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[1] Improved understanding of climate influences on tree ring stable carbon isotope (delta C-13) ratios for Qilian juniper (Sabina przewalskii Kom.) will improve prospects for long climate reconstructions in northwestern China’s Qaidam Basin, where weather stations are widely scattered with relatively short records. Here, we developed an annual-resolution delta C-13 series from 1800 to 2005 for trees in this extremely arid, high-elevation area. As expected, a significant decline in delta C-13 (of about 3.5 parts per thousand) occurred from 1850 to 2005 in response to increasing atmospheric CO2 concentrations and decreasing atmospheric delta C-13. High-frequency correlation analysis based on comparison of the tree ring delta C-13 chronology with recorded weather parameters revealed that mean temperature during the current growing season (April-August) most strongly influenced tree ring delta C-13 discrimination from 1956 to 2005. To clarify the climatic implications of the long-term trend, we systematically compared four previously published approaches to remove the effects of decreasing atmospheric delta C-13 from the climate signals. The optimal correction, which accounted for the decline in atmospheric delta C-13 (delta C-13(cor)) and for a discrimination rate of about 0.016% ppmv(-1) for the CO2 partial pressure, captured the strongest temperature signal (r = 0.75, P < 0.001). The historical mean April-August temperatures inferred from the correlations of tree ring delta C-13 with climate data revealed a persistent warming trend during the past two centuries, especially since the 1980s. Our results therefore reveal a high potential for reconstruction of growing season temperatures on a millennial scale in the northeastern Tibetan Plateau.