Comment on “Recent glacial recession in the Rwenzori Mountains of East Africa due to rising air temperature” by Richard G. Taylor, Lucinda Mileham, Callist Tindimugaya, Abushen Majugu, Andrew Muwanga, and Bob Nakileza

[1] The focal point – and new contribution to the community – of the study by Taylor et al. [2006] is the derivation of recent (1987, 1995, 2003) glacial extent in the central Rwenzori Mountains, Equatorial East Africa (EEA). This is done by a remote sensing technique, which involves analysis of Landsat satellite images, and shows strong shrinkage between 1987 and 2003. Ground-based measurements of two glacier termini retreats (Speke and Elena glaciers) complement the evidence of continuing glacier retreat on Rwenzori. [2] However, the main conclusion drawn by the authors is that there is a strong link between a climate signal, namely rising air temperature, and recent glacier retreat. In that context, they analyze meteorological station data which contain significant gaps, and have been recorded at low tropospheric levels (i.e., 3000 m or more below the altitude of the glaciers). While we do not doubt the persistently strong shrinkage of the glaciers on Rwenzori in agreement with previous work [Kaser and Osmaston, 2002], we show here that Taylor et al.’s [2006] main conclusion is based on (a) questionable data and (b) wrong assumptions for the interpretation of these data (items 1– 3 below). As the main conclusion is also expressed in the title of the paper (‘‘due to rising air temperature’’), we argue that its principal message is misleading and incorrect. In addition, the relevant part of the study (derivation of the new glacier extents) holds methodological deficiencies (item 4 below) that result in significant uncertainties of the reported and therefore predicted glacier extents. [3] 1. For small and fast-responding glaciers like on Rwenzori and other tropical mountains [e.g., Kaser et al., 2003], surface energy balance (SEB) and mass balance provide the best (and most direct) quantitative link between climate parameters and glacier change. Establishing this link requires a set of meteorological and glaciological measurements directly on (or very near) the glacier [e.g., Klok and Oerlemans, 2004], for input to a SEB model resolving all the glacier-atmosphere mass and energy fluxes that determine the surface mass balance. Such measurements and associated SEB modeling have never been performed in the Rwenzori mountains, but on the other two glacierized massifs in EEA; in the 1970s on Mount Kenya [Hastenrath, 1984], and presently on Kilimanjaro [Mölg and Hardy, 2004]. These studies, and also other SEB studies of tropical glaciers [e.g.,Wagnon et al., 1999; Favier et al., 2004], have demonstrated that parameters related to atmospheric moisture (air humidity, precipitation affecting also surface albedo, cloudiness affecting incoming solar radiation) dominate the SEB. Thus, looking at air temperature (Ta) as primary parameter, as done by Taylor et al. [2006], represents an oversimplified approach that does not meet the complexity of physical processes at the interface between atmosphere and tropical glaciers. [4] 2. Still, meteorological records provided by the national weather services can serve as useful basis for the interpretation of how climate may affect glacier mass balance, which Taylor et al. [2006] attempt in their section 4. Nonetheless they neglect essentials of atmospheric trend analysis and assume the trends in the available data, which have been recorded between 960 and 1869 m a.s. l. (as said, 3000 m or more below the glaciers), would also exist at the high, glacierized elevations. This assumption is crude and incorrect for two reasons. First, the term ‘‘thermal homogeneity’’ (used in their section 4) does not imply that trends at different tropospheric levels will be the same. It rather describes that – in tropical climate – the annual cycle in Ta is of a smaller amplitude than the diurnal cycle (temporal dimension), and that this seasonally almost constant Ta applies to all regions within latitudes experiencing the sun at the zenith (spatial dimension) [Kaser and Osmaston, 2002]. Secondly, trends at lower elevations do not necessarily have to occur in the midtroposphere, neither in magnitude nor in sign. This has been shown for the tropics many times through observations [e.g., Gaffen et al., 2000] and model simulations [e.g., Bradley et al., 2004]. The authors disregard this finding and persist in relying on the data collected far below the glaciers, which they use unreservedly for their interpretation. Changes of trends due to vertical dimension are ignored. Figure 1 in this comment demonstrates such changes by the widely used NCEP/NCAR reanalysis data [Kalnay et al., 1996], for the grid point almost perfectly GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L20404, doi:10.1029/2006GL027254, 2006