi: 10.1002/joc.1008
- Mölg, T., D. R. Hardy and G. Kaser. 2003. Solar-radiation-maintained glacier recession on Kilimanjaro drawn from combined ice-radiation geometry modeling. Journal of Geophysical Research 108(D23):4731. doi:10.1029/2003JD003546
- Mölg, T., and D. R. Hardy, 2004. Ablation and associated energy balance of a horizontal glacier surface on Kilimanjaro. Journal of Geophysical Research 109:D16104.
- Oerlemans, J. 2005. Extracting a climate signal from 169 glacier records. Science 308:675-677.
- Osmaston, H. 1989. Glaciers, glaciation and equilibrium line altitudes on Kilimanjaro. In Quaternary and Environmental Research on East African Mountains, ed. W. C. Mahaney. Rotterdam: Brookfield, pp. 7-30.
- Thompson, L. G., E. Mosley-Thompson and K. A. Henderson. 2000. Ice-core paleoclimate records in tropical South America since the Last Glacial Maximum. Journal of Quaternary Science 15:377-394.
- Thompson, L. G., et al.2002. Kilimanjaro ice core records: Evidence of Holocene climate change in tropical Africa. Science 298:589-593.
- Trenberth, K. E., et al. 2007. Observations: Surface and atmospheric climate change. Chapter 3 in Climate Change 2007: The Physical Science Basis. Contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, U.K., and New York: Cambridge University Press.