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Chapter 06
Climate and Study of Glaciers
imply that such a simple relationship may not be valid [57,58]. A visco-elastic model [59] predicts extension when the Tibetan Plateau exceeded 75% of its present altitude. The Tibetan Plateau, with its large extent and high elevation, acts as a major driving force for the present principle atmospheric circulation patterns (Fig. 1). It is located at the intersection of the Southwest Asian Monsoon and the East Asian Monsoon that bring moisture to the Tibetan Plateau during summer. During winter, the climate is controlled by the dry Asian Winter Monsoon. The Westerlies bring only little precipitation (< 50 mm) to the northernmost part of northwestern China [32]. Although there is evidence for an overall weakening of the global land monsoon precipitation during the last 56 years [60], an increase in precipitation and surface temperatures has been detected for the past decades in China and adjacent areas [61-62]. Global warming has led to a significant retreat of glaciers on the Tibetan Plateau [63] and consequently to a high production of melt water that will be followed by a decrease in water availability of 20-40 % over the next 50-100 years [64]. These changes present a major challenge for future engineering efforts and environmental protection. The shrinking of glaciers and lakes will continue and will be accompanied by a series of severe ecological changes including a decrease in total agricultural output of 10% by 2030-50, and land use change, particularly overgrazing [64] (see also chapter 3.3). Large threats to human livelihood and health, for example, could therefore be caused by unanticipated durations of extreme monsoon behaviour. Today, strong variations in the Asian Monsoons cause flooding and crop failures that impact nearly two thirds of the world's population [65]. Failures in the summer monsoon during the past were considered to have caused, for example, the collapse of Neolithic cultures around the Central Plain of China [66]. Hsu [67] suggests that extreme drought and untimely frosts caused the famine and peasant uprising that led to the demise of the Mongols in the late 14th century and to the demise of the Ming Dynasty in the 17th century. The Tibetan Plateau is an ideal test area to study past monsoon history in order to understand the dynamics of this important component of the global climate system as well as its impact on landscape, ecosystems, and societies. Over time scales older than instrumental data, proxies from palaeoclimate archives may provide an excellent opportunity to understand the full range of monsoon variability and allow to deduce long-term climate trends. Strategically selected climate archives may be used as tools to disentangle the regional monsoon dynamics and histories through time. A major challenge for palaeoclimate research is the heterogeneity of climate information in space and time [68]. Overall, high-resolution climate proxy records from the Tibetan Plateau are scarce and of limited representativeness [69]. Ice core evidence shows that glaciation on the Tibetan Plateau occurred asynchronously and that variation in monsoonal precipitation is a stronger driver of glaciation than cooling [70]. In addition, the timing of maximum monsoonal precipitation during the Late Pleistocene and Holocene across the Tibetan Plateau gives a heterogeneous picture [71-74] and suggests that the regional expression of climate is the result of local topography, heating and uplift of air, and of the interaction between different airstreams. Since the Little Ice Age, and particularly during the 20th century, glaciers have been progressively retreating. This pattern is likely to continue throughout the 21st century, exacerbated by human-induced global warming.
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Tibetan Plateau: Formation-Climate-Ecosystems
