We present a 483-year-long tree-ring chronology from a high-elevation Engelmann spruce stand in central Colorado. Over 800,000 density measurements produce a maximum latewood density (MXD) record, which together with tree-ring widths, is used to assess the climatic influences on radial tree growth. Variability in MXD is strongly related to local and regional August maximum temperatures (Tmax) for 127 years of overlapping instrumental climate data. A single-predictor reconstruction based on the MXD record explains > 50% of the variance in regional August Tmax is presented and considered robust for the period 1662-2021. The estimates of past temperature variability display phases of continuous cold conditions that are beyond the range of the instrumental record, including the 1830s. Our results further indicate that August Tmax have been higher since the early 1990s than for any other 30-year period in the past 350 years. This recent increase occurs on top of multi-centennial warming, and appears to be driven mainly by less frequent cold summers rather than an increase in warm extremes.

Department of Earth and Spatial Sciences University of Idaho Moscow ID USA

Department of Geography and Sustainability University of Tennessee Knoxville TN USA

Department of Geography Indiana University Bloomington IN USA

Department of Geography Johannes Gutenberg University Mainz Germany

Department of Geosciences University of Arkansas Fayetteville AR USA

Global Change Research Institute Czech Academy of Sciences Brno Czech Republic

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