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Abstract:Although global warming over the past century has been confirmed, the response of different regions to it is still uncertain. We developed a tree-ring width chronology based on tree-ring samples from juniper trees from the Xiqing Mountains in the northeast Tibetan Plateau, the central headwater area of the Yellow River. Using this tree-ring chronology, the minimum winter half-year (October-April) temperature for the research area was reconstructed for the past 425 years. The reconstruction shows that temperature variability was minimal over past four centuries prior to the warming that began in 1941. During the 50 years from 1941 to 1990, the minimum temperature of the winter half-year increased 2.5°C. This degree of warming relative to the past 400 years suggests that the eastern Tibetan Plateau is highly sensitive to global warming.

Drought reconstruction from 1794 to 2003 in the eastern part of NW China was developed by calibrating tree rings of Picea wilsonii on Xinglong Mountain with the Palmer drought severity index (PDSI). The reconstruction explains 40% of the actual PDSI variance during the period from 1951 to 2003. The most significant drought period over the reconstructed time span occurred during the 1920s to 1930s. The drought reconstructions show a climate transition in 1921 on a decadal scale and significant cyclic patterns that were identified in intervals of 10.5, 7.9-8.3, ~3.5, 3.0-3.2, 2.7-2.8 and ~2 yr. This reconstruction is representative of regional drought patterns in eastern NW China and demonstrates that they are different from patterns in western NW China.

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A juniper (Juniperus przewalskii Kom) tree-ring width chronology has been developed from the westernmost forest of the Qilian Mountains. Our analyses demonstrate both temperature and precipitation have significant effects on tree growth and that both should be considered in climate reconstruction. Thus a regional drought history (A.D. 1855-2001) is reconstructed by calibrating with a linear interpolation through four Palmer Drought Severity Index (PDSI) grid values nearest the sampling site. Our reconstruction extends the drought history of this area and also reveals that the most severe drought occurred in the 1920s. In the context of the drought history of western China, this extreme drought between 1925-1931 is consistent over a large surrounding region of Northwestern China. Multi-taper spectral analysis reveals the existence of significant 40-to 46-year, 29-year, and 2.1-to 3-year periods of variability. Overall, our study provides reliable information for the research of past drought variability in the Qilian Mountains, Northwestern China.