A 1100-Year Blue-Ring Record Reveals Sub-Annual Cooling Events Hidden in Tree-Ring Width Chronologies

Tree-ring width (RW) records primarily capture low-frequency temperature variability, yet resolving high-frequency signals is critical for testing climate models and contextualizing modern extremes. Blue rings (BRs)—bands of unlignified cells revealed by micro-anatomical staining—capture short-lived cooling events. Using 83 Pinus longaeva cores, we present the first millennial-length (900–2014 CE) BR chronology and explore its paleoclimatic significance. BRs are largely decoupled from growth reductions: they often occur in rings of normal or above-average width, preceding RW minima by one year, recording abrupt late-season cooling that does not immediately suppress growth but frequently triggers reduced growth the following year. Events include anomalies associated with major volcanic eruptions, whose impacts are delayed and smoothed in RW chronologies. By capturing transient temperature declines with annual precision, BRs provide a novel dendroclimatic proxy that bridges low-frequency and sub-seasonal climate signals, offering benchmarks for climate-model evaluation and attribution under past and contemporary climate change.