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Chapter 06
Climate and Study of Glaciers
the sensible and latent heat released by monsoon storms will cause increased glacier ablation. Second, the increase or decrease in surface temperature might result in ablation or accumulation by changing the snow/rain proportion of summer precipitation. In addition, we notice that temperature variations of the Tibetan Plateau correspond well with the SASM variability during the last 2000 years, indicating the great influence of thermal conditions in the TP on the variability of the SASM on a centennial timescale. This points to the important role that snow cover variations on the Tibetan Plateau might have played in inhibiting the formation of a strong monsoonal circulation in the subsequent summer during the last two millennia. Studies of modern observed meteorological records confirm that this relationship is also existent on annual and interannual timescales (Webster et al, 1998 and references therein).
It is exceptional that the widespread glacier expansion in the 19th century corresponded with a phase of intensive monsoon and thence high summer precipitation period, as indicated by the speleothem &O record. A possible reason to the question lies in the nature of the glacial advance evidence. In this paper, the timing for later LIA glacier activity was primarily based on ages of living trees or lichen ages, which provide minimum ages for moraine stabilization and fall into a time when the speleothem 8180 record suggests that monsoonal circulation was strengthening. The actual advance stage could predate these ages. Further investigation is required both in acquiring more precisely dated evidence for glacial advances and in understanding the climatological complexity of the speleothem 81O record of this special period.
Here it is noteworthy that an ice core net balance record from Dasuopu cap (28°N, 85°E) in the central Himalaya displays that the 19th century was the period of maximum snow accumulation during the last 600 years (Thompson et al., 2000; Duan et al., 2004), implying that the Dasuopu Glacier was also in an expansion state in the 19th century. Davis et al. (2005) examined the teleconnections between Tibetan ice cores and the North Atlantic and Asian monsoon circulation systems, and noted that the net accumulation values from the Dasuopu ice core were 30% higher than those of the north central India summer monsoon rainfall (NCISMR) during the 19th century. Since 1880 AD, the trends of the net accumulation time series and the NCISMR are similar. The increase in the net accumulation during the 19th century was possibly the result of increased winter snowfall in the central Himalayas, which might be correlated with lower temperatures in the North Atlantic (Davis et al., 2005). Therefore, besides the temperature decrease, the increase of winter snowfall was also responsible for the glacier expansion in the central Himalayas during the 19th century. To some extent, this explains why a slight temperature reduction of 0.1 °C during the 19th century corresponded with a widespread glacier advance on the southeastern Tibetan Plateau, as inferred from ELA depression of the Zepu Glacier. It can be inferred that the increased winter snowfall might have exerted some influence on the glacier advances that occurred in the southeastern Tibetan Plateau, in the central, eastern and southern Himalayas, and even in the Pamir Alay, Tien Shan and Altai Mountains.
The response pattern of glacier fluctuations to climatic change on a centennial timescale during the last two millennia seems to contradict with the hypothesis of Benn and Owen (1998). They emphasized that glacier advances culminated between 60 ka and 30 ka BP, corresponding to
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Late Holocene monsoonal temperate glacier Fluctuations...
