Maximum summer temperatures for the Western Tianshan Mountains of China inferred from tree rings over the past 300 years

    The trend of global climate warming is strengthening. During the 20^th century, climate warming has increased more than it was at any other time in history, seriously affecting water security and food supply. With global climate warming, extreme temperature events have occurred continuously in many regions of the world and have seriously damaged the ecological environment and human health. In this context, it is crucial to strengthen the understanding of regional extreme temperatures, analyze their changing characteristics and understand their impact on global climate warming. As one of the regions most affected by climate warming, the Tianshan Mountains has suffered several ecological crises, including retreating glaciers and water deficits. The climate warming in the Tianshan Mountains is considered to be mostly caused by increases in minimum temperature and winter temperature, while the influence of the maximum temperature is unclear. Here, a 300-year tree-ring chronology developed from Western Tianshan Mountains, China was used to reconstruct the summer (June–August) maximum temperature (T_max6-8) variations from 1718 to 2017. The reconstruction explained 53.1% of the variance in the observational records. Over the past 300 years, the T_max6-8 reconstruction showed obvious interannual and decadal variabilities and experienced roughly six warm periods (1723–1732 AD, 1768–1785 AD, 1818–1834 AD, 1841–1898 AD, 1911–1929 AD and 1973–2012 AD) and five cold periods (1733–1767 AD, 1786–1817 AD, 1835–1840 AD, 1899–1910 AD, and 1930–1972 AD). Our reconstructed T_max6-8 showed a significant warming trend (0.183℃/decade) after the 1950s, which was close to the increasing rates of the minimum temperature and mean temperature. We found that this significant warming of maximum temperature was also present in the whole Tianshan Mountains. The impact of the maximum temperature on climate warming has increased and cannot be ignored. Our reconstruction was found to be reliable and representative according to spatial correlation analysis. Additionally, the extremely cold years in the T_max6-8 reconstruction were due to the cooling effect of strong volcanic eruptions. The reconstructed T_max6-8 series was positively correlated with solar activity and negatively correlated with the summer North Atlantic Oscillation (SNAO) index. Combined with the periodic analysis, these results demonstrated that the T_max6-8 variations in the Western Tianshan Mountains, China was influenced by volcanic eruptions at high frequency and synergistically influenced by solar activity at low frequency. This study revealed the significant influence of maximum temperature variability on global climate warming and clarified the climate mechanism, which will aid in future climate change prediction.