A 200 year temperature record from tree ring δ¹³C at the Qaidam Basin of the Tibetan Plateau after identifying the optimum method to correct for changing atmospheric CO₂ and δ¹³C

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 δ ¹³C (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 (δ¹³C_cor) and for a discrimination rate of about 0.016‰ ppmv^-1 for the CO ₂ 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.