Reply to comment by T. Mölg et al. on “Recent glacial recession in the Rwenzori Mountains of East Africa due to rising air temperature”
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[1] Debate persists as to the extent to which recent glacial recession observed in tropical highlands is driven primarily by changes in air temperature [e.g., Bradley et al., 2006; Thompson et al., 2006] and atmospheric humidity [e.g. Kaser et al., 2004; Mölg and Hardy, 2004]. Uncertainty has also been expressed in the relationship between temperature trends at the surface and higher elevations in the tropical free troposphere [e.g., Christy et al., 2003; Christy and Norris, 2004; Douglass et al., 2004; Fu et al., 2004; Tett and Thorne, 2004] where alpine glaciers reside. We therefore welcome the constructive comments of Mölg et al. [2006] regarding our original paper and appreciate the opportunity to clarify arguments made therein [Taylor et al., 2006]. We agree with Mölg et al. that the surface energy balance and mass balance are best able to describe the relationship between climate parameters and glacier change [e.g., Wagnon et al., 1999; Mölg and Hardy, 2004]. For the Rwenzori Mountains, measurements that would form the basis of a glacier mass balance model do not exist. This point was recognized explicitly in the original paper, "The absence of continuous and proximate meteorological observations in the Rwenzori Mountains prevents direct analysis of the climatic factors driving observed glacial recession." Although a definitive, quantitative understanding of the climate variables responsible for glacier mass losses in the Rwenzori Mountains remains elusive, we dispute the assertion of Mölg et al. that air temperature (Ta) is unlikely to be the main driver of observed glacial recession and argue that trends of increasing air temperature are better supported by currently available evidence than decreasing humidity posited by Mölg et al. [2] The essential scientific criticism of our paper by Mölg et al. [2006] is the validity of the assumption that Ta trends observed in gridded CRU TS 2.0 climate data sets [New et al., 2002] and at meteorological stations between 960 and 1869 meters above sea level (masl), reflect Ta trends in the middle troposphere (4800 to 5100 masl) where glaciers in the Rwenzori Mountains occur. Mölg et al. suggest that we have disregarded evidence of inconsistencies between Ta trends at the surface and in the tropical troposphere, but the literature [Hense et al., 1988; Gaffen et al., 2000; Bradley et al., 2004] and evidence they cite is selective. 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. Indeed, linear Ta trends in the tropical troposphere can vary significantly based simply upon choice of start and end date as is the case in the paper by Gaffen et al. [2000] using MSU data in which at 500 hPa a cooling trend is detected between 1979 and 1997 but an overall warming trend occurs between 1960 and 1997. Nevertheless, recent studies that employ diurnal corrections to MSU observations between 1979 and 2003 [Mears and Wentz, 2005] and homogenized radiosonde data sets (HadAT2) between 1958 and 2002 [Thorne et al., 2005], show that the middle troposphere warmed at a similar or slightly greater rate to the surface in the tropics [Fu and Johanson, 2005; Santer et al., 2005], consistent with the sign and (within error) magnitude of Ta trends (+0.13°C per decade) at the surface from climate model (HadCRU2v) predictions [Jones and Moberg, 2003]. [3] Mölg et al. [2006] use NCEP reanalysis data [Kalnay et al., 1996] for the grid cell (30°E, 0°N) to support their claim that a discrepancy exists between Ta trends at the lower troposphere (850 hPa) and mid-troposphere (600 hPa) in the Rwenzori Mountains (their Figure 1). There is, however, widespread consensus within the climate community that reanalysis data are unsuitable for trend analysis in climate change studies as "…known discontinuities in reanalyzed data sets indicate that further research is required to reduce time-dependent errors to a level suitable for climate change studies" [Intergovernmental Panel on Climate Change, 2001, p. 120]. The existence of systematic, time-varying biases in reanalysis data is also highlighted by more recent studies [Bengtsson et al., 2004; Simmons et al., 2004; Sterl, 2004; Thorne et al., 2005]. Mölg et al. consider biases in the NCEP data associated with the introduction of satellite observations in 1979 to reanalysis data sets (see caption in their Figure 1) but not other inconsistencies that arise from the wide range of data sources including modeled processes [Pepin and Seidel, 2005]. In contrast to inferences drawn by Mölg et al. using NCEP data, upper air temperature records from gridded HadAT2 radiosonde data [Thorne et al., 2005] for the most proximate (and only) grid cell to the Rwenzori Mountains show consistent warming trends in the lower and middle troposphere (700 hPa, 500 hPa) from 1958 to 2005 (Figure 1). These warming trends coincide with increased Ta trends at the surface over the second half of the 20th century that have been detected in gridded (homogenized) CRU TS 2.0 data sets [New et al., 2002] at four locations in the East African Highlands by Pascual et al. [2006] and the Rwenzori Mountains [Taylor et al., 2006]. A comparison of temperature trends from surface observations at high elevations and free troposphere (radiosonde measurements) indicates more rapid warming of alpine surfaces than the free troposphere [Pepin and Seidel, 2005] though this discrepancy is reduced for mountain peaks and may stem from a systematic cooling bias arising from daytime heating of the radiosonde sensors [Sherwood et al., 2005]. Analyses of station data in the tropical Andes [Vuille and Bradley, 2000] and on the Tibetan Plateau [Liu and Chen, 2000] show that Ta trends between 1000 and 5000 masl remain constant in sign (i.e., increasing Ta) but can vary in magnitude (+0.1 to +0.3°C per decade). It is worth noting that a step-wise increase in Ta during the 1970s, noted globally at the surface [Jones and Moberg, 2003] and in the troposphere [Thorne et al., 2005] as well as in the tropical Andes [Vuille and Bradley, 2000], is also observed at the surface in CRU TS 2.0 data sets in the East African Highlands [Pascual et al., 2006, Figure 1] and station data in western Uganda [Taylor et al., 2006, Figure 3]. [4] Mölg et al. [2006] employ NCEP reanalysis data to indicate a trend of decreasing specific humidity in the mid-troposphere (600 hPa) from 1948 to 2005. Quite apart from the time-dependent biases in all NCEP data, the reliability of the specific humidity data is particularly questionable as NCEP humidity is a statistically derived parameter. The ability of NCEP humidity data to represent interannual precipitation anomalies associated with the dominant modes of climate variability in equatorial east Africa, highlighted by Mölg et al. (their Figure 2), does not bear on the reliability of these data sets for trend analyses. Radiosonde-derived humidity from 1965 to 1984 [Hense et al., 1988] cited in support of NCEP specific humidity trends from 1948 to 2005, are in fact uncorrected; systemic dry biases have been carefully removed from more recent corrected data sets [Guichard et al., 2000]. A decline in humidity over the 20th century is, furthermore, unsupported by surface CRU TS 2.0 precipitation and vapour pressure data sets (Figure 2). Mölg et al. additionally argue that observed glacial recession in the East African Highlands over the last century originates from a drastic reduction in moisture in the late 19th century. This drop in moisture, based on historical evidence of the levels of Lake Victoria and other East African lakes [Nicholson and Yin, 2001], is actually the descending limb of a brief, approximately decade-long high lake stand (Figure 3). Lake levels, a remote and indirect proxy of regional humidity, are variable during the 19th century prior to their peak in 1880 but comparable to lake levels throughout the 20th century. A modern comparison to the 19th century event is the 2.3 m rise in the level of Lake Victoria between October 1961 and May 1964 (Figure 3). The implied increase in humidity associated with this lake-level rise coincides with a very brief (one year) and very marginal advance (3 to 5 m) in the terminal positions of valley glaciers in the Rwenzori Mountains [Temple, 1968]. The humidity hypothesis proposed by Mölg et al. contends that (1) termination of a brief period of accumulation due to enhanced precipitation around 1880 led to continued glacial retreat into the latter half of the 20th century and (2) a trend of decreasing humidity, supported only by NCEP reanalysis data for which trend analysis is inappropriate, has driven glacial recession since 1970. Even ignoring concerns regarding this evidence, the argument that these climate events are responsible for the expected demise of small, fast-responding glaciers that have persisted for at least 5000 years [Thompson et al., 2006] is improbable. [5] Mölg et al. [2006] highlight several limitations in our analysis of glacial extent in the Rwenzori Mountains from 1987 to 2003 using Landsat imagery and field surveys. These include compensation for terrain-induced effects and limited rigor in our discrimination of snow and glacial cover using Landsat imagery. Small discrepancies noted in the mapped extent of glaciers on Mount Speke in 2003 with previous assessments [Kaser and Osmaston, 2002] arise, in part, from errors introduced through the reconciliation of data sets with different datums (i.e., WGS-84, 1950 Arc). We welcome the new estimate of glacial extent in 2005 from Mölg et al. using a similar approach but employing recently available ASTER imagery. They report combined glacial extents on Mounts Stanley, Speke, and Baker that are slightly larger (1.14 ± 0.10 km2) but within calculated error of our estimate in 2003 (0.96 ± 0.34 km2). Despite Mölg et al.'s objections to our reporting of a steady rate of decline in glacial extent in the Rwenzori Mountains over the last century, the apparent linearity in rate of retreat over the 20th century, which we acknowledge may partly result from the paucity of measurements, exists whether their analysis (r2 = 0.997) or ours (r2 = 0.999) is considered. [6] Both increasing air temperature and reduced air humidity remain plausible and likely related hypotheses to explain recent glacial recession in the Rwenzori Mountains of East Africa. There is agreement that glaciers in the Rwenzori Mountains continue to recede at a rate of ∼0.5 km2 per decade and presently occupy a total area of ∼1 km2. There is also agreement that there are insufficient data to represent the complex interactions of radiant energy and heat at the glacier's surface and thus quantify the link between changes in climate variables and glacial mass in the Rwenzori Mountains. We maintain that there is currently greater evidence of trends of increasing air temperature than decreasing humidity to explain deglaciation in the Rwenzori Mountains. This conclusion does not preclude, however, the likelihood that changes in humidity and radiative fluxes associated with rising air temperatures, have also contributed to observed glacial recession.Keywords:
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Recent evolutions and current status of glacial lakes in the central Chinese Himalayas were analyzed using Landsat satellite imagery acquired in 1990, 2000, and 2010. The datasets show that there are 604 glacial lakes with a total area of 85.17 km 2 in the central Chinese Himalayas in 2010, in which moraine-dammed lakes are the most represented typology (199 lakes, 54.92 km 2 ) in terms of area. From 1990 to 2010, the expansion rate of total glacial lake area was 0.57 km 2 /year in the central Chinese Himalayas and was significantly higher than in the Nepal-Bhutan and Western India-Pakistan-Afghanistan Himalayas (−0.08 to 0.45 km 2 /year ) between 1990 and 2009. Of all glacial lakes, moraine-dammed lakes experienced a rapid increase in size at a rate of 0.45 km 2 /year from 1990 to 2010, while the area of other types of glacial lakes grew more slowly with an expansion rate that did not exceed 0.05 km 2 /year (valley lakes at a rate of 0.003 km 2 /year and glacial erosion lakes at a rate of 0.006 km 2 /year ). In addition, 23 potentially dangerous glacial lakes (PDGLs) are identified and their area increased by 77.46% between 1990 and 2010 and the increase rate is higher than non-PDGLs (39%) in the same period.
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We have analyzed one rapidly expanding glacial lake and one stagnant glacial lake located in the central Himalaya to understand the impact of local topography on the expansion and evolution of glacial lakes using remote sensing data. The slope, aspect, incoming solar radiation and compactness ratio of glaciers associated with the glacial lakes have been studied and analyzed. Glacier topography play important role in the expansion of glacial lakes as observed from the study..
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<p>In recent years, the number and size of glacial lakes in mountain regions have increased worldwide associated to the climate-induced glacier retreat and thinning. Glacial lakes can cause glacial lake outburst floods (GLOFs) which can pose a significant natural hazard in mountainous areas and can cause loss of human life as well as damage to infrastructure and property.</p><p>The glacial landscape of the Jostedalsbreen ice cap in south-western Norway is currently undergoing significant changes reflected by progressing glacier length changes of the outlet glaciers and the formation of new glacial lakes within the recently exposed glacier forefields. We present a new glacier area outline for the entire Jostedalsbreen ice cap and the first detailed inventory of glacial lakes which were formed within the newly exposed ice-free area at the Jostedalsbreen ice cap. In detail, we explore (i) the glacial lake characteristics and types and (ii) analyse their spatial distribution and hazard potential.</p><p>For the period from 1952-1985 to 2017/2018 the entire glacier area of the Jostdalsbreen ice cap experienced a loss of 79 km<sup>2</sup>. A glacier area reduction of 10 km<sup>2</sup> occurred since 1999-2006. Two percent of the recently exposed surface area (since 1952-1985) is currently covered with newly developed glacial lakes corresponding to a total number of 57 lakes. In addition, eleven lakes that already existed have enlarged in size. Four types of glacial lakes are identified including bedrock-dammed, bedrock- and moraine-dammed, moraine-dammed and ice-dammed lakes. Especially ice- or moraine-dammed glacial lakes can be the source of potentially catastrophic glacier lake outburst floods. According to the inventory of glacier-related hazardous events in Norway GLOFs represent the most common hazardous events besides ice avalanches and incidents related to glacier length changes. Around the Jostedalsbreen ice cap several historical but also recent events are documented. The majority of the events caused partly severe damage to farmland and infrastructure but fortunately no people have been harmed by today.</p><p>Due to the predicted increase in summer temperatures for western Norway until the end of this century, it is very likely that the current trend of an accelerated mass loss of Norwegian glaciers will continue. As one consequence of this development, further new lakes will emerge within the newly exposed terrain. The development of new glacial lakes has diverse regional and global socio-economic implications. Especially in mainland Norway, where glaciers and glacier-fed streams have a high importance for hydropower production, tourism and climate research it is essential to gain a better understanding of the possible impacts of glacial lakes for being prepared for risks but also advantages arising from these newly emerging landscape elements.</p>
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Owing to intense glacial retreat and melting, it is anticipated that numerous glacial lakes will be formed in the next few decades. However, their development and distribution patterns in the Tibetan Plateau and its surroundings still need to be elucidated. In this study, a published glacier ice thickness distribution dataset was employed to fully detect overdeepened glacier beds as potential glacial lakes. We selected and expanded four morphological metrics to determine the formation probability of potential glacial lakes: surface slope, break in slope, lake area, and position on the glacier. The results revealed that 15,826 potential glacial lakes with areas >0.02 km2 exist in the Tibetan Plateau and its surroundings, covering an area of 2253.95 ± 1291.29 km2 with a water volume of 60.49 ± 28.94 km3 that would contribute to an equivalent sea level rise of 0.16 ± 0.08 mm. The experimental comparison and uncertainty assessment for the overdeepening processing showed that the different extraction methods and basic digital elevation models used could lead to non-negligible errors in the results (at least ±30%), which were ignored in previous studies, contributing to major divergences between the several current inventories of potential glacial lakes in the plateau. Notably, approximately 90% of the total area of the potential glacial lakes is concentrated in the lower half of the individual glaciers in the Tibetan Plateau and its surroundings. >70% of the potential glacial lakes and contemporary glacial lakes in this region were found to be concentrated within the 4000–5800 m elevation range. Moreover, the study identified 5361 potential glacial lakes with high or very high exposure probabilities, and their distribution was mostly determined by regional glacier resources. However, the numbers and sizes of some potential glacial lakes that are found in the Karakoram region are considered to be exaggerated because of the presence of numerous surge-type glaciers, which have not been discussed in previous studies. These results can improve our understanding of future glacial lake formation and distribution in the Tibetan Plateau and its surroundings and have implications for further implementation of effective prevention, mitigation, and adaptation measures for glacial lake outburst floods and water security.
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Global climate change is significantly triggering the dynamic evolution of high-mountain lakes which may pose a serious threat to downstream areas, warranting their systematic and regular monitoring. This study presents the first temporal inventory of glacial and high-altitude lakes in the Sikkim, Eastern Himalaya for four points in time i.e., 1975, 1991, 2000 and 2017 using Hexagon, TM, ETM+ and OLI images, respectively. First, a baseline data was generated for the year 2000 and then the multi-temporal lake changes were assessed. The annual mapping of SGLs was also performed for four consecutive years (2014-2017) to analyze their nature and occurrence pattern. The results show an existence of 463 glacial and high-altitude lakes (>0.003 km2) in 2000 which were grouped into four classes: supraglacial (SGL; 50) pro/peri glacial lake in contact with glacier (PGLC; 35), pro/peri glacial lake away from glacier (PGLA; 112) and other lakes (OL; 266). The mean size of lakes is 0.06 km2 and about 87% lakes have area 80%) are persistent in nature, followed by drain-out (15-20%) and recurring type lakes (7-8%). The new-formed lakes (9-17%) were consistently noticed in all the years (2014-2017). The results of this study underline that regional climate is accelerating the cryosphere thawing and if the current trend continues, further glacier melting will likely occur. Therefore, formation of new lakes and expansion of existing lakes is expected in the study area leading to increase in potential of glacial lake outburst floods. Thereby, persistent attention should be paid to the influences of climatic change in the region.
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Based on surveying data of glacial striae on roches mountonnees near the terminus of Glacier No.1 and Glacier No.7 at the head of Urumqi River, Tian Shan Mt., the statistical graduation character of glacial striae is discussed in this paper. It is shown that the statistical graduation character of glacial striae conforms to the exponent model, and the parameters (A and B) of this model can be used as indexes to describe the density of glacial striae and the glacial dynamics. The larger A and B are, the larger the density of glacial striae is. The spatial distribution of the parameters (A and B) of glacial striae is influenced by the size of glacier, location in the trough, and position on the roches mountonnee. It is shown in this area that the A and B values are larger in the larger glacier (Glacier No.1) than those in the smaller (Glacier No.7), and larger on the top side of the roches mountonnee than those on the lateral side. At the same time, the A and B values are also varied from the center to the edge in glacial troughs influenced by the micro forms in glacial valleys.
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Abstract This study is perhaps the first attempt to use satellite data (1990–2018) to analyze spatiotemporal changes in glacial lakes over the Kashmir Himalayas supplemented by field studies. Landsat images were used to delineate the spatial extent of glacial lakes at four time points, i.e., 1990, 2000, 2010 and 2018. The total count of lakes as well as their spatial extent showed a discernible increase. The number increased from 253 in 1990 to 322 in 2018, with a growth rate of 21.4%. The area has increased from 18.84 Km 2 in 1990 to 22.11 Km 2 in 2018 with a growth rate of 14.7 percent. The newly formed glacial lakes, including supra glacial lakes, were greater in number than the lakes that disappeared over the study period. All glacial lakes are situated at elevations of 2700 m asl and 4500 m asl. More than 78% of lake expansion in the study region is largely due to the growth of existing glacial lakes. Through area change analysis, our findings reveal that certain lakes show rapid expansion needing immediate monitoring and observation. The analysis of the meteorological variables reveals that minimum and maximum temperatures in the Jhelum basin have shown an increasing trend. T max showed an increase of 1.1°C, whereas T min increased to 0.7°C from 1990 to 2018. On the other hand, precipitation has shown a decreasing trend, which can be attributed to one of the major causes of glacier recession and the expansion of glacial lakes in the Upper Jhelum basin. Consequently, this study could play a significant role in devising a comprehensive risk assessment plan for potential GLOFs and developing a mechanism for continuous monitoring and management of lakes in the study region.
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