Unprecedented recent late-summer warm extremes recorded in tree-ring density on the Tibetan Plateau

Tree-ring records have shown a significant upward trend of late summer temperatures over the Tibetan Plateau (TP) in recent decades. More detailed instrumental observations over the TP have also shown an increase in warm extremes, especially in the high-altitude area. It is not clear whether the recent increase in occurrence of warm extremes on the TP reflects an amplification of recent temperature fluctuations or it relates to the long-term climate trend. This study aims to address the above question by analyzing long-term late summer (August-September) temperature extremes over the TP using temperature reconstructions derived from tree-ring maximum latewood density. Our results show that the recent frequency of late summer warm extremes was unprecedented over the past four centuries, while the occurrence of cold extremes in the instrumental interval was minimal. However, after the removal of long-term trend, the frequencies of both warm and cold extremes in the instrumental period are actually smaller than the pre-instrumental period, indicating a decreased variability of late summer temperatures. Thus, we conclude that the recent observed increase in warm extremes is related to the long-term warming trend, rather than an amplification of temperature variability. This finding implies that the persistent warming on the TP in the future might trigger much more frequent warm extremes with potential ecological and environmental effects.