Paleobotanical records from two high-altitude (>3300 m) sites in Colorado show a clear and immediate response to the Younger Dryas climate oscillation. The Black Mountain Lake and Sky Pond records indicate that alpine timberline migrated upslope to near-modern elevations during the late Bølling-Allerød (13.6–12.9 ka). Subsequent declines in arboreal pollen percentages and accumulation rates during the Younger Dryas interval (12.9–11.7 ka) reflect a downslope displacement of the alpine timberline ecotone of 60–120 m in elevation. This change translates to a cooling of summer temperature by ∼0.4–0.9 °C and is consistent with proposed Younger Dryas advances of alpine glaciers in the Rocky Mountains to positions close to Little Ice Age maxima. Alpine timberline readvanced upslope to elevations above both sites between 11.7 and 11.4 ka. The concomitant response of temperature-sensitive alpine timberline vegetation in Colorado and late-glacial changes in North Atlantic thermohaline circulation implicates a rapid, widespread atmospheric transmission of the Younger Dryas climate oscillation.

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