The identification of past climatic extremes and norms is important for a better understanding of the climate systems and the way they change. Here we present an almost continuous tree-ring and climate record from Vancouver Island, Canada for the last four millennia from Douglas-fir trees (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) that are sensitive to precipitation variation. Spring droughts more severe than that of the mid-1920s occurred in the late 1840s, mid-1460s AD, and ∼ mid-1860s BC. A remarkable climatic anomaly occurred in ∼ the 19th century BC during which strong pentadecadal oscillation prevailed and radial growth decreased by 71% in four years. This event could have been the final stage in the process of climatic and environmental transition beginning 2–3 centuries earlier that led to major cultural transformation in regions sensitive to climate change.