Earthshots: Satellite Images of Environmental Change

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» Mount Kilimanjaro, Tanzania

These images show the famous “snows of Kilimanjaro” in 1976, 2000, and 2010. 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. 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.

Map of the featured area.

Many of the world’s highest peaks are 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 (2), Mawenzi , and Shira.

In these Landsat images, concentric rings show the vegetation zones around this unusual peak. The bare summit is surrounded by low grass and shrubs. These transition to 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.

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.

As the 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.

Kilimanjaro’s glaciers have been shrinking since at least the 1880s. (Scientists measure the glaciers, not the fluctuating snow cover. Ground measurements are required in addition to aerial/satellite imagery, since the glaciers may be covered by snow or ash.) The ice has retreated both vertically and horizontally, but the loss of area has been more dramatic.

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. 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” (Bowen, 2005).

Area of Kilimanjaro’s glaciers, 1880–2007

188020.0 km2
191212.1 km2
19536.7 km2
19764.2 km2
19893.3 km2
20002.6 km2
20032.5 km2
20071.85  km2

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 lake levels started dropping. And since the early 1900s, at three weather stations on Kilimanjaro’s southern foothills, rainfall has decreased by 27 to 39 percent. 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. 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. The glaciers’ sharp edges and vertical walls (2) (3) 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.

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. 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. 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. At a station in Kilimanjaro’s northern foothills, the average daily high temperature has risen since 1976. And for the upper atmosphere near Kilimanjaro, calculations indicate warming at most heights since mid-century, but no trend at the peak’s approximate elevation. Some kind of mid-century change is suggested by the apparent timing of the glaciers’ vertical decline. 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.

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. “We are entirely against the black-and-white picture that says it is either global warming or not global warming,” said one scientist (Revkin, 2004).

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.

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, 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.

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.

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 the glaciers will soon disappear. Scientific estimates of that date vary from 2015 to mid-century, 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. 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.


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