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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

Phillip W. Mote, Georg Kaser

Time and Temperature

Figure%205.%20Surface-area%20measurementsClick to Enlarge ImageObservations of Kilimanjaro's ice from about 1880 to 2003 allow us to quantify changes in area but not in mass or volume. The early European explorers Hans Meyer and Ludwig Purtscheller were the first to reach the summit in 1889. Based on their surveys and sketches, but mainly from moraines identified with aerial photographs, Henry Osmaston reconstructed (in 1989) an 1880 ice area of 20 square kilometers. In 1912, a precise 1:50,000 map based on terrestrial photogrammetry done by Edward Oehler and Fritz Klute placed the area at 12.1 square kilometers. By 2003 that area had declined to 2.5 square kilometers, a shrinkage of almost 90 percent. Much of that decline, though, had already taken place by 1953, when the area was 6.7 square kilometers (down 66 percent from 1880). Over the same period, ice movement has been almost nil on the plateau and slight on the slopes. There are indications that the slope glaciers at least are coming into equilibrium.

This pacing of change is at odds with the pace of temperature changes globally, which have been strongly upward since the 1970s after a period of stasis. Other glaciers share this pacing, with many coming into equilibrium or even advancing around the 1970s before beginning a sharp retreat.

Temperature trends are difficult to evaluate, owing to the paucity of relevant measurements, but taken together the data presented in the 2007 report from the IPCC (Intergovernmental Panel on Climate Change) suggest little trend in local temperature during the past few decades. In the East African highlands far below Kilimanjaro's peaks, temperature records suggest a warming of 0.5-0.8 degree during 1901-2005, a nontrivial amount of warming but probably larger than the warming at Kibo's peak. For the free troposphere, a deep layer including Kibo's peak, the warming rate during the period 1979-2004 for the zone 20 degrees latitude north and south of the equator was less than 0.1 degree per decade—smaller than the surface trend for that time and not statistically different from zero. Averages over a deep layer of the atmosphere, however, may be a poor estimate of the warming at Kilimanjaro's peak, although it has been argued that the warming must be nearly the same at all longitudes in the tropics, given that rotational effects are small, imposing strong dynamical constraints.

Figure%206.%20Record%20of%20weather-balloon%20readingsClick to Enlarge ImageFocusing on measurements of air temperatures at the 500-millibar air-pressure level (roughly 5,500 meters altitude) from balloons, one paper suggests a warming trend in the tropical middle troposphere from about 1960 to 1979, followed by cooling from 1979 to 1997, although this study has not been updated.

Two of the data sets used to derive the tropical averages above are "reanalysis" data sets, in which observations are fed into a global dynamical model, thereby providing dynamically consistent fields of temperature, winds and so on, even where there are no observations. At the reanalysis point closest to Kilimanjaro's peak, there seems to be no trend since the late 1950s. But like the balloon and satel-lite data, the reanalysis data can be unsuitable for documenting trends over time .

When pieced together, these disparate lines of evidence do not suggest that any warming at Kilimanjaro's summit has been large enough to explain the disappearance of most of its ice, either during the whole 20th century or during the best-measured period, the last 25 years.

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