These images show the famous "snows of Kilimanjaro" in 1976 and 2000. Mount Kilimanjaro, almost 20,000 feet above sea level, is the highest peak in Africa. Though only about 200 miles from the equator it has been capped by glaciers and snow for 11,000 years.1 This white cap shrinks and grows almost daily, but over the last century or more its overall trend has been a steady decline. These images show durable, hard-ice glaciers, as well as the ephemeral snow on and around them. Scientists have focused on the glaciers, trying to understand why they are shrinking, how long they may last, and what they can teach us about the atmosphere today and the Earth long ago.
In these false-color images vegetation appears red, drylands a greenish tan, and glaciers and snow bright white.
An unusual mountain
Many of the world's highest peaks are merely the barely-tallest points of high ranges, but Kilimanjaro stands alone, 15,000 feet above the surrounding plains. It actually has three peaks, from three old volcanoes: Kibo, Mawenzi and Shira.2
In these Landsat images, concentric rings of red and gray show the vegetation zones around this unusual peak. The bare summit is surrounded by low grass and shrubs. These shade into a band of forest, protected as a reserve, which appears bright red. Below this is a densely populated agricultural area, appearing as speckled pink in the Landsat images. And at its base the mountain turns dry again, supporting fewer plants and people.3
People have farmed the lower slopes for at least 2,000 years. In the last century the mountain's population has grown rapidly— about tenfold between 1910 and 2000. These people raised cattle, fruits and vegetables and coffee. Prevented from migrating upslope by the forest/wildlife preserve, people have moved out into drier areas.4
The glaciers of Kilimanjaro
But Kilimanjaro's snow and ice, not its people, have gotten most of the international attention. Most famous was Ernest Hemingway's 1936 depiction of these snows as a symbol of death: "all he could see, as wide as all the world, great, high, and unbelievably white in the sun . . . the square top of Kilimanjaro."5
Global warming altered the meaning of this icon. As this 2000 image was being taken, climatologist Lonnie Thompson was camped on the glacier ice, drilling out samples for analysis and preservation. When a year later he publicly stated that the glaciers might disappear between 2015 and 2020, he raised these glaciers to a new level of public and scientific debate. For better or worse, Kilimanjaro became a poster child for global warming.6
Kilimanjaro's glaciers have been shrinking since at least the 1880s. (Scientists measure the glaciers, not the fluctuating snowcover. Ground measurements are required in addition to aerial/satellite imagery, since the glaciers may be covered by snow or ash.)7 The ice has retreated both vertically and horizontally, but the loss of area has been more dramatic:8
Year Area 1880 20 km2 1912 12.1 km2 1953 6.7 km2 1976 4.2 km2 1989 3.3 km2 2000 2.6 km2 2003 2.5 km2
Vertically, the glaciers don't appear to have lowered dramatically between 1880 and about 1960, but stereo aerial photographs indicate that from 1962 to 2000 they lowered almost 0.5 m per year.9 One member of Thompson's 2000 party described the melting he observed: "By day, water streamed off the top of the glacier and down its steep sidewalls. . . In some places, the rivulets wandering along the surface had converged to form streams, which catapulted in waterfalls off the top."10
Snowfall, sunshine, or warm air?
Why have these glaciers been shrinking? One factor is snowfall. We don't have weather measurements from the peak until very recently, but scientists have reconstructed a general picture. In the 1880s the East African climate became much drier. Glaciers started shrinking, and lakes started dropping.11 And since the early 1900s, at three weather stations on Kilimanjaro's southern foothills, rainfall has decreased by 27 to 39 percent.12 Glaciers need fresh snow for at least two reasons: to feed them with fresh ice, and to provide a bright white shield between the ice and the sun.
Sunshine can melt ice even when the ambient air temperature is below freezing, by warming the surface of the ice. And the same decrease in clouds that meant less snow for the glaciers also meant more sunshine. Even in 2000—2002, as scientists witnessed "strong melting" on Kilimanjaro, weather stations verified that the temperature never exceeded –1.6 degrees C.13 The glaciers' sharp edges and vertical walls also suggest that the melting is from sunshine, not above-freezing air temperatures. Warm air is a "rounder," melting evenly everywhere; sunshine is a "sharpener" that strikes selectively. Clear and cloudy seasons, solstices and equinoxes, and afternoon cloudiness all affect where the sun hits the ice.14
Sunshine can also remove ice at temperatures below freezing through sublimation. Sublimation is the mysterious process by which ice "evaporates" directly into water vapor without first passing through a liquid phase. It takes 8.5 times as much energy to sublimate ice than to melt it, but recent experiments suggest that a substantial portion of the ice loss on Kilimanjaro— perhaps two thirds— is through sublimation. Because sublimation is sensitive to humidity, the shift to drier air favored it at the expense of melting, a more energy-efficient mode of ice loss.15 Thus, dry weather strips a glacier of its protective shade and snow only to draw out its resulting death like an inefficient James Bond villain.
A number of scientists have concluded that snow and sun were the initial, and the largest overall, factors in the retreat of these glaciers.16 The difficult question is what effect the local air temperature also had. And as with snowfall and sunshine, temperature records were not kept at Kilimanjaro's peak. Over East Africa generally, there is no evidence of a temperature shift in the late 1800s, but over the 20th century surface warming did occur.17 At a station in Kilimanjaro's northern foothills, the average daily high temperature has risen since 1976.18 And for the upper atmosphere near Kilimanjaro, calculations indicate warming at most heights since midcentury, but no trend at the peak's approximate elevation.19 Some kind of midcentury change is suggested by the apparent timing of the glaciers' vertical decline, and by Thompson's report of radioactive snow from 1952 bomb tests still lying unmelted in 2000.20 On Mount Kenya, which is 200 miles north and 2,200 feet lower, studies have concluded that through the 1900s, warming became a greater cause for shrinking of the Lewis Glacier, and eventually its retreat pattern was unrelated to sun exposure.21
Those who have studied Kilimanjaro's glaciers agree that more research is necessary. They also agree that the Earth is warming, whether or not Kilimanjaro is good evidence for it.22 "We are entirely against the black-and-white picture that says it is either global warming or not global warming," said one scientist. "As a scientist I'm happy it's more complex, because otherwise it's boring."23
Beyond the ice
Meanwhile, to the people who live around Kilimanjaro, climate change involves more than a lofty icon. For example, scientists have examined whether climate is related to that region's recent rise in malaria.24
Others have looked at climate's effect on the forest. In many areas warming has led to vegetation belts migrating upward. But on Kilimanjaro the opposite has also happened. One study used ground observations and these Landsat images to map land cover in 1976 and 2000. The lower forest (shown in dark green) was nibbled away by illegal logging and other pressures. But the uppermost forests, dominated by trees of the genus Erica (in light green) also decreased by five sixths. In a warmer and drier climate, flammable Erica trees become more sensitive to fire; warming and drying both led to more fires, replacing forest with bushes and grass. Between logging and fire, Kilimanjaro has lost a third of its forest since 1929.25
The loss of high "cloud forest" can affect the water supply, because the trees catch significant amounts of water out of the air in the form of fog. For Kilimanjaro, the loss of this fog deposition may represent 20 times as much water as the runoff from the small remaining glaciers.26
Finally, tourism is now a major part of the local economy. Ten thousand visitors climb the mountain every year, employing local people as porters and guides. The snow and ice are part of what attracts the tourists, some of whom come as a "last chance" opportunity, believing that some day soon the glaciers will disappear. Scientific estimates of that date vary from 2015 to midcentury, and some believe the glaciers on the side slopes may survive. If the glaciers do disappear, seasonal snow will still whiten the peak, but perhaps not in the dry seasons when most tourists come.27 In the end, on this mountain both science and economics are always drawn back, as if by some powerful cultural gravity, to the famous white landscape on its peak.
Footnotes
1. Thompson, Lonnie G. and others, 2002, Kilimanjaro ice records: evidence of Holocene climate change in tropical Africa: Science, vol. 298, no. 5593, 18 October 2002, pp. 589-593.
2. Hastenrath, Stefan and Lawrence Greischar, 1997, Glacier recession on Kilimanjaro, East Africa, 1912-89: Journal of Glaciology, vol. 43, no. 145, 1997, pp. 455-459.
3. Maro, Paul S., 1988, Agricultural land management under population pressure: the Kilimanjaro experience, Tanzania: Mountain Research and Development, vol. 8, no. 4, 1988, pp. 273-282. Hemp, Andreas, 2006, Vegetation of Kilimanjaro: hidden endemics and missing bamboo: African Journal of Ecology, vol. 44, pp. 305-328.
4. Maro, 1988. Hemp, 2006. Population growth figure from Hemp, personal correspondence.
5. Hemingway, Ernest, 1936, The snows of Kilimanjaro: Esquire, August 1936.
6. Thompson, 2002. Revkin, Andrew C., 2004, Climate debate gets its icon: Mt. Kilimanjaro: New York Times, March 23, 2004.
7. Cullen, Nicolas J., 2006, Kilimanjaro glaciers: recent areal extent from satellite data and new interpretation of observed 20th century retreat rates: Geophysical Research Letters, vol. 33, 2006, L16502.
8. 1880 and 2003 data from Mote, Philip W. and Georg Kaser, 2007, The shrinking glaciers of Kilimanjaro: can global warming be blamed? American Scientist, vol. 95, no. 4, July-August 2007. 2000 datum from Thompson, 2002. 1912 through 1989 data from Hastenrath, 1997.
9. Thompson, 2002. Kaser, Georg, 2004, Modern glacier retreat on Kilimanjaro as evidence of climate change: observations and facts: International Journal of Climatology, vol. 24, 2004, pp. 329-339.
10. Bowen, Mark, 2005, Thin ice: unlocking the secrets of climate in the world's highest mountains: N.Y., Henry Holt and Co., 2005, p. 394.
11. Hastenrath, Stefan, 2001, Variations of East African climate during the past two centuries: Climate Change, vol. 50, 2001, pp. 209-217. Kruss, Phillip, 1983, Zeitschrift fur Gletscherkunde und Glazialgeologie, vol. 19, 1983, pp. 43-60.
12. Hemp, Andreas, 2005, Climate change-driven forest fires marginalize the impact of ice cap wasting on Kilimanjaro: Global Change Biology, vol. 11, 2005, pp. 1013-1023.
13. Kaser, 2004.
14. Kaser, 2004. Hastenrath, 1997. Mölg, Thomas and Douglas R. Hardy, 2004, Ablation and associated energy balance of a horizontal glacier
surface on Kilimanjaro: Journal of Geophysical Research, vol. 109, p. D16104.
15. Mölg, Thomas and others, 2007, Mass balance of a slope glacier on Kilimanjaro and its sensitivity to climate: International Journal of Climatology, 2007.
Hardy 2002
16. Mölg, Thomas and others, 2003, Solar-radiation-maintained glacier recession on Kilimanjaro drawn from combined ice-radiation geometry modeling: Journal of Geophysical Research, vol. 108, no. D23, 4731. Kaser, 2004. Mote 2007.
17. Mölg 2003. Mote, 2007. Cullen, 2006.
18. Hemp, 2005.
19. Cullen, 2006.
20. Thompson, 2002.
21. Kruss, 1983, Hastenrath, Stefan, 2006, Diagnosing the decaying glaciers of equatorial East Africa: Meteorologische Zeitschrift, vol. 15, no. 3, June 2006, pp. 265-271.
22. Mote, 2007.
23. Revkin, 2004.
24. Hay, Simon and others, 2002, Climate change and the resurgence of malaria in the East African highlands: Nature, vol. 415, Feb. 2002, pp. 905-909.
25. Hemp, 2005. Hemp, 2006.
26. Hemp, 2005.
27. Mölg, Thomas and others, 2008, Tropical glaciers, climate change, and society: focus on Kilimanjaro (East Africa), in Orlove, Ben, 2008, Darkening peaks: glacier retreat, science, and society. University of California Press.
LM2180062007602490 (Landsat 2 MSS, 24 January 1976)
LE7168062000005250 (Landsat 7 ETM+, 21 February 2000). Zero-fill in the southeast is from LE71670622000029EDC00 (Landsat 7 ETM+, 29 January 2000).
Thanks to Andreas Hemp for use of the land cover maps. Modified from: Hemp, Andreas, 2005, Climate change-driven forest fires marginalize the impact of ice cap wasting on Kilimanjaro: Global Change Biology, vol. 11, 2005, pp. 1013-1023. Legend from Hemp, personal correspondence.
Glacier area polygons redrawn from: Mölg, Thomas and others, 2003, Solar-radiation-maintained glacier recession on Kilimanjaro drawn from combined ice-radiation geometry modeling: Journal of Geophysical Research, vol. 108, no. D23, 4731.
Defense Mapping Agency, Operational Navigation Chart M-5, 1969, scale 1:1,000,000.
Thanks to these individuals and organizations for the use of their photographs.
Forest photo 1: Christian Lambrechts, UNEP.
Forest photo 2: Christian Lambrechts, UNEP.
Kibo photo 1: Christian Lambrechts, UNEP, 2002.
Kibo photo 2: Christian Lambrechts, UNF/UNEP/KWS/University of Bayreuth/WCST, 2001.
Kibo photo 3: Christian Lambrechts, UNF/UNEP/KWS/University of Bayreuth/WCST, 2001.
Kibo photo 4: Christian Lambrechts, UNF/UNEP/KWS/University of Bayreuth/WCST, 2001.
Kibo photo 5: Christian Lambrechts, UNF/UNEP/KWS/University of Bayreuth/WCST, 2001.
Kibo photo 6: NASA Goddard Space Flight Center, Scientific Visualization Studio. Simulated from a 17 February 1993 Landsat 5 TM image.
Mawenzi: Christian Lambrechts, UNEP, 2007.
Amboseli: Christian Lambrechts, UNEP, 2002.
Robert Wellman Campbell, ed. 2008. "Mount Kilimanjaro, Tanzania: 1976, 2000." Earthshots: Satellite Images of Environmental Change. U.S. Geological Survey. http://earthshots.usgs.gov. This article was released 14 August 2008.
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