Projected warming of global surface air temperatures will further exacerbate droughts, wildfires, and other agents of ecosystem stress. We use latewood blue intensity from high‐elevation Picea engelmannii to reconstruct late‐summer maximum air temperature for the Greater Yellowstone Ecoregion (GYE) spanning 770–2019 CE. Using a robust regression model (r2 = 0.60), the 1,250‐year reconstruction reveals 2016 as the single‐warmest year and the warming trend since ca. 2000 as the most intense. The Medieval Climate Anomaly contained the highest‐ranking warm event (1050–1070 CE) and was characterized by substantial multidecadal variability rather than a period of prolonged, homogeneous warming. We document regional expression of past warm and cool events, such as an anomalously warm period spanning the fifteenth to sixteenth centuries, and the Maunder and Dalton minima of the Little Ice Age. Summer temperature variability across the GYE shows multicentennial agreement with trends in solar irradiance, volcanic activity, snowpack, and other regional‐to‐hemispheric temperature records.

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Paleoclimate reconstructions for the western US show spatial variability in the timing, duration, and magnitude of climate changes within the Medieval Climate Anomaly (MCA, ca. 900–1350 CE) and Little Ice Age (LIA, ca. 1350–1850 CE), indicating that additional data are needed to more completely characterize late-Holocene climate change in the region. Here, we use dendrochronology to investigate how climate changes during the MCA and LIA affected a treeline, whitebark pine ( Pinus albicaulis Engelm.) ecosystem in the Greater Yellowstone Ecoregion (GYE). We present two new millennial-length tree-ring chronologies and multiple lines of tree-ring evidence from living and remnant whitebark pine and Engelmann spruce ( Picea engelmannii Parry ex. Engelm.) trees, including patterns of establishment and mortality; changes in tree growth; frost rings; and blue-intensity-based, reconstructed summer temperatures, to highlight the terminus of the LIA as one of the coldest periods of the last millennium for the GYE. Patterns of tree establishment and mortality indicate conditions favorable to recruitment during the latter half of the MCA and climate-induced mortality of trees during the middle-to-late LIA. These patterns correspond with decreased growth, frost damage, and reconstructed cooler temperature anomalies for the 1800–1850 CE period. Results provide important insight into how past climate change affected important GYE ecosystems and highlight the value of using multiple lines of proxy evidence, along with climate reconstructions of high spatial resolution, to better describe spatial and temporal variability in MCA and LIA climate and the ecological influence of climate change.

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