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

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 1 W m⁻². A warming of 0.0 to 0.2°C would translate into an additional downward-directed sensible heat transfer of 0.0 to 1.4 W m⁻². A 0.1 to 0.2 g kg⁻¹ increase in specific humidity would, through savings in the latent heat transfer, contribute 2 to 4 W m⁻². Long-term station records show little warming trend for East Africa itself. However, mid-tropospheric specific humidity trends of about 0.6 g kg⁻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.