Abstract:Atmospheric warming and enhanced melting of glaciers is already resulting in changes in the glacial contribution to run-off in mountain basins around the world. The enhanced melting of glaciers leads at first to increased run-off and discharge peaks and an increased melt season, while in the longer time frame glacier wasting can be so severe that it results in decreased run-off. Glacier basins with a decreasing run-off trend have been observed in south-central British Columbia, at low elevations in the Swiss Alps and in the central Andes of Chile, which is probably a combined effect of reduced melt from seasonal snow cover as the snow line rises, and relevant glacier area losses. In contrast, significant run-off increases are reported in Alberta, north-western British Columbia and Yukon in Canada, in highly glacierized basins in the Swiss and Austrian Alps, the Tianshan Mountains and Tibet in central Asia and in the tropical Andes of Peru. The run-off increase within these basins is closely related to observed temperature rise, indicating that there is an unequivocal signal of enhanced glacier melting under the present warming trends. In future warming scenarios, glacier run-off should start to decrease even in high-altitude basins, affecting water availability.
The Echaurren Norte Glacier is a reference glacier for the World Glacier Monitoring Service (WGMS) network and has the longest time series of glacier mass balance data in the Southern Hemisphere. The data has been obtained by the direct glaciological method since 1975. In this study, we calculated glacier area changes using satellite images and historical aerial photographs, as well as geodetic mass balances for different periods between 1955 and 2015 for the Echaurren Norte Glacier in the Central Andes of Chile. Over this period, this glacier lost 65% of its original area and disaggregated into two ice bodies in the late 1990s. The geodetic mass balances were calculated by differencing digital elevation models derived from several sources. The results indicated a mean cumulative glacier wide mass loss of −40.64 ± 5.19 m w.e. (−0.68 ± 0.09 m w.e. a−1). Within this overall downwasting trend, a positive mass balance of 0.54 ± 0.40 m w.e. a−1 was detected for the period 2000–2009. These estimates agree with the results obtained with the glaciological method during the same time span. Highly negative mass change rates were found from 2010 onwards, with −1.20 ± 0.09 m w.e. a−1 during an unprecedented drought in Central Andes of Chile. The observed area and the elevation changes indicate that the Echaurren Norte Glacier may disappear in the coming years if negative mass balance rates prevail.
Using daily suspended sediment and water discharge data, we calculated the current mean annual runoff and Specific Suspended Sediment Yield (SSY) for 66 mountainous and piedmont catchments in Chile. These catchments are located from the extreme north of Chile to Southern Patagonia and cover an exceptionally wide range of climates, slopes, and vegetation. The SSY ranges mainly between 0 and 700 t km -2 year -1 with some exceptions as high as 1780 t km -2 year -1 . The SSY increases between the extreme north and 33 S and then decreases toward the south. Sediment and water discharge north of 33 S occur mainly during summer. Farther south the contribution of winter precipitation increases and predominates. When the SSY database is correlated with topographic, climatic and vegetation indices, it is found to correlate significantly with runoff and mean slope only. In order to concentrate on erosion processes in the mountain range, 32 mountainous catchments were selected along a strong north-south SSY gradient between 27 S and 40 S. From north to south, SSY increases strongly with runoff and then decreases, even while runoff keeps increasing. In catchments where SSY is low, although runoff is high, the mean slope is less than 40% and the vegetation cover is greater than 8%. For the other catchments, runoff variations explain 67% of the variance in sediment yields. Thus, SSY seems to be controlled by vegetation cover and slope thresholds. In addition, SSY also correlates with glacier cover. However, a correlation between SSY and seismicity, although possible, is ambiguous.
Ectopic thymic tissue is rarely present as a neck mass or thyroid nodule on FNA biopsy. The ultrasound imaging findings reveal a well-defined fusiform lesion with punctate bright echoes that could be misinterpreted as papillary thyroid carcinoma. The aspirates show a small lymphoid population, immunophenotypically compatible with thymic T-cells, in addition to scattered epithelial cells. Therefore, knowledge of the typical ultrasonographic and cytopathological features can help make a definitive diagnosis and avoid more invasive procedures in paediatric patients.
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