As mentioned in the previous post covering the Rwenzori Mountains paper (Taylor et al. 2006), there were a number of conflicting theories over the primary forcing factor behind glacier retreat in the region of study. Scientific criticism came from Mölg et al. (2006) in the form of a post-publication comment; highlighting the author’s disagreement over several issues. As part of open peer commentary, Taylor et al. (2006) also replied to these criticisms in the same publication. It is hoped that this exchange can be documented clearly in the next two blog posts.
Mölg et al. begins by accepting the evidence of strong glacier retreat in the Rwenzori Mountains during the 20th century. However, they state disagreement with the main conclusion of Taylor et al., that there is a strong link between air temperature and this retreat. Part of this is due to the lack of a surface energy balance (SEB) model, as a result of having no meteorological or glaciological records from directly on or very near the glacier. Mölg et al. note that the two glacierised massifs in East Africa that have had SEB modelling performed, Mount Kenya and Kilimanjaro, showed that the parameters relating to atmospheric moisture (humidity, precipitation, surface albedo, cloudiness) are dominant. Thus Mölg et al. state that the focus of the authors on rising air temperature fails to identify these greater intricacies because of the lack of an SEB model and instead, the authors have over simplified the relationship between atmosphere and glaciers in the tropics.
Moving on from this, Mölg et al. addresses the issue of how local meteorological data was misused in this study to show how climate may affect glacier mass balance. The main contention here is how the key rules of atmospheric trend analysis were apparently neglected by Taylor et al. when stating that the measured trends found at much lower elevations, would be identical to those in the higher glacial elevations. Mölg et al. first states a problem with the use of the phrase “thermal homogeneity” by Taylor et al. to describe the same air temperature trends at different tropospheric levels. Instead, Mölg et al. state its use is more appropriately used to refer to seasonally constant temperatures across all tropical regions.
More importantly however, Mölg et al. move on to state that this concept is incorrect in any case. They cite several studies which show through observations and modelling that trends occurring in the lower elevations do not have to appear in the mid-troposphere. Mölg et al. state that the authors ignored these findings and therefore ignored changing trends in the vertical dimension. They then additionally attempt to show the existence of these vertical changes through the use of NCEP/NCAR reanalysis data of past and present atmospheric conditions over the Rwenzori Mountains. Whilst there are known problems with the use of NCEP/NCAR reanalysis data, Mölg et al. state that in these tropical areas, data of surface air temperature and moisture (even on the mountain tops) can be seen as reliable. Firstly, Mölg et al. displays the temperature trends at the elevations of the meteorological stations used by Taylor et al. This confirms a warming in air temperature throughout the 20th century. However, when they show the temperature trends from higher elevations (equivalent to the height of the glaciers), there apparently exists no significant temperature trend across the same temporal period. Mölg et al. therefore draws the conclusion that air temperature is unlikely to be behind the 20th century glacier retreat in the Rwenzori Mountains. Instead, using the same reanalysis data, humidity can be shown to have decreased over the same period. Accordingly, this would then increase sublimation, affect the energy flux of incoming longwave radiation and thus alter the net radiation budget. According to Mölg et al. the above mentioned lack of SEB model parameters in the study of Taylor et al. rendered them unable to consider these factors! Following this, a slightly contentious statement discounting the hypothesis of Taylor et al. perhaps puts too much reliance on the use of reanalysis data, which has known problems:
Moving on from this, Mölg et al. addresses the issue of how local meteorological data was misused in this study to show how climate may affect glacier mass balance. The main contention here is how the key rules of atmospheric trend analysis were apparently neglected by Taylor et al. when stating that the measured trends found at much lower elevations, would be identical to those in the higher glacial elevations. Mölg et al. first states a problem with the use of the phrase “thermal homogeneity” by Taylor et al. to describe the same air temperature trends at different tropospheric levels. Instead, Mölg et al. state its use is more appropriately used to refer to seasonally constant temperatures across all tropical regions.
More importantly however, Mölg et al. move on to state that this concept is incorrect in any case. They cite several studies which show through observations and modelling that trends occurring in the lower elevations do not have to appear in the mid-troposphere. Mölg et al. state that the authors ignored these findings and therefore ignored changing trends in the vertical dimension. They then additionally attempt to show the existence of these vertical changes through the use of NCEP/NCAR reanalysis data of past and present atmospheric conditions over the Rwenzori Mountains. Whilst there are known problems with the use of NCEP/NCAR reanalysis data, Mölg et al. state that in these tropical areas, data of surface air temperature and moisture (even on the mountain tops) can be seen as reliable. Firstly, Mölg et al. displays the temperature trends at the elevations of the meteorological stations used by Taylor et al. This confirms a warming in air temperature throughout the 20th century. However, when they show the temperature trends from higher elevations (equivalent to the height of the glaciers), there apparently exists no significant temperature trend across the same temporal period. Mölg et al. therefore draws the conclusion that air temperature is unlikely to be behind the 20th century glacier retreat in the Rwenzori Mountains. Instead, using the same reanalysis data, humidity can be shown to have decreased over the same period. Accordingly, this would then increase sublimation, affect the energy flux of incoming longwave radiation and thus alter the net radiation budget. According to Mölg et al. the above mentioned lack of SEB model parameters in the study of Taylor et al. rendered them unable to consider these factors! Following this, a slightly contentious statement discounting the hypothesis of Taylor et al. perhaps puts too much reliance on the use of reanalysis data, which has known problems:
“Thus, the scenario of a Ta -driven humidity increase in East African highlands, and associated accelerated melt... prior to the availability of reanalysis data, seems to be outdated and has not been reconsidered by Taylor et al. [2006]. However, this is their only causal explanation of how Ta changes would control glacier mass balance on Rwenzori.”
The final criticism from Mölg et al. relates to the use of remotely sensed imagery to show glacier extent. Here, they state that the authors unsatisfactorily discerned between the true glacier surface and transient snow. This issue stems from the use of a false colour composite, which does not apparently allow the differentiation between temporary snow and the perennial snow cover on top of the glacier surface. As a result, the original paper reports incorrect areal extents of each glacier and thus incorrect temporal retreat rates. As a result, Mölg et al. questions the validity of the observed steady rate of decline and perfect linear retreat in time. They instead conduct their own analysis using ASTER satellite imagery, with much clearer delineation of glacier boundary. The results are stated as such:
“The temporal retreat of glacier areas shows clear differences between the three Rwenzori mountains, depending on size and elevation of the glaciers. In particular during the last decades, the larger and higher elevated glaciers on Mount Stanley are retreating much slower...”
In summary, whilst Mölg et al. have critiqued some of the methodology employed in the original paper, their main concern involved the assertion that air temperature was the primary forcing factor in the retreat of the Rwenzori Mountain glaciers. Instead, they believe it to be a much more atmospherically complex issue than this, ultimately related to changes in humidity. Whilst air temperature cannot be denied to have an effect on glacier dynamics globally, it is the belief of Mölg et al. that the glaciers of East Africa may well have acted differently in the past; which is a very important issue if we are to fully understand the rapidly changing glacier conditions of the tropics in the future.
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