The dramatic retreat of Mount Kenya’s glaciers between 1963 and 1987: greenhouse forcing

Abstract

ABSTRACT. The retreat of the glaciers on Mount Kenya is quantitatively well documented for the intervals 1899-1963 and 1963-1987. The ice recession between 1899 and 1963 was strongly dependent on solar radiation geometry. By contrast, the ice thinning between 1963 and 1987 amounted to about 15 m for all glaciers regardless of topographic location. This suggests that climatic forcings other than solar radiation have become more prominent.Sensitivity analyses indicate that the energy supply of about 5 W m-2 , required to produce the observed ice thinning through melting, can be accounted for by a combination of climatic forcings. The direct effect of changing atmospheric composition ("greenhouse effect") on the net longwave radiation could have contributed less than I W m- Long-term station records show little warming trend for East Africa itself. However, mid-tropospheric specific humidity trends of about 0.6gkg-1 over the past two decades in the equatorial belt have been reported in the literature, and considered to be consequences of "global warming" and the "greenhouse effect".Viewed in perspective, the ice wastage on Mount Kenya between 1963 and 1987 appears to have been driven primarily by three climatic forcings, conceivably all steered by the "greenhouse effect": a direct forcing through the net longwave radiation; an indirect forcing through warming and therefore enhanced sensible heat transfer; and another indirect forcing through warming (not necessarily in the region itself), leading to increased (advected) atmospheric moisture, and hence to reduced latent heat transfer, this last line of control being the most important.

The contribution of increased incoming shortwave radiation to the retreat of the Rwenzori Glaciers, East Africa, during the 20th century

“…The plain ratio between category 1 and category 2 is maintained but weakens in the second half of the 20th century, denoting that the influence of ablation caused by shortwave radiation decreased. In its essential features, this aspect agrees with the findings of Hastenrath and Kruss (1992) for Mount Kenya. There, the authors assigned the retreat of the glaciers in the first half of the 20th century to shortwave radiation, and the retreat between 1963 and 1987 primarily to a combination of a 0.2 K warming and an increase in specific humidity of 0.3 g kg −1 .…”

Solar modulation of Little Ice Age climate in the tropical Andes

“…It is likely that this mechanism also may serve to amplify the effects of warming trends, irrespective of their origin, which raises concern that global warming will adversely affect highaltitude tropical montane regions (41). Supporting this concern, 20th-century temperature increases have raised the ELAs of tropical glaciers, leading to accelerated ablation and disappearance in many cases (12,(42)(43)(44). Our data suggest considerable sensitivity of tropical climate to small changes in radiative forcing from solar irradiance variability.…”

Observations: Cryosphere

“…• Molg et al (2003b) applied a radiation model to an idealized representation of the 1880 icecap of Kilimanjaro, concluding "modern glacier retreat on Kilimanjaro is much more complex than simply attributable to 'global warming only.'" Instead, and as reported by many other authors, the ice retreat has been "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;Wagnon et al, 2001;Kaser and Osmaston, 2002;Molg et al, 2003b], with humidity-related variables dominating this combination." report "all ice bodies on Kilimanjaro have retreated drastically between 1912-2003," but they add the highest glacial recession rates on Kilimanjaro "occurred in the first part of the twentieth century, with the most recent retreat rates (1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) smaller than in any other interval."…”