Improved understanding of climate influences on tree ring stable carbon isotope (δ13C) 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 δ13C series from 1800 to 2005 for trees in this extremely arid, high-elevation area. As expected, a significant decline in δ13C (of about 3.5‰) occurred from 1850 to 2005 in response to increasing atmospheric CO2 concentrations and decreasing atmospheric δ13C. High-frequency correlation analysis based on comparison of the tree ring δ13C chronology with recorded weather parameters revealed that mean temperature during the current growing season (April–August) most strongly influenced tree ring δ13C 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 δ13C from the climate signals. The optimal correction, which accounted for the decline in atmospheric δ13C (δ13Ccor) 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 δ13C 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.