Changes in African Glaciers since the 19th Century
Summary and Keywords
In equatorial East Africa, glaciers still exist on Mount Kenya, Kilimanjaro, and Ruwenzori. The decreasing ice extent has been documented by field reports since the end of the 19th century and a series of mappings. For Mount Kenya, the mappings are of 1947, 1963, 1987, 1993, and 2004, with more detailed mappings of Lewis Glacier in 1934, 1958, 1963, 1974, 1978, 1982, 1985, 1986, 1990, and 1993. For Kilimanjaro, the sequence is 1912, 1953, 1976, 1989, and 2000. For Ruwenzori (for which information is more scarce), the information is from 1906, 1955, and 1990. Photographs are valuable complementary evidence. At Lewis Glacier on Mount Kenya, measurements of mass budget and ice flow have been conducted over decades. The climatic forcing of ice recession in East Africa at the onset in the 1880s was radiationally controlled, affecting the most exposed locations. Later warming caused further ice shrinkage, except on the summit plateau of Kilimanjaro, above the freezing level. Whereas the ice recession in the Ecuadorian Andes and New Guinea began in the middle of the 19th century, plausibly caused by warming, the late onset in East Africa should be appreciated in the context of large-scale circulation changes evidenced by the historical ship observations in the equatorial Indian Ocean.
The high mountains of East Africa had long been of interest to explorers. The legendary “Mountains of the Moon” had been considered to be the source of the Nile since at least the time of the ancient Greek geographers, such as Ptolemy. The quest to discover these mountains attracted many European adventurers as well. Perhaps the first claims of snow and glaciers on the East African mountains were made by Johannes Rebmann in 1848 (Mount Kilimanjaro) and Johann Ludwig Krapf in 1849 (Mount Kenya). However, their reports were met with skepticism. Hans Meyer, who first visited Mount Kilimanjaro in 1887, made the first, noted measurements. Numerous expeditions by other scientists followed throughout the course of the 20th century. Reports of these are compiled in Hastenrath (1984).
Glaciers near the Equator still exist in the Ecuadorian Andes, New Guinea, and East Africa. Fossil glaciation is also evident from other parts of the continent, but glaciers now continue on Mounts Kenya, Kilimanjaro, and Ruwenzori (Fig. 1). Tropical glaciers are widely recognized as climate-sensitive components of the environment (IPCC, 2001, pp. 127–130, 647–656; IPCC, 2007, pp. 337–383; World Glacier Monitoring Service of IASH-ICSI-UNEP-UNESCO, 1996, 2005a, 2005b; Moelg, Georges, & Kaser, 2003a; Moelg, Hardy, & Kaser, 2003b; Moelg & Hardy, 2004; Kaser, Hardy, Moelg, Bradley, & Hyers, 2004; Hastenrath, 2006; Taylor et al., 2006). A drastic and continuing ice recession is apparent in the Ecuadorian Andes and New Guinea (Hastenrath, 1981, 2008; Klein & Kincaid, 2006) and particularly well documented for the cryosphere in East Africa (Hastenrath, 1984, 2005a, 2005b, 2006a, 2008; Hastenrath & Greischar, 1997; Kaser & Osmaston, 2002; Taylor et al., 2006). Mount Kenya, right on the Equator, and particularly Lewis Glacier, have the richest historical evidence of varying ice conditions in all the tropics (reviews in Hastenrath, 1984, pp. 108–142, 2005a; Rostom & Hastenrath, 2007). The relation of ice shrinkage to climate may appear obvious overall, but exploring the chain of causality remains a challenge. In that spirit, in the following article, we shall consider changes in ice extent, long-term field measurements on Lewis Glacier on Mount Kenya, climatic forcing, and circulation changes.
Changes in Ice Extent
For Mount Kenya (summit elevation 5,199 m), the glacier recession from the end of the 19th to the beginning of the 21st century has been documented in detail at a scale of 1:5,000 (Hastenrath, 2005a; Rostom & Hastenrath, 2007). A simplified version is presented in Figure 2. Observations from expeditions at the end of the 19th century into the first half of the 20th century have been evaluated (Gregory, 1894; Mackinder, 1900; Dutton, 1929; Hastenrath, 1984), and then there are photogrammetric mappings of the peak area at scale 1:5,000 taken in 1947, 1963, 1987, 1993, and 2004 (Rostom & Hastenrath, 1995; Forschungsunternehmen Nepal-Himalaya, 1967; Hastenrath, Rostom, & Caukwell, 1989; Rostom & Hastenrath, 1995, 2007; reviews in Hastenrath, 1984, 2005a, 2005b; Rostom & Hastenrath, 2007). For the end of the 19th century, expedition reports (Gregory, 1894; Mackinder, 1900; Hastenrath, 1984) indicate glacier termini close to the innermost large moraines. Of the former 18 ice entities, 9 have disappeared, and all have suffered drastic shrinkage. In particular, Lewis Glacier in 2010 lost connection with the Gregory, which has since disappeared (Prinz, Nicholson, & Kaser, 2012).
For the Kibo cone of Kilimanjaro (summit elevation 5,899 m), Figure 3 (Hastenrath & Greischar, 1997; Hastenrath, 2006a) shows the ice extent for 1912, 1953, 1976, 1989, and 2000. The appearance of a rock outcrop in the middle of the Northern Icefield after 1976 indicates ice thinning accompanying the decrease in area.
For the three glaciated mountains of East Africa, the time-series plots in Figure 4 display the drastic and progressive shrinkage of ice-covered area.
At fortunate occasions in the long history of research in this area, photographs have been taken and preserved, and they have become precious visual evidence of the fate of glaciers (Hastenrath, 2005b, 2008). A small selection is presented in Figures 5, 6, 7, and 8.
The photo in Figure 5 of Mount Kenya, taken from the west in 1986, should be appreciated in the context of the map in Figure 2, especially the 1987 status, showing the Lewis Glacier (to the right), the Diamond and Darwin (in the middle), and the Forel, Heim, and Tyndall Glaciers (to the left).
Likewise, the air photographs of Mount Kenya in Figure 6, which illustrate the drastic and progressive ice shrinkage from 1947 to 1987 to 1993, and then 2004, should be viewed with reference to the map in Figure 2.
The sequence of pictures in Figure 7 features the decay history and terminus retreat of the Lewis Glacier. The drawing of Gregory (1894) for the year 1893 shows the ice still in contact with the innermost large morraines. In 1926 and 1934, the ice still falls over the steep rock cliff. By 1971, however, the ice has retreated over the cliff, the Lewis Tarn has formed, and the ice has retreated farther, to just above the Tarn. The bare rock band between Tarn and the ice terminus increases further to 1973 and 1978, and drastic further retreat is shown for 1994 and 2004.
Lewis Glacier Field Measurements
In addition to the maps of the entire peak region referenced in the preceding section, detailed mappings have been conducted of Lewis Glacier. From field expedition in 1934, Troll and Wien (1949) published a map at scale 1:13,333. As part of the glaciological measurements, the 1957–1958 IGY Expedition (Charnley, 1959; Caukwell & Hastenrath, 2006) produced a map at scale 1:2,500. Further maps at scale 1:2,500 are available from 1974, 1978, 1982, 1985, 1986, 1990, and 1993 (Caukwell & Hastenrath, 1977, 1982; Hastenrath & Caukwell, 1979, 1987; Hastenrath & Rostom, 1990; Hastenrath, Rostom, & Hime, 1995).
The Troll and Wien (1949) expedition in 1934 and the IGY Expedition (Charnley, 1959) gave precious historic background. Then, following an exploratory field program in 1973–1974, we conducted a glaciological field project on Lewis Glacier from 1978 to 1996. This included the monitoring of precipitation, net balance, ice flow velocity, heat budget and hydrology, ice coring and pit studies, determination of bedrock topography and ice thickness, and repeated mappings of surface topography (Hastenrath, 1984, 2005b). Field observations have been complemented by numerical modeling of ice flows since the early part of the 20th century (Hastenrath & Kruss, 1982).
The time-sequence plots of indicative elements in Figure 9 detail the evolution and decay of Lewis Glacier over a century. Panel (A) shows that from the end of the 19th century to 1978, volume shrinkage was less than 20%, but there was near-disappearance by the beginning of the 21st century. Panels (B) and (C) exhibit the concomitant decreases in area and length of the glacier. Panel (D) illustrates the upward shift of the glacier terminus, which was particularly drastic from 1934 to 1947, when the tongue retreated to above a steep cliff, and after which the Lewis Tarn began to form. Panel E shows the drastic slowdown of maximum ice surface flow velocity. All the points represent actual measurements except for the 1899 value, which was obtained by numerical modeling of the ice flow dynamics (Hastenrath & Kruss, 1982). From the value of 15 m a−1, the flow decreased to a mere 2 m a−1 by 1990, when we discontinued the measurements of velocity.
Complementing Figure 9, Figure 10 displays in compact form our net balance measurements. The net balance for the glacier as a whole was negative throughout the 18 years, except for 1 year. Furthermore, during half of the years, the net balance was negative over the entire glacier, so there was no accumulation area, which diagnosed a dying glacier.
In the other glaciated mountain regions at the Equator, ice recession began in the middle of the 19th century. For the Ecuadorian Andes, this is shown by historical evidence (Hastenrath, 1981) and for New Guinea by numerical modeling of glacier dynamics (Allison & Kruss, 1977). In contrast to that, in East Africa, the glacier recession began only in the 1880s, a finding that should call attention to diversity in climatic forcing. Thus, decreased cloudiness/precipitation favors decreased albedo and enhanced solar radiation, which feeds ablation. On the other hand, the so-called greenhouse effect brings warming and enhanced longwave radiation, which feeds melting and evaporation. Solar radiation is an effective driver at topographically exposed locations and elevations above the freezing level; warming is particularly effective at the lower elevations, regardless of topography and exposure to radiation.
Climatic forcing of ice recession on Mount Kenya has been investigated in a series of studies (Kruss & Hastenrath, 1987; Hastenrath & Kruss, 1988, 1992a, 1992b) for the intervals 1899–1963 and 1963–1987. For the period 1899–1963, it was found that the drastic ice recession was largely due to an increase in absorbed solar radiation accompanying (and in part brought about by) changes in cloudiness and precipitation. In addition to the consequences for the overall ice amount, the spatial distribution of radiation totals, as influenced by topographical shading, appears to account for the remarkably diverse shrinkage of the glaciers on Mount Kenya from 1899–1963. By contrast, during 1963–1987, the ice thinning continued strongly at all glaciers regardless of topographic location, suggesting that climatic forcings other than solar radiation have become prominent. These may conceivably be steered by the greenhouse effect—namely, forcing through warming and therefore enhanced sensible heat transfer.
The map of Kilimanjaro in Figure 3 may illustrate the relevance of solar radiation for the distribution of ice extent. The local diurnal circulation on this great mountain has implications for radiation: in the morning, with a clear sky, the sun shines particularly on the east side, where ice becomes scarcer; in the afternoon, the sun stands to the west, with the sky cloudy, less radiation, and more extensive ice.
The glaciers flowing down the mountain slopes may well be affected by warming, but it should be noted that the summit plateau sits above the mean freezing level. Remarkably, the summit plateau has experienced both ice thinning and lateral retreat of vertical ice cliffs. There has been extensive discussion on the nature of climatic forcing (Moelg et al., 2003b; Moelg & Hardy, 2004; Thompson, Brecher, Mosley-Thompson, Hardy, & Mark, 2009, 2010; Moelg, Kaser, & Cullen, 2010; Mote & Kaser, 2012).
With this background, climatic forcing was explored by sensitivity analysis with a focus on temperature (Hastenrath, 2010). Given constant precipitation, net all-wave radiation, and relative humidity, as well as specific humidity riding on temperature, the sensible and latent heat transfer processes provide the substantial energy contribution to ablation. A precious source of temperature variations in the 20th century (Mitchell & Jones, 2005) provides the basis for the time-series plots for the areas of Mounts Kenya, Kilimanjaro, and Ruwenzori in Figure 11. For Lewis Glacier, it is found that with air some 0.7°C cooler, the mass budget could reach equilibrium. Observations on the secular evolution of air temperature and humidity in the areas of Mounts Kenya and Ruwenzori show compatible magnitudes, although contribution by radiative forcing cannot be excluded. For the summit of Kilimanjaro, above the mean freezing level, where ablation is limited to sublimation, turbulent heat transfer processes associated with temperature differences cannot account for the imbalance of the mass budget, and solar radiation forcing continues to be important for both the ice thinning and the lateral retreat of ice cliffs.
In the other two glacier regions under the Equator, ice recession began around the middle of the 19th century, which may seem plausible in relation to global warming. For the Ecuadorian Andes, this is borne out by historical evidence (Hastenrath, 1981) and for New Guinea by numerical modeling of glacier dynamics (Allison & Kruss, 1977). In contrast, the onset of glacier recession in East Africa was in the 1880s. This invites consideration of the circulation system and then long-term changes in the region.
The climate dynamics of equatorial East Africa and the Indian Ocean have been explored in a series of studies (Flohn, 1987; Hastenrath et al., 1993; Hastenrath, 2001, 2006b; Hastenrath, Polzin, & Mutai, 2010, 2011; Mutai, Polzin, & Hastenrath, 2012). Equatorial East Africa has two rainy seasons: boreal spring and boreal autumn. In boreal autumn, a powerful zonal-vertical circulation cell develops over the Indian Ocean Equator, which is manifested in the surface equatorial westerlies, of which we compiled an index UEQ (4oN-4oS, 60-90oE). This UEQ has a correlation of ‒0.85 with the boreal autumn precipitation over equatorial East Africa (Hastenrath et al., 1993). Already reported by Flohn (1987), an extreme example was 1961, with slow UEQ, and floods leading to great lake–level rise. Remarkably, this was a year with a high Southern Oscillation Index (SOI), although overall, the long-term correlation between UEQ and SOI is positive.
With this background, consider the long-term variations of UEQ apparent in Figures 12 and 13. In the decades 1866–1885, the equatorial westerlies, UEQ, were slow; after that, they became fast. Consider that finding in the context of the ‒0.85 correlation of UEQ versus precipitation. For the glacier mass budget, the accompanying radiation effect is quantitatively even more important than the snowfall. Plausibly, then, from the 1880s onward, the East African glaciers receded. Understandably, the onset of glacier recession in East Africa was much later than on the other glaciated mountain regions under the Equator. As detailed earlier in this article, warming became the driver later on, except for the summit of Kilimanjaro.
Thanks to observations, mappings, and measurements made and then preserved in the course of over a century, we can now gain some insight into the fate of ice on the three glaciated mountains of East Africa.
The varying ice extent on Mount Kenya has been documented by field reports since the end of the 19th century and mappings from 1947, 1963, 1987, 1993, and 2004, and in greater detail for the Lewis Glacier in 1934, 1958, 1963, 1974, 1978, 1982, 1985, 1986, 1990, and 1993. For Kilimanjaro, from ground surveys, satellite imagery, and air photographs, a sequence could be obtained of 1912, 1953, 1976, 1989, and 2000. For Ruwenzori, evidence is scarcer, with information from 1906, 1955, and 1990. The absolute amount of area decrease is largest for Kilimanjaro and smallest for Mount Kenya, but the percentage of shrinkage in the course of the 20th century is similar for the three mountains. Photographs of diverse times and locations give complementary testimony of the protracted cryosphere changes.
Lewis is the glacier with the most continuous and comprehensive field evidence in all the tropics. This included measurements, mass budget, and ice flow over decades. Consistent with the progressive decrease in area, the mass balance for the glacier as a whole was negative except in one year. The ice flow velocity progressively slowed, and then the ice became stagnant. The ice became ever thinner, and Lewis lost contact with the Gregory Glacier, which disappeared completely.
Regarding the climatic forcing of the cryosphere decay, one may think of global warming. Indeed, in the other two glaciated mountain regions at the Equator (namely, Ecuadorian Andes and New Guinea), the glacier recession began in the middle of the 19th century. In contrast, in East Africa, the onset was in the 1880s and first affected the radiationally most exposed locations. Warming may be the driver of later recession in most locations, but not the summit plateau of Kilimanjaro.
The late onset of ice recession in East Africa should be appreciated in the context of large-scale circulation changes. Surface westerlies over the equatorial Indian Ocean, the backbone of a powerful zonal-vertical circulation cell, were slower before and became faster after the 1880s, with consequences for precipitation, cloudiness, and radiation over equatorial East Africa. So much for the onset of glacier recession. With the ongoing warming, the glaciers of East Africa continue to suffer drastic decay.
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