Glaciers in Africa
From ClimateWiki
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
Model studies indicate that CO2-induced global warming will result in significant melting of earth’s glaciers, contributing to a rise in global sea level. Global data on glaciers do not support claims made by the IPCC that most claciers are retreating or melting.
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Africa
On the floor of the U.S. Senate in 2004, Arizona Senator John McCain described his affection for the writings of Ernest Hemingway, especially his famous short story, “The Snows of Kilimanjaro.” Then, showing photos of the magnificent landmark taken in 1993 and 2000, he attributed the decline of glacial ice atop the mount during the intervening years to CO2-induced global warming, calling this attribution a fact “that cannot be refuted by any scientist.”
New York Senator Hillary Clinton echoed Senator McCain’s sentiments. Displaying a second set of photos taken from the same vantage point in 1970 and 1999—the first depicting “a 20-foot-high glacier” and the second “only a trace of ice”—she said that in those pictures “we have evidence in the most dramatic way possible of the effects of 29 years of global warming.” In spite of the absolute certitude with which the two senators expressed their views on the subject, which allowed for no “wiggle room” whatsoever, both of them were wrong.
Modern glacier recession on Kilimanjaro began around 1880, approximately the same time the planet began to recover from the several-hundred-year cold spell of the Little Ice Age. As a result, a number of people, including the aforementioned senators, declared that the ice fields retreated because of the rising temperatures, encouraged in this contention by a few reports in the scientific literature (Alverson et al., 2001; Irion, 2001; Thompson et al., 2002). This view of the subject, however, is “highly simplified,” in the words of a trio of glaciologists (Molg et al., 2003b), who noted that “glacierization in East Africa is limited to three massifs close to the equator: Kilimanjaro (Tanzania, Kenya), Mount Kenya (Kenya), and Rwenzori (Zaire, Uganda).” All three sites experienced strong ice field recession over the past century or more. In that part of the world, however, they report “there is no evidence of a sudden change in temperature at the end of the 19th century (Hastenrath, 2001),” and that “East African long-term temperature records of the twentieth century show diverse trends and do not exhibit a uniform warming signal (King’uyu et al., 2000; Hay et al., 2002).” With respect to Kilimanjaro, they say “since February 2000 an automatic weather station has operated on a horizontal glacier surface at the summit’s Northern Icefield,” and “monthly mean air temperatures only vary slightly around the annual mean of -7.1°C, and air temperatures [measured by ventilated sensors, e.g., Georges and Kaser (2002)] never rise above the freezing point,” which makes it pretty difficult to understand how ice could melt under such conditions.
So what caused the ice fields of Kilimanjaro to recede so steadily for so many years? Citing “historical accounts of lake levels (Hastenrath, 1984; Nicholson and Yin, 2001), wind and current observations in the Indian Ocean and their relationship to East African rainfall (Hastenrath, 2001), water balance models of lakes (Nicholson and Yin, 2001), and paleolimnological data (Verschuren et al., 2000),” Molg et al. say “all data indicate that modern East African climate experienced an abrupt and marked drop in air humidity around 1880,” and they add that the resultant “strong reduction in precipitation at the end of the 19th century is the main reason for modern glacier recession in East Africa,” as it considerably reduces glacier mass balance accumulation, as has been demonstrated for the region by Kruss (1983) and Hastenrath (1984). In addition, they note that “increased incoming shortwave radiation due to decreases in cloudiness—both effects of the drier climatic conditions—plays a decisive role for glacier retreat by increasing ablation, as demonstrated for Mount Kenya and Rwenzori (Kruss and Hastenrath, 1987; Molg et al., 2003a).”
In further investigating this phenomenon, Molg et al. applied a radiation model to an idealized representation of the 1880 ice cap of Kilimanjaro, calculating the spatial extent and geometry of the ice cap for a number of subsequent points in time and finding that “the basic evolution in spatial distribution of ice bodies on the summit is modeled well.” The model they used, which specifically addresses the unique configuration of the summit’s vertical ice walls, provided “a clear indication that solar radiation is the main climatic parameter governing and maintaining ice retreat on the mountain’s summit plateau in the drier climate since ca. 1880.” Consequently, Molg et al. concluded that “modern glacier retreat on Kilimanjaro is much more complex than simply attributable to ‘global warming only’.” Indeed, they say it is “a process driven by a complex combination of changes in several different climatic parameters [e.g., Kruss, 1983; Kruss and Hastenrath, 1987; Hastenrath and Kruss, 1992; Kaser and Georges, 1997; Wagnon et al., 2001; Kaser and Osmaston, 2002; Francou et al., 2003; Molg et al., 2003b], with humidity-related variables dominating this combination.”
Kaser et al. (2004) similarly concluded that “changes in air humidity and atmospheric moisture content (e.g. Soden and Schroeder, 2000) seem to play an underestimated key role in tropical high-mountain climate (Broecker, 1997).” Noting that all glaciers in equatorial East Africa exhibited strong recession trends over the past century, they report that “the dominant reasons for this strong recession in modern times are reduced precipitation (Kruss, 1983; Hastenrath, 1984; Kruss and Hastenrath, 1987; Kaser and Noggler, 1996) and increased availability of shortwave radiation due to decreases in cloudiness (Kruss and Hastenrath, 1987; Molg et al., 2003b),” both of which phenomena they relate to a dramatic drying of the regional atmosphere that occurred around 1880 and the ensuing dry climate that subsequently prevailed throughout the twentieth century. Kaser et al. conclude that all relevant “observations and facts” clearly indicate that “climatological processes other than air temperature control the ice recession in a direct manner” on Kilimanjaro, and that “positive air temperatures have not contributed to the recession process on the summit,” directly contradicting Irion (2002) and Thompson et al. (2002), who, in their words, see the recession of Kilimanjaro’s glaciers as “a direct consequence solely of increased air temperature.”
In a subsequent study of the ice fields of Kilimanjaro, Molg and Hardy (2004) derived an energy balance for the horizontal surface of the glacier that comprises the northern ice field of Kibo—the only one of the East African massif’s three peaks that is presently glaciated—based on data obtained from an automated weather station. This work revealed, in their words, that “the main energy exchange at the glacier-atmosphere interface results from the terms accounting for net radiation, governed by the variation in net shortwave radiation,” which is controlled by surface albedo and, thus, precipitation variability, which determines the reflective characteristics of the glacier’s surface. Much less significant, according to the two researchers, is the temperature-driven turbulent exchange of sensible heat, which they say “remains considerably smaller and of little importance.”
Molg and Hardy conclude that “modern glacier retreat on Kilimanjaro and in East Africa in general [was] initiated by a drastic reduction in precipitation at the end of the nineteenth century (Hastenrath, 1984, 2001; Kaser et al., 2004),” and that reduced accumulation and increased ablation have “maintained the retreat until the present (Molg et al., 2003b).” Buttressing their findings is the fact, as they report it, that “detailed analyses of glacier retreat in the global tropics uniformly reveal that changes in climate variables related to air humidity prevail in controlling the modern retreat [e.g., Kaser and Georges (1997) for the Peruvian Cordillera Blanca and Francou et al. (2003) for the Bolivian Cordillera Real (both South American Andes); Kruss (1983), Kruss and Hastenrath (1987), and Hastenrath (1995) for Mount Kenya (East Africa); and Molg et al. (2003a) for the Rwenzori massif (East Africa)].” The take-home message of their study is essentially the same as that of Kaser et al. (2004): “Positive air temperatures have not contributed to the recession process on the summit.”
Two years later, Cullen et al. (2006) report that “all ice bodies on Kilimanjaro have retreated drastically between 1912-2003,” but they add that the highest glacial recession rates on Kilimanjaro “occurred in the first part of the twentieth century, with the most recent retreat rates (1989-2003) smaller than in any other interval.” In addition, they say no temperature trends over the period 1948-2005 have been observed at the approximate height of the Kilimanjaro glaciers, but that there has been a small decrease in the region’s specific humidity over this period.
In terms of why glacier retreat on Kilimanjaro was so dramatic over the twentieth century, the six researchers note that for the mountain’s plateau glaciers, there is no alternative for them “other than to continuously retreat once their vertical margins are exposed to solar radiation,” which appears to have happened sometime in the latter part of the nineteenth century. They also say, in this regard, that the “vertical wall retreat that governs the retreat of plateau glaciers is irreversible, and changes in twentieth century climate have not altered their continuous demise.” Consequently, the twentieth century retreat of Kilimanjaro’s plateau glaciers is a long-term response to what we could call “relict climate change” that likely occurred in the late nineteenth century.
In the case of the mountain’s slope glaciers, Cullen et al. say that their rapid recession in the first part of the twentieth century shows they “were drastically out of equilibrium,” which they take as evidence that the glaciers “were responding to a large prior shift in climate.” In addition, they report that “no footprint of multidecadal changes in areal extent of slope glaciers to fluctuations in twentieth century climate is observed, but their ongoing demise does suggest they are still out of equilibrium,” and in this regard they add that their continuing but decelerating demise could be helped along by the continuous slow decline in the air’s specific humidity. Consequently, and in light of all the facts they present and the analyses they and others have conducted over many years, Cullen et al. confidently conclude that the glaciers of Kilimanjaro “are merely remnants of a past climate rather than sensitive indicators of 20th century climate change.”
Two more recent studies, Mote and Kaser (2007) and Duane et al. (2008) additionally reject the temperature-induced decline hypothesis for Kilimanjaro, with Duane et al. concluding that “the reasons for the rapid decline in Kilimanjaro’s glaciers are not primarily due to increased air temperatures, but a lack of precipitation,” and Mote and Kaser reporting that “warming fails spectacularly to explain the behavior of the glaciers and plateau ice on Africa’s Kilimanjaro massif … and to a lesser extent other tropical glaciers.”
Clearly, the misguided rushes to judgment that have elevated Kilimanjaro’s predicted demise by CO2-induced global warming to iconic status should give everyone pause to more carefully evaluate the evidence, or lack thereof, for many similar claims related to the ongoing rise in the air’s CO2 content.
References
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Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html
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