Abstract
Summer maximum temperatures (Tmax) in the Sierra Nevada have risen rapidly since the turn of the 20th century, especially above 1,500 m where trends in the south exceed 3°C century 1. To place this warming into context, we developed a 504‐year reconstruction of growing‐season (April–September) Tmax (1520–2023 CE) from blue‐intensity and maximum latewood‐density data at nine high‐elevation conifer sites. The model explains 60% of instrumental variance (r = 0.77) and shows that the 20th–21st centuries were the warmest of the past five. The warmest year is 2021 (+2.38°C), while four of the five coldest years coincide with major volcanic eruptions. Since 1980, mean summer Tmax increased 1.14°C (p < 0.001; 0.026°C yr 1), concurrent with declining PDSI and a threefold rise in compound hot–dry‐fire years. Dynamic regression suggests a shift from snowpack‐buffered to temperature‐dominated soil‐moisture regimes after 1900. These results show that post‐1980 warming and unprecedented compound extremes mark a new era of temperature‐driven ecological vulnerability in the Sierra Nevada.