A paper published by Taylor et al. (2006) looks for the climatic causes of recent glacial recession in the Rwenzori Mountains, East Africa. The team, which comprised Richard Taylor of UCL Geography, colleagues from UCL and Makerere University and two others from Ugandan governmental departments used field surveys and analyses of remotely sensed imagery to report the reduction in area of the Rwenzori Mountains glaciers. The study saw an overall reduction in glacier area from 2.01 ± 0.56 km2 in 1987 to 0.96 ± 0.34 km2 in 2003. At the same time, air temperatures were seen to increase by approximately 0.5oC each decade; with no significant change in precipitation. By extrapolating trends from area data collected since 1906, it is suggested by the authors that the glaciers of the Rwenzori Mountains could disappear within the next two decades, primarily as a result of rising air temperatures.
The paper indicates the importance of alpine glaciers in the tropics as very sensitive indicators of tropical climate. This is of course very useful for areas such as East Africa, where meteorological records are limited. Many studies are referenced, which investigate the shrinking of glaciers in the highlands of East Africa over the 20th century; a trend shown by the satellite imagery utilised in this study. The authors indicate that debate exists over the nature of climate change in the highlands, as well as debate over the exact cause of glacial retreat in the tropics of East Africa.
The positions of terminal moraines were measured to show the reduced extent of the glaciers in this study. Two ‘indicator’ glaciers were used, which had been previously monitored, and could thus show any trends in glacial retreat. For the satellite data, LandSat5 and LandSat7 imagery were used to measure snow and ice cover. In slightly more detail, snow and ice were identified using a supervised classification on a false-colour composite, using both visible and infrared bands to do so. The accuracy of the classification was assessed by also comparing it with the results of a Normalised Difference Snow Index (NDSI) algorithm.
The results of this study were found to be in agreement over a clear trend of recent glacial retreat in the Rwenzori Mountains. The retreat of one of the indicator glaciers was shown to be quicker than the other; however, this was posited to be as a result of differences in elevation and bed morphology causing a contrast in the supply of ice to each glacier. Regardless, the authors state their findings clearly:
“Analyses of LandSat imagery... identify a 50% decrease in the total area of glaciers from 1987 (2.01 ± 0.11 km2) to 2003 (0.96 ± 0.34 km2). Broad agreement exists between estimates of glacial cover (<12% difference) derived from each method. The results of the NDSI-classified, Land-Sat image from 1987 are consistent with historical data derived from aerial and terrestrial photography.”
Whilst recent glacial retreat was clearly shown by the results, the paper also notes that when combined with measurements made over the last century, there is a steady rate of decline evident across this longer time period as well. As can be seen, the results shown in this paper are in agreement with many other studies, in stating that the glaciers of the Rwenzori Mountains have decreased in area. However, where much dispute still exists is in the debate over the primary climatic driver for this observed glacial retreat.
The knowledge of exactly what is controlling the retreat of glaciers in the tropics is, as always, very important. Not only do the glaciers in the Rwenzori Mountains have importance for local agriculture and drinking water through seasonal meltwater; but the glaciers also hold great cultural significance for the communities that live around them. If temperature alone is found to be controlling their quick retreat, then we can safely say that the glaciers will not exist for much longer. However, if more complex processes control these glacial dynamics, then perhaps some hope exists for their future – however futile. Putting local impacts aside though, there are many more glaciers in the highlands of the tropics, and thus by improving knowledge of how they react to anthropogenic climate change, we can better respond to protecting them.
As noted before, there are no continuous meteorological observations close to the Rwenzori Mountains. Therefore it is not possible to directly analyse the climatic factors which have been driving regional glacial retreat. The paper notes that that previous studies of glacial dynamics in East Africa, particularly including Mölg et al. (2003), stated that the observed 20th century retreat came as a result of a sudden decrease in humidity c.1880. As a result of this decrease in humidity, the authors state that cloud cover would decrease and thus increase the exposure of glaciers to solar radiation. On top of this, with reduced cloud cover; precipitation would be much lower and would therefore decrease accumulation, the subsequent lowering of albedo would also serve to increase the absorption of radiation. As a result of all of these effects, the rate of glacier mass loss would also increase over time.
However, to show the complexity of disagreement over this issue, the paper also cites another study which argues for a climatic driving factor that is completely anathema to that of Mölg et al. (2003). A paper by Hastenrath (2001) is described, which states that glacial recession in East Africa during the 20th century came as a result of a warming trend, which has caused an increase in atmospheric humidity. This increased humidity is then explained as causing a reduction in sublimation and therefore allowing more of the sun’s radiation to melt glacial ice (as a result of a saving of latent heat). The authors note that this process has been well documented in other tropical glaciers, such as those found in Bolivia, where increased humidity of the wet season causes higher melt rates as a result of inhibited sublimation.
After looking at both of these hypotheses, the authors of this paper reject both of them. In particular, the findings of Mölg et al. (2003) are questioned, with the proposed changes in humidity and solar incidence not being accepted as causal factors for the observed spatially uniform retreat of glaciers at lower elevations. Instead, Taylor et al. (2006) find this indicative of increased air temperature as being the primary driving factor for glacial retreat.
This emphasis on air temperature as the primary driving factor is the main conclusion of this paper, and the authors provide evidence of this using long term air temperature records. These records of daily air temperature show consistent trends, with confidence levels of 99% or greater, towards temperature rises of approximately 0.5oC per decade since the last period of glacial advance in the early 1960s. As well as this, the paper cites other gridded climate data which shows a smaller per decade surface temperature increase of 0.15oC over the same time period. Evidently, there is a significant regional warming trend identified in this study. One point worth noting is the relevance of this temperature data, which was measured at a lower elevation than that of glacial cover in the tropics. The authors of the paper address this issue however, stating that due to thermal homogeneity of the troposphere, this rise in temperature at lower elevations can be expected to occur in the much higher glacial areas. However, it must be noted that this point is still much debated. As Taylor et al. (2006) note in their reply to Mölg et al. (2003) (both of which will be discussed further in depth):
“Significant uncertainty persists in temperature data for the tropical troposphere whether these derive from satellite-borne Microwave Sounding Unit (MSU) observations or in situ measurements using radiosondes, particularly in data-poor regions like East Africa.”
Regardless of this issue, the use of lower elevation temperature data by Taylor et al. (2006) is taken to be reliable, and is supported by other studies of tropical troposphere temperature data. Rather than discussing these studies here however, an analysis of the scientific disagreements between Taylor et al. (2006) and Mölg et al. (2003) will be looked at in another blog post.
Finally, this paper also looks at the reduced precipitation claim made by Mölg et al. (2003) and finds it unsatisfactory. The Ugandan meteorological data used in this study fails to support any trend in reducing precipitation over the 20th century, despite there inevitably being much interannual variability present in the records. As well as this, whilst there does exist evidence of an abrupt decrease in humidity around 1880 in East Africa, there is no long term temperature record going back to this point. Therefore, the authors state it is not possible to test the relationship between humidity and temperature in the tropics, as stated by Mölg et al. (2003).
In summary, there are evidently several different proposed theories behind the retreat of glaciers in the Rwenzori Mountains. Whereas the ideas of Taylor et al. (2006) seem to consider fewer variables than the perhaps more complex hypothesis of Mölg et al. (2003), it would seem that the former proves more robust in standing up against the meteorological evidence. Sadly, if this is the case, then the near certain predictions of these mountain glaciers disappearing within a few decades will no doubt have a huge impact for the people of East Africa.
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